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Applied Catalysis B: Environmental 70 (2007) 330–334

Nitrous oxide formation in low temperature selective catalytic

reduction of nitrogen oxides with V2O5/TiO2 catalysts

Juan Antonio Martın, Malcolm Yates, Pedro Avila, Silvia Suarez, Jesus Blanco *

Instituto de Catalisis y Petroleoquımica, CSIC, C/Marie Curie 2, 28049 Madrid, Spain

Available online 12 June 2006

Abstract

Nitric oxide and nitric dioxide compounds (NOx) present in stack gases from nitric acid plants are usually eliminated by selective catalytic

reduction (SCR) with ammonia. In this process, small quantities of nitrous oxide (N2O) are produced. This undesirable molecule has a high

greenhouse gas potential and a long lifetime in the atmosphere, where it can contribute to stratospheric ozone depletion. The influence of catalyst

composition and some operating variables were evaluated in terms of N2O formation, using V2O5/TiO2 catalysts. High vanadia catalyst loading,

nitric oxide inlet concentration and reaction temperature increase the generation of this undesirable compound. The results suggest that adsorbed

ammonia not only reacts with NO via SCR, but also with small quantities of oxygen activated by the presence of NO. The mechanism proposed for

N2O generation at low temperature is based on the formation of surface V–ON species which may be produced by the partial oxidation of

dissociatively adsorbed ammonia species with NO + O2 (eventually NO2). When these active sites are in close proximity they can interact to form

an N2O molecule. This mechanism seems to be affected by changes in the active site density produced by increasing the catalyst vanadia loading.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Nitrous oxide; Vanadia-titania catalysts; Selective catalytic reduction; Dual site mechanisms

1. Introduction

Nitrous oxide has been considered a relatively innocuous

molecule until recently, receiving scarce interest by the

scientific community, engineers or politicians [1]. However,

during the last decade a growing interest has arisen, since

identification of this gas as a participant in the destruction of the

stratospheric ozone layer and in the greenhouse effect [2].

Although natural sources account for a large proportion of the

overall emissions of nitrous oxide, the estimated human

contribution of emissions to the atmosphere amounts to 4.7–7

million tonnes per year [3,4], equivalent to 30–40%. The main

anthropogenic sources of nitrous oxide are from adipic and

nitric acid production, and fossil fuel and biomass combustion,

however, quantities of nitrous oxide are also released to the

atmosphere from the control of nitric oxides emissions by

selective catalytic reduction (SCR) units.

SCR is a well-established and widely used process for the

abatement of NOx present in waste gases from stationary

* Corresponding author. Tel.: +34 91 585 48 02; fax: +34 91 585 47 89.

E-mail address: jblanco@icp.csic.es (J. Blanco).

0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2005.11.026

sources [5–7]. With these units, the NOx present in the stack

gases is reduced with ammonia to give nitrogen and water. The

catalysts most commonly employed in nitric acid plants are

V2O5/TiO2 anatase (Shell DeNOx system) [8], V2O5/g-Al2O3

(Rhone Poulenc DN 110 catalyst) [9] and CuNi oxides/g-Al2O3

(Espindesa DeNOx catalyst). When using these industrial SCR

catalysts, the formation of some N2O takes place, especially

with CuNi oxides catalysts [10].

The objective of this study was to obtain information about

the mechanism involved in the N2O formation in the SCR

processes with vanadia/titania catalysts, operating at low

temperature. Catalyst active phase loading and textural

properties were explored with this purpose. The results of

the study may also provide information to control nitrous oxide

emissions from nitric acid SCR units.

2. Experimental

2.1. Catalyst preparation and characterisation

The catalysts were conformed at laboratory scale by

extrusion (Bonnot single screw extruder) of doughs prepared

by kneading a mixture of anatase (Thann et Mulhouse) and

J.A. Martın et al. / Applied Catalysis B: Environmental 70 (2007) 330–334 331

natural magnesium silicate powders (Tolsa S.A.) with aqueous

solutions of vanadyl sulphate (Panreac) at different vanadium

salt concentrations. The addition of the magnesium silicate (a-

sepiolite) as inorganic binder improved both the rheological

characteristics of the slurry and the final mechanical properties

of the catalysts. The shaped extrudates were subsequently dried

at room temperature for 24 h, then at 100 8C and finally, treated

at 500 8C in air during 4 h. The extrudate catalysts have the

following geometric dimensions: average length 5 mm, mean

diameter 0.9 mm.

The vanadium contents were determined by inductively

coupled plasma (ICP) optical emission spectroscopy (Perkin-

Elmer Optima 3300 DV) of acid solutions of the ground

catalysts. The axial crushing strength of the extrudates were

determined with a Chantillon LTMC dynamometer; the

measurements were repeated 10 times to ensure statistically

significant values. The pore volumes were determined by use of

mercury intrusion porosimetry (MIP) using CE Instruments

Pascal 140/240 porosimeters, after drying the samples in an

oven at 150 8C overnight. For these measurements the values

recommended by the IUPAC [11] of contact angle 1418 and

surface tension 484 mNm�1 were used. The composition, axial

crushing strength and textural characteristics of the prepared

catalysts are collated in Table 1.

Raman spectra were determined with a single monochro-

mator Renishaw System 1000 equipped with a cooled CCD

detector (�73 8C) and holographic super-Notch filter. The

holographic Notch filter removes the elastic scattering while the

Raman signal remains very high. The samples were excited

with the 633 nm HeNe line; spectral resolution was ca. 3 cm�1

and spectrum acquisition consisted of 10 accumulations of 30 s.

The spectra were obtained under anhydrous conditions

(150 8C) in a hot stage (Linkam TS-1500) with a synthetic

air flow (30 ml/min).

2.2. Catalytic tests

Catalytic activity measurements were performed in a

reactor working close to an isothermal axial profile. The inlet

and outlet concentrations of NOx were determined by

chemiluminescence with a Signal NOx Analyser Series

4000. Analysis of N2O was performed by IR spectroscopy

with a Signal 7000FT GFC Analyser. The NH3 concentration

was determined with an ADC Double Beam Luft Type Infra-

red Gas Analyser.

Table 1

Composition, textural and physical properties of the catalysts

Sample V2O5

content

(wt.%)

TiO2

content

(wt.%)

a-Sepiolite

content

(wt.%)

Axial crushing

strength (MPa)

MIP surface area (m2 g�

1 1 57 42 1.17 69

2 3 55 42 1.46 68

3 5 53 42 1.51 60

4 6 52 42 1.57 57

5 8 50 42 1.71 38

The SCR operating conditions for the experiments carried

out for the series of catalysts prepared with different V2O5

loadings were as follows: temperature 210–310 8C, gas hourly

space velocity (GHSV, at normal conditions): 20,000 h�1,

linear velocity: 0.66 Nm/s, pressure: 0.12 MPa. Gas inlet

composition: [NO] = 1000 or 500 ppm, [NH3] = 1000 ppm,

[O2] = 3 vol.% and [N2] = balance; ammonia was fed directly

at the entrance of the catalytic bed to avoid ammonium nitrates

formation. At similar conditions ammonia oxidation tests were

also performed with the following gas inlet composition:

[NH3] = 500 ppm, [O2] = 3 vol.% and [N2] = balance.

A linked series of experiments were also carried out at

310 8C employing a stainless steel micro-reactor. In the first

step, the reactor was fed with ammonia (1000 ppm), using Ar as

carrier, during 2 h in order to reduce the oxidative state of the

active sites. In the second step, ammonia was removed from the

stream by a gas purge with Ar for 1 h. In the third step, the

reactor was fed with 1000 ppm of NO + Ar as carrier. Detection

of the reactants and products was made with mass spectrometry

(Balzers Omnistar) using a Channeltron detector.

3. Results and discussion

The pore volume distribution and mean pore diameters, of

the catalysts with different vanadia content are shown in

Table 1. Due to the bimodal pore size distribution, the results

are divided into a mesopore region, attributed to a-sepiolite

employed as permanent binder and a macropore region, due to

the titania content of the samples. Throughout the series, the

macropore volume remained largely unchanged, while the

volume of mesopores decreased as the V2O5 loading was

increased. These values reveal the possibility of a filling effect

of the active phase, which plugs the narrowest mesopores and

fills the bottom of the rest. A consequence of these phenomena

was a higher proximity of the active centres, which increased

their density in the mesopore domain of the catalyst surface.

The possibility of changes in the configuration of vanadia

surface species was investigated by FT-Raman spectroscopy.

The Raman spectra of the catalysts with 3, 5, 6 and 8

V2O5 wt.% of under anhydrous conditions are shown in Fig. 1.

The sharp Raman bands at 639, 515, 396, 195 and 143 cm�1,

characteristic of the anatase phase of titania [12] were clearly

present in all the samples. In these catalysts crystalline V2O5

was not observed nor were surface vanadia species on titania

detected. A broad feature in the 800–1070 cm�1 region

1) Pore volume (cm3 g�1) Mean mesopore

diameter (nm)

Mean macropore

diameter (nm)Mesopore

7.5–50 nm

Macropore

50 nm–10 mm

Total

0–10 mm

0.25 0.72 0.97 8 164

0.25 0.66 0.91 11 138

0.23 0.67 0.90 12 154

0.22 0.66 0.88 14 138

0.15 0.66 0.81 17 159

J.A. Martın et al. / Applied Catalysis B: Environmental 70 (2007) 330–334332

Fig. 1. Raman spectra of the catalyst series. Excitation: 633 nm HeNe line;

spectral resolution 3 cm�1. The spectrawere obtained under anhydrous conditions

(150 8C) in a hot stage (Linkam TS-1500) in a synthetic air flow (30 ml/min).

Fig. 3. Inverse linear correlation between the nitrous oxide produced in the

SCR reaction and the mesopore volume of the catalyst at different temperatures.

Feed composition: [NOx] = [NH3] = 1000 ppm, [O2] = 3 vol.% and [N2] = bal-

ance. Operating conditions: GHSV 20,000 h�1, LV = 0.66 Nm/s, P = 0.12 MPa.

suggests an incipient interaction of the surface vanadia species

with magnesium sites, present in the sepiolite component of the

support. Similar interactions have been reported in the literature

between vanadia species and other compounds, such as Sb [13]

and K [14]. Therefore, changes in the structure of the active

phase species could not be detected, even for the catalyst

prepared with the highest vanadia loading.

The nitrous oxide formation and nitric oxide conversion

values as a function of the vanadia loading are plotted in Fig. 2.

The nitric oxide conversion values were the difference between

the NO disappeared and the NO eventually produced. The nitric

oxide disappeared comprises the amount reduced by ammonia

and the quantity that may be consumed by other reactions, such

as N2O formation, while NO may be formed by ammonia

oxidation under certain SCR conditions [15].

As shown in Fig. 2, a significant increase of the NO

conversion values up to about 5 wt.% of vanadia was observed.

However, on further increasing the active phase loading from 6

to 8 wt.%, lower nitric oxide conversions were observed. As the

conversion values for the 8 wt.% V2O5 catalyst were lower at

temperatures between 210 and 250 8C, they could not be related

with ammonia oxidation, which takes place only at tempera-

Fig. 2. NO conversion and N2O formation as a function of the catalyst vanadia

loading at different temperatures. Feed composition: [NO] = [NH3] =

1000 ppm, [O2] = 3 vol.% and [N2] = balance. Operating conditions: GHSV

20,000 h�1, LV = 0.66 Nm/s, P = 0.12 MPa.

tures above 250 8C. The sharp decrease observed for the

catalyst surface area of the 8 wt.% V2O5 catalyst (Table 1) was

probably the main reason for the lower SCR activity.

An increase in N2O generation with the vanadia content was

observed, which could be related to the textural properties of

the catalysts. A linear increase in the N2O formation with the

mean mesopore diameter was observed for this series of

catalysts. The inverse correlation, between the mesopore

volume and nitrous oxide formation is shown in Fig. 3. Thus,

the increase in the active phase may lead to a higher active sites

density in the mesopore region, which seems to favour N2O

formation.

Several authors [1,10,12] have reported the presence of N2O

in the exit gas of SCR units. The ammonia oxidation reaction

with catalysts, such as: CuO/Al2O3 [16], V2O5-WO3/TiO2 [17],

MoO3/SiO2 [18] or V2O5 [15], has been identified as a nitrous

oxide source, and thus, the formation of this compound in the

SCR process has been usually related with that reaction. In

ammonia oxidation, experiments carried out with the prepared

catalysts, N2O formation was also observed at conditions where

ammonia oxidation to form nitrogen and water takes place. In

Fig. 4 nitrous oxide formation values for the ammonia oxidation

and the SCR processes, obtained at similar conditions, with the

6 wt.% V2O5 catalyst are compared. As can be observed the

presence of NO in the feed enhanced the nitrous oxide

formation at all temperatures. The amount of N2O formed

during ammonia oxidation was much lower than that observed

in the SCR experiments.

Consequently, although under certain conditions, NH3

oxidation would be implicated in the appearance of N2O,

there should be another mechanism involved in the SCR

process, which could justify the higher values of N2O observed.

Similar results have been reported with a V2O5-MoO3/TiO2

catalyst for which the direct participation of nitric oxide in the

formation of nitrous oxide was suggested [12]. Other authors

have indicated that the formation of N2O could involve a

mechanism similar to that of NO photodecomposition

comprising the reoxidation of reduced metal ions by NO

adsorption [19–21].

J.A. Martın et al. / Applied Catalysis B: Environmental 70 (2007) 330–334 333

Fig. 4. N2O formation for SCR (~) and the ammonia oxidation reaction (*)

with the 6 wt.% V2O5 catalyst, versus reaction temperature. Feed composition

in SCR experiments: [NOx] = [NH3] = 1000 ppm, [O2] = 3 vol.% and

[N2] = balance; feed in ammonia oxidation experiments: [NH3] = 500 ppm,

[O2] = 3 vol.% and [N2] = balance. Operating conditions: GHSV 30,000 h�1,

LV = 1 Nm/s, P = 0.12 MPa.

Fig. 5. Programmed experiments changing the feed in the reactor carried out

with the 6 wt.% V2O5 catalyst. First step feed: 1000 ppm of NH3 for 2 h. Second

step feed: Ar for 1 h. Third step feed: (a) [NO] = 1000 ppm and [Ar] = balance

and (b) [Ar] = balance, increasing the temperature from 310 to 500 8C. Oper-

ating conditions: T = 310 8C, GHSV 20,000 h�1, LV = 0.11 Nm/s,

P = 0.12 MPa.

Taking into account these considerations an experiment was

carried out in a stainless steel micro-reactor, to check the

viability of the reoxidation of the reduced active sites of the

catalyst by NO molecules. The catalyst sample was previously

reduced with ammonia and after purging with argon, was

exposed to NO. As shown in Fig. 5(a), two small peaks of N2O

were detected after the exposition of the catalyst to NO. The

existence of two shoulders in the NO signal simultaneously to

the N2O peaks, also shown in the magnified inset, should be

noted. The test shown in Fig. 5(b) demonstrates the total

absence of adsorbed ammonia species after the treatment with

Ar as there was no nitrogen containing species detected on

heating the sample up to 500 8C. Therefore, these results

support the possibility of the reoxidation of V/Ti catalysts by

NO under certain conditions, in accordance with previously

reported results that demonstrated the adsorption of NO

molecules on reduced active sites over these catalysts [22].

Nitrous oxide formation would be produced by the interaction

of two V–ON species on neighbouring active sites:

However, at the SCR conditions, the presence of ammonia

would hinder the existence of two adjacent reduced centres

(oxygen vacancies) that could be reoxidised by NO. Thus, the

formation of V–ON species should take place by another way,

based on the interaction of the dissociatively adsorbed

ammonia species with other surface species, formed by the

presence of NO and O2 (eventually NO2) in the gas stream. The

relevance of NO2 in this process has been pointed out by

temperature programmed desorption experiments carried out

after reaction of NO2 + NH3 + O2 at 200 8C, in which

significant quantities of N2O were detected in the temperature

range of 250–350 8C [10]. Kantcheva et al. [23] described the

strong adsorption of NO2 on Lewis acid sites (V O) over titania

supported vanadia catalysts as responsible for nitrate formation

(NO3�) on the catalyst surface.

Taking into account that these catalysts have Lewis acid sites

where small quantities of NO2, present in the gas stream, may

be adsorbed giving rise to nitrate species, which could react

with ammonia adsorbed on the Bronsted acid sites, the

following nitrous oxide formation mechanism is proposed.

According with these considerations, in the presence of

oxygen the formation of nitrate species on the catalyst surface

and consequently N2O generation will be enhanced when the

partial pressures of NO + NO2 in the gas stream are increased.

Experiments carried out to confirm this possibility are shown in

J.A. Martın et al. / Applied Catalysis B: Environmental 70 (2007) 330–334334

Fig. 6. NO conversion and N2O formation with the 6 wt.% V2O5 catalyst as a

function of the inlet NO concentration at different temperatures. Feed composi-

tion: [NO] = 1000 ppm (~) and 500 ppm (*), [NH3] = 1000 ppm,

[O2] = 3 vol.% and [N2] = balance. Operating conditions: GHSV 20,000 h�1,

LV = 0.66 Nm/s, P = 0.12 MPa.

Fig. 6, where a direct relationship between NO inlet

concentration and N2O formation may be observed.

The adsorbed ammonia species not only are able to react

with NO of the gas stream via Eley–Rideal SCR mechanism,

but also with small quantities of NO2 adsorbed species,

producing H2O and N2O. This dual site mechanism is also

supported by the fact that the N2O generation was favoured by

higher vanadia catalyst loading, which increased the density of

active centres on the catalyst surface, as can be observed in

Fig. 2. Also in this figure, the N2O generation increased with

reaction temperature; as the Lewis acid centres (V O)

formation in V2O5 catalysts is favoured with this parameter

[15], the increase in N2O generation has been assigned to a

higher presence of Lewis acid sites on the catalyst surface.

Odenbrand et al. [24] or Lintz and Turek [25] have also

studied the formation of nitrous oxide over vanadia-titania

catalysts in the selective reduction of nitric oxide with

ammonia. In these works a reduction in the formation of

nitrous oxide was observed with the addition of water vapour to

the reaction. These results strongly agree with the reaction

mechanism proposed here, as a high quantity of Lewis acid sites

(V O) favours the formation of nitrous oxide molecules. The

presence of water would lead to an increase in the

hydroxylation degree of the catalyst and the subsequent

formation of Bronsted (V–OH) sites instead of Lewis (V O)

sites. Therefore, the dehydroxylation required for nitrous oxide

formation in this mechanism is favoured by high temperatures

and inhibited by the presence of water.

4. Conclusions

In the SCR of nitrogen oxides with ammonia at low

temperature by vanadia/titania based catalysts some nitrous

oxide formation may be observed. The generation of nitrous

oxide increases with the active phase loading, nitric oxide inlet

concentration and reaction temperature. The nitrous oxide

generation, even at conditions for which ammonia oxidation

reactions are not produced, indicated the existence of a

mechanism in which nitric oxide plays an important role. The

dual site mechanism proposed is based on the interaction of

ammonia species adsorbed on Bronsted acid sites with other

surface species, formed by the adsorption of NO + O2

(eventually NO2) on Lewis acid sites. This interaction would

give rise to the formation of neighbouring V–ON centres, and

subsequent production of nitrous oxide molecules.

Acknowledgements

The authors would gratefully acknowledge the CICYT

project Ref: CTQ2004-02206/PPQ for financial support and

M.A. Banares for Raman Measurements and his valuable

discussions.

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