Indian Journal of Chemistry Vol. 41A, August 2002, pp. 1554-1561

Oxidation and decomposition of NH3 over combustion synthesized Ah03 and Ce02 supported Pt, Pd and Ag catalysts

Parthasarathi Bera & M S Hegde*

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India

Received 9 April 2002

The catalytic oxidation and decomposition of NH3 have been carried out over combustion synthesized Al20 3 and Ce02

supported Pt, Pd and Ag catalysts using temperature programmed reaction (TPR) technique in a packed bed tubular reactor. Metals are ionically dispersed over Ce02 and fine metal particles are found on Al 20 3. NH3 oxidation occurs over I% Pt/AizOJ, 1% Pd/Alz0 3 and 1% Ag/Al20 3 at 175, 270 and 350 °C respectively producing N2, NO, N20 and H20, whereas I% Pt/Ce02, I% Pd/Ce02 and I% Ag/Ce02 give N2 along with NO, N20 and H20 at 200, 225 and 250 °C respectively . N2 predominates over other nitrogen-containing products during the reaction on all catalysts. At less 0 2 concentration, N2 and H20 are the only products obtained during NH3 oxidation. NH3 decomposition over all the catalysts occurs above 450 °C.

Oxidation of NH3 has been studied for a long time over various catalysts as either a primary reaction or as a side reaction. It is well established that NH3 oxidation plays an important role in the selective catalytic reduction (SCR) of NO by NH3 in the presence of 0 2. But here oxidation of NH3 to NO or N2 is an unwanted side reaction. The selectivity of SCR cannot be understood without investigating the role played by oxidation of NH3. Past and present investigations on NH3 oxidation over noble metals1-7

and various oxides, zeolites8-14 have shown that either NO or N2 is the selective oxi~ation product over most of the catalysts. In a few cases, N20 has been found as a trace product but at higher temperatures it disappears. Ozkan et al. 15 and Ramis et al. 16 have studied the role of NH3 oxidation in SCR of NO over vanadia, CuO-Ti02 and Mg0-Fe20 3 based catalysts. Supported Mo, V and Cu catalysts 17-19 are active and selective to N2 for NH3 oxidation. Gang et a/ 19 have shown that CuA]z04 like phase is more active than a CuO phase for NH3 oxidation whereas [Cu-O-Cu]2

+

like species or small copper oxygen aggregates in copper zeolites are the active centers at low temperature reaction. Yahiro et al. 20 have reported NH 3 oxidation over SiOrpillared oxy-compound where N2, N20 and H20 are the main products.

Recently, Burch and Southward21-24 have established internal selective catalytic reduction (iSCR) through NH3 oxidation during combustion of gasified biomass under 0 2-limited environment. They have shown the selective oxidation of NH3 to N2 in

biomass derived gases by a coupled NH3 oxidation to NO and subsequent reduction of NO by CO or H2 over 2% Rh/A]z03, 2% Rh/10% Ce02/Al20J, 1% Pt/20% CuO/ A]z03 I% Pt/20% BaO/ A]z03 and A]z03 supported precious metal catalysts. Ayyoob and Hegde25 have shown that high concentration of dissociatively chemisorbed oxygen on potassium covered Pt surface favors the formation of NO from NH3 in contrast to N2 over pure Pt at lower oxygen concentration. Bradley et al. 26 and Kim et al. 27 have reported that NO is the major product on pure Pt metal at high oxygen coverage (80 > 0.2 ML) and N2

is formed at lower oxygen coverage (80 < 0.2 ML) . In the present study, A]z03 and Ce02 supported Pt, Pd and Ag catalysts have been synthesized by a combustion method. Here Pt, Pd and Ag metals are ionically dispersed over Ce02 whereas fine nano size metal particles have been formed over a-Ah03 support. These catalysts are examined for NH3 oxidation using temperature programmed reaction (TPR) technique in a packed bed tubular reactor. We report herein a comparative study among Pt, Pd and Ag catalysts over Al20 3 and Ce02 supports.

Materials and Methods Synthesis and characterization

Pt, Pd and Ag metals supported on A]z03 and Ce02 have been synthesized by combustion technique. The details of the preparation have been described elsewhere28-30

. The combustion mixture for the preparation of 1% Pd/Ce02 contained

BERA eta/. : DECOMPOSITION OF NH3 OVER Pt, Pd & Ag CATALYSTS 1555

(NH4hCe(N03)6, PdCI2 and C2H6N402 (oxalyl­dihydrazide) in the mole ratio 0.99 : 0.01 : 2.33. In a typical preparation of I% Pd/Ce02, I 0 g of (NH4hCe(N03) 6 (E. Merck India Ltd., 99%), 0.033 g of PdCh (Glaxo India Ltd., 99%) and 5.175 g of C2H6N402 were dissolved in the minimum volume of water in a borosilicate dish of 130 cm3 capacity. The dish containing the redox mixture was introduced into a muffle furnace maintained at 350 °C. Initially the solution boiled with frothing and foaming and underwent dehydration. At the point of complete dehydration, the surface ignited, burning with a flame ( -1000 °C) and yielding a voluminous solid product within 5 min. Similarly alumina and ceria supported Pt and Ag catalysts are prepared by the same method. These catalysts have been characterized by XRD, TEM and XPS techniques.

XRD patterns of the catalysts were recorded on a JEOL JDX-8P diffractometer using CuKa radiation with Ni filter and scan rate of 2° min·'. TEM of the catalysts was carried out in a JEOL JEM-200CX transmission electron microscope operated at 200 kV.

XPS of the catalysts were recorded on an ESCA-3 Mark ll spectrometer (VG Scientific Ltd., England) using AIKa radiation (1486.6 eY). Binding energies calibrated with respect to C(ls) at 285 eY were accurate within ± 0.2 eY. For the XPS analysis the powder samples were made into pellets of 8 mm diameter and kept in an ultra high vacuum (UHV) chamber at 10-9 Torr housing the analyzer. Prior to mounting the sample into the analyzing chamber it was kept in the preparation chamber at 10'9 Torr for 5 h in order to desorb any volatile species present on the catalyst.

Temperature programmed reaction (TPR) The catalytic reactions were carried out in a

temperature programmed reaction system equipped with a quadrupole mass spectrometer QXK300 (VG Scientific Ltd., England) for product analysis. The details of the TPR experiments have been reported earlier28

-31. Typically 0.1-0.2 g of the catalyst was

loaded in a quartz tube reactor of 20 em length and 6 mm diameter. The gases were passed over the catalyst at a flow rate of 25 !J.mol s· ' and it was varied from 10-40 !J.mol s· '. Accordingly, space velocity was in the range of 5 x 10·5 to 2 x 10-4 mol g _, s·'. 0 2 to NH3 volume ratio was L5: I in the excess 0 2 condition whereas it was 0.7: I in 0 2 lean condition. Concentrations of reactants and products are obtained

by dividing the relative intensities of molecular ions from the mass spectrometric analysis with their respective ionization cross-sections32 . A series of gas mixtures (eg. NH3 + 0 2, NO+ N2) of different volume ratios were analyzed in the mass spectrometer and intensities of molecular ions were confirmed to be proportional to the respective ionization cross­sections. Errors in concentrations reported in our results are within 5%. NH3 and 0 2 gases were obtained from Bhoruka Gases Ltd., Bangalore. Their purities were better than 99% as analyzed by the quadrupole mass spectrometer.

Results Structural studies

XRD patterns of I% M/ Ah03 (M= Pt, Pd and A g) show the broad peaks of Pt, Pd and Ag metals along with a-AI20 3 peaks. Metal particle sizes are in the range of 6-20 nm as calculated from Debye-Scherrer method. On the other hand, the diffraction lines agree well with Ce02 in fluorite structure and there are no reflections corresponding to the metal in I % M/Ce02 patterns. Crystallite sizes of Ce02 calculated from the Debye-Scherrer method are in the range of 15-30 nm. In all the cases, peaks corresponding to these metal oxides are not seen. Typical XRD patterns of Ah03

and Ce02 supported catalysts are shown in Fig. I . TEM studies show that the crystallites of Ce02 are

in the range of 15-25 nm. Very few Pt particles of 3-5 nm size are seen on Ce02 crystallite surface in I% Pt/Ce02 catalyst. If all the platinum in I%

Fig. 1-XRD patterns of (a) I% Pt/ Al 10 3, (b) I% Pd/ Al 10 3• (c) I% Pt/ Ce02 and (d) I% Pd/ Ce01.

I556 INDIAN J CHEM, SEC A, AUGUST 2002

Pt/Ce02 have been formed as Pt metal particles of 3-5 nm size, at least 10 times more number of Pt particles should have been observed in the TEM images. Therefore, over 90% of Pt in l % Pt/Ce02 are dispersed in ionic form. However, corresponding Pd and Ag particles are absent on I % Pd/Ce02 and I % Ag/Ce02 catalysts. In contrast, dispersed metal particles of 7-20 nm have been obtained on a-Ah03

support. Analysis of electron diffraction patterns does show rings due to Pt, Pd and Ag metals in M/Al203.

XPS studies The XPS of Pt( 4f) core level region in I %Pt/ Ah03

and I% Pt/Ce02 are given in Fig. 2. As binding energies of Pt(4j) and Al(2p) core levels are in closer region, a broad spectrum is obtained in the case of I % Pt/ Al20 3 [Fig. 2 (a)]. This spectrum can be deconvoluted into Pt(4f712• 512 ) and Al(2p) peaks. Here Pt( 4/712. 512) peaks at 7l.l and 7 4.3 e V can be attributed to Pt metal only and an intense peak at 73.9 eY corresponds to Al(2p) in a-Ah0/3. Similarly, Pd and Ag metal peaks are observed in the case of Al203 supported Pd and Ag catalysts28. The XPS of I% Pt/Ce02 could be resolved into three sets of spin-orbit

600

400

200 ..-

I en en c ::I 0 () 65 70 75 80 85

~ en Pt2+ c (b) Q)

c

... .. .. 70 75 80 85

Binding Energy;eV

Fig. 2-XPS of Pt(4f) core level region in (a) I% Pt/AI20 3 and (b) I% Pt/Ce02• In I% Pt/AI20 3, Al(2p) of a-AI20 3 is also shown.

doublets. Accordingly, Pt(4f712• 512 ) peaks at 71.0, 74.2; 71.9, 75.1 and 74.3, 77.5 eY could be attributed to Pt metal, Pt2+ and Pt4+ respectively [Fig. 2 (b)]. Here Pt is found to be dispersed mostly in +2 (72%) and +4 (2I %) oxidation states on Ce02 crystallites with only 7 % Pt present as Pt metal particles. This observation agrees well with the TEM studies. However, Pd and Ag are present in +2 and + l oxidation states in I% Pd/Ce02 and l % Ag/Ce02 respectively. The respective surface concentrations of Pt, Pd and Ag have been estimated by the relation

XM _ IM ace Ace DE (Ce) Xce - Ice aM AM DE(M)

... (I)

where X, I, a, A and DE are the surface concentration, intensity, photoionization cross section, mean escape depth and geometric factor respectively. Integrated intensities of metal core level and Ce(3d) peaks have been taken into account to estimate the concentration. Photoionization cross sections and mean escape depths have been obtained from the literature34' 35. Accordingly, surface concentrations of as-prepared Pt, Pd and Ag in Ce02 are 15, 5 and 12 %

800 ~

(a)

600

400

..- 200 I en en c ::I 0

0 ()

~ 800 02 "(ij (b)

c Q)

c 600 --~0

400

200

N

0 100 200 300 400 500 600 700

Temperature ,OC

Fig. 3--TPR profiles of NH3 + 0 2 reaction over (a) I% Pt/ Al 20 .1 and (b) 1% Pt/ Ce02.

BERA et al. : DECOMPOSITION OF NH3 OVER Pt, Pd & Ag CATALYSTS 1557

respectively against I% taken in the preparation. Similarly, surface concentrations of as-prepared Pt, Pd and Ag in A}z03 support are 0.7, 0.8 and 0.7 % respectively which are close to the bulk concentrations of the respective metals in A}z03.

Thus, higher dispersion of metals is achieved over Ce02 support.

TPR studies The catalytic properties of these materials have

been investigated for the oxidation and decomposition of NH3. Oxidation of NH3 by 0 2 occurs at 175 and 170 °C over 1% Pt/ Al20 3 and 1% Pt/Ce02 respectively giving N2, N20 and H20 as the reaction products. Typical TPR profiles of NH3 + 0 2 reaction on I% Pt/Ce02 and 1% Pt/ A}z03 are shown in Fig. 3. In Pt/ A}z03, N2 is formed selectively as the main product at lower temperature region, whereas NO concentration increases in the temperature range of 250-450 °C in Pt/ A}z03. At a higher temperature (> 450 °C) NO intensity decreases and accordingly, increase in N2 intensity is observed. In the case of Pt/Ce02, NO concentration decreases above 300 °C and N2 concentration increases. In both the cases N20 disappears at higher temperature. On Pt/ A}z03 the reaction can be written as follows

6NH3 + 60z ----7 Nz + 2NO + NzO + 9Hz0 ( <450°C) ... (2)

4NH3 + 30z ----7 2N2 + 6H20 (> 450°C) ... (3)

NH3 oxidation over 1% Pd/ AI20 3 starts at 270 °C and N2, NO, N20 and H20 are the reaction products. Here NO intensity increases and N20 disappears at

Table l--NH .1 conversion and N2 selectivities during NH3

oxidation over Al20 3 and Ce02 supported catalysts.

Catalyst Temp. NH3 Nz (OC) conversion selectivity

(%) (%) Pt/AI20-' 450 93 68

500 100 89 Pd/AI20 .1 350 86 74

400 93 82 Ag/AI20 3 400 49 92

450 77 80 Pt/Ce02 450 100 71

500 100 77 Pd/Ce0 2 300 94 73

350 100 70 Ag/Ce02 350 71 91

400 81 95

higher temperatures (> 400 °C). In 1% Pd/Ce{h, NH3

oxidation starts at 250 °C giving N2 as the main nitrogen-containing product. Above 350 °C NO concentration increases, N2 intensity decreases and N20 decomposes. But at a higher temperature N2

intensity increases. In the case of 1% Ag/ A}z03, N2 starts at 350 °C

during NH3 oxidation. Above 400 °C NO and trace amount of N20 are observed along with N2. In the case of 1% Ag/Ce02, N2 and a very less amount of N20 are obtained at 275 °C. Above 400 °C NO comes up as a product along with N2 and accordingly the intensity of N2 decreases. N20 decomposition occurs in both cases at higher temperatures.

In NH3 oxidation under excess oxygen condition, N2, NO, N20 and H20 are the products from the reaction over all the catalysts. Complete NH3

conversion occurs at higher temperatures in all the cases. NH3 conversion and N2 selectivity at a particular temperature during NH3 oxidation over all the catalysts are given in Table 1. The selectivity to

N2 formation ( s N ) is calculated as follows20: 2

S N = 100 x [Amount of ~ 2 formed] 2 [Amount of NH 3 consumed]

= IOO X ____ _:_[A_m_ou_n_t_of_N..:.2_fo_rm_ed_]:...._ ___ _

[Amount of N2 formed]+ [Amount of NO formed I+

[Amount of N2 0 formed]

. .. Ci a: c

.Q iii E 0 u.

160 r-------------,

1-40

120

100

80

1-40

120

100

80

60

-40

20

200250300350400450500550600

Temperature, •c

Fig. 4-Rates of N2, NO and N20 formation over (a) 1% Pt/AI20 3

and (b) 1% Pt/Ce02 in NH3 + 0 2 reaction.

1558 INDIAN J CHEM, SEC A, AUGUST 2002

During NH3 oxidation over all the catalysts N2 predominates and selectivity to N2 is higher in relation to other nitrogen-containing products. Rates of formation of all nitrogen-containing products such as N2, NO and N20 over Pt/Al20 3 and Pt/Ce02 catalysts are given in Figs. 4 (a) and (b) respectively. It is important to note that N2 formation rates are higher than NO and N20 formation rates. Above 500 °C selectivity to N2 is nearly 100% for NH3 oxidation over Pt/Aiz03. In Figs. 5 (a) and (b), N2, NO and N20 formation rates are given for PdfAI20 3 and PdfCe02 respectively. Below 400 °C, NO formation rates are much lower than N2 over Pdf Alz03, but above 400 °C, NO formation rates increase. Rates of NO formation increase over I% PdfCe02 above 300 °C. Similarly, rates of N2, NO and N20 formation for Ag/Al20 3 are given in Fig. 6. Rates of N2 formation are higher at higher temperatures over both Pt/ Al20 3 and Pt/Ce02 catalysts. Accordingly, NO formation rates are lower over these catalysts in the higher temperature region. On the other hand, N2 formation rates decrease over Pdf Alz03 and Ag/ Alz03 catalysts and the rates of NO formation increase in the case of PdfAI20 3, PdfCe02 and Ag/Ce02 catalysts. But NO formation rates do not exceed N2 formation rates in all the cases in the entire range of temperature. N2 formation rates are much higher in comparison with NO and N20 over all the catalysts.

150.---------------,

125

100

75

-: Ul 50 -: OJ

0 25 E ::1.

600

60

40

20

0~====~~:!::::::::=-:::,_.,l 200 300 400 500 600

Temperature, •c Fig. 5---Rates of N2, NO and N20 formation over (a) I% Pd/AI20 3 and (b) I% Pd/Ce02 in NH3 + 0 2 reaction.

NH3 oxidation at low 0 2 concentration was also carried out over Pt/Ce02 and Pd/Ce02 catalysts. When NH3 and 0 2 gases in I :0.7 ratio were passed over Pt/Ce02 catalyst, N2 and H20 are the only products over the entire range of temperatures. A sharp increase in N2 and H20 intensities is observed at 250 °C. Accordingly, 0 2 is fully utilized in the reaction. Other nitrogen-containing products like NO and N20 are absent during the reaction. Here excess NH3 remains in the products indicating 'NH3 slip' 36

.

The NH3 + 0 2 reaction can be written as follows:

... (4)

Similarly, N2 and H20 are seen as the NH3 + 0 2

reaction products over I% PdfCe02 catalyst in Or lean condition. TPR profiles of NH3 + 0 2 reaction over I% Pt/Ce02 and I% PdfCe02 in Orlean condition are shown in Fig. 7.

In order to understand the role of lattice oxygen 10

in NH3 oxidation, NH3 was passed over all the catalysts and N2 and H2 are obtained as the products, not N2 and H20. Complete NH3 decomposition occurs at 7(}()-750 °C over Pt/Ce02 catalyst. Over PdfCe0 2

catalyst, H2 and N2 are obtained at 650 oc whereas over PdfA1z03 the decomposition occurs at 500 °C. The rates of N2 and H2 formation in NH3 decomposition over Pt/Ce02 are shown in Fig. 8. The concentration ratio of N2 to H2 of 1:3 is maintained during NH3 decomposition and rate of formation of H2 is three times higher than that of N2 in entire range of temperature. Thus, absence of H20 formation indicates that lattice oxygen from Ce02 as well as Alz03 is not utilized during the reaction. Therefore, in M/Ce02 and M/Al20 3 as catalysts, it is the feed oxygen that is utilized in the oxidation of NH3.

=.-------------------------,

·., -;D 150

0 E "-., .. 100 c;; ~ c:

.Q c;; 50

E 0 u.

ot-~~-=~~~~====~--~~~ 300 350 400 450 500 550 600

Temperature, °C

Fig. 6---Rates of N2, NO and N20 formation over I% Ag/AI20 3 in NH3 + 0 2 reaction.

BERA et al. : DECOMPOSITION OF NH3 OVER Pt, Pd & Ag CATALYSTS 1559

800 (a)

600

400 N2 '------NH3

•., 200

Ill ... c jo 'J: ,I

~ ~ 0 u 100 200 300 400 500 600 700

~ ... . iii ;aoo .s NHJ

(b)

600 ¥

f 400

200

N2

\ NH3

r' N2 _) 1 02

0 100 200 300 400 500 600 700

Temperature ,oc

Fig. 7-TPR profiles of NH3 + 0 2 reaction in Or lean condition over (a) 1% Pt/Ce02 and (b) 1% Pd/Ce02•

Discussion

NH3 oxidation is of great importance regarding SCR of NO and HN03 synthesis. The possible reactions involved in NH3 oxidation are as follows: 4NH3 + 302 ~ 2N2 + 6H20 . .. (5) 4NH3 + 502 ~ 4NO + 6H20 .. . (6) 2NH3+ 202~ N20 + 3H20 ... (7)

4NH3+ 702~ 4N02+ 6H20 .. . (8)

Although the decomposition of NH3 and NO are also possible reactions that could take place during NH3 oxidation, they become significant only at a temperature above 500 °C. The reaction between NH3 and 0 2 produces some N20 at low temperatures, but above 400 oc the only nitrogen-containing species observed are N2 and NO. In fact, N02 was not observed during the reaction in any of the above experiments.

It is widely proposed that NH3 oxidation is a multi­step process involving different intermediate species. Bodenstein 1, Andrassow2 and Zawadzki3 have advocated the formations of hydroxylamine, nitroxyl and imide respectively during the first step of the reaction. Fogel et a/.4 and Nutt and Kapur have

~----------------------------~

":'. 200

·.,. 0 'i 150

0-~~~==~~L_--~~----~-----~~--~~ Temperature, •c

Fig. 8-Rates of N2 and H2 formation over I% Pt/Ce02 in NH3

decomposition .

observed NO as the product of first reaction step which occurs on the catalyst surface. They suggested that N2 production at low temperatures can be interpreted more simply as a bimolecular reaction between NO and NH3 to produce N2. However, at high temperatures the unimolecular decompositions of NH3 and NO also produce N2. Recently, Carley et al. 37 have shown a number of distinct reaction pathways during NH3 oxidation in the formation of either chemisorbed imide or nitrogen adatoms.

In the present study, metals are ionically dispersed on Ce02 support, whereas nano size metal particles are homogeneously dispersed on a-Ah03 matrix28.30

.

In I% Pt/CeO~. Pt is in +2 and +4 states along with small amount of Pt particles whereas Pd and Ag are in +2 and +I states in I% Pd/Ce02 and I% Ag/Ce02 respectively. Oxidation of NH3 over Pt/Ce02 catalyst gives N2, NO and N20 as the reaction products. Above 300 °C N20 decomposes to give N2 and 0 2. The formation of N20 may be due to partial oxidation of NH3 and above 300 °C it decomposes. The NO concentration decreases at higher temperatures over both Pt/Ce02 and Pt/ Ah03. Accordingly, N2 concentration increases. Product NO may react with NH3 leading to N2 indicating a parallel reaction. One can write the reactions as follows:

2N20 ~ 2N2 + 02 6NO + 4NH3 ~ 5N2 + 6H20

. .. (9)

. . . (10)

In 1% Pd/Ah03 and in 27Q-370 °C region, N2 concentration increases and at higher temperatures (> 400 °C) it decreases. It is worthy to mention our earlier results of NH3 oxidation over Cu!Ce02 cata1yst38. NH3 oxidation over 5% Cu/Ce02 gives N2 as the main product at lower temperatures ( < 400 °C).

1560 INDIAN J CHEM, SEC A, AUGUST 2002

But NO is the main nitrogen-contammg species at higher temperatures (> 400 °C) along with less content of N2. Therefore, SCR of NO with NH3 in the presence of 0 2 over Cu/Ce02 works at 150-350 °C region. Above 350 °C NO predominates over N2. On the other hand, in the entire range of temperature N2 is the main product of NH3 oxidation over Ce02 and Al20 3 based Pt, Pd and Ag catalysts. In the case of less 0 2 concentration, N2 is the sole nitrogen­containing product. Formation of NO and N20 requires more 0 2. Due to unavailability of sufficient 02 in the Orlean condition, NO and N20 are not observed during the reaction in the entire range of temperature. These observations are in complete agreement with BHK modef6

•27. However, NH3

oxidation over Ah03 and Ce02 supported Pt, Pd and Ag catalysts shows that N2 is the major product during the reaction compared to NO, N20 and N02. Relatively low temperature of NH3 oxidation over Pd/Ce02 and Ag/Ce02 can be due to higher dispersion of metals on Ce02 support compared to Al20 3.

However, a similar increase in the catalytic activity is not prominent in the case of Pt/Ce02 in relation to Pt/Ah03. Pt/Ah03 and Pt/Ce02 catalysts show relatively low temperature NH3 oxidation compared to supported Pd and Ag catalysts. Rates of N2 formation are higher in the case of Pt/ Ah03 and Pt/Ce02 catalysts compared to Pdf Ah03, Pd/Ce02, Ag/ Ah03

and Ag/Ce02 catalysts. NH3 decomposition over these catalysts predicts

the role of lattice oxygen in NH3 oxidation. There is no losely bound 0 2 in the lattice which can react with NH3

10. As NH3 decomposition to N2 and H2 occurs at

a much higher temperature over these catalysts compared to NH3 oxidation, formation of N2 as a product during NH3 oxidation is only due to oxidation. The unavailability of labile oxygen in the catalysts suggests that the reaction is unlikely to follow Mars-Van Krevelen pathway39

. On the other hand, lower reaction temperature in the presence of 0 2 in a catalytic reaction indicates that the catalyst

·, surface does provide suitable active sites for both NH3

and 0 2 adsorption to adopt a Langmuir-Hinshelwood pathway40

.

Conclusion The important findings of the study are as follows: (i) combustion synthesized alumina and ceria

supported Pt, Pd and Ag catalysts are all active in NH3 oxidation at low temperature; (ii) ionic as well as

metal sites are the active centers for NH3 oxidation; (iii) Pt/Ce02 and Pt/ Ah03 show low temperature NH3

oxidation among all the catalysts; (iv) in all cases N2

predominates over other nitrogen-contammg products; (v) N2 and H20 are the only products under 0 2-lean condition; and (vi) lattice oxygen is not utilized during the oxidation, only feed oxygen reacts with NH3 to produce nitrogen-containing species.

Acknowledgement PB is thankful to the CSIR, Government of India,

for the award of a research fellowship. We thank Professor K S Gandhi, Department of Chemical Engineering, Indian Institute of Science, for useful discussions. Financial support from the Department of Science and Technology (DST), Govern ment of India is gratefully acknowledged.

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