The role of ceria in catalysis

B. Murugan

National Centre for Catalysis Research

IITM, Chennai-36.

18-12-2007

Rare earths: 15 lanthanide elements – divided into two groups

First four elements – ceric (or) light rare-earths

Remaining elements – yttric (or) heavy rare-earths

Bastnasite, Monazite and Loparite – principle cerium ores

Monazite – most abundant

Ce – two stable valence states; Ce4+ and Ce3+

Ce is the unique rare-earth for which dioxide is the normal stable

phase contrary to the others for which Ln2O3 is the normal

stoichiometry.

Why do we need to talk about ceria?

Owing to number of application – catalysis, chemicals, glass

and ceramics, phosphors and metallurgy

The applications of ceria based materials are related to a

potential redox chemistry involving Ce(III) and Ce(IV), high

affinity of the element for oxygen and sulfur and

absorption/excitation energy bands associated with its

electronic structure.

Applications of cerium in catalysis and chemicals

1. Fluid Catalytic Cracking – huge amounts consumed for refinery

operations – convert crude oil to lower molecular weight fractions.

2. TWC – major technological application – vehicle emission control – to

remove pollutants from vehicle (auto-exhaust) emissions – significant

portion of cerium consumed annually.

3. Oxidizing agent – potential use as additives to aid combustion – to

reduce the particle emissions from Diesel engine.

4. SOx control agent.

5. Eletrode material in SOFC.

6. EB dehydrogenation – ceria addition improves activity for styrene

formation.

7. Supports the ammoxidation of propylene to produce acrylonitriles.

Fluorite has a very simple structure – space group Fm3m

The structure can be viewed as a face-centered cubic array of

Cerium (green) ions with the oxygen (purple) ions residing in

the tetrahedral holes.

Crystal Structure of ceria: The Fluorite structure

Consider the stoichiometry of single unit cell.

Each of the corner cerium ions is 1/8 inside the cell; since there

are eight corners these add up to one ion inside the cell.

There are six faces to a single cell, each with a cerium ion one-half

inside the cell.

Therefore a single cell contains four cerium ions.

A single cell also contains eight oxygen ions, each one located

entirely within the unit cell.

Since there are four cerium ions and eight oxygen ions inside the

cell, the 1:2 stoichiometry is maintained.

1. We can also view the structure as a simple cubic array of

oxygen with a cerium in the center of alternate cubes.

2. Considered that way, there are obviously diagonal planes of

cubes containing no cations.

3. These planes will obviously be planes of weakness, accounting

for fluorite's excellent octahedral cleavage.

Octahedral Holes

Regardless of whether hexagonal layers

are stacked in an AB or ABC fashion, there

exist two types of spaces or holes between

the layers.

One type of space is called an octahedral

hole, and is formed between three atoms

in one layer and three atoms in the layer

immediately above or underneath.

Although it takes six spheres to form an

octahedron, the name is derived from the

fact that the resulting shape has eight

sides.

Tetrahedral Holes

A second type of space

which can exist between

stacked hexagonal layers is

called a tetrahedral hole.

Tetrahedral holes are

formed between three

atoms in one layer and a

single atom immediately

above or underneath.

Octahedral Holes in the Fluorite

Structure

In the fluorite structure, the fluoride

ions reside within the tetrahedral

holes formed by the face-centered

cubic array of calcium ions, and the

octahedral holes are vacant. In this

illustration the green cylinders

outline eight of the vacant

octahedral holes.

This illustration shows the vacant

octahedral holes in the fluorite

structure, outlined by the green

spheres, as seen from the top.

Tetrahedral Holes in the Fluorite Structure

This illustration shows the location of the tetrahedral holes in the

fluorite structure.

Why the fluoride ions would reside in the tetrahedral holes

rather than the octahedral holes?

The most obvious answer to this question is, of course,

stoichiometry.

There are two oxygen atoms for every one cerium atom, and

since an array of N atoms results in the formation of N

octahedral holes, there would simply not be enough spaces

for all oxygen atoms.

If the ions were reversed, with the oxygen ions forming the

face-centered cubic array, there would be enough cerium

ions to fill only 1/4 of the tetrahedral holes or 1/2 of the

octahedral holes; this would be terribly inefficient.

Technically, the descriptions of the fluorite structure

given above are inaccurate in the sense that

because the oxygen ions are in fact larger than the

cerium ions, they therefore do not "fit inside" the

tetrahedral holes.

As can be seen here, the cerium ions form a sort of

"expanded" face-centered cubic structure and do not

physically touch each other.

Nevertheless this does represent the most efficient

packing arrangement.

Defect Structure of Ceria

1. Defects in ceria – intrinsic or extrinsic

2. Intrinsic defects – due to thermal disorder or by the redox process

3. Extrinsic defects – by impurities or by the introduction of aliovalent

dopents.

Three possible thermally generated intrinsic disorder in ceria

CeCe + 2 OO V’’’’Ce + 2VÖ + CeO2 E = 3.53 eV Schottky

CeCe Cei•••• + V’’’’Ce E = 3.53 eV Frenkel

OO O’’I + VÖ E = 3.20 eV Frenkel

From variation in E, it is evident that the predominant defect category

is the anion Frenkel-type.

Results obtained from X-ray, neutron diffraction and combined

dilatometric and X-ray lattice parameter measurements proved that the

predominant defects in ceria are anion vacancies.

Faber et al. examined the electron density distribution using XRD and

concluded that the amount of interstitial Ce is less than 0.1% of the

total defect concentration in CeO1.91.

The process of ceria reduction may be written as:

Oo + 2CeCe = V••o + 2Ce’Ce + 1/2O2 (gas)

In the case of H2 reduction:

Oo + 2CeCe + H2 (gas) = V••o + 2Ce’Ce + H2O (gas)

Oxide vacancies may also be introduced by doping with oxides of

metals with lower valencies, e.g. dissolution of CaO and Gd2O3

CaO = Ca’’Ce + V••o + Oo

Gd2O3 = 2Gd’Ce + V••o + 3Oo

Already existing oxide vacancies may be removed by doping with

oxides of higher valency than 4

Nb2O5 + V••o = 2Nb•

Ce+ Oo

Electrical behavior of ceria

Ceria can be classified as mixed conductor showing both electronic and ionic

conduction. Its electrical properties are strongly dependent upon T, oxygen

partial pressure and presence of impurities or dopents.

For general case in CeO2-x the total conductivity is given by

t = [Ce’Ce]ee + [h]eh + [VÖ]2eÖ

At high temperatures and low oxygen partial pressures, ceria behaves as an n-

type semiconductor and electrons liberated following the reduction are the

primary charge carriers.

Oo VÖ + 2e- + 0.5O2 (g)

Transition from n-type to p-type conduction is observed at lower temperatures

and higher oxygen partial pressures near stoichiometric composition, where

electronic conductivity arises from holes introduced by impurities

IO I’Ce + VÖ + Oo

VÖ + 0.5O2 Oo + 2 h h indicates an electron hole

Ionic conductivity – due to the mobility of oxide ion vacancy

It is always much lower than the electronic conductivity in pure reduced

ceria.

However, the situation is different in ceria doped with oxides of two or

three-valent metals due to the introduction of oxide ion vacancy.

The electronic conductivity in air may be very low and the doped ceria

under these conditions are excellent electrolytes.

The conductivity mechanism is the hopping of oxide ions to the vacant

sites and the ionic conductivity i may be expressed as

i = (o /T) exp (-EH/kT),

EH is the activation energy for small polaron hopping.

The ionic conductivity increases with increasing ionic radius, from Yb to

Sm, but decreased at rdopant > 0.109 nm.

The most important parameter for ionic conductivity in fluorites is the

cation match with the critical radius, rc.

Highest conductivity – ionic radius of the dopant is as close to rc as

possible

Lattice Defects and Oxygen Storage Capacity of Nanocrystalline

Ceria and Ceria-Zirconia

1. Ceria-based oxides - automotive exhaust emission control systems as

catalyst supports and oxygen promoters.

2. Three-way automotive catalytic converters - oxidize CO and

hydrocarbons and at the same time reduce nitrogen oxides.

3. A high rate of simultaneous conversion of all the pollutants can only be

achieved within a narrow operating window near the stoichiometric air-

to-fuel ratio.

4. CO-NOx conversions are strongly affected by the local oxygen partial

pressure at the catalyst surface.

5. At high oxygen partial pressures (under lean conditions), the NOx

conversions drop off precipitously, whereas at low oxygen partial

pressures (under rich conditions), the CO conversions are low.

1. The role of ceria, and more recently ceria-zirconia, is to act as

an oxygen storage-and-release component to stabilize the

local oxygen partial pressure at the catalyst surface even

when the air-to-fuel ratio in the engine exhaust fluctuates with

time.

2. Pure ceria has a serious problem of degradation in

performance with time at elevated temperatures.

3. Traditionally, this degradation has been attributed to decrease

in its surface area and in turn its oxygen storage capacity

(OSC).

1. However, recent experimental observations on pure ceria

suggest that the surface area may not be the only parameter

that determines the effectiveness of ceria.

2. It has been proposed that in pure ceria "active" weakly bound

oxygen species are present, which belong to the bulk rather

than to the surface.

3. It is likely that these weakly bound oxygen species undergo

fast exchange with the environment and provide OSC. Such

"active" oxygen species become deactivated following a high-

temperature treatment.

1. Pulsed neutron diffraction data both in the reciprocal space by

the Rietveld refinement and in the real space by the atomic

pair-distribution function (PDF) analysis - presence of the

vacancy-interstitial (Frenkel-type) oxygen defects in CeO2.

2. These defects were found to disappear following a high-

temperature treatment of 1073 K (800 C). It is possible that

the interstitial oxygen ions are the "active" species that

provide necessary oxygen mobility crucial in the functioning

of ceria as a catalyst support

3. Decreasing concentration of the Frenkel-type oxygen defects

at high temperatures contributes to deterioration of the

oxygen storage properties in thermally aged ceria.

1. Zirconia is known to alleviate partially the degradation of ceria

at high temperatures. The beneficial effect of doping ceria with

zirconia is believed to be due to stabilizing the surface area by

suppressing thermal sintering.

2. However, it has been observed that ceria-zirconia mixed

oxides with low surface area still maintain a high oxygen

storage capacity compared to undoped ceria, and therefore

other mechanisms must be present.

3. Zirconia keeps ceria slightly reduced, and preserves oxygen

defects up to high temperatures.

4. The enhanced stability of oxygen defects in ceria-zirconia

accounts for the improved oxygen storage capacity and

thermal stability of ceria-zirconia systems.

Temperature dependence of the neutron diffraction patterns

Temperature dependence of the crystallite size in ceria

(filled circles) and ceria-zirconia (open diamonds)

(a) Perfect fluorite structure. All the Td

sites are filled by oxygen ions, and all

the Oh sites are empty.

(b) Oxygen defects in fluorite structure.

Some oxygen ions (filled circle) occupy

the interstitial Oh sites, leaving

vacancies in the Td sites (not shown).

The interstitial oxygen ions are

displaced from the centers of the

interstitial Oh sites in the <110>

directions.

In the general case, the concentration of vacancies may exceed that

of interstitial ions, resulting in oxygen non-stoichiomety.

O

Ce

Temperature dependence of

the oxygen defect

concentration. Filled circles:

oxygen interstitial ions, open

circles: oxygen vacancies.

The EPR spectra obtained from the as prepared samples at 77 K

CO2 output profiles in the

temperature-programmed

reduction experiment using CO.

Temperature dependence of defect concentration

in ceria and ceria-zirconia

1. High temperature treatment: Ceria exhibits a dramatic drop in

the concentrations of vacancies and interstitial ions, these

concentrations remain virtually constant in ceria-zirconia.

2. Interstitial oxygen ions in ceria-containing compounds are likely

to form during sample processing.

3. When oxygen-deficient material is oxidized to CeO2 or (Ce,Zr)O2,

absorbed oxygen ions may at first enter the roomier octahedral

sites, rather than fill the spatially tight tetrahedral sites.

4. If annealing temperature is not high enough they may not be

able to overcome a potential barrier to get into the regular

tetrahedral sites, and remain in the octahedral sites.

5. Only when the sample is treated at sufficiently high

temperature thermally activated interstitial ions may enter

regular tetrahedral sites and recombine with vacancies.

6. Because of the smaller ionic radius of zirconium ions, mixing

zirconia with ceria will reduce the lattice constant and produce

the atomic-level pressure at the smaller tetrahedral sites,

making them even more difficult to reach for the interstitial

oxygen ions than in pure ceria.

7. This may explain the enhanced stability of oxygen defects

against thermal aging in ceria-zirconia, where the

recombination of interstitial ions with vacancies may be

expected to occur at higher temperatures compared to pure

ceria.

8. The interstitial oxygen ions are the "active" ions that provide

necessary mobility crucial to the function of ceria as an oxygen

storage medium.

9. Apart from decreasing surface area the annihilation of the oxygen

Frenkel-type defects might contribute to deterioration of the

oxygen storage capacity in thermally aged automotive catalyst

supports.

10. Doping ceria with zirconia may improve the oxygen storage

properties of ceria at three different levels. At the level of the

microstructure, it inhibits surface diffusion and in turn the loss of

surface area at high temperatures. At the mesoscopic level,

substantial doping may result in the formation of an interface

structure that facilitates the oxygen transport from bulk to the

surface. Besides, as demonstrated by the above study, at the

atomic-level, it stabilizes the oxygen defective structure.

Activation energy for oxygen migration as a function of the

composition

Computer simulation studies further proved that,

1. Ce4+/Ce3+ reduction energy is significantly reduced even by small

amounts of zirconia; this effect is magnified when the association

between Ce3+ ions and oxygen vacancies is taken into account,

resulting in the bulk reduction energies becoming comparable

with values calculated for pure ceria surfaces.

2. Activation energy for oxygen migration in the bulk is found to be

low and decreases almost monotonically with the zirconia

content; this indicates facile oxygen diffusion through the bulk

catalyst.

Ceria based fuel electrodes for SOFC

1. The electrolyte in SOFC must consist of a good ion conductor and no

electronic conductivity – often YSZ is used.

2. Electrodes must possess good electron conductivity in order to

facilitate the electrochemical reaction and to collect the current from

the cell.

3. Anodic oxidation of the fuel (H2 or CO) can take place in the vicinity

of the three-phase boundary, where oxide ions, gas molecule and

electrons are present.

4. TPB should therefore be extended.

5. One way is to use mixed ionic and electronic conductor – partially

reduced ceria can be used as part of the SOFC anode.

6. Ceria based anodes have important advantages over conventional Ni-

based anodes – ability to endure repetitive redoxing and ability to

avoid (or tolerate) carbon deposition from hydrocarbon fuels.

1. In the temp. range 700-1000 oC ceria undergoes a change of volume

when the oxygen partial pressure is varied from air to that of the

operating SOFC anode.

2. The electronic conductivity of doped ceria is not sufficient to take care of

the current collection in an SOFC stack.

3. Sintering of doped ceria anode on YSZ electrolyte – limits the oxide ion

conductivity due to the radii misfit of Ce4+ and Zr4+.

Problems associated with ceria as anode in SOFC and ways to overcome

Current collector

Ceria

YSZ-scales YSZ

Provides sufficient adhesion

Ceria thin layer – governs volume instability during redoxing

Maintains high electronic conductivity

Diesel Soot Abatement Technology

1. Diesel engine exhaust Particulate matter (soot) + NOx

2. Pt + Ce fuel additives with Pt treated filter lowest temp. activity

(595 K)

3. The oxidation of soot with NO2 is catalyzed by cerium present in the

activated soot and not by Cu (or) Fe-activated soot.

Pt Ce

O2 + 2NO 2NO2 + soot 2NO + CO2

Continuously Regenerating – Diesel Particulate Filter (CR-DPF).

• When Pt and Ce additives are applied, there is a synergistic effect

resulting in a high oxidation rate.

5. This synergy can enhance the use of the proposed oxidation cycle

because the reactions involving NO are kinetically coupled.

6. If the rate at which NO2 oxidize soot is high, the NO2 concentration is

lowered, which facilitates the formation of NO2 from NO. At high NO2

concentrations, this formation is limited by thermodynamics.

7. The resulting ash from the cerium does not plug the filter, in

contrast to copper, where serious filter plugging are reported.

8. When 25 ppm of Ce additive is used for a typical heavy duty truck,

the filter will be 50% filled after 75,000 to 150,000 miles.

9. Cu deteriote ceramic fibre-wound filters.

10. Cu-regeneration problem – high temp. required.

Ceria based Wet-Oxidation catalyst

Mn-CeO2 composites and Ru/CeO2 – best catalysts

The function of the wet-oxidation catalysts should be confined to

1. Activation of O2

2. Direct electron transfer with the reactants (redox reaction) in the

first step of the reaction.

Ceria seems to effectively contribute to both factors

The very mobile nature of the oxygen on CeO2 is one of the critical causes

for the high performance of ceria-containing wet-oxidation catalysts.

The sole function of the wet-oxidation catalyst is to produce active

radicals via interaction with the pollutants in the first step of the reaction.

This rxn. involves free radical mechanism.

Ceria in catalytic combustion

Noble metal associated with ceria and ceria-zirconia are used as

catalysts

Several studies showed clearly the participation of oxygen atoms

from the bulk of ceria for both combustion of CO and HC.

Ceria stabilizes noble metal in high oxidation states leading to the

superior interaction in the case of O-Pt-O-Ce-

There are some surface oxygen anionic vacancies. These vacancies

induce the formation of surface oxygen peroxide or superoxide close

to the metal-ceria interface and might be the true active species.

So the role of the metal might be only that of donor/acceptor of

electrons.

Fluid Catalytic Cracking

1. Heavy hydrocarbons to gasoline-range hydrocarbons

2. Catalyst: mixture of zeolite and SiO2-Al2O3 – fast coke formation on catalyst

– regeneration required.

3. If the feed contains higher sulfur content then part of (< 10%) sulfur

remains trapped in the coke which builds up on the catalyst.

4. This sulfur is to be oxidized to SO2/SO3 in the regeneration step.

5. A highly effective and less costly approach is incorporation of SOx

adsorption/reduction additive

6. The function of this additive is to transform SOx back to H2S which will be

treated in Claus plant.

7. Commercial catalytic system : Ceria/Mg-aluminate spinel-MgO solid

solution.

8. This catalyst contains basic site for SOx adsorption, active site for oxidation

of SO2 to SO3 and redox properties for the conversion of sulfates to H2S

under reducing atmosphere.

9. The role of ceria in this catalytic formulation derives from its basic/redox

character.

MgO CeO2

CeO2/Mg2Al2O5

Ce2O3

MgO CeO2

O2

MgSO4 Ce2(SO4)3

H2S

H2

SO3

SO2

A mechanism proposed for the action of CeO2-MgO based catalyst in

the treatment pf SO2 in FCC plants

Ceria can also have an important role in the reduction of sulfates to give H2S

Under FCC conditions, ceria also reduce NOx emissions from cracking unit.

Here the role of ceria is to provide oxygen vacancy for the reduction of NO to N2.

de-SOx processes

Ceria with its double functionality (redox material with basic sites) represents a more versatile solution

CeO2 + SO2 Ce2O2S + SO2 S2 (elemental sulfur) + CeO2

CeO2 + SO2 sulfated CeO2 + CO (or) CH4 H2S + Ce2O2S

2CeO2 + H2S + H2 Ce2O2S + 2H2O

Ce2O2S + SO2 2CeO2 + S2

The presence of Cu and Ni in ceria based catalyst significantly increases

the performance at low temperature.

This may be attributed to the promotional effect of metal on the redox

activity of ceria.

Moreover the presence of metal favors the decomposition of sulfate

species and decreases the breakthrough temperature of the reaction.

Cu is selective to S2 whereas, Ni favors H2S.

Syn-gas production

Reforming reaction – application in fuel cell technology

Alternative process for syn-gas production

CH4 + CO2 2CO + 2H2

CH4 + 0.5O2 CO + 2H2

Ceria-zirconia based catalysts – high reducibility and oxygen storage

capacity

Two pathway mechanism

HC/CH4 decomposition to carbon then the carbon atom react with oxygen

from ceria based support.

Oxygen replenished by dissociation of CO2 in dry reforming or by H2O in

steam reforming

M-Ce-ZrO2

Ce-ZrO2

M M M

CH4 H2O/O2/CO2

O2-

CO + 2H2

O*