chapter 2 literature review -...
TRANSCRIPT
Chapter 2
Literature review
2.1 Automotive emissions
Automotive exhaust gas emission is one of the main sources of air pollution in cities
(Sartipi et al., 2008) and contributes significantly to the air pollution (Labhsetwar et al., 2006;
Pundir, 2007). Air pollution generated from mobile sources such as automobiles is a problem
of general interest (Kaspar et al., 2003). Emissions from automobiles were identified first time
as a major culfit for urban air pollution in California during 1950s. The automobiles
contribute toward air pollution was first recognized by Prof. Hazen Smith who discovered that
the two vehicular emissions namely HC and NOx were responsible for the famous Los
in the Los Angeles area because of the photochemical reactions between HC and NOx that
lead to the formation of secondary pollutants including O3, PAN, NO2 and other oxidants
(Wei, 1975; De Nevers, 2000; Pundir, 2007; Rao and Rao, 2007).
Today, automobiles are the most popular mode of transportation in our day to day life
all over the world. In the year of 2011, a total of 80 million cars and commercial vehicles
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were produced worldwide led by China with 18.4 million motor vehicles manufactured
followed by the U.S. with 8.6 million and Japan with 8.4 million (Website:
http://en.wikipedia.org/wiki/Motor_vehicle). Initially, the problem of vehicular air pollution
was limited to the U.S. and European countries. Due to increase in vehicle population all over
the world, automotive emissions control are now placed in most of the countries and are being
gradually strengthen (Uenishi et al., 2005b; Pundir, 2007; Bera and Hegde, 2010).
to be used. Commonly used fuels are gasoline and diesel. Various fuels like natural gas,
liquefied petroleum gas (LPG), compressed natural gas (CNG), methanol, ethanol, hydrogen
and biodiesel are also used as alternative fuels in vehicles. There are two type of engines run
on road: spark-ignition (SI) and compression-ignition (CI) engines (Heywood, 1988; Neeft et
al., 1996; Koltsakis et al., 1997; Srinivasan, 2001; Gupta, 2006; Bera and Hegde, 2010).
Spark-ignition (gasoline) engines work either on two-stroke (the cycle is completed in two-
strokes of the piston) or four-stroke design (the cycle is completed in four-strokes: (1) suction
(2) compression (3) expansion or power and (4) exhaust strokes of the piston). Compression-
ignition (diesel) engines work on four-stroke design (Srinivasan, 2001; Gupta, 2006; Pundir,
2007; Bera and Hegde, 2010). There are three main types of automotive vehicles being used
in market are: (1) passenger cars powered by four-stroke gasoline engines (2) motor cycles,
scooters and auto-rickshaws powdered by two-stroke gasoline engines and (3) large buses
and trucks powered by 4-stroke diesel engines (Kaspar et al., 2003; Rao and Rao, 2007; Bera
and Hegde, 2010).
Vehicular transportation use the fuels derived from crude oil such as gasoline, diesel,
natural gas etc. as a source of energy. When fuel burns inside the engine in the presence of air,
large amount of chemical energy is released and transformed into thermal energy. This
thermal energy is converted to mechanical work (power) that run the vehicles (Srinivasan,
2001; Kaspar et al., 2003; Gupta, 2006; Rao and Rao, 2007; Bera and Hegde, 2010). Ideally,
an engine operates at stochiometric air to fuel ratio i.e. about 14.7 on weight basis for gasoline
engine and completes the combustion process to carbon dioxide, water vapor and nitrogen
(equation 2.1) (Wei, 1975; Neeft et al., 1996; Kaspar et al., 2003; Prasad and Singh, 2012).
2C8H17 (gasoline) + 24.5O2 2 + 17H2O + heat (2.1)
The combustion process is never complete so that exhaust contains poisonous gases such as
CO (equation 2.2), HC, NOx and PM which are released through the tail pipe of the vehicle to
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the atmosphere (Wei, 1975; Neeft et al., 1996; De Nevers, 2000; Heck and Farrauto, 2001;
Srinivasan, 2001; Kaspar et al., 2003; Bera and Hegde, 2010; Prasad and Singh, 2012).
2C8H17 (Gasoline) + 24.5O2 2 + 17H2O + 2CO + O2 + heat (2.2)
More than 50% of the man-made emissions of CO, HC and NOx are due to
automobiles (Wei, 1975; Prasad and Singh, 2012).
2.2 Formation of CO and their adverse effects
The formation of engine exhaust emissions can be understood by combustion process
and chemistry of combustion. The sources of exhaust emissions in a conventional spark-
ignition engine are shown schematically in Fig. 2.1.
Fig. 2.1. Sources of pollutants formation in spark-ignition engines (Pundir, 2007).
Air and fuel are mixed before introducing into the cylinder using fuel injection system.
The air to fuel ratio is maintained constant at stochiometric i.e. about 14.7 (weight basis). The
mixture is compressed in the cylinder and ignited by the spark plug (Heywood, 1988; Neeft et
al., 1996; De Nevers, 2000; Gupta, 2006; Pundir, 2007). After the spark ignites the
compressed fuel air mixture in a combustion chamber, the flame propagates across the
combustion chamber and burns the mixture in the engine cylinder. CO and nitric oxide (NO)
are formed during combustion and post combustion reactions (De Nevers, 2000; Pundir,
2007). CO is a product of incomplete combustion which occurs when carbon in fuel is
partially oxidized (due to unavailability of O2 during combustion) instead of completely
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oxidized to CO2 (Heck and Farrauto, 2001; Srinivasan, 2001; Pundir, 2007; Prasad and
Pratichi, 2012). NO is formed due to oxidation of N2 present in inducted air when high
temperature attains in combustion chamber. NOx consists of mainly NO and relatively small
amount of NO2 in the spark-ignition engines (Pundir, 2007). As the flame approaches the cold
combustion chamber walls, it is quenched leaving a very thin layer of the unburned fuel air
mixture. During the compression stroke, fuel-air mixture enter to the narrow passages
between piston crown and cylinder wall, around spark plug and cylinder head gasket called as
stion chamber. The flame is not able to propagate the mixture in the
cervices and left mixture unburned. HC emission results from flame quenching at the
combustion chamber walls, cervices and from adsorption and desorption in lubricating oil
film and combustion chamber deposits (Wei, 1975; Acres, 1996; Fritz and Pitchon, 1997;
Heck and Farrauto, 2001; Srinivasan, 2001; Pundir, 2007).
During the expansion stroke, piston travels downwards and rapidly cool the
combustion products by expansion that freezing the reactions of CO and NO formation (De
Nevers, 2000; Pundir, 2007). The fuel-air mixture being rich during cold start, engine warm
up and transients like acceleration, engine operation and these modes contribute significantly
to CO emission (Heck and Farrauto, 2001; Pundir, 2007). At the time of starting the engine,
CO emission is high. When speed increases, the percentage of CO emission decreases. At
steady speeds, CO emission also decreases. Even when lean mixture is used, some amount of
CO always present in the exhaust (Srinivasan, 2001). Finally, the exhaust valve(s) open and
the combustion products along with CO, NOx and part of the entrained unburned
hydrocarbons form quench layers and cervices leave the engine and are exhausted (Heywood,
1988; Pundir, 2007). NOx is formed within the combustion chamber and also in the engine
exhaust. At relatively high temperature, N2 and O2 react and form NO and NO2. NOx are also
formed when NO combines with NO2 and this formation occurs only in the engine exhaust
(Srinivasan, 2001).
Emissions from automobiles are generally classified as: (1) Exhaust emissions - The
tail pipe of vehicle releases exhaust gases along with the pollutants to the atmosphere. It may
constitute around 60% of total emissions (2) Crank case emissions (also called running loss
emissions) - These are unburned or partially burned fuel components that under the pressure
escape from the combustion chamber pass the piston and enter to the crankcase. This mixture
is called blow-by. It emits blow-by gases and fuel vapors into the atmosphere. It may
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constitute 13-25% of total emissions and (3) Evaporative emissions - The fuel tank and fuel
lines emit fuel (HC) vapors into the atmosphere due to volatile nature of fuel. The emissions
depend on fuel composition, engine operating temperature and ambient temperature. It may
constitute 20-32% of total emissions (Acres, 1996; De Nevers, 2000; Srinivasan, 2001;
Bartholomew and Farrauto, 2006; Rao and Rao, 2007). CO emission takes place only through
the engine exhaust (Srinivasan, 2001; Zhu et al., 2005; Pundir, 2007). The emissions from
gasoline vehicles are reported in Table 2.1.
Rao and Rao (2007) reported that diesel powered vehicles create relatively less
pollution problems compared to gasoline powdered vehicles. Crankcase emissions are
negligible in case of diesel engines because the cylinder contains only air during compression
stroke. Due to low volatility of diesel and use of closed injection fuel system, evaporative
emissions are also non-significant in case of diesel engines (Rao and Rao, 2007).
Table 2.1 Emissions from gasoline vehicles (Prasad and Pratichi, 2012)
Sr. No. Source Amount of emissions (%)
Four-stroke Two-stroke
1 Crankcase (blow-by) emissions 20 -
2 Evaporative emissions 20 3
3 Exhaust emissions 60 97
Engine exhaust composition depends on a variety of factors such as: types of
engine (e.g. two or four-stroke, spark or compression-ignition), driving conditions (e.g. urban
or extra urban) and vehicle speed (e.g. acceleration, deceleration, ideal and constant speed
cruising) (Silveston, 1995; Heck and Farrauto, 2001; Kaspar, 2003; Rao and Rao, 2007; Bera
and Hegde, 2010). Air to fuel ratio (A/F) is one of the most important parameter that
influences the engine exhaust emissions (Wei, 1975; Kaspar, 2003; Pundir, 2007). A/F is
generally defined as:
engine by the consumed fuel of massengine by the consumedair of mass
=F
A
(2.3)
a and is defined as
A/F engine tricstoichiome
A/F engine actual=al
(2.4)
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The stoichiometric value of A/F ratio is 14.7 (weight basis) for gasoline engines (De Nevers,
2000; Bera and Hegde, 2010).
In actual practice, an engine is operated at fuel rich conditions gives the highest power
output by consuming high fuel (if the A/F ratio is below 14.7) and the exhaust gas contains
more reducing agents like CO and HC. On the other hand, it is also operated at fuel lean
condition (if A/F ratio exceed 14.7) and the exhaust gas contains more oxidizing agents like
NO and O2 (Wei, 1975; Kaspar, 2003; Rao and Rao, 2007; Bera and Hegde, 2010). The
emission levels of CO, HC and NOx are as a function of fuel composition, engine type and the
power/load conditions on the engine (Acres, 1996).
The vehicle operation is divided into four modes or driving cycles: (1) Idle/Start mode
when the engine of vehicle has been started. It is yet stationary. At this stage, there is high
concentration of CO and HC and very low level of NOx concentration (ii) Acceleration the
emission of CO and HC decrease but at the same time NOx concentration levels may increase
(iii) Cruise/Steady mode steady speed produces low concentration of CO and HC but high
concentration of NOx and (iv) Deceleration (a) free and (b) while applying brake slow speed
released more pollutants (Pundir, 2007; Prasad and Singh, 2012). Table 2.2 reports the typical
compositions of exhaust gases for different types of engines.
The main pollutants emitted by spark-ignition (gasoline) engines are CO, HC and NOx
(collectively NO and small amount of NO2) in addition to other compounds such as SO2,
acetaldehyde (CH3CHO), CO2, H2O, smoke and PM etc. The principal pollutants emitted by
compression-ignition (diesel) engines are NOx, PM, CO and HC in addition to other
compounds such as SO2, CH3CHO, CO2, H2O, smoke. Thermal efficiency of diesel-powered
vehicles is high and due to that it emits less amount of CO, CH3CHO and HC than gasoline-
fueled vehicles but the higher amount of PM (soot particulates) and NOx are emitted (Acres,
1996; Koltsakis et al., 1997; Srinivasan, 2001; Fino et al., 2003a, 2003b; Kaspar, 2003; Milt
et al., 2005; Russo et al., 2005; Fino et al., 2006b, 2007; Fino and Specchia, 2008; Liu et al.,
2008; Bera and Hegde; 2010).
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Table 2.2 Example of exhaust conditions for two and four-stroke, diesel and lean four-stroke engines (Kaspar, 2003)
Exhaust components
and conditionsa
Diesel engine Four-stroke spark ignited
engine
Four-stroke lean burn
spark ignited engine
Two-stroke spark ignited
engine
NOx 350 1000 ppm 100-4000 ppm 100-200 ppm
HC 50-330 ppm C 500-5000 ppm C 20,000-30,000 ppm C
CO 300-1200 ppm 0.1-6% 1-3%
O2 10-15% 0.2-2% 4-12% 0.2-2%
H2O 1.4-7% 10-12% 12% 10-12%
CO2 7% 10-13.5% 11% 10-13%
SOx 10-100 ppmb 15-60 ppm 20 ppm
PM 65 mg/m3 - - -
Temperature (Test cycle)
RT - 650 0C
(RT - 420 0C)
RT - 1100 0Cc RT- 850 0C RT - 1000 0C
GHSV (h-1) 30,000-100,000
30,000-100,000 30,000-100,000
30,000-100,000
a (A/F)d (14.7) e
RT stands for room temperature a N2 is remainder. b For comparison: diesel fuels with 500 ppm of sulfur produce about 20 ppm of SO2 c Close-coupled catalyst. d a defined as ratio of actual A/F to stoichiometric A/F, a = 1 at stoichiometry (A/F = 14.7). e Part of the fuel is used for scavenging of the exhaust which does not allow to define a precise definition of the A/F.
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CO is highly toxic and unwanted gas in the environment (Wang and Zhong, 2010;
Prasad and Pratichi, 2012). CO is a colorless, odorless gas and slightly denser than air
(Srinivasan, 2001; Pundir, 2007; Bera and Hegde, 2010; Royer and Duprez, 2011; Prasad and
Pratichi, 2012). On inhalation, it is absorbed by lungs and combines with hemoglobin in the
blood forming carboxy-hemoglobin. The carboxy-hemoglobin complex is more stable than
oxy-hemoglobin because CO has 210 times more affinity to combine with hemoglobin in the
blood than the oxygen. Thus, exposure to CO reduces oxygen carrying capacity of the blood
and it will not reach to the various part of the body that damage tissue, cells and also affected
rapidly to the brain and nervous system (Wei, 1975; Pundir, 2007; Bera and Hegde, 2010;
Royer and Duprez, 2011; Prasad and Pratichi, 2012). The symptoms of rapid breathing,
headache, dizziness, confusion etc. indicate the poisoning effect of CO. These signs and
symptoms result due to supply of less amount of oxygen to brain tissue a condition called
hypoxia. Nausea, vomiting and diarrhea may also appear later on. CO becomes life
threatening if exposure of CO is very high. Exposure to high CO concentrations or for a
longer period may cause cardiac arrest (heart disease), pulmonary edema, loss of
consciousness, damage the nervous system and finally cause death. High concentration of CO
will cause illness to animals and may cause low birthrates in pregnant women. It affects the
N2 fixation ability of bacteria, causes leaf curling, reduction in leaf size and chlorophyll with
premature ageing etc. (Pundir, 2000; Rao and Rao, 2007; Prasad and Pratichi, 2012). In
addition, CO increases the amount of greenhouse gas by oxidizes into CO2 which is
responsible for global warming (Pundir, 2007; Prasad and Pratichi, 2012).
2.3 Automotive CO emission control technology
Environmental, ecological and health concern result in increasing stringent norms for
pollutants emissions from the vehicles (Neeft et al., 1996; Fino et al., 2006b; Millet et al.,
2009; Bera and Hegde, 2010). Several developed and later developing countries have adopted
legislation for the control of automotive exhaust gas emissions to reduce air pollution. Each of
these countries has emission standards which set specific limits to the amounts of pollutants
that could be released into the environment for different types of vehicles according to their
(Srinivasan, 2001; Bera and Hegde, 2010). The developed countries
like U.S., Europe and Japan have set their own emission standards (Neeft et al., 1996;
Srinivasan, 2001, Wang et al., 2007b). India set their emission standards based on European
(Srinivasan,
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2001). As these regulations imposed, the automakers have to reduce or control the emissions
from the vehicles (Fritz and Pitchon, 1997). These regulations require the need for more
active and durable emission control system. The automotive emissions can be controlled at
three stages: (i) Stage 1 or pre combustion stage where the quality of fuel can be improved by
using alternative fuels or by adding fuel additives (ii) Stage 2 or combustion stage
(combustion improvement which is aimed at lowering their formation) where engine
modification, combustion of lean air-fuel mixture, multistage injection of fuel, exhaust gas
recirculation (EGR) and loading (injection) of additional water into cylinder etc. are needed
and (iii) Stage 3 or post combustion stage (post treatment which is aimed at reducing their
emission) where exhaust treatment device like catalytic converter is required (Acres, 1996;
Postrzednik et al., 2004; Peng et al., 2006).
Vehicles are fitted with various emission control system which are positive crank case
ventilation (PCV) system, carbon canister, EGR and catalytic converter to abate automotive
emissions (De Nevers, 2000; Srinivasan, 2001; Rao and Rao, 2007). Peng et al. (2006)
reported that the improvement in combustion reduces the formation of pollutants which might
not be sufficient to meet the more stringent emission standards expected in the future. So
exhaust post-treatment based on catalytic converter which reduces the emissions should be
adopted. This satisfies the regulation for reduction of the CO, HC, NOx and PM emissions
(Neeft et al., 1996; Fino et al., 2003c; Postrzednik et al., 2004; Fino et al., 2006b, 2007; Bera
and Hegde, 2010).
Catalytic converters have been used for automotive emissions control since 1970s
(Nishihata, 2002; Screen, 2007; Sartipi, 2008; Khanfekr et al., 2009). The catalytic converter
(colloquially called catcon) is a device installed in the engine exhaust system of vehicles that
removes 90% of the poisonous gases (CO, HC and NOx) to harmless gases (Mouza et al.,
1995; Fabbrini et al., 2003; Matsumoto, 2004). It was first invented by Eugene Houdry, a
French Mechanical Engineer and later on further developed by John J. Mooney and Carl D.
Keith at the Engelhard Corporation (Bera and Hegde, 2010). The catalytic converters are
developed in the U.S. in the year of 1970s and in Japan in the year of 1990s (Royer and
Duprez, 2011). It was first time used in automobiles of U.S. in 1975 to satisfy the emission
regulations imposed by Environmental Protection Agency (EPA) (Matsumoto, 2004).
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The catalyst is the heart of catalytic converter and responsible for treating poisonous
gases (Bartholomew and Farrauto, 2006; Labhsetwar et al., 2006; Pundir, 2007). Several types
of materials used as catalysts that promote the desire reactions to convert poisonous gases to
harmless gases at low temperature. The catalytic materials used for CO oxidation are base
metals (Cu, Mn, Cr, Co, Ni and Fe etc.), oxides of base metals (Cu, Mn, Cr, Co, Ni, V and Fe
etc.), mixture of oxides, noble metals (synonymous to precious metals or platinum group
metals include silver, gold, ruthenium, rhodium, palladium, osmium, iridium and platinum),
supported noble metals, spinel structure materials (copper chromite, manganese cobaltites,
cobalt magnetite etc.), perovskite structures (mixed oxides), alloys and zeolites etc. (Wei,
1975; Bartholomew and Farrauto, 2006; Labhsetwar et al., 2006; Zhang et al., 2006a; Cheng
et al., 2007; Pundir, 2007; Zhang et al., 2008a; Bera and Hegde, 2010; Royer and Duprez,
2011; Biabani-Ravandia and Rezaei, 2012; Prasad and Pratichi, 2012).
Base metal catalysts show a lower catalytic activity compared to noble metals but still
exhibit sufficient activity as oxidation catalysts. Their main advantage of using base metals as
CO oxidation catalysts compared to noble metals is their lower costs (Zhang-Steenwinkel et
al., 2002; Sui et al., 2011).
Copper, iron, manganese and nickel are used as base metal catalysts for CO oxidation
although each has its own limitations. Nickel reacts with CO and forms hazardous nickel tetra
carbonyl (Ni(CO)4) which inhibits the legal use of Ni in the European Union. Copper can be
used all over the world except North America where its use is illegal because of the formation
of dioxin (Website:https://en.wikipedia.org/wiki/Catalytic_converter). Transition (base) metal
oxides (CuO, NiO, Cr2O3, MnO2, Co2O3, Co3O4 and Fe2O3 etc.) are not expensive and are
used for CO oxidation (Cheng et al., 2007; Biabani-Ravandia and Rezaei, 2012). Relative CO
oxidation activities at 300 0C of noble metals are compared to those for base metal oxides in
Table 2.3. It shows that the precious metals are considerably more active than the base metal
oxides and perovskites and that activity depends on the species to be catalyzed (Bartholomew
and Farrauto, 2006; Royer and Duprez, 2011).
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Table 2.3 Relative activities of noble metal, base metal oxide and perovskite catalysts for
CO oxidation at 300 0C (Bartholomew and Farrauto, 2006; Royer and Duprez, 2011).
Catalyst Relative activity Catalyst Relative activity
Au 15 CuO/Cr2O3 40
Pd 500 NiO 0.013
Pt 100 MnO2 4.4
Co2O3 80 Fe2O3 0.4
CuO 45 Cr2O3 0.03
LaCoO3 35
Reaction conditions : 1% CO in O2 excess
Among the base metal oxides, Co, Fe and Cu-based oxides present an interesting
catalytic activity for CO oxidation (Cheng et al., 2007; Royer and Duprez, 2011; Biabani-
Ravandia and Rezaei, 2012). Biabani-Ravandia and Rezaei (2012) reported that iron-cobalt
mixed oxide (Co/Fe = 1/1) shows 100% CO oxidation at low temperature below 100 0C due
to synergistic effect of the composite oxides. Due to the synergistic effect, the composite
oxides of transition metal exhibit higher catalytic activity for CO oxidation than those of
individual transition metal and their oxides (Cheng et al., 2007). Cheng et al. (2007) reported
that copper iron composite oxide catalysts prepared by co-precipitation method after
calcination at 300 0C exhibit 100% CO conversion below 100 0C. Copper chromite, mixture of
oxides of copper and chromium, is also active in oxidation of CO and HC as well as in the
reduction of NOx in the presence of CO (Wei, 1975; Prasad and Pratichi, 2012).
Base metal oxides or mixture of oxides exhibit good catalytic activity for CO
oxidation but at the same time they sinter and deactivate when subjected to high-end exhaust
gas temperature of conventional spark-ignition engine (Wei, 1975; Bartholomew and
Farrauto, 2006; Punidr, 2007; Biabani-Ravandia and Rezaei, 2012). Kirchnerova et al. (2002),
Liotta et al. (2005) and Biabani-Ravandia and Rezaei (2012) reported that the Co3O4 exhibits
good catalytic activity for CO oxidation and CH4 combustion but in most of the cases their
thermal stability is lower even at less than 727 0C than that of the noble metals. Since the
24
beginning of the 20th century, oxides such as Hopcalitie (binary amorphous Mn Cu oxide)
exhibits CO oxidation activity at room temperature but deactivates in the presence of water
and heat make its durability less (Wei, 1975; Royer and Duprez, 2011) If base metal oxides
expose to high temperature greater than 700 0C in presence of air, most of the base metal
oxides react (solid-state transformations) with the support materials and form less active
phase as well as the active surface area reduce as a result of sintering. For example, reaction
between the metal oxide (CuO) and Al2O3 support forms an inactive aluminates causing
irreversible deactivation.
CuO + Al2O3 CuAl2O4 (2.5)
Their catalytic activity is inhibited by SO2 resulting from sulphur in the fuel. Exposure of base
metals to small amounts of SO2 and/or SO3 present in the exhaust causes formation of
catalytically inactive sulfates which deactivate the catalysts.
CuO + SO3 CuSO4 (2.6)
There is no base metal oxide catalysts available having acceptable thermal stability and
resistance to SO2 poisoning (Bartholomew and Farrauto, 2006; Pundir, 2007).
Due to sintering and poisoning problems faced by base metal oxides, its catalytic
activity substantially decreases so it would require larger volume of reactors. It requires more
space for reactor and also increases the weight of emission control system which limits their
use because space and weight are important in automotive design. Base metal oxides have
high thermal inertia so they require longer time to heat up and also to achieve light-off
(operating) temperature (Pundir, 2007; Royer and Duprez, 2011). Finally, the main reasons
for not using the base metal oxides as catalytic converter formulations are their undesirable
behavior in cycled transient conditions and high susceptibility to deactivation by sulfur and
water (Cheng et al., 2007; Royer and Duprez, 2011; Biabani-Ravandia and Rezaei, 2012).
Precious metals have high catalytic activity and stability for CO oxidation at low
temperature (Cheng et al., 2007; Biabani-Ravandia and Rezaei, 2012; Prasad and Pratichi,
2012). However, high cost, need for complex pretreatment and limited availability of precious
metals search for the substitutes. Even though in practice only more stable, poison resistant
precious metals are used as active catalytic material for both oxidation of CO and HC and
reduction of NOx as they have:
¶ High specific activity
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¶ High resistance to thermal degradation
¶ Superior cold start performance and
¶ Low deactivation caused by fuel sulfur (Wei, 1975; Mouza et al., 1995; Shelef and
McCabe, 2000; Bartholomew and Farrauto, 2006; Labhsetwar et al., 2006; Pundir,
2007; Bera and Hegde, 2010).
The activity of platinum group metals inhibit due to the presence of sulfur compounds which
effect is quite smaller than the base metals.
Platinum group metals are used as oxidation catalysts for oxidation of carbon
monoxide, methane and olefins for application in exhaust gas clean up (Song et al., 1999;
Zhang-Steenwinkel et al., 2002). They show higher catalytic activity for CO oxidation even at
lower temperature because they are efficient materials for promoting oxygen dissociation at
lower temperature (Wang and Zhong, 2010). Wang and Zhong (2010) reported that platinum
group metals catalysts for example gold (Au) exhibits 100% CO conversion at ambient
temperature.
The precious noble metals platinum (Pt), palladium (Pd) and rhodium (Rh) are used in
catalytic converters that convert CO into CO2, NOx to N2 and O2 and HC into CO2 and H2O in
the (Andrew, 1976; Guilhaume et al., 1996; Fritz and
Pitchon, 1997; Belton and Taylor, 1999; Heck and Farrauto, 2001; Srinivasan, 2001;
Nishihata, 2002; Kaspar et al., 2004; Labhsetwar et al., 2006; Shinjoh, 2006; Tanaka et al.,
2006; Screen, 2007; Saetipi et al., 2008; Khanfekr et al., 2009; Bera and Hegde, 2010).
Catalytic converters based on platinum group metals are work effectively only under normal
working conditions for a stochiometric A/F ratio within a narrow operating window of about
±0.05 around the A/F ratio of about 14.7 (wt basis) (Wei, 1975, Andrew, 1976; Fritz and
Pitchon, 1997; Shelef and McCabe, 2000; Srinivasan, 2001; Shinjoh, 2006; Bera and Hegde,
2010).
The pollutants present in exhaust having either oxidizing or reducing agents so it is
necessary to simultaneously carry out both reduction and oxidation reactions over the exhaust
catalysts. Some of these reactions are summarized in Table 2.4 which indicates only desirable
reactions. Apart from desirable reaction, many undesirable reactions like reduction of NOx to
ammonia (NH3), partial oxidation of HC to methanol (CH3OH) and other toxic compounds
etc. occur (Kaspar et al., 2003).
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Table 2.4 Reactions occurring on the automotive exhaust catalysts (Silveston, 1995;
Koltsakis et al., 1997; Heck and Farrauto, 2001; Kaspar et al., 2003; Labhsetwar et al.,
2006; Bera and Hegde, 2010; Acres, 1996)
Oxidation: 2CO + O2 2CO2
H2 + ½ O2 H2O
CO2 + H2 2 + H2
HC + O2 CO2 + H2Oa
Reduction: 2CO + 2NO 2CO2 + N2
HC + NO CO2 + H2O + N2a
2H2 + 2NO 2H2O + N2
Water gas shift (WGS): CO + H2O CO2 + H2
Steam reforming (SR): HC + H2O CO2 + H2a
a unbalanced reaction
All above reactions are required minimum temperature on the catalyst surface for the
reaction to occur. When the vehicle starts first time, engine and catalyst are cold. After
startup, the heat of combustion is transferred from the engine to the exhaust pipe and begins to
heat up the pipe. Finally, temperature is attained by the catalyst bed and catalytic reactions
begin. Typically, the CO reaction begins first followed by the HC and NOx reaction (Heck
and Farrauto, 2001). The technology that simultaneously convert all three pollutants (CO, HC
and NOx) into harmless compounds is referred to as three way catalysis (He et al., 2001; Heck
and Farrauto, 2001; Fabbrini et al., 2003; Kaspar, 2003; Giannakas et al., 2006; Ozawa and
Urashima, 2006; Screen, 2007; Khanfekr et al., 2009). The diagram of catalytic converter and
metallic honeycomb is shown in Figs. 2.2(a) and 2.2(b) respectively. A typical automotive
catalyst structure design is also shown in Fig. 2.3.
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Fig. 2.2(a). Diagram of catalytic converter (Kaspar et al., 2003).
Fig. 2.2(b). Metallic honeycomb (Kaspar et al., 2003).
Fig. 2.3. Automotive catalyst structural design [Heck and Farrauto, 2001].
28
Heck and Farrauto (2001) reported that by installing catalytic converter based on
noble metals removes 800 million tons of combined pollutants of HC, CO and NOx.
The catalytic converters are the reactors that consist of monolith honeycombs structure
made of ceramic usually made of cordierite (magnesium alumina silicate) or metallic
materials (chromium-nickel stainless steel). The Monolith honeycomb structure is most
widely used because it provides high geometric surface area with lower pressure drop, low
cost, lighter weight, excellent high temperature and thermal shock resistance, mass and heat
transfer in the thin catalyst layer is fast and allows high conversion efficiencies (Bardhan et
al., 1997; Farrauto and Heck, 1999; Schneider et al., 2000; Shelef and McCabe, 2000; Szabo
et al., 2003b; Labhsetwar et al., 2006; Tomasic and Jovie, 2006). Monolith is first wash
-Al2O3 (70-80%) modified by the use of oxide stabilizers i.e.
La, Si, Ba, etc.(10-20%) to maintain high surface area after experiencing high exhaust gas
temperature up to 1100 0C in presence of water vapor. After wash coating, the three way
catalyst Pt, Pd and other platinum group metals usually called active phase (10%) is coated
over monolith (Farrauto and Heck, 1999; Shelef and McCabe, 2000; Srinivasan, 2001;
Fabbrini et al., 2003; Labhsetwar et al., 2006; Shinjoh, 2006; Sharma et al., 2008; Seyfi et al.,
2009).
Pt and Pd are used for oxidation of CO, all HC except CH4 and oxygenated organic
compounds such as aldehydes (R-CHO) and alcohols. Under reducing conditions, Pt converts
NO to N2 and NH3. Pt is also used for both simultaneously oxidation and reduction reaction.
Rh is used for NOx reduction. The other four element of the platinum group metals are not
used in catalytic converter as they are in short supply. Ruthenium (Ru) produces the least NH3
concentration during NOx reduction in comparison with other catalysts but it forms volatile
toxic oxides that avoid their use in catalytic converter (Wei, 1975).
Certain kinds of rare earth metals can be used as oxygen storage and oxygen release
materials. They promote noble metal dispersion, increase thermal stability of the alumina
support, promote the water gas shift and steam reforming reactions, store and release oxygen
under conditions fluctuating between oxidizing and reducing (Andrew, 1976; Ciambelli et al.,
2002; Shinjoh, 2006; Eyssler et al., 2010). Three way catalysts works efficiently under
atmospheres with fluctuating air to fuel ratio (A/F) because the average A/F is maintained
close to the stochiometric value. Cerium is used as oxygen storage material that store excess
of oxygen in an oxidative atmosphere and release oxygen in a reductive atmosphere (Fritz and
29
Pitchon, 1997; Farrauto and Heck, 1999; Shelef and McCabe, 2000; Fabbrini et al., 2003;
Shinjoh, 2006; Alifanti et al., 2007; Sharma et al., 2008). Oxygen storage capacity (OSC) and
thermal stability of CeO2 are insufficient for automotive use. Addition of lanthanum or
zirconia ions into CeO2 improves OSC by increasing the number of oxygen defects under
reductive conditions (Belton and Taylor, 1999; Farrauto and Heck, 1999; Kaspar, 2003;
Matsumoto, 2004; Shinjoh, 2006; Bera and Hegde, 2010). Sharma et al. (2008) reported that
when ceria is used as oxygen storage materials, the metal support interaction with precious
metals such as Pt, Pd and Rh is also observed that enhances their catalytic activity.
The catalytic converter is installed in the exhaust system between the exhaust
manifold and the muffler (Srinivasan, 2001). The size and shape of the final catalyst
configuration varies with each automobiles company but typically they are about 5-6 in. in
diameter and 3 in. long with multiple honeycomb catalysts (Mouza et al., 1995; Farrauto and
Heck, 1999). Temperature of the gas entering the catalyst bed in catalytic converter is around
1000 0C and space velocities reach over 100,000 h-1 when vehicle operates at full throttle
(Silveston, 1995).
2.4 Limitations of platinum group metal catalysts
Platinum group metal catalysts exhibit higher oxidation catalytic activity even below
350 0C but their application is hampered in actual practice due to number of reasons that have
been already discussed in section 1.2. Platinum group metal catalysts mainly Pd and Rh are
highly toxic and carcinogenic in nature (Wei, 1975; Kalavrouziotis and Koukoulakis, 2009).
Most of the noble metal compounds can only escape from the exhaust system when very fine
airborne dust particles form due to catalyst attrition. A few compounds such as highly toxic
metal carbonyls are not present in the original catalyst but may be formed by reaction with the
exhaust gases and emit into the atmosphere in the vapor state (Wei, 1975). Road traffic is
responsible for metallic and organic pollutant emissions due to the abrasion of automotive
catalyst washcoat which contaminates the environment (Wei, 1975; Kalavrouziotis and
Koukoulakis, 2009).
2.5 Alternative to platinum group metal catalysts
Efforts are being put for the replacement of supported noble metals for automotive
exhaust gas emissions control because of limitations of noble metals like high cost, volatility
of noble metals at high operating temperature, sintering at high temperature, poisoning
30
problem and limited resources of noble metals etc. (Shu and Kaliaguine, 1998; Song et al.,
1999; Ciambelli et al., 2000; Forni and Rossetti, 2002; Zhang-Steenwinkel et al., 2002;
Cimino et al., 2003; Merino et al., 2005; Qis et al., 2005; Royer et al., 2005b; Zhu et al., 2005;
Barbero et al., 2006; Labhsetwar et al., 2006; Shinjoh, 2006; Zhang et al., 2006d; Singh et al.,
2007; Sartipi, 2008; Popescu et al., 2009; Seyfi et al., 2009; Deng et al., 2010; Ziaei-Azad et
al., 2011).
Various catalysts have been investigated as discussed in section 2.3 for automotive CO
oxidation reaction as a substitute of noble metals in catalytic converter. Most of the catalysts
posed limitations except pervoskites for abatement of automotive CO emission as discussed in
section 2.3. In that regards, perovskites (ABO3 where where A and B are rare earth, alkaline
earth, alkali metal and transition metal cations respectively) have been considered as
promising alternatives to the precious noble metals as TWCs which can exhibit good activity,
have low cost, thermally and mechanically more stable at high temperature (900-1100 0C),
better resistant to lead, sulfur, phosphorus and halogens poisoning, less affected by the
oscillation of the air/fuel ratio, diverse physicochemical properties, promote noble metal
dispersion, increase thermal stability of the support, flexibility of their compositions for
possible tailoring of specific catalytic properties, promote the water gas shift and the steam
reforming reactions, store and release oxygen under conditions fluctuating between oxidizing
and reducing, method of preparation is simple etc. (Shu and Kaliaguine, 1998; Ciambelli et
al., 2000; Cimino et al., 2003; Rosso et al., 2003; Szabo et al., 2003b; Fino et al., 2004;
Kalinguine and Neste, 2004; Nishita et al., 2005; Royer et al., 2005b; Labhsetwar et al., 2006;
Luod and Liu, 2007; Singh et al., 2007; Tzimpilis et al., 2008; Zhang et al., 2008b; Dacquina
et al., 2009; Russo et al., 2009a; Seyfi et al., 2009; Abdolrahman et al., 2010; Doggali et al.,
2010; Ghasdi et al., 2010; Ziaei-Azad et al., 2011).
Noble metal catalysts (e.g. Au) exhibit 100% CO conversion at ambient temperature
but they are expensive and sinter at high temperature and catalytic activity decrease with
reaction time. On the other hand, perovskite type catalysts exhibit 100% CO conversion at
relatively high temperature (> 300 0C) but they have long catalyst life (Wang and Zhong,
2010). Therefore, perovskites have received increased attention in the last decades for
emissions control (Zhang-Steenwinkel et al., 2002). It is reported by many researchers
(Ciambelli et al., 2002; Zhang-Steenwinkel et al., 2002; Ran et al., 2005; Song et al., 2006;
He et al., 2007; Screen, 2007) that perovskites can efficiently oxidize CO and HC and also
31
reduce NOx simultaneously. It is used as TWCs effectively for controlling automotive exhaust
gas emissions (Szabo et al., 2002; Zhang-Steenwinkel et al., 2002; Zhang et al., 2006c; Karita
et al., 2007; Wen et al., 2007). Some of the perovskites are used as active CO oxidation
catalysts as early as in 1952 (Zhang et al., 2006c). Later on their potential application as
catalysts for automobiles exhaust purification was pointed out by Libby (Ramadas, 1978;
Zhang et al., 2006a, 2006e).
2.6 Perovskites
2.6.1 About perovskites
Perovskite based catalysts have been investigated as automobiles exhaust catalysts in
catalytic converter since early 1970s and also for variety of other reactions (Zhang-
Steenwinkel et al., 2002; Ran et al., 2005; Nishita et al., 2005; Ran et al., 2005; Royer et al.,
2006; Singh et al., 2007; Levasseur and Kaliaguine, 2008; Pecchi et al., 2008a, 2008b;
Tzimpilis et al., 2008; De Lima et al., 2009).
Perovskites are any oxides materials with well defined crystalline structure similar to
calcium titanium oxide (CaTiO3 - generally in its orthorhombic form) (Ramadas, 1978; Tejuca
et al., 1989; Pena and Fierro, 2001; Tanaka and Misono, 2001; Ciambelli et al., 2001a; Luod
and Liu, 2007; Screen, 2007). It was first described in the area of Ural Mountains of Russia
by Gustav Rose in 1830 and is named after Russian mineralogist, Count Lev Aleksevich von
Perovski who discovered the naturally occurring mineral CaTiO3 (1792-1856) (Tanaka and
Misono, 2001; Screen, 2007).
The general chemical formula for perovskite compounds is ABO3 or ABO where A
and B are cations of very different sizes and O is an anion
nonstochimetric oxygen. The size of A cation is larger than size of B cation (Tejuca et al.,
1989; Ciambelli et al., 2000; Falcon et al., 2000; Ciambelli et al., 2001a; Pena and Fierro,
2001; Tanaka and Misono, 2001; Forni and Rossetti, 2002; Civera et al., 2003; Royer et al.,
2004; Merino et al., 2005; Giannakas et al., 2006; Labhsetwar et al., 2006; Zhang et al.,
2006c, 2006e; Iwakuni et al., 2007; Karita et al., 2007; Screen, 2007; Pecchi et al., 2008a;
Sartipi et al., 2008; Khanfekr et al., 2009; Russo et al., 2009a; Seyfi et al., 2009). A cation can
be rare earth, alkaline earth, alkali and other large ions such as Pb2+, Bi3+ and B cation can be
3d, 4d and 5d transition metal. The following metals Y, Na, K, Rb, Cs, Pb, La, Sr, Ba, Cr,
Ag, Ca, Pr, Nd, Bi etc. are selected for A cation and Al, Ga, In, Zr, Nb, Sn, Ru, RH, Pd, Re,
32
Os, Ir, Pt, u, Co, Fe, Ni, Mn, Cr, Ti, Cu, Mg, V, Nb, Ta, Mo,W etc. are selected for B cation
in ABO3 perovskites. B cation surrounded by six oxygen in octahedral coordination and A
cation surrounded by twelve oxygen in dodecahedral coordination in ideal perovskite
structure. (Tejuca et al., 1989; Porta et al., 1999; Ciambelli et al., 2000; Leanza et al., 2000;
Poplawski et al., 2000; Schneider et al., 2000; Ciambelli et al., 2001ª, 2001b; He et al., 2001;
Labhsetwar et al., 2001; Forni and Rossetti, 2002; Zhang-Steenwinkel et al., 2002; Rosso et
al., 2003; Kalinguine and Neste, 2004; Merino et al., 2005; Barbero et al., 2006; Labhsetwar
et al., 2006; Rida et al., 2006; Song et al., 2006; Zhang et al., 2006c, 2006e; Alifanti et al.,
2007; De Lima et al., 2009; Khanfekr et al., 2009; Russo et al., 2009a, 2009b). (The oxygen
ions are connected to six cations (4A+2B). If the unit cell is selected with a B ion at the body
center then oxygen ion occupies the face center and the A ion occupies the corners of the unit
cell. When the A ion is assigned to the body center position then oxygen ion are at the middle
of the edges with the B ion at the corners of the unit cell. The perovskite structure can view
also as ReO3, CsCl and Cu3Au-type of structure (Ramadas, 1978; Pena and Fierro, 2001). The
structures of perovskites are shown in Fig. 2.4.
Fig. 2.4. Structure of Perovskite (ABO3) [Screen, 2007]
In perovskite structure, 2p orbit of oxygen is valence band and d orbit of cation B is
conduction band (Qisheng et al., 2007). These oxides are typically p-type of semiconductor
(Furfori et al., 2009; Khanfekr et al., 2009). The ideal structure of perovskite is cubic with
unit cell of side about 3.9 Å, space group Pm3m and contains one formula unit (Ramadas,
1978; Tejuca et al., 1989; Pena and Fierro, 2001; Zhang-Steenwinkel et al., 2002; Ran et al.,
2005; Rida et al., 2008).
33
In general, the perovskite structure can be stable if tolerance factor (t) is defined by
Goldschmidt ( ( )( )A
A
rrrrt
++=
Bcation
Acation
2) is in the range of 0.8 to 1 where rcation A, rcation B
and rA are size (ionic radii) of cation A, cation B and anion O respectively (Ramadas, 1978;
Tejuca et al., 1989; Ciambelli et al., 2001a; Tanaka and Misono, 2001; Colonna et al., 2002a;
Zhang-Steenwinkel et al., 2002; Ariafard et al., 2003; Cimino et al., 2003; Royer et al., 2004;
Batis et al., 2005; Qis et al., 2005; Royer et al., 2005b; Zhang et al., 2006e; Levasseur and
Kaliaguine, 2008; Royer et al., 2008; Khanfekr et al., 2009). The size of the cations A and B
play important role because that decide the stability of the perovskite structure (Ferri and
Forni, 1998). Pena and Fierro (2001), Batis et al. (2006) and Zhang-Steenwinkel et al. (2002)
reported that t is very close to unity then perovskite exhibits ideal cubic structure. In most of
the cases, deviation from ideal structure in form of distorted perovskite structures such as
orthorhombic, rhombohedral, tetragonal, monoclinic and triclinic are also observed. The latter
three ones are rarely observed and poorly characterized (Pena and Fierro, 2001; Zhang-
Steenwinkel et al., 2002). LaFeO3 exhibits orthorhombic type of distorted structure and
LaMnO3 and LaCoO3 exhibit rhombohedral type (Colonna et al., 2002a; Merino et al., 2006;
Khanfekr et al., 2009). The tolerance factor is important to measure the A-O and B-O bond
lengths which decide the stability of perovskite structure (Royer et al., 2005b). In the ideal
structure the atoms are touching each other and B-O distance is calculated by a/2 (a is the
cubic unit cell parameter) while A-O distance will be calculated by 2
a (Pena and Fierro,
2001). Due to its wide range of t, large numbers of metallic elements having the size of cation
A (rcation A) greater than 0.90 Å and cation B (rcation B) greater than 0.51 Å are incorporated in
the ABO3 perovskite structure without changing its original structure of perovskites (Porta et
al., 1999; Ciambelli et al., 2000, 2001a; Spinicci et al., 2001; Tanaka and Misono, 2001;
Ciambelli et al., 2002; Colonna et al., 2002b; Cimino et al., 2003). Pena and Fierro (2001)
reported that 90% of the metallic elements of the periodic table are stable in perovskite
structure. Precious metals such as Pd, Pt and Rh can also be incorporated into the crystal
lattice (Royer et al., 2005c; Khanfekr et al., 2009; Ghasdi et al., 2010).
Perovskite compounds can tolerate significant partial substitution (A and/or B with
metals of different oxidation states i.e. A1-xA'xBO3 or AB1-xB'xO3 or A1-xA'xB1-xB'xO3 and
oxygen non- subscript in
the formula A1-xA'xB1-xB'xO ) without destroying the perovskite structure. A large number
34
of metal ions having different oxidation state can replace both A and B ions (Guilhaume et al.,
1996; Ferri and Forni, 1998; Ciambelli et al., 2000; Leanza et al., 2000; He et al., 2001; Pena
and Fierro, 2001; Forni and Rossetti, 2002; Ariafard et al., 2003; Cimino et al., 2003; Rosso et
al., 2003; Zhu et al., 2005; Merino et al., 2006; Song et al., 2006; Khanfekr et al., 2009; Russo
et al., 2009a, 2009b). Multi component perovskites can be synthesized by partial substitution
of cations in the position of A and B-site which exhibit interesting catalytic properties (Ferri
and Forni, 1998; Pena and Fierro, 2001; Forni and Rossetti, 2002; Campagnolia et al., 2005;
De Lima et al., 2009; Ghasdi et al., 2010) and also enhance the activity for the oxidation of
CO (Zhang-Steenwinkel et al., 2002), combustion of CH4 (Cimino et al., 2000; Tian et al.,
2009), soot particles (Fino et al., 2003c), reduction of NO by CO (Zhang et al., 2006c) and
oxidation of volatile organic compounds (Barbero et al., 2006).
The catalytic properties of perovskite-type oxides for oxidation of pollutants depend
on the nature of A and B ions and on their oxidation (valence) state (Labhsetwar et al., 2001;
Liu et al., 2002; Rida et al., 2006; Sartipi et al., 2008; Seyfi et al., 2009; Abdolrahman et al.,
2010; Furfori et al., 2010; Ghasdi et al., 2011). The selection of cations A and B are important
in the design of perovskite catalysts because they are responsible for modification of the
catalytic properties (Zhou et al., 2002; Abdolrahman et al., 2010; Furfori et al., 2010). The A-
site ions are catalytically inactive but responsible for stability of perovskite whereas B-site
cations responsible for the catalytic activity (Zhong et al., 1997; Ferri and Forni, 1998;
Labhsetwar et al., 2001; Forni and Rossetti, 2002; Liu et al., 2002; Ariafard et al., 2003; Rida
et al., 2006; Zhang et al., 2006b; Seyfi et al., 2009). Qi et al. (2005) reported that the B-site
metals in perovskite structure play important role for catalytic activity by forming the primary
active sites while the metals on A-site affect the activity and stability of the perovskite by
synergistic interaction with the B-site metal (Labhsetwar et al., 2001; Qis et al., 2005).
The partial substitution of A-site with another metal cations A' or B-site with another
metal cations B' having lower or higher oxidation state (valence) can strongly affect the
catalytic activity due to formation of structural defects such as anionic or cationic vacancies
and/or change in the oxidation state of the transition metals (cations B) to maintain the electro
neutrality of the compounds (Zhong et al., 1997; Ferri and Forni, 1998; Leanza et al., 2000;
Ciambelli et al., 2001a; Forni and Rossetti, 2002; Ariafard et al., 2003; Cimino et al., 2003;
Merino et al., 2005; Barbero et al., 2006; Pecchi et al., 2008a ; Russo et al., 2009b; Magalhaes
et al., 2010). The reduction of the transition metal to a lower valency and its re-oxidation are
35
also affected by these substitutions. The properties such as oxygen mobility, oxidation state
and reducibility of the transition metal are responsible for the catalytic activity of the
perovskites (Porta et al., 1999; Song et al., 1999; Ciambelli et al., 2000; Forni and Rossetti,
2002; Liu et al., 2002; Cimino et al., 2003; Civera et al., 2005; Goldwasser et al., 2005; Ran et
al., 2005; Barbero et al., 2006; He et al., 2007; Screen, 2007; Wen et al., 2007; Pecchi et al.,
2008b; Seyfi et al., 2009). Ferri and Forni (1998) and Rida et al. (2008) reported that the
structural defects are responsible not only for catalytic activity but also for the oxygen
mobility within crystal structure due to the non-stoichiometry created by substituting A' for
metal cations A. Due to partial substitution, the oxidation state of B cation increases that
makes the redox process easy which generates larger quantities of oxygen at low temperature
which enhance the overall oxidation activity (Barbero et al., 2006). The oxygen vacancies
created due to partial substitution favor the catalytic activity for oxidation reaction because it
provides the active sites for adsorption of reactants and also increase the lattice oxygen
mobility (Ferri and Forni, 1998; Cimino et al., 2003; Barbero et al., 2006; Rida et al., 2008).
Partial substitution with the metal cations having the same valence state should not modify the
physicochemical properties due to the unchanged charge balance but at the same time the
bond strength of M-O become weak due to synergistic effects (Ciambelli et al., 2002; Cimino
et al., 2003). The deviation from stoichiometry can also be modified by partial substitution of
the A and B cations which significantly change the physical and chemical properties
(Abdolrahman et al., 2010). Voorhoeve et al. (1977) reported that by appropriate partial
substitution many desirable properties like the valence state of transition metal ions, the
binding energy (BE), diffusion of O in the lattice, the distance between active sites and the
magnetic and conductive properties of the catalyst can be modified.
There are two mechanisms such as suprafacial and intrafacial proposed by Voorhoeve
et al. which is widely used for the explanation of oxidation reaction over perovskite catalysts
(Zhong et al., 1997; Ciambelli et al., 2000; Leanza et al., 2000; Forni and Rossetti, 2002; Fino
et al., 2004; Merino et al., 2005; Russo et al., 2005; Zhu et al., 2005; Royer et al., 2006;
Morales et al., 2007; Liu et al., 2009; Russo et al., 2009a, 2009b). There are two types of
oxygen species released depending on the temperature. The oxygen desorbs in the range of
300-600 0 - -O2 i.e. surface oxygen) i.e. low temperature species
(O or O2 ) bonded weakly on the surface and interact with the reactants known as suprafacial
mechanism and when it desorbs at higher temperature at 600-900 0 -
36
-O2 i.e. bulk oxygen) i.e. high temperature species (O with simultaneous reduction
of some M3+ to M2+ i.e. Co3+ to Co2+) known as intrafacial mechanism and involves a Mars-
Van Krevelen redox cycle. When the concentration of adsorbed species becomes low on
surface, intrafacial mechanism occurs. In intrafacial mechanism, bulk oxygen migrates from
lattice towards the surface and available for the oxidation of the adsorbed reactant whose
desorption increases the formation of oxygen vacancies without destroying the perovskite
structure and it is quickly replaced by oxygen coming from the gaseous phase. The mobility
of O ions within the crystalline framework determines the mechanism of the catalytic
reaction (Zhong et al., 1997; Ciambelli et al., 2000; Leanza et al., 2000; Tanaka and Misono,
2001; Forni and Rossetti, 2002; Fino et al., 2003a, 2004; Russo et al., 2005, 2005a, 2005b;
Fino et al., 2006a; Royer et al., 2006; Ifrah et al., 2007; Zhang et al., 2007; Fino and Specchia,
2008; Liu et al., 2009; Rousseau et al., 2009; Russo et al., 2009a; Ziaei-Azad et al., 2011).
Further, -O2 1-O2 2-O2 species according to their desorption
temperature (Zhang et al., 2007). The first peak observes in temperature programmed
desorption (O2-TPD) referred to as 1-O2 is ascribed to physically adsorbed oxygen or O2
(ad) species (Merino et al., 2005; Petrovic et al., 2006; Song et al., 2006). The second peak 2-
O2 is associated with the desorption of O species (Merino et al., 2005; Song et al., 2006). The
-O2 desorbs mainly dependant on the specific surface area of the samples and
stay lower than one monolayer (0.7 0.8) (Royer et al., 2005b; Song et al., 2006). -O2
dependent on the
morphology of the samples (Merino et al., 2005; Royer et al., 2005b; Levasseur and
Kaliaguine, 2008).
-O2 desorption peak is not always observable in TPD curve and strongly depends
on the concentration of surface oxygen vacancies. Its onset and intensity of the peak depend
on the nature of the metal cations B of the ABO3 structure as well as mainly on the degree of
substitution of the A ion with ions of lower valence. -O2 peak is observable at high
temperature and its onset depends mainly on the nature of the B ion and its partial reduction to
a lower oxidation state (Leanza et al., 2000; Fino et al., 2003a; Russo et al., 2005; Petrovic et
al., 2006; Ifrah et al., 2007; Ziaei-Azad et al., 2011). A-site replacement mainly affects the
amount of sorbed oxygen whereas B-site replacement influences the nature of sorbed oxygen
which modifies the catalytic behavior of the perovskites (Ferri and Forni, 1998; Merino et al.,
2005).
37
Suprafacial mechanism (participation of a surface oxygen species adsorb on the
-O2) is responsible for oxidation of CO (Zhong et al., 1997;
Royer et al., 2005a; Zhu et al., 2005; Royer et al., 2006; Hueso et al., 2009; Levasseur and
Kaliaguine, 2009; Ghasdi et al., 2011), propane (Merino et al., 2005) and n-hexane
(Kaliaguine, 2007) -O2 originating
from the bulk of the catalyst) oxidation of CH4 (Royer et al., 2005a; Zhu et al., 2005; Royer et
al., 2006; Ifrah et al., 2007; Kaliaguine, 2007) and reduction of NO over perovskites (Zhu et
al., 2005; Ifrah et al., 2007). It is also observed that suprafacial mechanism is responsible for
soot oxidation (Fino et al., 2003a; Russo et al., 2005; Ifrah et al., 2007).
The efforts are made in order to increase catalytic activity by partial substitution of A
cation by cations of different valancies such as Sr2+ and Ce4+ (Viswanathan, 1992; Ciambelli
et al., 2001b; Ran et al., 2005; Royer et al., 2005c; Furfori et al., 2009; Seyfi et al., 2009).
Perovskite catalyst with La at the A-site and Co at the B-site provides the excellent catalytic
activity for CO oxidation. Such activity is further improved by the replacement of La at the A
site with ions that have a different valence state such as Sr or Ce (Seyfi et al., 2009).
Ciambelli et al. (2001a) and Viswanathan (1992) reported that the partial substitution of La
with divalent ions in particular Sr+2 increases the average oxidation state of B cation in case of
Co and Fe based perovskites with minute anion vacancies. Due to partial substitution, the
transition metals reduce easily and larger quantities of oxygen are available at low
temperature which enhances CO oxidation activity of the catalysts. Rida et al. (2008) reported
that Sr or Ce substitution for La in LaCrO3 perovskites improves the C3H6 combustion
catalytic activity due to the formation of defects like oxygen vacancies or change in the
chromium valence state (from Cr3+ to Cr4+).
In general, the properties of ABO3 perovskites can be easily modified by partial
substitution of the A-site cation La3+ with other cations of different oxidation states like Sr2+
or Ce4+. These substituted catalysts enhance the oxidation catalytic activity of the catalyst by
creating defect structure, modifying the vacancy concentration, by facilitating oxygen
mobility or by enhancing the redox activity of the Bn+ cation (Ferri and Forni, 1998;
Ciambelli et al., 2001b; He et al., 2001; Forni and Rossetti, 2002; Royer et al., 2005c; Qis et
al., 2005; Giannakas et al., 2006; Rida et al., 2006; Song et al., 2006; Niu et al., 2007; Royer
et al., 2008; Furfori et al., 2009; Levasseur and Kaliaguine, 2009; Rousseau et al., 2009; Seyfi
et al., 2009; Ghasdi et al., 2011).
38
The partial substitution of La3+ in LaMnO3 with Ba2+ or Sr2+ results in the
modification of surface properties of the catalysts which significantly increase the thermal
stability and catalytic activity of the perovskite oxides for CH4 oxidation (Liu et al., 2002). Li
et al. (2008) observed that the substitution of Sr at A-site and Fe at B-site in LaCoO3
perovskite (i.e.La1-xSrxCo1-yFeyO3) improve the catalytic activity for toluene because partial
substitution enhances the amount of weakly adsorbed oxygen which requires for combustion
of toluene. Substitution of La+3 with Ag+, Sr+2 and Ce+4 in LaMnO3 perovskites i.e. La1-
xMxMnO3 (M = Ag, Sr, Ce) improve the catalytic activity for CO and CH4 oxidation (Song et
al., 1999). It is also reported that substitution of either Ce4+ or Eu2+ for La3+ in case of
LaCoO3 affect the desorption of both types of oxygen (Leanza et al., 2000). Liu et al. (2002)
reported that A-site substitution with other cations may be possible route for producing high
surface area perovskites. They pointed out that the surface area of La1-xAxMnO3 (A = Sr, Ce,
Eu, K) are much higher than that of LaMnO3 as calcined at the same temperature and also
exhibit high oxidation activity. Doggali et al. (2010) showed that improved catalytic activity
for CO oxidation even below 200 0C as well as for carbon/PM oxidation above 370 0C is
achieved over La0.9Ba0.1CoO3 because of substitution of Ba at A-site is released higher
-O2 which may be responsible for enhance catalytic activity. Zhong et al., (1997)
reported that the catalytic activity for CH4 combustion over ultrafine LaFe1-yByO3 (B = Mn,
Al, Co) oxides are much higher than that of LaFeO3 sample due to increase of high valence B-
site cations and lattice oxygen content by the substitution of B-site elements.
Fino et al. (2003b) pointed out that when A-site cation is partially substituted with the
metal of different oxidation state for example La0.9K0.1Cr0.9O3- then compensation in charge
is required to maintained electro neutrality. This can be attained by formation of oxygen
vacancies due to change of oxidation state of B cation (e.g. Cr3+ to Cr4+). This might increase
the amount of both and -types of O2. The presence of Cr4+ is responsible for increase of
catalytic activity toward CH4 combustion.
It is reported (Barbero et al., 2006) that La0.8Ca0.2FeO3 perovskite shows increase in
Fe4+ concentrations by partial substitution of Ca2+ for La3+ which enhance the catalytic
activities for C3H8 and ethanol (C2H5OH) combustion compared to LaFeO3 perovskite. These
enhance catalytic activities are due to large number of active sites associated with the Fe4+and
also anion vacancies form to maintain electro neutrality which is essential for oxidation
reaction.
39
The basic strategies for designing perovskite based catalysts to enhance their catalytic activity
are as follows:
¶ Selection of transition metal at B-site of perovskite structure (ABO3) that primarily
determines the catalytic activity (Tanaka and Misono, 2001; Batis et al., 2005;
Misono, 2005).
¶ Control of oxidation state of B-site elements (and vacancy control at B-site and of
oxygen). Redox property of perovskite depends on oxidation state of transition metal
(Misono, 2005).
¶ Synergistic effect of two different B-site ions. For example, synergistic effect is due to
bi-functional effect of Mn and Cu located at B-site in 1:1 atomic ratio and exhibits
higher catalytic activity for CO oxidation (Tanaka and Misono, 2001; Misono, 2005;
Labhsetwar et al., 2006).
¶ Enhancement of specific surface area by forming thin layers or nano-particles of
perovskite on support materials or use of modified alumina with suitable pre-coat to
make it inert towards perovskite. Perovskites prepared by ordinary methods usually
have very small specific surface area (Misono, 2005; Labhsetwar et al., 2006).
¶ Addition of precious metals and their regeneration. In the case of perovskite catalysts
for automotive applications, addition of small amount of precious metals greatly
enhances the catalytic activity (Tanaka and Misono, 2001; Misono, 2005; Labhsetwar
et al., 2006).
Perovskites exhibit good catalytic activity for CO and HC oxidation but they cannot
remove NOx effectively (He et al., 2001). Their application is limited because of low specific
surface area due to sintering and lower resistance to poisoning by water and sulfur (discussed
in detail in section 2.10). Precious metals most widely used as TWCs due to high activity and
resistance to water and sulfur poisoning but these catalysts suffer from the noble metal
sintering and metal volatilization at high operating temperature of engine. The limitations of
perovskites and also noble metals can be minimize at some extent by incorporating small
amount of precious metals into a perovskite structure because that can prevent their sintering,
reduce losses due to volatilization at high operating temperature and avoid reactions with the
support that lead to the catalyst deactivation (Nishihata, 2002; Zhou et al., 2002; Nishihata, et
al., 2003, 2005; Ran et al., 2005; Petrovic et al., 2006; Song et al., 2006; Tanaka et al., 2006;
40
Screen, 2007; Singh et al., 2007; Sartipi et al., 2008; Khanfekr et al., 2009; Ziaei-Azad et al.,
2011) and in addition to that the noble metal doped perovskites enhance the catalytic activities
of NO reduction (by CO and HC) as well as NO decomposition (He et al., 2001).
Pd is used for doping or substitution in perovskites rather than Pt and Rh because Pd
exhibits high stability, lower price than Pt and Rh and also has good activity for oxidation of
CO and HC (Zhou et al., 2002; Sartipi et al., 2008). The modern gasoline engines use close-
loop feedback control system that uses an oxygen sensor for three-way catalysts to function in
optimum conditions. Therefore, the environment of automotive exhaust gas fluctuates
between oxidative and reductive atmospheres through the operation of catalytic converter. Pd-
doped catalyst (for ex. LaFe0.95Pd0.05O3) is also known as a regenerative catalyst or intelligent
catalyst which keeps a high catalytic activity through the redox fluctuations by adapting its
original structure to the environment. It is reported by many researchers that in oxide form, Pd
may diffuse into the perovskite lattice and replaces the B-site cation. In reducing atmospheres,
Pd segregates (metallic Pd0) out and disperses as metallic nano particles (1 to 3 nm) on the
surface of the perovskites. Transfer of the Pd between the bulk and the surface occurs in
response to the redox conditions of the environment. This back and forth movement of Pd as
per redox fluctuation of engine exhaust suppresses the agglomeration and growth of the Pd
(Misono, 2005; Uenishi et al., 2005b, 2005b;
Tanaka et al., 2006; Screen, 2007; Singh et al., 2007; Sartipi et al., 2008; Eyssler et al., 2011a,
2011b). Schematic of the operation of self-regeneration in noble metal based perovskite
catalysts is presented in Fig. 2.5.
Noble metal containing perovskites have been used for CO oxidation (Singh et al.,
2007; Sartipi et al., 2008), oxidation of C3H8 (Sartipi et al., 2008), catalytic combustion of
CH4 (Eyssler et al., 2010, 2011a, 2011b; Ziaei-Azad et al., 2011), NO reduction (Uenishi et
al., 2005a), NO reduction by H2 (Uenishi et al., 2005b). Among the precious metals, Pd in its
oxidized form (PdO) is to be the most active catalyst for CH4 oxidation from natural gas
fueled vehicles (Ziaei-Azad et al., 2011). Some of the noble metal doped perovskites
(LaFe0.95Pd0.05O3) are now in commercial use as three-way catalysts for gasoline engine in
order to control automotive emission since October 2002 in Japan. The noble metal doped
perovskites require 70% less amount of precious metals compared to conventional automotive
catalysts (Misono, 2005; Tanaka et al., 2006; Screen, 2007; Singh et al., 2007).
41
Fig. 2.5. Schematic of the operation of self-regeneration in noble metal based perovskite catalysts (Nishihata, et al., 2003; Misono, 2005; Tanaka et al., 2006; Screen, 2007; Royer and Duprez, 2011).
Perovskites exhibit wide range of physical properties such as ferroelectricity,
piezoelectricity, semiconductivity, metallic conductivity, superconductivity that can be
controlled by changing the ion size, electronic configuration and also using specific
preparative conditions (Ramadas, 1978). Because of wide range of composition, perovskites
are used as a prototype material in the field of solid state chemistry, physics, catalysis, solid
oxide fuel cells, oxygen separation membranes, chemical sensors for the detection of
humidity, alcohol and gases such as oxygen, CO, HC, CH4 and NO (Ramadas, 1978; Brosha
et al., 2000; Labhsetwar et al., 2001; Cimino et al., 2003; Ito et al., 2004; Ajami et al., 2006;
Labhsetwar et al., 2006; Alifanti et al., 2007; Pokhrel et al., 2007; Vaz and Salker, 2007;
Shetkar and Salker, 2008; Khanfekr et al., 2009; Ghasdi et al., 2011). It is also used in the
field of electro catalysis, optics, electrics and magnetic etc. (Porta et al., 1999; Pena and
Fierro, 2001; Ciambelli et al., 2002; Kalinguine and Neste, 2004; Vaz and Salker, 2007;
Najjara et al., 2011).
Several perovskite-type mixed oxides have been used as active oxidation/reduction
catalysts for various reactions: oxidation of CO (Voorhoeve et al., 1977; Viswanathan, 1992;
Song et al., 1999; Ciambelli et al., 2000, 2001a, 2002; Colonna et al., 2002a, 2002b; Zhang-
Steenwinkel et al., 2002; Cimino et al., 2003; Petrovic et al., 2006; Rida et al., 2006; Singh et
al., 2007; Vaz and Salker, 2007; Sartipi et al., 2008; Shetkar and Salker, 2008; Taguchi et al.,
2008; Seyfi et al., 2009; Abdolrahman et al., 2010; Wang and Zhong, 2010), HC (Forni and
42
Rossetti, 2002; Sartipi et al., 2008; Deng et al., 2010), combustion of natural gas or CH4
(Zhong et al., 1997; Ferri and Forni, 1998; Song et al., 1999; Ciambelli et al., 2000; Cimino et
al., 2000; Ciambelli et al., 2001a, 2002; Cimino et al., 2003; Civera et al., 2003; Fabbrini et
al., 2003; Rosso et al., 2003; Alifanti et al., 2005; Batis et al., 2005; Civera et al., 2005; Royer
et al., 2005c; Popescu et al., 2009; Russo et al., 2009a, 2009b; Eyssler et al., 2011a), oxidation
of VOC (Soni, 1998; Spinicci et al., 2003; Barbero et al., 2006; Alifanti et al., 2007;
Levasseur and Kaliaguine, 2009), oxidation of C3H8 (Doshi et al., 1993; Merino et al., 2005,
2006; Rida et al., 2006, 2008), reduction of NO (Viswanathan, 1992; Teraoka and Kagawa,
1998; Peng et al., 2006; Iwakuni et al., 2007; Wen et al., 2007; Liu et al., 2008; Mescia et al.,
2008), selective catalytic reduction of NO with H2 (Furfori et al., 2009; Furfori et al., 2010),
selective reduction of NO with CO (Shen and Weng, 1998; Zhang et al., 2006c, 2006e; Karita
et al., 2007; De Lima et al., 2009), selective catalytic reduction of NO with C3H6 (Zhang et al.,
2006a, 2006c, 2006d, 2007, 2008b), selective reduction of NO with NH3 (Bukhtiyarova et al.,
2009), combustion of soot (Teraoka and Kagawa, 1998; Fino et al., 2003a, 2004; Russo et al.,
2005; Milt et al., 2005; Fino et al., 2006a; Peng et al., 2006; Liu et al., 2008; Mescia et al.,
2008), oxidation of 1,2-dichlorobenzene (C6H4Cl2) (Poplawski et al., 2000), ethyl acetate
(C4H8O2) (Niu et al., 2007), C2H5OH (Merino et al., 2006) and oxidation of chlorinated
hydrocarbon (Schneider et al., 2000) etc. It is also used as a catalysts in different reactions
such as steam reforming of CH4 (Pecchi et al., 2008b), CO2 reforming of CH4 (Goldwasser et
al., 2003), ethane (C2H6) reforming (Goldwasser et al., 2005), hydrogen peroxide (H2O2)
decomposition (Ariafard et al., 2003), CH4 coupling (Spinicci et al., 2001), Oxidative
coupling of methane (Spinicci et al., 2001) and partial oxidation of CH4 to syngas (Guo et al.,
2004). Perovskites are also used for regeneration of DPF and as diesel catalytic converter
(DCC) (Fino et al., 2003b, 2003c; Labhsetwar et al., 2004; Milt et al., 2005; Fino et al.,
2006b; Cauda et al., 2007; Mescia et al., 2008).
2.6.2 Preparation of perovskites
The functionality of a catalyst in a specific reaction is strongly affected by the method
of preparation. The method of preparation is important to develop texture of perovskite
catalysts with specific characteristics and also to achieve pure phase of perovskite. The
physico-chemical properties and catalytic activity of perovskites depend on three factors:
chemical composition, degree of crystallinity and the crystals morphology which remarkably
influence by the preparation route and the specific synthesis operating conditions (Forni and
43
Rossetti, 2002; Royer et al., 2005b; Labhsetwar et al., 2006; Seyfi et al., 2009). The
morphological properties such as particle size, specific surface area, size of crystal domain
etc. of perovskites catalysts are directly related to the synthesis conditions (Seyfi et al., 2009).
The preparing procedure and parameters affect the physico-chemical characteristics and the
catalytic activity of perovskites (Labhsetwar et al., 2006; Tzimpilis et al., 2008). The
durability of a catalyst is also affected by the method of preparation (Forni and Rossetti,
2002). Therefore a suitable catalyst preparation method is needed in order to achieve the
desired properties.
Perovskites are available naturally or produced synthetically (Screen, 2007). Pena and
Fierro (2001), Asada et al. (2008), Ito et al. (2004) and Rougier et al. (2002) reported two
classical methods for synthesis of perovskites (i) solid state reaction i.e. equimolar oxides
mixing synthesis method (ii) chemical solution method i.e. decomposition synthesis or wet
processing.
Various methods of preparation are reported in the literature for the synthesis of
perovskites include solid state reaction, solution combustion synthesis, reactive grinding,
citrate complexation, co-precipitation, sol-gel and other methods such as microemulsion,
pyrolysis, flame-hydrolysis, spray-drying and freeze-drying etc. (Shu and Kaliaguine, 1998;
Pena and Fierro, 2001; Civera et al., 2005; Zhang et al., 2006c; Iwakuni et al., 2007; Screen,
2007; Levasseur and Kaliaguine, 2008; Petrovic et al., 2008; Rida et al., 2008). A high surface
area and good chemical homogeneity of perovskites can be achieved by the freeze-drying,
sol-gel and citrate complexation method among various preparatory methods (Song et al.,
1999; Rida et al., 2008).
2.6.2.1 Solid state reaction (SSR)
Solid state reaction namely solid-to-solid reaction in which both the starting materials
(the catalyst precursors) and the catalyst are in solids form and it is a convenient method to
prepare several industrial catalysts especially those containing two or more metallic elements
or their oxides (Campagnolia et al., 2005; Wang et al., 2007a). The solid state reaction
involves high temperature ceramic process in which the salt precursors (oxides, hydroxides or
carbonates) mainly oxides are heated at high calcination temperature generally above 1000 0C
(Zhang and Saito; 2000a; Rougier et al., 2002; Kalinguine and Neste, 2004; Screen, 2007). In
this case, the obtained specific surface area was less than 5 m2/g (Porta et al., 1999).
44
Iwakuni et al. (2007) synthesized doped BaMnO3 and SrMnO3 by a conventional
solid-state reaction method in which calculated amount of metal nitrates Ba(NO3)2, Sr(NO3)2,
Mn(CH3COO)2 were mixed and first calcined in presence of air at 400 0C for 2 h followed by
calcined in air at 1000 0C for 6 h.
Typical procedure used by Ghasdi et al. (2010) for preparation of LaCoO3 perovskite
by solid state reaction is as follows: Dehydrated amount of La2O3 and Co3O4 were weighed by
a molar ratio of 1:1 (La:Co), well-mixed and heat treated at 1100 0C for 4 h in presence of air.
The specific surface area of LaCoO3 was measured by BET method and obtained lower value
of 3.9 m2/g. Doshi et al. (1993) prepared other perovskite compositions such as
La0.8Sr0.2CoO3- and La0.8Sr0.2CrO3- using the same procedure mentioned above. The specific
surface area of La0.8Sr0.2CoO3- and La0.8Sr0.2CrO3- synthesized by solid state reaction after
calcination at 1200 0C for 12 h exhibited lower values of 0.19 and 0.91 m2/g respectively.
Royer et al. (2005b) synthesized LaCoO3 by solid state reaction in which La2O3 and
Co3O4 were hand grinded, mixed properly. Then homogeneous mixture of the single oxides
was calcined at 1000 0C (ramp = 5 0C/min) for 10 h. XRD analysis showed that other minor
phase Co3O4 was detected in addition to the major ABO3 perovskite phase for the SSR
sample. The LaCoO3 exhibited low specific surface of 0.4 m2/g.
La1-xSrxCrO3 (x = 0.0-0.3) catalyst synthesized by Rida et al. (2006) using solid state
reaction in which the amount of nitrate salts as per stochiometric were dissolved in water and
heated till dryness. The homogeneous powder mixtures were dried at 200 0C for 24 h and
calcined in air at 800 0C for 2 h and finally heated under air to 1200 0C for 10 h.
The solid state reaction requires a high calcination temperature to ensure the formation
of the crystalline phase and to remove the organic precursors which form coarse aggregates
and limited degree of chemical homogeneity. The grain size of the powder obtained by this
method is relatively large and the specific surface area is too small which limits their use for
commercial purpose (Doshi et al., 1993; Porta et al., 1999; Ciambelli et al., 2000; Zhang and
Saito; 2000a; Cimino et al., 2002; Colonna et al., 2002b; Rougier et al., 2002; Ito et al., 2004;
Royer et al., 2004; Zhang et al., 2006d, 2006e; Layla et al., 2008; Zhang et al., 2008b;
Abdolrahman et al., 2010).
45
2.6.2.2 Solution combustion synthesis (SCS)
Solution combustion synthesis method is highly exothermic, fast and self-sustaining
reaction between metal salts and suitable organic fuel. This method is used to synthesize
nano-sized catalyst particles (Fino et al., 2003a, 2004; Civera et al., 2005; Russo et al., 2005;
Fino et al., 2006a, 2006b; Mescia et al., 2008; Russo et al., 2009a, 2009b; Ziaei-Azad et al.,
2011). It is also used to produce homogeneous, very fine, crystalline powders without the
intermediate decomposition and/or calcination steps require by conventional synthesis routes
(for example in case of citrate method, the heat treatment is necessary in order to burn out the
carbonaceous residue) (Civera et al., 2003, 2005).
Ignition of a combustible component provides the energy for the formation of the
perovskite materials (Fino et al., 2004; Royer et al., 2005b; Cauda et al., 2007; Screen, 2007;
Mescia et al., 2008; Russo et al., 2009a). To produce any metal oxide by solution combustion
synthesis method, the stoichiometric amount of the metal salt precursors and the fuel are
dissolved in distilled water; resulting solution stirred thoroughly to ensure complete
dissolution of all reagents then transferred into a ceramic or porcelain dish and to be kept in
muffle furnace or electric or microwave oven; shortly thereafter the mixture is ignited and fast
self-sustaining redox reaction take places which forms final catalyst composition (Fino et al.,
2003b, 2004, 2005; Furfori et al., 2009; Russo et al., 2009b; Furfori et al., 2010; Ziaei-Azad et
al., 2011). Spongy morphology of catalyst prepared by solution combustion synthesis method
provides the rough interfaces for a catalyst which maximize contact between catalyst and
carbon (soot) and promoting carbon combustion and consequently NO reduction (Fino et al.,
2003a; Russo et al., 2005; Fino et al., 2006b).
Metal nitrates are selected as best starting materials because NO3- groups are oxidizing
agents and their solubility is high in water which provides a good homogenization (Civera et
al., 2003; Fino et al., 2004; Cauda et al., 2007). The various organic fuels such as urea (Civera
et al., 2003; Fino et al., 2003a, 2004; Russo et al., 2005; Fino et al., 2006a, 2006b; Cauda et
al., 2007; Mescia et al., 2008; Furfori et al., 2009; Russo et al., 2009a, 2009b; Furfori et al.,
2010), Sorbitol (Ziaei-Azad et al., 2011), glycien (Najjara et al., 2011) etc. are used in
solution combustion synthesis method. Urea is used as most preferred fuel among all because
of low cost and readily available (Civera et al., 2003). NH4NO3 (1 g NH4NO3 per g
perovskite) is to be added during perovskite preparation because it decomposes (NH4NO3
N2 + 2H2O + 1/2O2) and forms gaseous products which provide additional heat requires for
46
synthesis of perovskites. Moreover, ammonium nitrate has a low cost compared to other
nitrates and brings more availability of oxygen into the system (Fino et al., 2004).
Cauda et al. (2007), Furfori et al. (2009) and Fino et al. (2003c) prepared various
compositions of perovskite by solution combustion synthesis method. Typical procedure
followed is: A homogeneous aqueous solution of metal nitrates (oxidizers) and urea (fuel) was
placed in an oven at a constant temperature selected in the range of 400-800 0C for few
minutes. NH4NO3 was also added to (1 g per g of catalyst prepared) the solution. It quickly
began to boil and ignition took place which synthesized perovskites. Urea combustion
provided the heat necessary for the endothermic transformation of nitrates into the desired
oxide. The whole process was over in a few minutes.
In the literature, various perovkite compositions have been reported such as LaCoO3
(Ziaei-Azad et al., 2011), LaFeO3 (Fino et al., 2003a; Furfori et al., 2009; Russo et al., 2009a,
2009b; Furfori et al., 2010; Ziaei-Azad et al., 2011), La0.8Sr0.2FeO3, La0.8Sr0.2Fe0.9Pd0.1O3,
La0.7Sr0.2Ce0.1FeO3, La0.7Sr0.2Ce0.1Fe0.9Pd0.1O3 (Furfori et al., 2009, 2010), La1-xAxFe1-yByO3
(A = Na, K, Rb and B = Cu) (Mescia et al., 2008), CeO2-promoted PrCrO3 (Cauda et al.,
2007; Furfori et al., 2009), LaMnO3 (Civera et al., 2003; Russo et al., 2009a, 2009b; Ziaei-
Azad et al., 2011), LaCrO3 (Fino et al., 2003a, 2006b; Russo et al., 2009a, 2009b), LaNiO3
(Ziaei-Azad et al., 2011), La0.9K0.1Cr0.9O3- (Civera et al., 2005), LaMn0.9Pd0.1O3,
LaFe0.9Pd0.1O3, LaCr0.9Pd0.1O3 (Russo et al., 2009a, 2009b), LaMnO3·17MgO (Civera et al.,
2003), LaB0.95Pd0.05O3 (B =Mn, Fe, Co and Ni) (Ziaei-Azad et al., 2011) prepared by solution
combustion synthesis method for NO reduction with H2 or CO or C3H8 and soot oxidation.
Solution combustion synthesis method has potential advantages because it requires
low cost starting reactants (metals nitrates), organic molecules as fuel and also the preparation
procedure is simple (Fino et al., 2003a; Civera et al., 2005; Russo et al., 2009b). However, the
main limitation of this method is that hazardous or polluting compounds such as NH3 or NOx
are emitted during the synthesis and release to the atmosphere which create air pollution
(Civera et al., 2005).
2.6.2.3 Reactive grinding (RG)
The reactive grinding method is discussed in detail in Section 2.7.
47
2.6.2.4. Citrate complexation (CT)
The citrate complexation method is simple and gives pure single phases by avoiding
segregation and/or contamination of impurities (Barbero et al., 2006). Citrate method was first
proposed by Zhang et al. to prepare high surface area perovskites (Ciambelli et al., 2000).
This method allows homogeneous dispersion of the precursor salts which requires lower
calcination temperature than other methods to form perovskites and hence avoid sintering
problem which produces high specific surface area and good chemical homogeneity of
perovskites (Song et al., 1999; Ciambelli et al., 2000, 2001a; Alifanti et al., 2005; Niu et al.,
2007; Rousseau et al., 2009). The lower calcination temperature is required because the heat
requires for the formation of perovskite crystalline structure is supplied by the exothermic,
fast and self-sustaining chemical reaction between metal nitrates and citric acid (Song et al.,
2006).
This method produces catalysts with a higher degree of crystallinity compared to co-
precipitation method (Seyfi et al., 2009). This method involves addition of citric acid to the
precursor nitrate solution which form citrate complexes and undergone several decomposition
steps followed by breaking of the citrate complexes which eliminate residual CO32- and NO3-
ions and finally perovskite structure is obtained (Ciambelli et al., 2000). In the citrate method,
metal salts (nitrates) of the required perovskite were dissolved in water. Metal ions were
complexed by adding 1 mol of citric acid per mole of metal cations to the aqueous solution
followed by evaporation under vacuum to obtain a spongy amorphous citrate precursor. The
obtained spongy material was finely crushed and calcined to form perovskites (Ferri and
Forni, 1998; Shu and Kaliaguine, 1998; Ciambelli et al., 2002; Cimino et al., 2003; Royer et
al., 2005b; Sartipi et al., 2008; Hammami et al., 2009; Sui et al., 2011).
Seyfi et al. (2009) prepared perovskites with composition of LaCoO3, LaCo0.8Cu0.2O3,
La0.8Sr0.2Co0.8Cu0.2O3, La0.8Ce0.2FeO3 and La0.8Sr0.2FeO3 by citrate complexation method. A
typical procedure is as follows: Metal nitrates were first dissolved in distilled water (50 ml).
Citric acid (2 times the number of mole of metals cations) was separately dissolved in distilled
water (50 ml) and then added to the precursor solution under vigorous stirring. Excess water
was evaporated under slow stirring at ambient temperature. The viscous gel was then dried at
100 0C for 5 h. The obtained spongy material was finely ground and calcined under air
atmosphere at 700 0C for 5 h. They reported that perovskite samples prepared by the citrate
48
method have been capable of producing nano-size perovskite crystals close to 100 nm verified
by scanning electron microscopy (SEM).
Royer et al. (2005b) synthesized LaCoO3 by complexation of the nitrate salts with
citric acid. Salt precursors (lanthanum nitrate and cobalt nitrate) were first dissolved in water
(40 ml). Citric acid (1.5 times the number of mole of Co+ La) was dissolved in water (20 ml)
and slowly added to the precursor solution under vigorous stirring. Excess water was
evaporated under slow stirring at ambient temperature. The viscous gel was then dried at 80 0C for 1 night and at 110 0C under vacuum for 1 day. The obtained spongy material was finely
ground followed by calcination at 600 0C (ramp = 1 0C/min) for 8 h under air. XRD result
confirmed the single-phase perovskite and specific surface area of perovskite was observed
6.6 m2/g. They also prepared LaCo1-xFexO3 (x = 0.03, 0.10, 0.20) perovskites (Royer et al.,
2005a).
Citrate method has limitations that the procedure is complex and expensive (Song et
al., 1999). Various compositions of perovskites such as LaCoO3, LaCo0.95Pd0.05O3 (Sartipi et
al., 2008), LaMnO3, NdMnO3, Sm1-xSrxMnO3 (x = 0.0, 0.1, 0.3, 0.5) (Ciambelli et al., 2000),
La1-xSrxMO3 (M = Co0.77Bi0.20Pd0.03 and x = 0.0, 0.2, 0.4, 0.6, 0.8), La0.8Sr0.2Co0.8Bi0.2O3 (He
et al., 2001), La1-xSrxCoO3 (x = 0.0, 0.2, 0.4), La0.8Sr0.2BO3 (B = Fe and Ni), La0.9A CoO3 ( A¡
= Eu and Ce) (Ferri and Forni, 1998), LaAl1-xMnxO3 (x = 0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0)
(Cimino et al., 2003), La1-xSrxNiO3 (x = 0.0-0.20) ( de la Cruz et al., 2001) , LaAl1-
xFexO3 (0 x 1) (Ciambelli et al., 2002), La1-xCaxCoO3 (x = 0.0, 0.2, 0.4, 0.5) (Merino et al.,
2005), ABO3 (A= La, Y, Nd or Gd, B = Co, Fe, Mn or Cr) (Poplawski et al., 2000), La1-
xCaxFeO3 (x = 0.1, 0.2, 0.3, 0.4, 0.5) (Ciambelli et al., 2001a), LaFe1-xPdxO3 (x= 0.05, 2 wt %
of Pd) (Eyssler et al., 2010, 2011b) and LaFeO3 (Eyssler et al., 2010, 2011a) were prepared by
citrate method for oxidation of CO and HC (mainly CH4 or C3H6) or as a three way catalytic
performance to remove CO, C3H6 and NO.
2.6.2.5. Co-precipitation (COP)
The objective of this method is to precipitate the catalyst precursor in the form of a
hydroxide or carbonate in the pores and on the surface of the support by adjusting the pH of
the metal salt solution-support slurry by addition of base such as aqueous NH3 (Labhsetwar et
al., 2006), NaOH (Liu et al., 2002; Zhang-Steenwinkel et al., 2002; Pecchi et al., 2008a;
Shetkar and Salker, 2008), H2O2 (Zhang-Steenwinkel et al., 2002), ammonium hydroxide
(NH4OH) (Sefyi et al., 2009), sodium carbonate (Na2CO3) (Liu et al., 2002; Morales et al.,
49
2007; Liu et al., 2009), ammonium bicarbonate (NH4HCO3) (Song et al., 1999; Zhou et al.,
2002; Iwakuni et al., 2007) and potassium carbonate (K2CO3) (Shen and Weng, 1998) etc.
The following steps are followed for preparation of catalyst by co-precipitation
method: Soluble precursors of the constituent metals are dissolved in suitable solvent and then
co-precipitated as a gel by adding an acid/base or another reagent to cause the precipitation.
Precipitation has three stages: super saturation, nucleation and growth. An amorphous or
crystalline precipitate or a gel is obtained which is aged, filtered, washed thoroughly until salt
free. The further steps: drying, shaping, calcination and activation are also followed. Various
characterization techniques confirm that the co-precipitation method produces lower
crystallinity and higher surface area than the citrate method (Labhsetwar et al., 2001; Liotta et
al., 2005; Royer et al., 2005b; Labhsetwar et al., 2006; Sefyi et al., 2009). In addition, the
final material obtained from the alkali co-precipitation route suffers from the contamination of
alkaline metals as well as the formation of vast amount of environmental wastes (e.g. salts
from hydrolysis and wash water) (Wang et al., 2007a).
Labhsetwar et al. (2001, 2006) prepared unsupported and supported La1-xSrxMn1-
yPtyO3 by co-precipitation method as follows: Mixed metal hydroxides were co-precipitated
using aqueous NH3. The precipitate was washed, dried in oven then Pt was added in the co-
precipitated mass. The dried precipitate was subsequently heated at about 850 0C to obtain the
perovskite phase. They found that single-phase perovskite having distorted cubic or tetragonal
crystal system formed which was confirmed by XRD. Specific surface area of perovskite was
observed in the range of 1.0 5.0 m2/g.
Seyfi et al. (2009) prepared perovskites with composition of LaCoO3, LaCo0.8Cu0.2O3,
La0.8Sr0.2Co0.8Cu0.2O3, La0.8Ce0.2FeO3 and La0.8Sr0.2FeO3 by co-precipitation method. In
synthesis method, the following salt precursors were used: La(NO3)3 6H2O, Sr(NO3)2,
Fe(NO3)3 9H2O, Co(NO3)2 6H2O, Ce(NO3)3 6H2O Cu(NO3)2 3H2O. A certain amount of
nitrate salts containing the following cations La+3, Ce+3, Sr+2, Co+2 and Fe+3 were dissolved in
distilled water. Precipitating agent NH4
solution under vigorous stirring until pH 10. The obtained precipitate was filtered and washed
with distilled water until a pH of 7 was reached. The compound was then dried at 100 0C
overnight and calcined at 700 0C for 5 h (ramp = 3 0C/min) under air atmosphere.
50
Royer et al. (2005a, 2005b, 2006) synthesized LaCoO3 by co-precipitation method and
typical procedure is as follows: The nitrate salt precursors (La(NO3)3 2O and
Co(NO3)2 2O) were first dissolved in distilled water. NaOH (2M) was quickly added as
precipitating agent to the precursors solution under vigorous stirring until pH = 10.5. The
obtained precipitate was filtered followed by washing with distilled water until a pH of the
precipitate reached around 7. The compound was then dried at 80 0C for one night and
calcined at 700 0C (ramp = 3 0C/min) under air for 12 h. XRD result confirmed the single-
phase perovskite and specific surface area of perovskite was observed 3.5 m2/g.
Various compositions of perovskite such as LaFe0.77Co0.17Pd0.06O3 (Zhou et al., 2002) ,
La1-xSrxBO3 (B = Mn, Fe, Co, Ni; x = 0.0, 0.3, 0.5, 0.7, 1.0) (Shen and Weng, 1998), La1-
xCaxFeO3 (x = 0-0.4) (Pecchi et al., 2008a), Zn1-xNixMnO3 (x = 0.0-1.0) (Shetkar and Salker,
2008), LaNi1-xCoxO3 (x = 0.0, 0.3, 0.5, 0.7, 1.0) (Vaz and Salker, 2007), La1-xAxMnO3 (A =
Ba, Sr or Ca; x = 0.2, 0.4) (Liu et al., 2002), La1-xMxMnO3 (M = Ag, Sr, Ce, La; x = 0.0, 0.1-
0.4 for Ag; x = 0.3 for Sr and Ce) (Song et al., 1999) and La1-xCexMnO3 (x = 0.0 to 0.3)
(Zhang-Steenwinkel et al., 2002) were prepared by co-precipitation method for oxidation of
CO and HC (CH4 or C3H6), NO reduction with NH3 and as a three way catalytic performance
to remove CO, C3H6, propane (C3H8) and NO.
2.6.2.6. Sol-gel (SG)
Sol-gel method is one of the most efficient method because it permits a better control
of stoichiometry, high reactivity, controlled dimensions, produce highly crystalline,
homogeneous structures and prepare samples at low temperature (Forni and Rossetti, 2002;
Campagnolia et al., 2005). Sol-gel method is most promising one from efficiency and scale up
point of view (Ghasdi et al., 2010).
The sol-gel method involves the formation of a sol (which is a liquid suspension of
solid particles ranging in size from 1 nm to 1 micron) followed by gel. The precursor in a sol-
gel preparation can either be a metal salt/alkoxide dissolved in an appropriate solvent (organic
solvent) or a stable colloidal suspension of preformed sols. Sol-gel method involves
hydrolysis and condensation of alkoxide based precursors.
MOR + H2O = MOH + ROH (hydrolysis) (2.7)
MOH + ROH = M-O-M + ROH (condensation) (2.8)
51
In sol-gel method, the gel must be heated to enhance solid state diffusion in order to
form a perovskite structure. Due to finely dispersed oxides, the lower calcination temperature
requires than the conventional ceramic method. It is still high (> 600 0C) which reduces the
specific surface area by enhancing grain growth and limits their catalytic activity
(Campagnolia et al., 2005; Ghasdi et al., 2010).
Uenishi et al. (2005a), Tanaka et al. (2006) and Screen (2007) prepared
LaFe0.95Pd0.05O3 by alkoxide method in which the metal salts (metal ethoxyethylates)
precursors were dissolved in toluene in the desired proportions. Hydrolysis of the salts with
water gave a precipitate which was isolated, dried and finally calcined at 700 0C for 4 h in air
to obtain the perovskites.
Sol gel citrate method is used to obtain catalysts of high specific surface area up to
two orders of magnitude higher than that for the samples prepared by calcination milling
method (Forni and Rossetti, 2002; Campagnolia et al., 2005; Merino et al., 2006) but it has
the drawback of rapid sintering depending on the temperature (Campagnolia et al., 2005).
Ghasdi et al. (2010) synthesized LaCoO3 by sol gel citrate method in which metal salts
(La(NO3)3·6H2O, Co(NO3)2·6H2O) and acetic acid (molar ratio of 1:1:1) were dissolve in
distilled water and agitated the solution at 60 0C. Then citric acid was added slowly and
temperature was increased to 100 0C and maintained at for 2 h, a gel was obtained followed
by calcination at 600 0C for 24 h. The XRD spectra of that catalyst showed that pure
perovskite phase (LaCoO3) was developed. Other minor phase such as La2O3 was detected in
addition to the major ABO3 perovskite phase which indicates the presence of a small quantity
of unreacted precursors. The specific surface area and particle size of LaCoO3 perovskite after
calcination at 600 0C were found to be 3.5 m2/g and 20 nm respectively.
LaFe1-xCoxO3 (x = 0.0, 0.2, 0.4, 0.6, 0.8) (Goldwasser et al., 2005) and Sr- or Ce-
doped PrMnO3 (Pr:Sr/Ce=1- x:x, x=0.0, 0.2, 0.4, 0.6, 0.8, 1) (Ran et al., 2005) were prepared
by sol gel citrate method.
2.6.2.7. Other methods
The other methods such as reverse microemulsion, flame hydrolysis, pyrolysis, spray-
drying/freeze-drying etc. are also reported in the literature to prepare perovskite based
catalysts.
52
Reverse microemulsion method is promising method over other conventional methods
to prepare nano-size metals, metal oxides as well as perovskite-type oxides (Wallin et al.,
2004; Giannakas et al., 2006; He et al., 2007) in which the surfactant molecules aggregate
spontaneously with water located in the core of the aggregates forming nanometer sized water
droplets that act as reactors for the formation of perovskite precursor particles (Wallin et al.,
2004). The typical procedure to prepare La0.8Ce0.2Cu0.4Mn0.6O3 catalysts by microemulsion
used by He et al. (2007) is as follows. A microemulsion containing lanthanum, cerium, copper
and manganese nitrates in the cetyltrimethyl ammonium bromide (CTAB)/1-
butanol/water/heptanes was mixed with the another emulsion containing precipitating agent
NaOH and continuously stirred at 30 0C for 2 h. During stirring, mixed microemulsion formed
which was destructed by adding acetone and generated precipitates. The resultant precipitates
were subjected to washing with acetone and water repeatedly to remove the oil, Na+ and
surfactant molecules adsorbed on the surfaces of the particles, drying and calcination. The
catalyst had high specific surface area of 14.2 m2/g with nano-particles size of 40-50 nm
which was used as a promising catalyst for reduction of NO with CO.
Shu and Kaliaguine (1998) prepared LaCoO3 using pyrolysis in which stoichiometric
amounts of lanthanum nitrate and cobalt nitrate were mixed well then heated to 300 0C for 2 h
followed by calcination at 850 0C in air for 2 h for the formation of crystalline perovskite
catalysts.
A high surface area and good chemical homogeneity of perovskites can be obtained by
the freeze-drying (Song et al., 1999). Freeze-drying is relatively complex and expensive
techniques (Ghasdi et al., 2010).
2.6.3 Limitations of perovskites prepared by conventional methods
Perovskites are mainly prepared by ceramic and wet chemical methods. One major
problem of perovskites prepared by ceramic method is that a high calcination temperature
around 800 0C requires to ensure the formation of the crystalline phase and to remove the
organic precursors that results in large grain size and low specific surface area which inhibits
their applications in automobiles field (Shu and Kaliaguine, 1998; Porta et al., 1999;
Kalinguine and Neste, 2000; Xiulan and Yuan, 2000; Kalinguine et al., 2001; Zhang et al.,
2002; Royer et al., 2004; Zhang et al., 2006c, 2006b, 2006c, 2006e; Niu et al., 2007; Zhang et
al., 2007, 2008b; Dacquina et al., 2009; Abdolrahman et al., 2010; Ghasdi et al., 2010; Ghasdi
53
et al., 2011; Sui et al., 2011). To improve the specific surface area of perovskites requires
lower calcination temperature during synthesis. In order to improve specific surface area,
various preparation methods including microemulsion (Wallin et al., 2004; Giannakas et al.,
2006), citrate complexation (Ferri and Forni, 1998; He et al., 2001; Ciambelli et al., 2002;
Colonna et al., 2002a; Qis et al., 2005; Giannakas et al., 2006; sartipi et al., 2008), co-
precipitation (Giannakas et al., 2006; Sefyi et al., 2009), sol-gel process (Rida et al., 2008;
Rousseau et al., 2009; Ghasdi et al., 2010) and flame-hydrolysis (Forni and Rossetti, 2002;
Fabbrini et al., 2003) have been developed. All of these methods require slight higher
calcination temperature in order to obtain crystalline perovskite catalysts that result low
specific surface area of the catalyst thus yielding a limited catalytic activity (Shu and
Kaliaguine, 1998). The specific surface area of perovskites prepared by above all mentioned
method observed rarely exceeds 25 m2/g (Levasseur and Kaliaguine, 2009). Wet chemical
methods such as sol-gel, co-precipitation, citrate compexation etc. are complicated, the
reagents used are normally expensive and the use of large amount of water as well as
formation of large amounts of gaseous or liquid wastes which create additional environmental
problems (Zhang and Saito; 2000b, 2001; Rougier et al., 2002; Ito et al., 2004; Liu et al.,
2009; Ghasdi et al., 2010, 2011).
Spray-drying/Freeze-drying method requires minimum calcination temperature
(around 500 0C) and time (15 h) to obtain single phase crystalline perovskite but it requires
complicated equipment (Xiulan and Yuan, 2000). A method based on citrate precursors
allows the production of perovskites even with a few tens (m2/g) of specific surface area even
after calcination at 800 0C but rapid sintering at higher temperature cannot be avoided
(Cimino et al., 2002; Colonna et al., 2002b).
To overcome the limitation of lower specific surface area, the researchers have
synthesized the perovskite phase on support like alumina, metallic and ceramic monolith or
other supports (Labhsetwar et al., 2001; Cimino et al., 2002; Colonna et al., 2002a; Liu et al.,
2002; Nguyen et al., 2002; Alifanti et al., 2007; Popescu et al., 2009; Sefyi et al., 2009).
Perovskites are not well dispersed on alumina washcoat (Labhsetwar et al., 2001, 2006;
Alifanti et al., 2007). Larger particle size of perovskites and their reactivity with alumina are
the major limitations (Labhsetwar et al., 2006). Several perovskite compositions and their
precursors (in case of in situ synthesis) are found to be reactive towards alumina at higher
54
temperature leading to the formation of undesired phases like mulite and metal aluminates
(Labhsetwar et al., 2001, 2006; Cimino et al., 2002; Liu et al., 2002; Alifanti et al., 2007).
To avoid limitations of perovskites prepared by conventional methods, preparation
method called reactive grinding has been proposed for preparation of perovskites (Zhang et
al., 2006d Nguyen et al., 2002; Ito et al., 2004; Merino et al., 2006; Zhang et al., 2006a,
2006d, 2006e, 2008b; Dacquina et al., 2009). A reactive grinding (high energy
mechanochemical synthesis) method produces pure crystallization perosvkite phase at
ambient temperature without using the thermal treatment (Szabo et al., 2002, 2003b; Royer et
al., 2004). This method uses high energy ball milling resulting in relatively high specific
surface area (exceeding 150 m2/g) for perovskites (Szabo et al., 2002, 2003b; Kaliaguine,
2007; Zhang et al., 2008b).
2.7 Reactive grinding method
2.7.1 About reactive grinding method
In last two decades, a number of mechanical routes such as mechanical alloying (MA),
mechanical alloying combined with annealing, reactive milling and mechanical milling (MM)
has been developed in order to produce nanocrystalline or amorphous alloys or intermetallic
compounds. All these techniques use mechanical energy to achieve chemical reactions and
structural changes (Grugeon-Dewaele et al., 1998; Suryanarayana, 2004; Chicinas, 2006).
Mechanochemical processing (MCP) or mechanochemical synthesis is a powder
processing techniques in which due to severe and intense mechanical action on solid surfaces
cause physical and mechanical changes. These mechanically initiated chemical and
physicochemical effects in solids are termed as mechanochemical effect (Grugeon-Dewaele et
al., 1998; Soni, 1998; Venkataraman and Narayanan, 1998; Suryanarayana, 2004; et
al., 2007). It is also known as reactive (reaction) grinding/milling because chemical reactions
and phase transformations take place between solid phases of powder compounds inside the
ball mill due to the application of mechanical energy. Mechanical energy is converted to
chemical energy that cause chemical reaction which is normally occurs at very high
temperature. The silent feature of the process is that plastic deformation and chemical
processes occur almost simultaneously (Lu and Lai, 1997; Grugeon-Dewaele et al., 1998;
Soni, 1998; Suryanarayana, 2004; Chicinas, 2006; Chaira et al., 2007; Wang et al., 2007a).
55
It is used to synthesis various nano-structured metal, metal oxides, carbides, nitrides
and borides (Wang et al., 2007a; Chaira et al., 2009). The materials produced by reactive
grinding method are used in various fields such as gas absorbers, fertilizers, electrochemistry
and catalysis etc. and also use as hydrogen storage materials (Rougier et al., 2002;
Suryanarayana, 2004). Reactive grinding is dry chemical method (Wang et al., 2007a). There
are two type of milling carried out in reactive grinding: (1) Wet milling (grinding) in which
metal powder mixtures are milled with a liquid (2) Dry milling in which no liquid is involved.
Dry milling is more effective than wet milling (Pourghahramani et al., 2008).
Reactive grinding is a process which involve synthesis of materials by high energy
ball milling e.g. SPEX shaker mills, planetary mills, vibratory mills, attritors and tumbling
ball mills in which elemental blends of salts precursors (or pre-alloyed powders, oxides,
nitrides etc.) are milled and high energy induce due to intense milling cause the chemical
reaction among oxide powders and thereby synthesize the crystalline perovskite catalysts
(Kalinguine and Neste, 2004; Suryanarayana, 2004; Chicinas, 2006). During milling, there are
three processes occur such as cold welding, plastics deformation and further fragmentation of
the particles (Lu and Lai, 1997; Suryanarayana, 2004; Chicinas, 2006; Pei-Zhong et al.,
2008).
In reactive grinding method, powder materials and balls are called charge materials
which are placed in a jar (also called as vial, bowl or pot) as shown in Fig. 2.6. These charge
materials are move in vial or jar along with the motion of the vial or jar. This motion makes
charge materials in colliding positions. The repeated collision between balls and powders with
very high impact energy create crack and fracture which break the powder materials into
smaller size. Due to repetitive cold welding and fracturing mechanism, cold welding of
overlapping particles occurs between new and clean surfaces formed due to fractures. The
competing process of plastic deformation, fracture and cold welding during milling process
produces a microstructure refinement and finally some composition changes (Chicinas, 2006;
Chaira et al., 2007; Budina et al., 2009).
56
Fig. 2.6. Schematic of the collision as the main event of energy transfer (Chaira et al.,
2007).
There are two types of actions occur during milling: (1) Collision (create impact
energy) (2) Rubbing (create friction). Both are responsible to produce fracture in powder
materials. The action of rubbing creates friction and generates heat in the system (Budina et
al., 2009).
Reactive grinding method is simple (Kalinguine and Neste, 2000, 2004; Wang et al.,
2007a; Schneider et al., 2009; Ghasdi et al., 2010), efficient (Kalinguine and Neste, 2000,
2004), cheaper, no need of solvent (Kalinguine and Neste, 2000, 2004; Wang et al., 2007a;
Schneider et al., 2009; Ghasdi et al., 2010), proceed in close environment, no generation of
waste during synthesis (Xiulan and Yuan, 2000; Ghasdi et al., 2010), reduce the time requires
for reaction (Ghasdi et al., 2010) and can be carried out at nearby ambient temperature with
no need of heat treatment in subsequent steps to produce crystalline perovskites mixed oxides
consisting of agglomerates of nanocrystalline or nanocomposites (Kalinguine and Neste,
2000; Szabo et al., 2002; Kalinguine and Neste, 2004; Kaliaguine, 2007; Wang et al., 2007a)
with no formation of hazardous airborne nanoaerosols (Kaliaguine, 2007). It is energy saving
dry chemical method for example the better energy balance in comparison with microwave-
assisted reactions (Wang et al., 2007a; Schneider et al., 2009). During milling, precursors
crystallite size reduce to nano-scale due to mechanical impact which make mixture more
homogeneous and enhances solis state diffusion. A number of perovskites composition with a
57
crystallite size down to 10 nm and a specific surface area varying between 4 and 100 m2/g
have been successfully synthesized for catalytic applications (Kaliaguine, 2007; Levasseur
and Kaliaguine, 2008; Ghasdi et al., 2010). It produces the perovskite catalysts with high
density of nano-scale grain boundaries which is highly desirable in their applications as a
catalyst for redox reactions because of oxygen mobility is usually higher in grain boundaries
(Kalinguine and Neste, 2000, 2004; Ghasdi et al., 2010). The Que´bec firm, Nanox Inc.
installed a demonstration unit for the production of perovskite prepared by reactive grinding
method with capacity of 15 t/year (Levasseur and Kaliaguine, 2008).
2.7.2 Equipment for reactive grinding method
Various types of high energy milling equipments are used to produce milled powders.
The high energy milling equipments are classified according to their design, capacity,
efficiency of milling, speed of operation and capacity to control the operation by varying the
temperature of milling and the extent to which minimize the contamination of the milled
powders and additional arrangements for cooling, heating etc. There are two types of grinding
devices: (1) self-rotation and (2) agitation types. Typical devices of the former type are
tumbling, vibrating and planetary mills and the latter includes bead and agitation mills (Soni,
1998; Kano et al., 2001; Suryanarayana, 2004; Pourghahramani, et al., 2008; Budina et al.,
2009; Schneider et al., 2009).
Various types of ball mills are used such as (drum) ball mills, jet-mills, bead-mills,
vibration ball mills, planetary ball mills, attritors, SPEX shaker mills and horizontal rotary
ball mills (Soni, 1998; Suryanarayana, 2004). All of these mills are working based on the
principle that a starting material is placed between two surfaces and crushed due to the impact
or frictional forces that are generated by collisions between these surfaces. The various mills
are classified based on the method by which these collisions are caused. Besides the intensive
grinding effect, these collisions lead to an energy transfer that increases the internal
temperature and pressure. In general, ball mills are able to produce materials with a particle
(Schneider et al., 2009). SPEX shaker mill is more energetic, ball mills are
under the category of least energetic and planetary mill is in between (Suryanarayana, 2004).
58
2.7.2.1 Planetary ball mills
Planetary ball mills referred to as Pulverisette as shown in Fig. 2.7 are smaller in size
than common ball mills and mainly used in laboratories for grinding the sample material with
the capacity of 100 g and down to very small sizes (Suryanarayana, 2004).
Fig. 2.7. Fritsch Pulverisette P-5 four-station planetary ball mill (Lu and Lai, 1997; Soni,
1998; Suryanarayana, 2004).
A planetary ball mill shown in Fig. 2.8 consists of pots (grinding jars) on a sun wheel
(disk) and both are rotated in clock or anticlockwise direction at high speed. Due to high-
speed rotation of both the pots and the sun wheel makes the balls to move violently which
create large impact energy and that improves grinding performance. Planetary ball mill is
used for mechanochemistry (MC), mechanical alloying (MA) and the mechanical milling
(MM) for last ten years (Mio et al., 2004a, 2004b). The planetary ball mill owes its name due
to its planet-like movement of its jar.
Fig. 2.8. Schematic diagram of the planetary ball mill (Mio et al., 2004a, 2004b).
59
The grinding pot rotates about its own axis and also in the direction around the
common axis of the sun wheel. Due to rotation of the grinding pot and the sun wheel in
opposite directions, the centrifugal forces alternatively act in the same and opposite
directions. The grinding balls and the powder material in the grinding pot experience
centrifugal forces which make grinding ball to move with a high pulverization energy. The
centrifugal forces acting on the grinding pot wall and initially carry the grinding balls in the
direction in which the grinding jar is rotating. Due to differences occur between the speed of
the grinding pot wall and the balls results in strong frictional forces acting on the sample. As
the rotational movement of pot increases, coriolis forces developed which displaces the balls
from the grinding pot walls. The balls fly through the grinding pot interior and impact against
the sample on the opposite grinding pot wall. This releases large amount of dynamic impact
energy. The combination of the frictional forces and impact forces causes the high degree of
size reduction of materials inside the planetary ball mills. Planetary ball milling can induce
chemical reactions simultaneously with size reduction (Lu and Lai, 1997; Suryanarayana,
2004; Zhang et al., 2006b).
At the beginning of the milling process, reaction does not occur because it requires
critical milling time for combustion reaction to be initiated. However, it becomes possible
after a critical milling time (the time at which sudden an increase in temperature occurs is
referred to as the igniting time or critical milling time). During milling the components are
mixed thoroughly because it reduces the particle size and increases the number of chemically
active defect sites. The active defect sites increase due to repeated cold welding and fracturing
of the powder particles which increases the contact area between the reactant powder particles
by repeatedly bringing fresh surfaces created by reduction of particle size during milling. This
allows the reaction to proceeds without the necessity for diffusion through the product layer
because intimate contact between reactant powder particles requires for combustion to occur.
As a consequence, reactions that normally require high temperatures to occur will occur at
low temperature in a planetary mill without any need of heat treatment (Grugeon-Dewaele et
al., 1998; Suryanarayana, 2004; Chaira et al., 2007).
Hosseini et al. (2011) observed that no reaction is occurred between Co3O4 and Al
before 45 min. When the powder mixtures of Co3O4 and Al milled for 50 min (at which time
combustion took place), large exothermic heat released due to reaction between two
60
components which suddenly increased the temperature of vial from 29 to 43 0C and finally
Al2O3/Co nano-composites formed.
In planetary ball mill, grinding of materials depends on various grinding conditions
such as pot diameter, pot depth, ball diameter, ball-filling ratio, revolution radius and
rotational speed etc. (Mio et al., 2004a, 2004b). These all are the parameters important for
designing of planetary ball mill (Mio et al., 2004b). Mio et al. (2004b) scale up the method for
the design of the planetary ball mill using the impact energy of balls. They concluded that the
grinding rate increases with increasing the revolution speed of the mill and by reducing the
weight of the sample. The impact energy of the balls in the planetary ball mill is proportional
to cubed of diameter of the pot, depth of the pot and revolution radius.
Specific impact energy increases with an increase in rotation to revolution speed ratio
(r =NPot/Nr where NPot is rotation speed of the pot, Nr is revolution speed of the disk) and
subsequently falls down. The motion of ball as shown in Fig. 2.9 changes inside the mill from
rolling when rotation to revolution speed ratio is considerably a large due to the centrifugal
force caused by the rotation of the pot. This rolling motion is ineffective for grinding
performance because it reduces the rate of collision of balls each other or against the mill
wall.
Fig. 2.9. Schematic view of motion of the balls and powder mixture inside the planetary ball mill (Lu and Lai, 1997; Budina et al., 2009).
The speed ratio at which significant large specific impact energy is imparted on the
materials is defined as the critical speed ratio and at which no rolling motion of balls takes
place. Thus, the mill pot is rotated at around the critical speed ratio rcs ( 1ClrR
csr -= where rcs
61
is critical speed ratio, Rr is revolution radius and lc is distance from the rotating shaft to the
centroid of ball that contacts with the mill wall as shown in Fig. 2.10 and in the counter
direction to the disk revolution reported by Mio et al. (2004b).
Fig. 2.10. Model of centrifugal force (Mio et al., 2004b).
They also pointed out that if pot rotates in the counter direction to the disk is effective
for improving the grinding performance by imparting large specific impact energy on the balls
rather than the normal rotational direction because the vector of the relative velocity in the
counter direction is found to be significantly larger than that in the normal one (as shown in
Fig. 2.11).
Fig. 2.11. Difference in the vector of relative velocity of balls between counter and
normal directions (Mio et al., 2004b)
62
U.S. Pat. No. 6,126,097 (Chen et al., 2000) disclosed high energy planetary ball
milling apparatus and method for the preparation of nano-sized powders. The disclosed
planetary ball milling was single motor operated, main rotary wheel run clockwise and at the
same time mill pots run anticlockwise vice versa. The mill pots speed depends on main rotary
wheel speed. The disclosed mill was used for milling to produce ultrafine and nano-sized
powders. This apparatus provide improved crushing forces and intensity with which the
grinding balls impact the powder materials inside the mill pots which make the process fast
and effective and make it possible to mass produce nano-sized ceramic powders.
U.S. Pat. No. 7,744.027 B2 (Nagao, 2010) disclosed planetary ball mill in which mill
pots were supported on the revolution turning arm which rotated about a rotation shaft
inclined (inclination in the range of 15 to 40 degrees) from the vertical direction toward the
side of revolution shaft. They disclosed the planetary ball mill for grinding a matter by
bringing a mill pot containing medium balls and the matter into rotation concurrently with
revolution. They claimed that inclination of mill pot and concave spherical shape of mill pot
was generated tornado motions which create high grinding efficiency.
2.7.2.2 SPEX shaker mills
Shaker mills such as SPEX mills as shown in Fig. 2.12 which has the capacity of
milling about 10-20 g of the powder at a time. The SPEX mill has one vial that filled with
powder sample and grinding balls, secured in the clamp and swung energetically back and
forth several thousand times a minute. The back-and-forth shaking motion is combined with
lateral movements of the ends of the vial so that the vial appears to be describing a figure of 8
or infinity symbol as it moves. With each swing of the vial the balls impact against the sample
and the end of the vial which create both milling and mixing of the sample. Because of the
amplitude (about 5 cm) and speed (about 1200 rpm) of the clamp motion, the ball velocities
Therefore, these mills can be categories under high energy type of mill (Suryanarayana, 2004;
Zhang et al., 2006a).
63
Fig. 2.12. SPEX 8000 mixer/mill (Suryanarayana, 2004).
There are two vials provided in the most recent design of the SPEX mills in order to
increase the yield by milling the powder sample has simultaneously in two. This machine
incorporates forced cooling to permit extended milling times. The sizes of grinding vials are
21/4 in. wide and up to 3 in. long. They have an internal volume of 50-60 ml with a grinding
capacity of about 10 ml and a blending capacity of 25 ml. They are available in tool steel,
hardened steel, tungsten carbide, alumina ceramic, zirconia ceramic, silicon nitride, agate and
methacrylate. Vials are used in pairs to maintain the balanced motion of the clamps
(Suryanarayana, 2004). A typical example of a tungsten carbide vial, gasket and grinding
balls and other vials for the SPEX mill is shown in Figs. 2.13(a) and (b) respectively.
In case of SPEX shaker mill, the balls may roll around the end of the vial rather than
hitting it which decreases the intensity of milling and the powder may collect in the eyes of
(Suryanarayana, 2004).
Fig. 2.13(a). Tungsten carbide vial set consisting of the vial, lid, gasket and balls
(Suryanarayana, 2004).
64
Zirconia ceramic vial set Stainless steel vial set Silicon Nitride vial set
Agate vial set Methacrylate vial set Round ended hardened steel vial set
Fig. 2.13(b). Vial sets (Suryanarayana, 2004).
2.7.2.3 Attritor mills
The Attritor (as shown in Fig. 2.14) is often referred to as an agitated or stirred ball
mill in which large quantities of powder with capacity of around 45.3 kg and can be milled at
a time. An attritor consists of a vertical drum containing a series of impellers which rotated
with the help of powerful motor. The impellers agitate the steel balls in the drum. The
material to be ground is charged into a vertical tank filled with the grinding media. Both the
material and grinding media are then agitated by a shaft with arms that exert shearing and
impact forces on the material with the help of media. This action reduces the materials into
extremely fine material. No pre mixing is required. All attritors are provided with cooling
jackets. Recirculation facility is available as optional. The velocity of the grinding medium in
the attritors is much lower (about 0.5 m/s) than in the planetary or SPEX mills and
consequently the energy of milling in the attritors is low (Lu and Lai, 1997; Suryanarayana,
2004).
65
Fig. 2.14 Schematic diagram of attritor mill (Lu and Lai, 1997; Suryanarayana, 2004)
2.7.3 Process variables in milling
Number of process variables affect on the desired product phase, microstructure
and/or properties during reactive grinding method and needs to optimize. During high energy
ball milling, various parameters have an effect on the final constitution of the milled powder
are types of mill, milling container, milling speed, milling time, type, size and size
distribution of grinding medium, ball to powder weight ratio (BP), extent of jar filling, milling
atmosphere, process control agent and temperature of milling etc. (Soni, 1998;
Suryanarayana, 2004; Pei-Zhong et al., 2008; Pourghahramani, et al., 2008; Budina et al.,
2009; Schneider et al., 2009).
2.7.3.1 Type of mill
Depending on the types of powder, the quantity of powder to be milled and the final
composition required, a suitable mill can be selected. Most commonly used mills are SPEX
shaker mills, the Fritsch pulverisette planetary ball mills and attritors (Suryanarayana, 2004;
Soni, 1998).
2.7.3.2 Milling container
The material used for the milling container (grinding vessel, vial, jar and bowl are
some of the other terms used) is important because due to impact of the grinding medium on
the inner walls of the container some material will be dislodged and be incorporated into the
66
powder. This can be contaminated the powder and/or alter the composition of the milled
powder. If the material of the grinding vessel is different from that of the powder being milled
then the powder may be contaminated with the material of the grinding vessel. On the other
hand, if the materials of the container and the powder being milled are the same then the
composition of the final powder may be altered (Suryanarayana, 2004). Milling containers are
made up of various materials such as hardened steel, tool steel, hardened chromium steel,
tempered steel (TS), stainless steel (SS), tungsten carbides (WC), WC-lined steel and bearing
steel etc. (Soni, 1998; Suryanarayana, 2004).
2.7.3.3 Milling speed
The kinetic energy imparted by the grinding medium to the powder being milled is
calculated by 2
21
mvE = where m is the mass of the grinding medium and v is the relative
velocity of the grinding medium (Chicinas, 2006; Pei-Zhong et al., 2008). If the mill rotates at
high speed, relative velocities of the grinding medium increases which impart higher kinetic
energy to the powder. However, depending on the design of the mill there are certain
limitations to the maximal speed that could be used. The mill is rotated at around the critical
speed ratio as discussed in section 2.7.2.1. As the speed of the mill increases, due to intense
impact of the grinding medium, temperature of the jar may increase which may contaminate
the powder and also cause excessive wearing of the milling tools (grinding medium and
container) that may also increase the powder contamination (Suryanarayana, 2004; Budina et
al., 2009). The powder yield could also be lower if the powder gets stuck to the inner walls of
the milling container.
The term milling intensity rather than milling speed or milling energy is used which is
the function of ball to powder weight ratio (BP = Mb/MP) and velocity and frequency of the
balls as defined as I = MbVmaxf /MP Where Mb is the mass of the ball, Vmax is the maximum
velocity of the ball, f is the impact frequency and MP is mass of the powder in the jar. Thus,
the milling intensity increases rapidly with the impact frequency, velocity and mass of the
balls (Suryanarayana, 2004). It has been observed that high mill energies increase the degree
of crystallization and with low energies amorphization occurs (Soni, 1998).
67
2.7.3.4 Milling time
Milling time is an important variable during milling and selected in such a way that
steady state can be achieved between fracturing and cold welding of the powder particles to
avoid agglomeration of the particles. The time required may vary depending on the types of
mill used, mill settings and intensity of milling, BP and temperature of milling. If the powder
is milled for longer time them it increases the level of contamination or some undesirable
phase formation. Therefore, it is desirable that the powder is to be milled just for the required
duration.
As a general rule, milling time is short for high-energy mills and longer for low-
energy mills in order to achieve steady. Also milling time is shorter for high BP values and
longer for low BP values (Soni, 1998; Suryanarayana, 2004).
2.7.3.5 Types, size and size distribution of grinding medium
The size of the balls affects the size, morphology, recrystallization temperature and
enthalpy of the powder produced. Cold welding and fracturing events can be enhanced by the
use of a different size of balls rather than using the same size of balls (Lu and Lai, 1997;
Kaliaguine, 2007). Hardened steel, tempered steel, hardened chromium steel, tempered steel,
stainless steel, zirconium oxide, tungsten carbides and bearing steel etc. are the most common
types of materials used for the grinding medium. More impact force imparted on the powder
by selecting high density grinding medium. The density of the grinding medium higher which
imparted more kinetic energy on the powder during milling. Thus, WC balls are frequently
used instead of steel to generate higher impact energy (Lu and Lai, 1997; Soni, 1998;
Suryanarayana, 2004; Kaliaguine, 2007; Schneider et al., 2009). The number of milling balls
require for each grinding should be calculated based on the extent of filling the jar (grinding
vessel) criteria (Schneider et al., 2009).
2.7.3.6 Ball to powder weight ratio (BP)
The ratio of the weight of the balls to the powder (BP) sometimes referred to as charge
ratio (CR) is an important variable in the milling process (Suryanarayana, 2004).
Reactive grinding process is fast if BP is higher because the number of collision per
unit time increases as the number of balls increases. At the same time, collision frequency
also increases that result in an increasing in milling temperature which makes diffusion
process faster. In general, the BP used in the range of 5:1 to 30:1 (Lu and Lai, 1997; Soni,
68
1998). If BP increases, due to intense impact of balls cause wearing of the milling tools which
increase the contamination in the powder (Soni, 1998).
BP depends on types of mills used for milling. BP of 10:1 is most commonly used if
milling the powder in a small capacity. In case of high energy mill such as SPEX shaker mill,
BP values are in the range of 4:1 to 30:1. For planetary ball mill, BP value is around 20:1.
When milling is carried out in low energy mill for example an attritor then BP is up to 50:1
(Lu and Lai, 1997; Soni, 1998). BP can be increased either by increasing the number (weight)
of balls or by decreasing the powder weight. Higher BP can also be obtained (for the same
number of balls) either by increasing the diameter of the balls or by using higher density
materials such as WC rather than steel (Suryanarayana, 2004).
2.7.3.7 Extent of filling the jar
The amount of charge materials in the jar or vial will change the amount of free space
available in the jar or vial. It is necessary that there will be enough free space available for the
balls and the powder particles to move around freely in the milling container to create severe
impact forces on powder particle. Therefore, the extent of filling the jar or vial with the
powder and balls is important (Suryanarayana, 2004; Pei-Zhong et al., 2008; Budina et al.,
2009; Schneider et al., 2009).
If the quantity of the balls and the powder is very small, then the production rate is
very low. On the other hand, if the quantity is large then there is not enough space for the
balls to move around and so energy of the impact is less (Suryanarayana, 2004; Pei-Zhong et
al., 2008; Budina et al., 2009; Schneider et al., 2009). Suryanarayana (2004) suggested that
the care must be taken not to overfill the jar generally about 50% or a little more of the jar
space is left empty.
2.7.3.8 Milling atmosphere
The milling process is normally carried out under vacuum or in an inert atmosphere in
order to prevent or minimize oxidation and/or contamination of the milled powder (Lu and
Lai, 1997; Soni, 1998; Suryanarayana, 2004).
The major effect of the milling atmosphere is on the nature and extent of
contamination of the powder. Therefore, powders are milled generally in containers that have
been evacuated or filled with an inert gas. In most of the case Ar is used. Other gases He and
69
N2 are also used (Calka and Wulliams, 1992; Lu and Lai, 1997). H2 atmosphere was used to
produce hydrides (Chen and Williams; 1996).
Normally, loading and unloading of the powders into the jar or vial is carried out
inside evacuated or atmosphere-controlled glove boxes. These glove boxes are usually
repeatedly evacuated and refilled with an inert gas such as argon (Lu and Lai, 1997;
Suryanarayana, 2004). The presence of air in the vial produces oxides and also nitrides if the
powders are reactive in nature. Thus, care must be taken to use an inert atmosphere during
milling (Calka and Wulliams, 1992). Kalinguine and Neste (2004) reported that the milling
atmosphere was controlled by using various gases such as H2O, He, Ar, N2, O2, H2, CO, CO2,
NO2, NH3, H2S and mixture thereof in order to control the nano-crystalline structure and
stochiometric oxygen content of the perovskites.
2.7.3.9 Process control agents (PCAs)
It is necessary to maintain a balance between cold welding and fracturing of powder
particles to increase the specific surface area of the prepared materials by avoiding
agglomeration. A process control agent (PCA) (also referred to as lubricant or surfactant or
leachable additive) is added to the powder mixture during milling which covers the whole
surface area of the powder particles and then excessive cold welding does not take place
which improve the specific surface area of materials (Chen and Williams; 1996; Lu and Lai,
1997; Suryanarayana, 2004). Process control agents (PCAs) can be solid, liquid or gaseous.
They are mostly organic compounds which act as surface-active agents but not necessary
every time. The choice of a PCA for milling depends on the nature of the powder being milled
and the purity of the final product desired (Suryanarayana, 2004). It should be selected in such
a way that they can be removed easily during further leaching and washing process for
example zinc oxide (ZnO) or sodium chloride (NaCl) is used as PCA, the obtained powders
are repeatedly washed with diluted ammonium nitrate (NH4NO3) if the additive is ZnO or
with distilled water if the additive is NaCl (Kalinguine and Neste, 2004; Tien-Thao et al.,
2007b). The nature and amount of PCA used during milling would determine the final powder
particle size and powder yield (Suryanarayana, 2004). A wide range of PCAs has been used in
practice at a level of about 1-5 wt% of the total powder charge. The most important PCAs
include stearic acid (C18H36O2), hexane (C6H14), methanol (CH3OH), ethanol (C2H5OH),
alkali chloride (lithium chloride-LiCl, NaCl, rubidium chloride-RbCl, cesium chloride-CsCl,
ammonium chloride-NH4Cl), sodium acetate (CH3COONa) or nitrate (NaNO3), ZnO (Chen
70
and Williams; 1996; Tien-Thao et al., 2007b), triisopropanolamine (C9H21NO3), ethylene
glycol (C2H6O2) and propylene glycol (C3H8O2) etc. (Campagnolia et al., 2005).
2.7.3.10 Temperature of milling
The temperature of milling is another important variable in determining final phase of
the powder produced. Sometimes temperature of milling affects on the milling time require
for formation of final phase of the powder (Suryanarayana, 2004; Budina et al., 2009). For
example, it was noted that the X-ray diffraction (XRD) pattern of Si milled in anhydrous
ammonia for 48 h at 100 0C was similar to that of milled for 168 h at room temperature
(Suryanarayana, 2004). The effect of milling temperature has been already discussed in the
section 2.7.3.3.
Schneider et al. (2009) carried out Suzuki Miyaura reaction by mechanochemical
reported that the yield of 4-acetylbiphenyl depends on number of parameters such as
revolutions per minute (rpm), milling time (t), number and size of milling balls and the
grinding material. They investigated that the various parameters have positive effect on the
formation of 4-acetylbiphenyl as per following order: rpm > milling time > size of milling
balls > number of milling balls > grinding material.
Chaira et al. (2007) synthesized silicon carbides (SiC) by reactive grinding at room
temperature from elemental silicon and graphite powder. They reported that the amount of
impact increases with increasing ball diameter hence size of ball affect the reaction milling.
As the milling time increases, powder particles become finer and contain more defects which
are prerequisite for the synthesis of crystalline SiC at ambient temperature.
2.7.4 Preparation of perovskites by reactive grinding method
It is reported in the literature that pervoskites can be prepared by reactive grinding
method using high energy ball mills such as SPEX shaker mills, planetary ball mills and
vibration mills etc. Neste (2004) (U.S. Pat. No. 6,713,037B2) disclosed the mechanochemical
synthesis method for the synthesis of lithium based mixed oxides from their elementary
oxides by subjecting them to a high energy milling process using SPEX mill at room
temperature. They claimed that high energy milling process used for synthesize lithium based
mixed oxides is simple, economical and requires no heating steps.
71
Kalinguine and Neste (2000, 2004) disclosed the mechanochemical synthesis method
using high energy ball milling to synthesize perovskites with high specific surface area in
U.S. Pat. No. 6,017,504 and U.S. Pat. No. 6,770,256B1. They reported two steps milling
process comprised of development of pure crystalline perovskite in SPEX shaker mill
followed by second milling step in which leachable process control agent was added as an
additive to increase specific surface area which was further removed in a subsequent step. The
product in form of slurry obtained was subjected to drying followed by calcination. The
improved specific surface area greater than 40 m2/g was obtained with this method.
Alamdari et al. (2009) invented the process for producing activated perovskite based
washcoat formulation using high energy ball milling suitable for reduction of CO, VOC, PM
and NO emission from an exhaust stream in U.S. Pat. No. 0324470 A1. They disclosed three
steps (a) activation of a perovskite structure (b) mixing with a dispersing media and (c)
obtaining the washcoat formulation. They disclosed the process in which fully synthesized
perovskite prepared by co-precipitation, sol-gel or citrate method after calcination in the range
of 700 to 1200 0C was subjected to horizontal ball milling and milling was carried out at a
speed in the range of 50 to 1000 rpm for 1 to 7 h to obtain activated nanocrystalline
perovskite (step (a)). Activated nanocrystalline perovskite was mixed with dispersing media
(5 to 60 wt% of total charge) such as water or alcohols or amines or any other compatible
solvents (combination of water and triethanolamine C6H15NO3) in order to increase the
specific surface area and subjected to further milling. The dispersing media was partially or
totally removed from the product obtained after milling was subjected to drying followed by
calcination (step (b)). Then slurry of activated pervoskite based catalysts was coated on a
metallic or ceramic substrate to obtain a perovskite based catalytic converter (step (c)). The
activated perovskites based (La0.6Sr0.4Co0.99M.01O3 where M is Pt or Pd) washcoat formulation
had a specific surface area varying between 10 and 200 m2/g and exhibited 100% CO
conversion to CO2 at a temperature lower than 150 0C.
Ito et al. (2004) synthesized LaCoO3 perovskite by two methods (1) direct synthesis
from oxides and (2) synthesis from chlorides (indirect synthesis) route using planetary ball
mill (Fritch Pulverisette-7) operated at 700 rpm at room temperature. They used zirconia pot
and zirconia balls as grinding medium. In the direct method for synthesis, starting oxides
(La2O3 and Co2O3) were mixed and grinded in planetary ball mill. During milling, the
grinding caused reaction between two oxides and synthesized LaCoO3. The yield of LaCoO3
72
was obtained after 3 h milling and the specific surface area was only 3 m2/g due to strong
agglomeration. The synthesis method from chlorides (indirect) route was composed of three
steps: the first step was to grind a mixture of LaCl3, CoCl2 and NaOH and carried out solid
state reaction to form La(OH)3, Co(OH)2 and NaCl. In the second step, the mixture was
calcined at 600 0C under atmospheric condition that converted hydroxide into LaCoO3
followed by washing with water to remove NaCl and finally drying in an oven. The product
formed weak agglomerates and its specific surface area was about 10 m2/g larger than that in
the direct process due to dehydration of hydroxides formed and dispersed in the ground
product. This indirect method is recommended as a novel mechanochemical approach for
preparing nano-particle materials.
Sompech et al. (2012) also prepared LaCoO3 by the same indirect method. The
starting materials (LaCl3 2O, CoCl2 and Na2CO3) were mixed in molar ratio of 1:1:2.5 in
planetary ball mill which operated at 300 rpm for 3 h. The milled sample was calcined at 600,
700 and 800 0C for 90 min followed by washing with distilled water several times to remove
NaCl phase and subsequently filtered and dried at 110 0C for 3 h. XRD spectra show that the
pure LaCoO3 perovskite phased was developed in all the samples calcined at 600, 700 and
800 0C. SEM results show that the particles were in an agglomeration form with mean
primary particle sizes in the range of 0.3-0.6 µm.
Zhang et al. (2002) prepared LaCrO3 using a planetary ball mill operated at 700 rpm at
room temperature in which first hydrous amorphous chromium oxide Cr2O3 nH2O and
crystalline Cr2O3 powders prepared by heating Cr2O3 nH2O at different temperature were
separately ground with La2O3 powder. Hydrous amorphous chromium oxide reacted with
La2O3 and formed LaCrO3. No reaction was took place in the case of crystalline Cr2O3. The
perovskite phase LaCoO3 was obtained after 3 h milling with low specific surface area of 5
m2/g. The prepared catalyst had agglomerates and large particles with size up to around 100
µm due to water existing in the starting hydrous oxides. They concluded that that reactivity of
starting oxides depends on the preparative conditions and plays significant role during
synthesis of perovskite using high energy ball milling.
Zhang and Saito (2001) reported that crystal sizes of starting oxides are important to
synthesis perovskite phase. Fe2O3 powders with different crystallite sizes prepared by heating
FeOOH at various temperature were ground with La2O3 powder using a planetary ball mill
operated at 650 rpm. Fe2O3 powder with smaller crystallite size (20 nm) obtained by heating
73
at lower temperature reacted more easily with La2O3 than that of with larger size and formed
LaFeO3. Specific surface area of the LaFeO3 powder synthesized after 240 min milling was
obtained 11 m2/g.
Nitadori et al. (1986) -Al2O3 is not reacted with La2O3 during
mecahnochemical synthesis of LaAlO3 -
Al2O3) is stable against grinding to react with other oxides. They also observed that single
phase LaAlO3 with a large surface area was obtained when transition alumina was reacted
with La2O3. They attempted to synthesize RMO (R = Nd, Pr, Sm, M = Fe, Cr) (Zhang and
Saito; 2001; Zhang et al., 2002), LaMnO3 (Zhang and Saito; 2000b), SrMnO3, La0.7Sr0.3MnO3
(Zhang et al., 2000), RAlO3 (R = La, Nd, Sm, Dy, Ce, Pr) (Zhang and Saito; 2000a) by
grinding the mixture of constituent oxides using high energy ball mill i.e. planetary ball mill
at room temperature.
Kalinguine and Neste (2004) prepared LaCoO3 from component oxides using high
energy ball mill i.e. SPEX shaker mill. The typical procedure is as follows: 3.3 g of La2O3 and
1.7 g of Co3O4 were introduced in a cylindrical tempered steel vial with three tempered steel
balls (two of 11mm diameter and 1 of 14 mm diameter). The container was closed with a
thick cover and O-ring. The container was inserted horizontally in a SPEX shaker mill. The
milling was carried out at the speed of 1000 cycles per minute. After 16 h of milling in
presence of air, perovskite phase was formed and exhibited specific surface area of 16 m2/g
slightly higher than the conventional methods. They reported that milling time to form
LaCoO3 reduced from 16 h to 14 h by changing the milling atmosphere with pure oxygen
because of speed of reaction enhances in the presence of pure oxygen. They also synthesized
various perovskites composition CeCuO3, La0.6Sr0.4Co0.8Fe0.2O3, La0.6Sr0.4CoO3, YCoO3,
La0.6Sr0.4MnO3.
Royer et al. (2004, 2005b) synthesized LaCoO3 by reactive grinding method and
followed the procedure in two steps: In first step, the starting oxide precursors were ground
for 4 h under O2 in a SPEX mill at 1040 rpm and at the end of 4 h single oxides were
convertered to perovskites. In the second step, refinement was made with an additive (ZnO or
NaCl) added to the perovskite. After the second grinding step, the obtained compound
(perovskite + additives ZnO or NaCl) was washed repeatedly with water or solvent to free the
sample from any trace of additive. This step was performed in order to increase the specific
74
surface area of the LaCoO3. BET result shows that specific surface area of the LaCoO3 was
17.1 m2/g.
LaCo1-xCuxO3 (x = 0.0, 0.1, 0.2), LaMn1-xCuxO3 (x = 0.0, 0.1, 0.2), LaFe1-xCuxO3 (x =
0.0, 0.1, 0.2), LaFe1-xPdxO3 (x = 0.03) (Zhang et al., 2006c, 2006b, 2006c, 2006d, 2007,
2008b), LaCo1-xFexO3 (x = 0.0, 0.2, 0.4, 1) and La0.9Ce0.1CoO3 (Levasseur and Kaliaguine,
2009) prepared by reactive grinding method in which starting oxides precursors (pre-calcined
La2O3 at 600 0C for 24 h, CuO or PdO, Co3O4 or Mn3O4
or Fe2O3 or Ce2O3) in desired
proportions were milled using a SPEX grinder rotated at 1100 rpm equipped with the
tempered steel vial and three tempered steel balls. Grinding was conducted in two steps of 8 h
for synthesis and 10 h for refining with ZnO as the grinding additives in order to enhance the
specific surface area of the product. The product was washed with solvent to remove ZnO and
finally calcined at 500 0C for 5 h. The specific surface area of 20-35 m2/g for cobalt
lanthanates and approximately 40 m2/g for manganese lanthanates and 20 m2/g for ferrite
lanthanates were obtained even after calcination.
LaCoO3 with additives ZnO (Royer et al., 2005b) for CO oxidation, LaCoO3 with
additives ZnO (Royer et al., 2005b, 2005c), LaCo1-xFexO3 (Szabo et al., 2003b ; Royer et al.,
2004), LaCoO3, LaMnO3, La1-xSrxMnO3 (x = 0.2, 0.4), La1-xCexMnO3 (x = 0.05, 0.1) with
additives ZnO (Royer et al., 2005c) for CH4 oxidation, LaBO3 (B = Co, Mn, Fe) for CH3OH
oxidation (Levasseur and Kaliaguine, 2008), LaCoO3, LaCo0.8Fe0.2O3, LaCo0.6Fe0.4O3,
LaFeO3, La0.9Ce0.1CoO3) (Levasseur and Kaliaguine, 2009) and LaCo1-xFexO3 (Szabo et al.,
2002) for VOC oxidation, LaCo0.7Cu0.3O3 with additives NaCl (Tien-Thao et al., 2007a,
2007b) and LiCl, KCl, RbCl, and CsCl (Tien-Thao et al., 2007a) for conversion of syngas to
higher alcohols were synthesized by reactive grinding using SPEX shaker mill. The catalytic
reduction of NO by propene over series of LaFe0.8Cu0.2O3 (Zhang et al., 2008b), LaCo1-
xCuxO3 (x = 0.0, 0.1., 0.2) (Zhang et al., 2006a), LaMn1-xCuxO3 (x = 0.0, 0.1., 0.2) (Zhang et
al., 2006d), La(Co, Mn, Fe)1-x(Cu, Pd)xO3 ( x = 0.2 for Cu & x = 0.03 for Pd) (Zhang et al.,
2006b, 2007), LaGa1-xCuxO3 (x = 0.0, 0.2) (Zhang et al., 2008a) and reduction of NO by CO
over LaCo1-xCuxO3 and LaMn1-xCuxO3 (x = 0.0, 0.1, 0.2) (Zhang et al., 2006c), LaFeO3,
LaFe0.97Pd0.03O3, LaFe0.8Cu0.2O3 (Zhang et al., 2006e) were synthesized by reactive grinding
using SPEX shaker mill.
The reactive grinding method has the limitations which include it requires long
processing time to obtain final catalyst with good catalytic activity, high contamination of
75
impurities in the final product and also difficulties in bulk production ( et al., 2007;
Wang et al., 2007a; Budina et al., 2009; Chaira et al., 2009).
Royer et al. (2005b) observed that iron impurities are present in the perovskite
structure after milling which lead to a decrease of its intrinsic activity for methane oxidation.
et al. (2007) synthesized Ca1/3La2/3MnO3 perovskite at room temperature by milling
a mix of Mn2O3, La2O3 and CaO powders using high energy ball mill (SPEX 8000 D
mixer/mill) in which ball to powder weight ratio was maintained 12:1. They found that the
synthesis reaction was completed within 3 h. They also observed that powder mixture was
highly contaminated with iron after prolonged milling time (> 9 h) due to dislodging of iron
from the vial and balls that modify the catalytic properties of perovskite.
2.8 Catalytic activity for CO oxidation
Perovskites of LaMO3 (M = Co, Mn, Fe, Cr, Ni) are used for catalytic oxidative
reductive reactions and also remove the poisonous gases emitted from the vehicle (Zhang and
Saito; 2001, 2000b; Ito et al., 2004; Kucharczyk and Tylus, 2008; Layla et al., 2008; Ziaei-
Azad et al., 2011). Among the perovskite type oxides, cobalt and manganese based
perovskites are used as most efficient catalysts for oxidation reaction mainly for CO oxidation
and they are also proposed as alternative to the noble metal supported catalysts (He et al.,
2001; Nguyen et al., 2002; Zhang-Steenwinkel et al., 2002; Zhou et al., 2002; Cimino et al.,
2003; Royer et al., 2005b, 2005c, 2006; Labhsetwar et al., 2006; Song et al., 2006; Levasseur
and Kaliaguine, 2008, 2009; Doggali et al., 2010; Eyssler et al., 2010). It is reported in the
literature that the non-stochiometric structure of perovskites improve the redox ability and
also enhance desorption of adsorbed and lattice oxygen at low temperature (Niu et al., 2007).
Lanthanum cobaltites and lanthanum manganites exhibit oxygen deficiency (Zhang et al.,
2006c; Pecchi et al., 2008a) and oxygen excess non-stochiometric respectively (Porta et al.,
1999; Batis et al., 2005; Zhang et al., 2006a, 2006c; Pecchi et al., 2008a; Hammami et al.,
2009; Abdolrahman et al., 2010). Fe and Ni-based perovskites exhibit intermediate behavior
in terms of non-stochiometric oxygen (Pecchi et al., 2008a).
LaCoO3 is reported as most active perovskite among all for oxidation of CO, CH4,
C3H8, C6H14 and toluene (C7H8) (Ramadas, 1978; Vaz and Salker, 2007; Li et al., 2008;
Sartipi et al., 2008; Taguchi et al., 2008; Magalhaes et al., 2010; Sompech et al., 2012).
LaCoO3 exhibits good catalytic activity for CO oxidation because it is one of the most
76
reducible among all perovskites structure (Khanfekr et al., 2009). Doggali et al. (2010)
reported LaCoO3 as highly active catalyst for CO oxidation which depends upon preparation
method and calcination temperature used. LaMnO3 perovskite is known as active oxide
catalysts for the deep oxidation of CH4 (Najjara et al., 2011). Rousseau et al. (2009) reported
that the catalytic performance of LaMnO3 is related to the specific surface area which strongly
depends on the synthesis conditions. They also pointed out that the specific surface area is not
only the factor which affects the catalytic activity. Perovskite type oxides containing
transition metals Fe is also good for complete oxidation of HC and reduction of NO with CO
(Zhang and Saito; 2001; Nguyen et al., 2002; hang et al., 2006e). The copper lanthanates is
also most active among the lanthanum perovskites for the reduction of NO with CO (Zhang et
al., 2006e).
Based on extensive literature, the catalytic activity for oxidation of CO is a function of
various factors such as:
¶ The specific surface area influencing the activity (Tanaka and Misono, 2001; Niu et
al., 2007; Levasseur and Kaliaguine, 2009)
¶ The structure defects such as cation and anion vacancies which determine different
oxygen mobility in the perovskite lattice (Ferri and Forni, 1998; Batis et al., 2005;
Giannakas et al., 2006; Singh et al., 2007; Rida et al., 2008)
¶ The electronic configuration of the transition metal cation in the B-site affecting the
energy of the metal-oxygen bond (Ciambelli et al., 2002; Cimino et al., 2003; Batis et
al., 2005)
¶ The oxidation number (valence) change of B cation influencing the redox properties,
i.e. easy change of cation oxidation state with fast bonding/release of oxygen
(Ciambelli et al., 2002; Cimino et al., 2003; Batis et al., 2005; Zhang et al., 2006c)
¶ The degree of substitution in the A and/or in the B-site influencing the dilution and the
stability of active sites (Ciambelli et al., 2001a; Batis et al., 2005)
Ciambelli et al. (2002) prepared LaAl1-xFexO3 (0 x 1) perovskites by citrate
method and calcined at 800 0C for 5 h in order to study the effect of non transition metal at B-
site for catalytic CO oxidation. The catalytic activity results for CO oxidation showed that all
the Fe-containing perovskites were more active than LaAlO3. The results revealed that the
transition metals are essential for developing highly active catalysts for CO oxidation.
77
Ciambelli et al. (2001a) studied CO oxidation over AFeO3 (A = La, Nd, Sm) and
LaFe1-xMgxO3 (x = 0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined
at 800 0C for 5 h. The order of activity for CO oxidation over AFeO3 catalysts was NdFeO3 >
LaFeO3 > SmFeO3 while the activity (per unit SA) of the LaFe1-xMgxO3 catalysts decreased at
high magnesium content. NdFeO3 was the most active catalyst among the un-substituted
AFeO3 catalysts because higher density of active sites available over NdFeO3 for CO
oxidation. The strongly bonded oxygen species was observed for LaFe1-xMgxO3 catalysts
which required higher temperature to start redox reaction and hence decreased the CO
oxidation activity.
Seyfi et al. (2009) prepared LaCoO3, LaCo0.8Cu0.2O3, La0.8Sr0.2Co0.8Cu0.2O3,
La0.8Ce0.2FeO3 and La0.8Sr0.2FeO3 perovskites by co-precipitation and citrate methods in order
to understand the effect of Sr and Ce substitution for cation A and Cu for cation B in the
ABO3 perovskite and also tested for CO oxidation using a gas mixture containing 2% CO, 1%
O2 and balance N2. The prepared catalysts showed good stability up to 600 0C and higher
activity for CO oxidation. Among all compositions, La0.8Sr0.2Co0.8Cu0.2O3 prepared by citrate
method showed the higher activity for CO oxidation and achieved 100% CO conversion at
355 0C. Various characterization techniques confirmed that the co-precipitation method
produces lower crystallinity and higher surface area than the citrate method. The samples
prepared by the citrate method exhibited higher catalytic activity for CO oxidation than the
samples prepared by co-precipitation due to higher degree of crystallinity because crystallinity
plays important role for CO oxidation than the internal surface area of these catalysts. Taguchi
et al. (2008) also reported that metal ion content and surface crystallinity (regularity of ions)
play an important role for catalytic activity of CO oxidation. They also pointed out that CO2 is
produced by the reaction of CO with oxygen adsorbed on the metal ions of the outmost
surface and the amount of adsorbed oxygen depends on the surface crystallinity.
Xiulan and Yuan (2000) used new technique to synthesis ultrafine LaBO3 (B = Co or
Mn) and La1-xSrxBO3 (x = 0.0, 0.1, 0.2, 0.3) in which Na2CO3-NaOH was used as the
precipitating agent and also supercritical drying method was adopted effectively to decrease
the calcination temperature for the formation of perovskites. They pointed out that the
catalytic activity for CO oxidation depends on the calcination temperature as well as specific
surface area. They observed that the catalytic activity for CO oxidation increases by lower
down the calcination temperature because it decreases the size of particles which increases the
78
specific surface area responsible for CO oxidation. Partial substitution of Sr for La in LaBO3
increases the activity markedly. Niu et al. (2007) also pointed out that the reaction rate over
bulk perovskites is directly proportional to the BET (Brunauer-Emmett-Teller) surface area
for oxidation reaction.
Zhang-Steenwinkel et al. (2002) pointed out that the numbers of cation/anion
vacancies formation are required for adsorption of CO which is essential for higher catalytic
activity for CO oxidation. The adsorption of oxygen species over Co3+ sites and oxygen
vacancies in perovskite structures play an important role for oxidation of CO also reported by
Magalhaes et al. (2010). The preparation method strongly affects the oxygen species mobility
of perovskite catalysts reported by Ifrah et al. (2007). Viswanathan (1992) reported that lower
binding energy for the surface oxygen species is favorable for the oxidation of CO.
Royer et al. (2005b) reported that LaCoO3 prepared by CT (citrate complexation)
method converted 100% CO to CO2 at a temperature below 230 0C whereas only 90% CO
converted at 300 0C for LaCoO3 prepared by RG (reactive grinding of the single oxides)
method. The COPRG (reactive grinding of an amorphous precursor) sample had shown
similar specific activity like CT sample. The lower activity of RG for CO oxidation was due
to the strong contamination of its surface by Fe. They also pointed out that the COPRG
sample was slightly contaminated by Fe even exhibited good activity which showed that
catalytic activity is mainly dependant on specific surface area and only surface oxygen
participates in the oxidation reaction.
Ziaei-Azad et al. (2011) investigated the CO and CH4 oxidation activity over LaBO3
(B = Mn, Fe, Co, Ni) perovskite catalysts for pollution abatement from natural gas fueled
vehicles. They pointed out that the presence of PdO in Pd containing catalysts is responsible
for higher catalytic activity for CO and CH4 oxidation. They also reported that Pd substitution
significantly facilitates the reducibility of cation B in LaB0.95Pd0.05O3 perovskite which
increases the mobility of lattice oxygen and hence enhances the catalytic activity of Pd
containing catalysts.
Sartipi et al. (2008) carried out catalytic activity for oxidation of CO and C3H8 over
LaCoO3, LaCo0.95Pd0.05O3 together with the pre reduced one using mixture of 6% CO, 0.2%
C3H8 in Ar. They observed that by partial reduction of Pd in LaCo0.95Pd0.05O3 catalyst at 180 0C for 30 min gave higher catalytic activity for CO and C3H8 oxidation over LaCoO3 and
LaCo0.95Pd0.05O3. The enhanced catalytic activity of pre-reduced LaCo0.95Pd0.05O3 catalyst for
79
CO oxidation is due to segregation of Pd from the perovskite lattice, its dispersion on the
catalyst surface as metallic nano-particles and also its easy exposure to the reactants.
Zhou et al. (2002) reported that the catalytic activity of Pd/LaFe0.8Co0.2O3 higher than
that of LaFe0.77Co0.17Pd0.06O3 for elimination of automotive pollutants. This enhanced activity
is due to impregnation, Pd localized on the perovskites surface and easily reduced to Pd nano-
particles and available for the reactant. For substituted perovskite, Pd is in the lattice of
LaFe0.77Co0.17Pd0.06O3 and makes strong interaction between Pd and O which cause difficulty
in reduction of Co.
He et al. (2001) synthesized La1-xSrxMO3 (M = Co0.77Bi0.20Pd0.03) catalysts by citrate
method. The results showed that La0.8Sr0.2MO3 (M = Co0.77Bi0.20Pd0.03) exhibited good three-
way catalytic activity for the elimination of CO, C3H6 and NO among all. The best activities
of 100% CO conversion at 160 0C, 80% C3H6 conversion at 390 0C and 96.7% NO conversion
at 260 0C were observed over the La0.8Sr0.2MO3 catalyst at = 1.00 and 60 000 h-1. These
enhanced activities were attributed due to incorporation of Sr2+ into the A-sites and Bi5+ into
the B-sites which alter the density of oxygen vacancies, the mobility of lattice oxygen and
oxygen non-stoichiometry. The substitution of Pd at B-site also improves the oxygen
vacancies and over-stoichiometric oxygen which is also responsible for three-way catalytic
activity.
Zhang et al. (2006e) repared LaFeO3, LaFe0.8Cu0.2O3 and LaFe0.97Pd0.03O3 perovskites
by reactive grinding method and calcined at 500 0C for 5 h and carried out activity for NO
reduction with CO. Large number of anion vacancies were generated after partial substitution
of Cu2+ at Fe3+ ions in the B-sites of the lattice due to a positive charge deficiency. The
nitrosyl species formed via NO chemical adsorption on these anion vacancies was highly
reactive toward CO. On the other hand, Cu substitution also enhanced the reducibility of
lanthanum ferrite and accelerated the regeneration of anion vacancies on the surface.
Therefore, a significant improvement in catalytic performance of NO + CO reaction was
achieved over LaFe0.8Cu0.2O3 compared to LaFeO3. The reducibility of LaFeO3 was
remarkably enhanced after incorporation of Pd into the lattice which was responsible for good
performance of NO reduction and CO oxidation at low temperatures over LaFe0.97Pd0.03O3.
Zhang et al. (2006a) pointed out that the catalytic activity for reduction of NO by CO
over LaCoO3 was improved by 20% Cu substitution at B-site leading to a 97% N2 yield and
nearly complete CO conversion at 450 0C. This improvement in activity is due to easy
80
generation of anion vacancies after Cu substitution which provides the place for NO
adsorption and desorption. In addition, Cu substitution enhances lattice oxygen mobility
which promotes CO oxidation and also recovers anion vacancies which would be available for
further activity. They also pointed out that reduction of NO by C3H6 was improved after
substitution of Cu at B-site of Co, Mn and Fe based perovskites because Cu substitution
-oxygen which are highly active for
reaction with adsorbed C3H6 and formed organo nitrogen compounds which further reduce to
N2 (Zhang et al., 2006a, 2006c, 2006d). Incorporation of Pd into B-sites significantly
improves the mobility of lattice O2 and the reducibility of Fe-based perovskites. C3H6
transformation over Pd-substituted samples involves not only complete oxidation into CO2 via
-O2 but also partial oxidation into CO due to highly mobile lattice oxygen reported by Zhang
et al. (2006d). NO and CO interacted at low temperature and formed isocynate which reduce
to N2 (Zhang et al., 2006b).
Szabo et al. (2002) carried out catalytic activity for oxidation of n-hexane over LaCo1-
xFexO3 perovskites prepared by reactive grinding using SPEX laboratory grinder and citrate
method. All catalysts prepared by reactive grinding exhibited higher activity for oxidation of
n-hexane compared to catalysts prepared by conventional synthesis using amorphous citrate
method. The enhanced activity for oxidation of n-hexane was associated with both the high
surface area and high defect density achieved by the reactive grinding. Cobalt based
perovskites were shown more active for n-hexane oxidation per unit surface area than iron
based perovskites calcined at the same temperature. The similar observations were obtained
by Szabo et al. (2003b) over LaCo1-xFexO3 prepared by reactive grinding using SPEX
laboratory grinder for oxidation of CH4.
La1-yCeyCo1-xFexO3 (y = 0.0, 0.1; x = 0.0, 0.2, 0.4) perovskite-type mixed oxides
prepared by reactive grinding method and catalytic activities were carried out for CH3OH, CO
and CH4 oxidation. The activity results showed that the Ce-substituted catalyst exhibited
superior catalytic activity due to easy reducibility of the B-site cations in perovskite structure
-O2 desorbed responsible for CO and CH3 -O2
desorbed responsible for CH4 oxidation while Fe-doped catalysts were inferior in catalytic
activity because of drop in oxygen desorption, oxygen mobility and B-site cation reducibility
(Levasseur and Kaliaguine, 2009).
Table 2.5 summarized the catalyst developed, preparation method and performance.
81
Table 2.5 Summary of catalysts used for automotive CO oxidation
Catalyst *Preparation method
Calcination temperature
Operating conditions
T50 (0C)
T100 (0C)
LaFeO3 CT 800 0C, 5 h w = 0.5 g CO = 1%, O2 = 20%, He = 79% GHSV = 12000 Ncm3/g h
348 T90 = 400 Ciambelli et al. (2001a)
NdFeO3 321 T90 = 388 SmFeO3 366 T90 = 442 LaFe0.9Mg0.1O3 325 T90 = 396 LaFe0.8Mg0.2O3 333 T90 = 382 LaFe0.7Mg0.3O3 342 T90 = 394
LaCoO3 RG 550 0C, 5 h w = 0.1 g CO = 5%, O2 = 20% in He 30 ml/min VHSV = 22500 h-1
162 229 Levasseur and
Kaliaguine (2009)
LaCo0.8Fe0.2O3 237 331 LaCo0.6Fe0.4O3 256 371 LaFeO3 342 - La0.9Ce0.1CoO3 85 104
Before each run flushed with a 20 ml/min flow of He for 1 h at room temperature BaCe0.95Pd0.05O3- as prepared
RG 1000 0C, 10 h w = 0.1±5 g 50 cm3/min CO = 1000 ppm, O2 = 10% in N2 SN =200 (Lean burn diesel engine)
83 Not reported
Singh et al. (2007)
BaCe0.95Pd0.05O3- reduced with 5% H2 at 1000 0C
165
BaCe0.95Pd0.05O3- reoxidized with O2 for 1 h at 1000 0C
85
BaCeO3 225 Pretreatment with N2 (60 cm3/min ) at 350 0C for 3 h LaCoO3 CT
800 0C, 5 h w = 0.5 g
CO = 1%, O2 = 20%, He = 79% 100 cm3
STP/min GHSV=12000 cm3 STP/g h
142 227 Colonna et al. (2002b)
LaFeO3 327 427 Ciambelli et al. (2002), Colonna et al. (2002a)
LaAlO3 477 577 Cimino et al. (2002,
2003)
LaAl0.95Mn0.05O3 187 317 LaAl0.90Mn0.10O3 277 337 LaAl0.80Mn0.20O3 219 317 LaAl0.6Mn0.4O3 182 277 LaAl0.4Mn0.6O3 152 237 LaAl0.2Mn0.8O3 87 217 LaMnO3 307 377 Before each run pretreatment at 500 0C with O2 in flow at 100 cm3 STP/min for 0.5 h LaCoO3 Not mentioned 600 0C, 2 h w = 0.1 g
CO = 1.8%, O2 = 0.9%, Ar = balance 50 ml/min
190 270 Tana and Zhua (2005)
82
Table 2.5 Continued
Catalyst *Preparation method
Calcination temperature
Operating conditions
T50 (0C)
T100 (0C)
LaCoO3 CP
700 0C, 5 h w = 0.5 g CO = 2% O2 = 1% N2 = 97% 250 cm3/min
312 440 Seyfi et al. (2009) LaCo0.8Cu0.2O3 345 422
La0.8Sr0.2Co0.8Cu0.2O3 359 400 La0.8Ce0.2FeO3 355 411 La0.8Sr0.2FeO3 357 401 LaCoO3 CT
700 0C, 5 h 280 400
LaCo0.8Cu0.2O3 303 382 La0.8Sr0.2Co0.8Cu0.2O3 314 355 La0.8Ce0.2FeO3 327 386 La0.8Sr0.2FeO3 332 374 La0.7Sr0.3CrO3 annealing a
mixture of metal oxides
and carbonates
1000 0C, 10 h
CO = 1%, O2 = 1% in N2 The feed flow rate = 10 l/h, the overall recycle flow rate =500 l/h, Recycle ratio = 50
- T20 = 300 Petrovic et al. (2006) La0.7Sr0.3Cr0.075Ru0.025O3 234 T80 =
220 La0.7Sr0.3Cr0.05Ru0.05O3 165 T96 =
160 La0.7Sr0.3Cr0.025Ru0.075O3 150 T96 =
155 La0.7Sr0.3Cr0.09Ru0.01O3 140 T96 =
150 LaCoO3 CT 700 0C, 5 h w= 0.250 g
particle size = 0.125-0.297 mm 6% CO, 0.2% C3H8 in Ar 50 cm3 STP/min SN =1.1
205 250 Sartipi et al. (2008)
LaCo0.95Pd0.05O3 188 240 LaCoO3 reduced at 180 0C for 30 min
180 230
LaCo0.95Pd0.05O3reduced at 180 0C for 30 min
162 165
LaCoO3 Na2CO3-NaOH
Supercritical drying
590 0C w = 0.26 g CO = 6%, O2 = 3%, N2 = 91% SV =15000 ml/gcat h
160 210 Xiulan and
Yuan (2000) LaCoO3 700 0C 180 230 LaCoO3 800 0C 200 260 LaCoO3 900 0C 250 270 La0.9Sr0.1CoO3 700 0C 110 130 La0.8Sr0.2CoO3 700 0C 150 200 La0.7Sr0.3CoO3 700 0C 190 210 ZnMnO3 CP 700-800 0C,
10-12 h w = 1 g 5% CO and 5% O2 in N2
5000 ml/h
325 T85=400 Shetkar and Salker ( 2008)
Zn0.8Ni0.2MnO3 200 300 Zn0.6Ni0.4MnO3 190 325 Zn0.4Ni0.6MnO3 275 T80=400 Zn0.2Ni0.8MnO3 300 T72=400 NiMnO3 175 300 LaMnO3 Pechini 750 0C, 6 h w = 0.2 g
2% CO, 20% O2 in N2
50 cm3/min (STP)
240 290 Abdolrahman et al. (2010) LaMn0.8Cu0.2O3 220 270
LaMn0.6Cu0.4O3 225 270 LaMn0.4Cu0.6O3 245 300 LaMn0.2Cu0.8O3 255 310
83
Table 2.5 Continued
*CT: Citrate complexation; CP: Co-precipitation; RG: Reactive grinding, SN (stochiometric number)
2.9 Kinetic and reaction mechanism
In order to obtain intrinsic kinetic data, a number of exploratory experiments are first
made in order to obtain the kinetic data in chemical reaction controlled regime by eliminating
film diffusion and pore diffusion (Idem and Bakshi, 1996; Wojciechowski, 2003; Prasad and
Pratichi, 2012). The plug flow conditions are maintained by providing catalyst bed height to
catalyst particle size ratio Lc/Dp
particle size ratio D/Dp (Froment and Bischoff,
1990). The precautions should be taken to avoid a temperature rise of the catalyst bed to
assure isothermal behavior. It is recommended that for intrinsic kinetic studies, data should be
collected in the surface reaction regimes (Prasad and Pratichi, 2012).
There are few studies in the literature on the kinetics and mechanism of oxidation over
perovskite based metal oxides. Cimino et al. (2003) and Ciambeli et al. (2002, 2001a) studied
Catalyst *Preparation method
Calcination temperature
Operating conditions
T50 (0C)
T100 (0C)
LaMnO3 Sol-gel 750 0C, 6 h w = 0.2 g 2% CO, 20% O2 in N2
50 cm3/min (STP)
250 300 Abdolrahman et al. ( 2010) LaMn0.8Cu0.2O3 240 290
LaMn0.6Cu0.4O3 235 285 LaMn0.4Cu0.6O3 250 320 LaMn0.2Cu0.8O3 260 330
LaNiO3 CP 237 352 Vaz and Salker (2007) LaNi0.7Co0.3O3 202 352
LaNi0.5Co0.5O3 152 312 LaNi0.3Co0.7O3 142 227 LaCoO3 87 147 LaMnO3 Spray
decomposition 700 0C, 4 h w = 2.5 g
(3 ml) CO = 1% in air GHSV = 10000 1/h
167 207 Song et al. (1999) La0.7Ag0.3MnO3 57 77
La0.7Ce0.3MnO3 107 157 La0.7Sr0.3MnO3 137 177
La0.8Sr0.2CoO3- Solid state 1200 0C , 36 h W = 0.4 g CO = 2%, O2 = 98% 100 ml/min
177 T90=300 Doshi et al. (1993) Pechini 700 0C, 6 h 146 170
Pechini 500 0C, 6 h 150 190 Solid state 1200 0C, 36 h 126 T72- 270
Pechini 1200 0C, 12 h 121 T65=245
84
the oxidation of CO over different compositions of perovskite based catalysts. They reported
that the kinetic is the first order with respect to CO concentration and zero order for O2 (fed in
large excess). The values of pre-exponential factor (A0), activation energy (Ea) and rate
constant obtained for perovskites from the Arrhenius plots are summarized in Table 2.6.
Table 2.6 Oxidation of CO: first order rate parameters
Catalyst Kinetic parameters Ea
(kcal/mol) A0
(l/m2 h) ³ 10-7 *kw (200 0C)
(l/g h) LaCoO3 19 (range of
150-250 0C) - - Yao (1975)
3 (range of 300-400 0C)
- -
LaAlO3 21.4 - 5.1 ³10-3 Cimino et al. (2003) LaAl0.95Mn0.05O3 21.5 - 0.23
LaAl0.9Mn0.1O3 21.7 - 0.42 LaAl0.8Mn0.2O3 13.9 - 6.64 LaAl0.6Mn0.4O3 14.1 - 13.9 LaAl0.4Mn0.6O3 13.4 - 32.1 LaAl0.2Mn0.8O3 10 - 71.2
LaMnO3 14.7 - 11 LaFeO3 18.1 1.44 - Ciambelli et al.
(2001a, 2002, 2002a)
LaFe0.9Mg0.1O3 17.3 0.82 - LaFe0.8Mg0.2O3 18.1 0.87 - LaFe0.7Mg0.3O3 18.1 0.52 -
NdFeO3 18.3 3.36 - SmFeO3 15.9 0.11 -
LaAl0.95Fe0.05O3 19.6 - **1.96 LaAl0.1Fe0.9O3 23.5 - **1.31
* first order kinetic constant for CO oxidation; ** at 200 0C
Cimino et al. (2003) reported that adsorption plays an important role in kinetic study
for CO oxidation. Ciambeli et al. (2001a) pointed out that the Arrhenius plots obtained over
perovskites by assuming first order rate equation with respect to CO and Arrhenius plots was
linear which suggests that a single kinetic regime observed. Vaz and Salker (2007) studied
CO oxidation reaction over different compositions of Co and Ni-based perovskites. The
85
values of activation energy, frequency factor and rate obtained for Co and Ni-based
perovskites from the Arrhenius plots are summarized in Table 2.7.
Table 2.7 Kinetic parameters of CO oxidation over LaNi1-xCoxO3 catalyst at 160 0C (Vaz
and Salker, 2007)
Catalysts Rate (molecules/m2 g)
Ea (kcal/mol)
Frequency factor (molecules/m2 g)
LaNiO3 4.02×1017 12.58 1.905×1019
LaNi0.7Co0.3O3 6.83×1017 12.71 3.204×1019
LaNi0.5Co0.5O3 8.43×1017 11.44 4.394×1019 LaNi0.3Co0.7O3 9.95×1017 8.99 6.602×1019
LaCoO3 6.63×1018 12.48 3.171×1020
It was observed that the rate of reaction and frequency factor increases with the cobalt
substitution in the B-site of LaNiO3 indicating that the rate increases with the concentration of
active sites of the catalyst. Thus, catalytic activity for CO oxidation was also increased with
the increase in the concentration of cobalt ions.
Hueso et al. (2009) carried out the kinetic studied over La0.5Sr0.5CoO3-d by taking
assumption (i) first-order mechanism for the oxidation of carbon monoxide (ii) the
concentration of oxygen is in excess in comparison with CO which indicates that the kinetic
depends only on the concentration of CO and (iii) the variation of the CO oxidation rate is
linked to the variation of the overall rate constant which directly depends on temperature.
They also mentioned that if zero and second-order rate equations were considered for CO
oxidation then larger discrepancies and errors above 20% were obtained.
Ouyang et al. (2010) developed the rate laws for CO oxidation over BaCe0.90Pd0.10O2.90
perovskite under CO-lean, CO-rich as well as near-stoichiometric conditions. They proposed
the empirical rate law for CO oxidation over Pd-substituted BaCeO3 with the simple power
form
rate = kobsP(CO)mP(O2)n
(2.7)
86
In equation 2.7, m and n are the apparent reaction order with respect to CO and O2
respectively. The kinetics study showed that Langmuir Hinshelwood (LH) mechanism is
contributed for CO oxidation if CO is in lean conditions and hence m = -1 and n = +1 which
indicates that both the reactants compete for the same adsorption sites. The activation energy
was found low with 7.8 ± 0.3 kcal/mol due to weak adsorption of CO on ionic surface Pd(II)
sites. Mars-van Krevelen mechanism used for the reaction under CO-rich conditions with m =
n = 0 reveals that the reaction orders for both CO and O2 strongly depend on P(CO)/P(O2)
and then gradually become independent of both P(CO) and P(O2) at high P(CO). This is due
to the presence of oxygen vacancies created on the surface of BaCeO3 which facilitates
oxygen activation independent of CO adsorption. The value of activation energy was of 9.7 ±
0.3 kcal/mol which was slightly higher due to slow mobility of oxygen in oxide phase. Both
the mechanisms contributed to the overall reaction under P(CO)/P(O2
The oxidation of CO over perovskite type oxides has been widely studied. On the
basis of kinetics and spectroscopic data the following mechanism proposed a simple reaction
mechanism for CO oxidation on LaCoO3
)(2
)(2)(2 2 adadg OOO --
(2.8)
)()( adg COCO
(2.9)
)(2 )(23)()( slowCOOCO adadad-- +
(2.10)
)(2
)(2)(2
)(2)(3 adgadadad OCOOCOCO -- ++
(2.11)
(Tejuca et al., 1989; Tabata et al., 1998; Pena and Fierro, 2001; Zhu et al., 2005; Royer and
Duprez, 2011).
Where equation 2.11 should be the rate determining step. Oxygen is adsorbed as molecular
-2O species on Co2+ ions which subsequently dissociated yielding atomic oxygen (O-)
adsorbed on the same site. On the other hand CO adsorbs on surface oxide ions leading to a
liable species that interact with adsorbed atomic oxygen producing carbonate structure that
then decomposed to adsorbed CO2 and O2 (Tabata et al., 1998; Pena and Fierro, 2001).
87
2.10 Deactivation studies
Perovskites deactivate due to easy poisoning of sulfur present in the fuels (Rosso et
al., 2003; Song et al., 2006; Zhang et al., 2007; Tzimpilis et al., 2008; Zhang et al., 2008b).
The presence of phosphorous in lube oil deactivates perovskite catalysts by poisoning the pore
mouth of catalysts particles and destructing the perovskite structure (Tana and Zhua, 2005).
The SO2 adsorbed on B-sites in perovskite that deactivate the catalyst by blocking the
surface sites which are necessary for CO adsorption and lattice oxygen replenishment. This
attributed lower catalytic activity for CO oxidation. The poisoning is due to the stable sulfate
formation on B-site with SO3 which forms by oxidation of SO2 in O2 (Yoo et al., 1992; Royer
et al., 2004; Zhang et al., 2008b). The sulfate formation modifies the catalysts structure by
destructing the perovskite phase into oxide which suppresses quickly their activity (Royer et
al., 2004; Zhang et al., 2008b). Zhang et al. (2008b) studied the effect of SO2 poisoning over
LaFe0.8Cu0.2O3 perovskite prepared by reactive grinding method for NO reduction by C3H6.
They observed that low concentration of SO2
competitive adsorption of SO2 and reactants as well as the coverage of active sites by surface
sulfite and sulfate species. If concentration of SO2 > 80 ppm, SO2 causes severe sulfatation
and the perovskite structure destroy due to formation of La2(SO4)3 and Fe2O3 phases result in
loss of activity for NO reduction by C3H6.
Steam is always present with high concentration in exhaust gas of vehicles due to
complete combustion of fuel which deactivate the perovskites by occupying the active sites
only. There is no modification in perovskite structure and morphology after water poisoning
is reported. The presence of water does not make perovskite permanently poison (Song et al.,
2006; Zhang et al., 2007; Tzimpilis et al., 2008) This deactivation attribute due to strong
competition between water and reactant for the active sites which decreases the catalytic
activity (Zhang et al., 2007; Tzimpilis et al., 2008). Substitution of A and/or B cations and
also by incorporating the noble metals in the perovskite structure improve the SO2 and H2O
poisoning (Rosso et al., 2003; Zhang et al., 2007; Tzimpilis et al., 2008). Zhang et al. (2007)
reported that substitution of Cu at B-site promotes the resistance to water deactivation for the
88
parent lanthanum perovskites by enhancing density of anion vacancies. They also pointed out
that by substitution of Pd in LaFeO3 (LaFe0.97Pd0.03O3) makes the perovskite more resistance
to H2O poisoning. This is due to enhancement in stem reforming of adsorbed C3H6 which
produces H2. This H2 can be dissociated over Pd into H2 atoms which reduced the NO. Such
H2 dissociation does not take place over oxide catalysts.
Colonna et al. (2002b) carried out deactivation study over LaCoO3 perovskite
prepared by citrate method for CO oxidation. They studied time-on-stream activity for CO
oxidation over LaCoO3 at isothermal temperature of 25 0C. LaCoO3 exhibited 10% CO
conversion at 25 0C. However, catalyst undergone continuous deactivation with time-on-
stream after many hours because product strongly chemically adsorbed in the form of CO2
and/or carbonates which is not easily decomposed at room temperature and block the active
sites available for CO oxidation. Levasseur and Kaliaguine (2008) observed same observation
during their deactivation study over LaBO3 (B = Co, Mn, Fe) perovskite prepared by reactive
grinding for methanol oxidation.
Sartipi et al. (2008) reported that catalytic activity of pre-reduced LaCo0.95Pd0.05O3
perovskite at 180 0C for 30 min for oxidation of CO declines in lean reaction conditions after
a period of time because Pd is oxidized again and diffuse back into the perovskite lattice
which prevents the exposure of highly active Pd nano-particles to the CO for oxidation. The
catalytic activity again regenerates by reducing the catalyst which again disperses Pd nano-
particles on the catalyst surface.