J. Phys. Chem. 1995,99, 17032-17042
17032
Control of a Biphasic Surface Reaction by Oxygen Coverage: The Catalytic Oxidation of Ammonia over Pt{ 100) J. M. Bradley, A. Hopkinson? and D. A. King* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEW, U.K. Received: August 8, 1995@
The ammonia oxidation reaction on Pt{ 100) has been investigated over the temperature range 300-800 K, using molecular beams under UHV conditions. The reaction is biphasic, with N2 being the major product below 600 K and NO being the major product above 600 K. It is found that product selectivity can be controlled by varying the beam composition as well as by varying the surface temperature. The efficiency of the reaction to NO can be significantly increased by preadsorption of oxygen on the crystal. Coadsorption and isothermal experiments translate the beam composition dependence into a surface oxygen coverage dependence, with high oxygen coverages resulting in the suppression of N2 production. N2 is believed to be produced mainly from the dissociation of NO produced by oxidation of adsorbed N H 3 . The observed oxygen coverage dependence of product formation is explained by a sharp fall in the heat of adsorption of the dissociated N(a) and O(a) with increasing O(a) coverage. At high surface oxygen coverages the suppression of NZ production arises from the resulting inhibition of NO dissociation. The observed surface temperature dependence of product formation is explained by competition between NO desorption and NO dissociation.
1. Introduction Ammonia is oxidized over platinum to form NO as the first step in the industrial synthesis of nitric acid, a process that has been in large-scale use for over 70 years1 The reaction is strongly exothermic, enabling reactors to be run adiabatically, and it results in high conversions of NH3 to NO (94-98%).2 Despite this efficiency, problems are encountered with Pt loss, which is believed to be due to the formation of PtOz(g) at the high operation temperatures (> 1000 K).3 These high temperatures are necessary both because the reaction exhibits a temperature dependent product selectivity (below -800 K the oxidation reaction mainly produces Nz) and also for the activation of new Pt gauzes. A new gauze is inactive until it is at temperature in the presence of the reactants; the wires restructure causing their surfaces to become highly facetted. This facetting is believed to arise from the surface energy and activity differences between the different crystal plane^.^ Academic interest in the reaction lies in the challenge of elucidating details of the reaction mechanism over which, despite numerous studies, there is still much uncertainty. With regard to investigating the facetting process, studies on single-crystal planes are of obvious importance. In general, the ammonia oxidation reaction on platinum has been observed over the temperature range 300-1700 K.2 Studies have been canied out on supported Pt? polycrystalline Pt,6-11and single-crystal P t . I 2 - l 4 A product selectivity is observed as a function of surface temperature. At low temperatures Nz is the main product, but above -850 K this is superseded by NO production, and above 1473 K, N2 again becomes the major product. Most of the reaction mechanisms proposed to explain the product temperature dependence below 1473 K are based on that originally suggested by Fogel et a1.I5 Fogel et al. used polycrystalline Pt wire, with reactant partial pressures of Torr, and worked over the temperature range 300- 1473 K. Using secondary ion mass spectrometry (SIMS), they concluded that the intermediate species HNO, NHzOH, + Present address: Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, L63 3JW, U.K. Abstract published in Advance ACS Abstracts, October 15, 1995. @
0022-3654/95/2099-17032$09.00/0
H N 0 2 , and N20 were not formed during the oxidation reaction and therefore proposed a reaction mechanism which had simple steps, with none of the above intermediates involved, based on the two reactions
NH,(a)
+ NO(a)
-
N,(g)
+ H,O(g) + H(a)
(2)
NZ was thus believed to be formed in a bimolecular surface reaction between NO and NH3, with the transition from NZto NO formation occurring at a surface temperature when NO has too short a surface residence time to take part in reaction 2, i.e. when NO starts to desorb from the surface. Nutt et aL6 and Pignet et aL7 investigated the kinetics of ammonia oxidation over polycrystalline Pt wire over the pressure range 0.1- 1.O Torr and temperature range 400- 1600 K. Gland et al. moved the kinetic investigation into the regime of UHV and single crystals, working on a stepped Pt{lll} crystal at pressures of 10-8-10-'o Torr over the temperature range 200873 K,I2 and UHV studies have also been done on planar Pt{lll}.'3 These authors all agreed with the mechanisms proposed by Fogel et al. Similarities in the temperature profile of the reaction between NO and NH3 and that between 0 2 and NH3 to produce N2 were used as support for step 2 of Fogel's m e ~ h a n i s m . ~However, ,'~ it should be noted that this is not self-evident; if the temperature profile is the same, this suggests there is an elementary step that involves NH3 alone. Using Auger electron spectroscopy (AES),Gland et al. related the kinetic data to quantitative surface compositional analysis.I2 When N2 is the only product (Ts 673 K), an N-containing adsorbate is believed to be the only surface species, although this species is not identified. When NO is the only product (Ts > 673 K), the primary species detected by AES is adsorbed oxygen. The product surface temperature dependence was thus shown to be related to the dependence of surface composition on surface temperature. Both Pignet et al.' and Gland et a1.I2 observed that product selectivity at constant surface temperature was a function of gas phase composition as well, with only N2 formed over the whole temperature range in excess NH3 and 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 46, 1995 17033
Catalytic Oxidation of Ammonia over Pt{ 100) with NO being the primary reaction product over the whole temperature range in excess 02.7J2 In all, a qualitative picture, based on the competition for active sites between the reactants NH3 and 0 2 and the intermediate NO, compatible with that proposed by Fogel et al., was developed to explain these observations. These models for the overall behavior of the ammonia oxidation reaction on Pt appear reasonable. They do, however, address only the overall reaction mechanism and do not delve into the details of the elementary reaction steps. Two further studies have been made into the mechanism of NO formation, at surface temperatures greater than 700 K, with the conclusion that the key steps involved are NH(a)
+ O(a) - NO(g) + H(a)
N(a)
-
+ O(a>
NO(g)
(3) (4)
Selwyn et aL8 reported the desorption of NH and OH radicals during the course of the reaction over polycrystalline Pt in the temperature range 800- 1330 K, detected by laser-induced fluorescence. To account for the production of these radicals, they proposed that the reaction takes place via atomic and radical pathways at these temperatures. This is not an unusual proposal since both NH3 and 0 2 dissociatively adsorb at these temperatures. They also found that selectivity at high temperatures was related to the relative coverage of nitrogen to oxygen atoms on active sites. The above reaction scheme is supported by the work of Asscher et al.I4who used a laser multiphoton technique, combined with molecular beams under UHV conditions, to probe NO production during the reaction on a Pt{ 111) crystal. By analyzing the decay in the NO signal after pulsing NH3, they identified two different NO production kinetics and assigned them to the mechanisms detailed above. They found reaction 3 to be the faster mechanism, which is enhanced at high O2/NH3 ratios and had a higher activation energy than reaction 4, the slower mechanism. In the most recent study of this reaction system Mieher et a1.I6 examined coadsorbed oxygen and ammonia on Pt{ 11l} using EELS and TPRS (temperature-programmed reaction spectroscopy). Using EELS they identified OH, NH, and NH2 as intermediate species but were unable to comment on NO due to ambiguities in the assignment of the NO frequency. They concluded that the reaction proceeded via the simple stripping of NH3 by oxygen atoms followed by the combination of nitrogen atoms with oxygen, to form NO, or with other nitrogen atoms, to form N2. They account for N2 formation without considering the action of an NO intermediate. Finally, high-temperature NZ formation, above 1500 K, is believed to be due to NH3 dissociation or perhaps to NO di~sociation.~
scattering plane. LEED optics, an ion-sputtering gun, and a second QMS are mounted onto rhe main chamber. The Pt{ l00} crystal was cleaned with sputter-anneal cycles and oxygen treatments at 600 K and a final annealing up to 1200 K. This cleaning procedure produced a clean Pt{ 100}-hex-R0.7" surface that exhibited excellent LEED and helium diffraction patterns" and reproducible spectra for CO" and 02.l8 The unreconstructed surface was prepared using the method prescribed by Griffiths et al.*O The 0 2 and N H 3 gases used in the experiments had purities of 99.998 and 99.999%, respectively, as quoted by the suppliers (Distillers M.G. Ltd.). For most of the experiments the reaction products NO, Nz, and H20 were followed using the fixed main-chamber mass spectrometer (difficulties, however, were encountered with the H2 signal). Oxygen coverage was calibrated to the known saturation value of 0.63 monolayers (ML) on Pt{100},20 where one monolayer is defined as the number density of Pt atoms in the (1 x 1) surface. The ammonia beam flux and all the products, expressed in MLh, were calibrated by preparing a known mixture of 0 2 and He in a mixing cylinder and noting the partial pressures measured by the fixed QMS when the mixture is beamed into the main chamber. The 0 2 flux is calculated from the area under the beam pulse trace, and knowing what percentage of the beam is 0 2 , the total beam flux can be worked out and the mass spectrometer signal for He and N2 can be calibrated in terms of MLls. For NH3 He beams, the He signal was calibrated in terms of MLJs as detailed above, and knowing the percentage of NH3 in the beam, the NH3 flux can be calculated. For the calibration of NO signals the coadsorption results (section 3.2) are used; all the O(a) that reacts is assumed to form NO and H20. The H20 signals were calibrated by saturating the Pt{ loo} surface with O(a) and exposing the surface to H2. The resulting H20 signal was assumed to be due to 0.63 ML of O(a) reacting. Details particular to each set of experiments are included in the relevant results sections.
+
3. Results and Discussion
Previous studies of ammonia oxidation over Pt show that for surface temperatures 900 K two parameters control product selectivity: beam composition and surface temperature. In the present study three types of experiment were carried out to investigate the influence of these parameters. To gain an overall view of the reaction profile, steady-state measurements were made of reaction products as a function of beam composition and of surface temperature. The beam composition dependence was then translated into a surface oxygen coverage dependence through temperature-programmed reaction measurements and also through a series of isothermal experiments. 3.1. Temperature Dependence of Steady-State Products. These experiments were aimed at investigating the product selectivity as a function of surface temperature and gas phase 2. Experimental Section composition. For each measurement a mixed NH3 0 2 beam The experimental apparatus has been described previ~usly.l~.'~ was incident normal to the crystal surface and the steady-state reaction products were recorded. The efficiency of formation The Pt{ loo} sample is mounted centrally on a manipulator in of reaction products is quoted in terms of a reaction probability, an ultrahigh-vacuum main chamber. The main chamber is 5 (=product fluxheactant NH3 flux), which gives the probability pumped by a 6 in. oil diffusion pump with a liquid nitrogen that an NH3 molecule incident on the surface leads to that cold trap plus a titanium sublimation pump (base pressure 2 product. Variations were made of the crystal temperature, the x Torr). A molecular beam is directed at the sample relative amounts of NH3 and 0 2 in the beam mixture, the crystal and can be modulated using a rotating chopper. The beam surface structure (hex-R or (1 x l)), and oxygen precoverage. passes through two pumping stages before entry into the main 3.1.1. Initially Clean Pt{lOO}-hex-R. The Pt{ 100) surface chamber, the first stage consists of a nozzle source, skimmer, is most stable in the reconstructed hex-R form. Figures 1, 2, and collimating aperture and is pumped by a 9 in. oil diffusion and 3 display 5 for the formation of the products N2, NO, and pump, and the second stage is pumped by a 6 in. oil diffusion H20, respectively, for beams of varying composition incident pump. A rotating differentially pumped quadrupole mass on Pt{ 100}-hex-R at various surface temperatures. The reaction spectrometer (QMS) is located in the main chamber in the
+
17034 J. Phys. Chem., Vol. 99, No. 46, 1995 2.0
,
Bradley et al. I
1
v
8
4.0-
LP
3.0-
0.0 Temperature I K
+ 02 beams of varying composition incident on clean Pt{lW)-hex-R at various surface temperatures. The percent of 0 2 in the mixed beam is indicated in the figure.
Figure 1. Reaction probability for N2 formation from NH3
,
Pt(100kk-R T,=SOOK
6.0-
\
1.0
ma+%-
6.0-
1.5
'Q .Oxygen-precovered
_____
oI - - - - . - - I 1 % r 9 0 9 2 . ? 4 9 3 9 8 1 0 0 % 0,in mixed beam
Figure 4. Reaction probability for Nz formation on oxygen-precovered and clean Pt{lOO}-hex-R at 500 K for NH3 0 2 beams of varying
+
composition.
I
Ts ISOOK
Clean ~..----~--------------a 0.0
2003004005006007oOaN00900 Temperature / K
Figure 2. Reaction probability for NO formation from NH3
+
0 2
beams of varying composition incident on clean R(lOO)-hex-R at various surface temperatures. The percent of 0 2 in the mixed beam is indicated in the figure.
As the proportion of 0 2 in the mixed beam is increased from 30 to 90%, there is a noticeable increase in the reaction probabilities for N2, NO, and HzO formation. This indicates that the dissociative adsorption of 0 2 plays an important role in the formation of each of these products. Similarly, the catalytic activity of stepped Pt{ 111) for the ammonia oxidation reaction was found to be greater than that of planar Pt{ 11l}, and this was attributed to a greater efficiency for 0 2 dissociation.I3 On the hex-R surface, at temperatures below 500 K, the reaction is probably enhanced due to 02 dissociation at surface defects (1% steps on the surface). We have shown22 that the initial dissociative sticking probability of 0 2 on the 200 300 400 500 600 700 800 900 hex-R surface increases significantly between 500 and 600 K, Temperature I K although it is never high. This effect is probably directly Figure 3. Reaction probability for H20 formation from NH3 0 2 correlated with the increase in NO production which occurs over beams of varying composition incident on clean Pt{100)-hex-R at various surface temperatures. The percent of 0 2 in the mixed beam is exactly the same temperature range (Figure 2). A higher steadyindicated in the figure. state oxygen coverage during the reaction results in a higher NO production at these temperatures. This, in turn,suggests probabilities indicate that reactivity is low with, at most, 3% of that the efficiency of dissociative adsorption of 0 2 is a major the beam reacting (at 500 K). The observed low catalytic factor in determining the rates of product formation. activity of the hex-R surface is not surprising, as this surface is The lower curves in Figure 4, 5, and 6 show the reaction known to be relatively inert to both 0 2 and N H 3 dissociation.18,21 probabilities for N2, NO, and H20 formation, respectively, on Such a low conversion has also been observed on Pt{ 111) a clean surface with a temperature of 500 K and for higher @%).I4 These yields are considerably smaller than those proportions of 0 2 in the beam. As shown in Figures 1 and 2, obtained under industrial conditions (94-98% conversion), this surface temperature is associated with maximum N2 where multiple impingement under high-pressure industrial production and with low NO production. However, as the conditions results in much higher yields. We shall see (in percentage of 0 2 in the mixed beam is increased above 90%, section 3.3.3 below) that these high yields can be reproduced I;N~ is observed to decrease from 0.02 at 90% 02 to 0 at 98% under specific conditions in the present work on Pt{ 100). 0 2 , while 5 ~ 0 increases from almost 0 at 90% to 0.06 at It is clear from Figures 1 and 2 that there are two N-containing 98% 0 2 . Thus, at a surface temperature of 500 K and with product regimes which are defined by the surface temperature. high-percentage 0 2 beams, Nz production becomes suppressed Below -600 K the major N-containing product is N2, and this and a reaction pathway that favors NO production takes over. is superseded by NO production at surface temperatures greater In terms of steady-stateoxygen coverage, it appears that a higher than -600 K. At and below 400 K no NO is produced. This steady-state oxygen coverage (due to higher percentage 02 is in keeping with the observations of previous s t u d i e ~and ~ , ~ ~ beams) promotes NO production and discourages N2 production. will be discussed later in terms of reaction mechanism. This effect, a change in reaction route with gas phase composi-
+
Catalytic Oxidation of Ammonia over Pt{ 100)
J. Phys. Chem., Vol. 99, No. 46, 1995 17035 33,
1
Clean .__.*-
9 3 9 2 9 ? S 6 9 e l W % 0,in mixed beam
0, 20
h - R -.......----, , a; , - -_ _ _ - - $~
33
60 70 m % 0,in mixed beam
40
50
90
loo
Figure 6. Reaction probability for HzO formation on oxygenprecovered and clean Pt{ 100)-hex-Rat 500 K for NH3 0 2 beams of varying composition.
Figure 7. Reaction probabilities for Nz and H20 formation from NH3 and 0 2 beams of varying composition on clean Pt{100)-(1xl) and hex-R surfaces at 380 K.
tion (or surface adlayer composition), has also been observed by Pignet et al.' and Gland et a1.I2over the temperature range 400-1600 K. The explanations offered, however, are vague and show that this behavior is not fully understood. Pignet et al. attribute the suppression of NO production to high NH3 pressures inhibiting NO desorption from the surface. Gland et al. discuss this in terms of competition of reactants for active surface sites. This coverage dependence is further investigated in section 3.1.2 below and in sections 3.2 and 3.3. 3.1.2. Initially Clean Pt{1OO}-{Ix1). The clean Pt{lOO} surface can be prepared in a metastable (1x1) state, but measurements on this surface are limited to a maximum temperature of 380 K since the structure reverts to hex-R at higher temperatures. Figure 7 displays the reaction probabilities measured for the products N2 and H20 for beams of varying composition incident on Pt{ 100)-(1 x 1) at 380 K. No NO(g) is produced, as this temperature is below the desorption temperature of NO on clean Pt{lOO}. Figure 7 also displays corresponding 380 K data for the hex-R surface showing that the (1x 1) surface is much more reactive (by -2 orders of magnitude). The much greater activity exhibited by the (1 x 1) surface may perhaps be attributed to the greater ability of this surface to dissociate both reactants. The initial dissociative sticking probability of 0 2 is 0.2 on the (1 x 1) surface at 380 K, which is 3 orders of magnitude greater than that on the hex-R surface (4 x 10-4).23 Furthermore, the (1x1) surface is able to dissociate NH3.21 This suggests that the dissociation of both reactants is rate limiting, at least on the hex-R surface. Increasing the percentage of 0 2 in the beam mixture causes an increase in reactivity (increase in f~~and 5 ~ ~ for 0 )the (1 x 1) surface, providing further support for the proposal that the dissociative adsorption of 0 2 plays a critical role in NZand H20 production. This also indicates that N2 formation arises predominantly from a reaction of N H 3 with oxygen and that the following reaction mechanism is not important for NZ production:
However, alternative routes with oxygen atoms involved either in stripping the ammonia molecules of hydrogen or in forming an NO intermediate must be more important:
+
NH,(a)
-
N(a)
+ 3H(a)
Later we see that H20 production is very limited; the balance must be made up by H2(a) evolved. Therefore, these reaction steps cannot be ruled out, as they may simply be promoted by oxygen. Oxygen is an electron acceptor and N H 3 adsorbs through lone pair donation, so increased binding of NH3 to the surface must be expected in sites neighboring 0 adatoms, and this in tum may lead to an increased rate of NH3 dissociation.
+ 20(a)- NH(a) + 20H(a) NH(a) + O(a) - NO(a) + H(a) NO(a) - N(a) + O(a)
NH3(a)
(1) (2) (3)
This reaction scheme includes N2 production from an NH fragment, from NO, or from both. It is of note that the ammonia oxidation reaction shows similar trends on the hex-R surface at 500 K and the (1 x 1) surface at 380 K, with oxygen adsorption as a rate-limiting step and higher steady-state oxygen coverages suppressing N2 production. 3.1.3. O(a)-PrecoveredPt{ 100). Oxygen preadsorption on Pt{ 100)-hex-R produces a structural transition to a (1 x 1)-like surface.20 0 2 in the gas phase should be able to maintain a high enough adatom coverage to keep the surface in the (1 x 1) phase (SO is much higher on the (1 x 1) surface than on the hex-R surface). It was therefore decided to examine the influence of this transition on the reaction by pretreating the surface with oxygen. Figures 4, 5, and 6 show the products resulting from using a mixed beam of varying composition incident on an almost O(a)-saturated, initially hex-R surface held at 500 K. The reaction probabilities, to NO in particular, are considerably higher than those observed on the hex-R surface. With a 98% 02 beam mixture, we note that the 0-pretreated surface produces NO with about 100% eficiency. Again, the same reaction route beam mixture dependence exhibited by the hex-R and (1 x 1) surfaces is observed, with N2 production suppressed with the higher percentage oxygen beams. If differences in absolute rates are ignored, this implies that the reaction occurs by the same mechanism on small areas of nominal hex-R surface (perhaps defects or step edges where the dissociative sticking probability of 0 2 is high), on the whole of the oxygenprecovered surface and on areas of the (1x 1) surface. In the case of reaction after oxygen precoverage the steady-state reaction is maintained, probably because the surface does not revert to hex-R. This suggests that a high coverage of reactants, presumably mostly O(a), is maintained. At this stage we reach the following conclusions: (1) Dissociative adsorption of 0 2 is the rate-limiting step for the formation of NO for beams with up to 98% 0 2 . Dissociative adsorption of 0 2 is the rate-determining step in some cases for the formation of Nz;for example, in Figure 4 a different step is
17036 J. Phys. Chem., Vol. 99, No. 46, 1995 obviously rate limiting. This implies that N2 is principally produced from NH3 molecules stripped of hydrogen by O(a), from NO(a) dissociation, or from both. (2) The surface temperature and beam composition dependence observed is similar to that reported in previous s t ~ d i e s . ~ , ' ~ (3) Similar trends are observed on hex-R, ( l x l ) , and 0-precovered surfaces, suggesting the same mechanisms on each. (4) The (1 x 1) and the (1 x 1)-like surface produced by 0 2 pretreatment are considerably more effective in the production of NO than the hex-R surface. 3.2. Temperature-Programmed Reaction (TPR). In this set of experiments, mixed adlayers of NH3(a) and O(a) were heated from 150 to '1000 K at a constant rate of 5 Us, and the reaction products N2, NO, and H20 were followed as a function of surface temperature. The relative amounts of reactants in the adlayer were varied so as to investigate what effect surface composition changes had on the reaction. To achieve a mixed adlayer, it was necessary to first adsorb oxygen onto the crystal surface by exposure at 573 K, where the sticking probability is high. The crystal was then cooled in the oxygen beam to 200 K. After further cooling in vacuo to 150 K, the crystal was exposed to NH3. As oxygen adsorbs on the hex-R surface, it forms islands, lifting the reconstruction, and at a coverage of about 0.3 ML these islands begin to coalesce.18 Sticking versus coverage measurements of NH3 on an oxygen-covered surface at 1500 K show that oxygen atoms do not block NH3 adsorption.2' This indicates that NH3 molecules occupy different surface sites from oxygen atoms; the likelihood is that oxygen atoms occupy hollow sites, whereas NH3 molecules occupy on-top positions. We conclude that under conditions of excess oxygen NH3 adsorption is not blocked. Figure 8 shows the TPR products resulting from temperature ramping an adlayer of -0.3 ML of NH3 coadsorbed with varying amounts of O(a). At oxygen precoverages of 0.45 ML and above, not all the oxygen reacts, and there is at least 0.2 ML of O(a) left on the surface (determined by following the 0 2 signal during the TPR). At the lowest O(a) precoverage (0.07 ML) the coadsorbed oxygen and ammonia react to produce NZ and H20 (and H2) only; no NO is formed. The NH3 molecules are stripped of H by O(a), and H20 is formed in preference to gaseous NO. This observation was also made by Mieher et al. in similar coadsorption experiments on Pt{ 11l} under conditions where NH3(a) was in excess of O(a).I6 As the oxygen precoverage is increased, gaseous NO is formed and becomes the major product, increasing in amount as oxygen precoverage is increased. Concurrently, the amount of N2 produced decreases (the amount of NZ produced is relatively small, however). The NO thus formed is observed to desorb from the crystal surface with a peak temperature of -460 K. In studies of molecular NO adsorption on Pt(lOO}, the NO desorption spectrum is complex, but there is a comparable peak at -460 K.24 By 400 K, all the H20 formed in the reaction has desorbed from the surface and does not accompany the NO or N2 desorption at -460 K. This indicates that once the surface has reached -400 K, adsorbed ammonia molecules have been completely stripped to N(a) or have been converted to an NO intermediate. Any N(a) formed on the surface would simply combine and desorb as N2 (from Figure 8b N2 desorbs at temperatures lower than 400 K). We therefore conclude that NO(a) has already been formed on the surface at lower temperatures, and the NO peak is desorption, not reaction, limited. It is also noted that, at the higher oxygen precoverages,
Bradley et al.
a
0.10
4
A
0.08
1
0.56hU 045ML 0.30ML 0.23ML 0.20ML
,
0.02
100
300 400 500 Temperature / K
200
600
700
h
Oxygen Precoverage
,
0.018
h
4
Ic
5
0.012
3
0.010
h
I
/-c
0.01ej 0.014
.
Oxygen Precoverage 0.63ML
..A , 0.63 ML 0.56ML
-.
'
2" 0.008 0.006
4
0.002 0.0°4 100
,
,
,
I
2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 Temperature / K
C
Oxygen Precoverage
0.022
1
0.63ML
0.018
0.56 ML
5
0.016
3
0.014
0.45ML 0.30ML 0.23ML
a e :
0.012
h
. c
i
:::::l 0.006 100
I
200
'
0.20 ML
0.13 ML 0.09 ML
0 07 ML I
'
I
'
I
300 400 500 Temperature / K
'
I
600
'
700
Figure 8. (a) TPRD spectra of NO (mass 30) desorbing from an adlayer of -0.3 ML NH3 coadsorbed with varying amounts of O(a). The oxygen precoverages are indicated in the figure. (b) TPRD spectra of N2 (mass 28) desorbing from an adlayer of -0.3 ML NH3 coadsorbed with varying amounts of O(a). The oxygen precoverages are indicated in the figure. (c) TPRD spectra of HzO (mass 18) desorbing from an adlayer of -0.3 ML NH3 coadsorbed with varying amounts of O(a). The oxygen precoverages are indicated in the figure.
when not all the oxygen reacts, the coadsorbed oxygen has a small effect on the desorption peak temperature of NO; that is, repulsive interactions cause the peak temperature to decrease. In Figure 9 this effect is further demonstrated. As the ammonia coverage is increased and the surface oxygen coverage decreases during heating, less repulsive interactions are experienced in the adlayer and the NO peak temperature increases. The variation of NO desorption energy with O(a) coverage is, however, small. The observed peak temperature shift of -20 K is consistent with a desorption energy change of about 6 kJ mol-'. In the coadsorption study carried out on Pt{ 11l} by Mieher et a1.,I6NO production is also found to increase with increasing oxygen coverage. However, NO is found to desorb at 500 K, a temperature higher than expected for molecular NO desorption
Catalytic Oxidation of Ammonia over Pt{ loo} ~
accompanied by a peak in H20 production at the same Tp. Coincidence of a H20 desorption peak with this a-N2 peak suggests that this reaction provides a different route for H2O production, perhaps via an NH2 fragment rather than an NH3 fragment. At such high oxygen coverages NO dissociation is impossible since the empty sites required are blocked, so there exists a route to N2 formation, possibly given by the scheme below, that involves O(a) but has no NO(a) intermediate. This distinction was not possible before these particular results.
Oxygen Precoverage= 0.63ML
h
0.08i
. g
J. Phys. Chem., Vol. 99, No. 46, 1995 17037
0.04 0.02
30 s NH,
J'L
~
4
0.m 100
200
300 400 500 Temperature / K
600
+ O(a) - NH(a) + OH(a) NH(a) + OH(a) - N(a) + H20(g)
700
NH2(a)
Figure 9. TPRD spectra of NO from an adlayer of 0.63 ML O(a) coadsorbed with varying amounts of NH3. NH3 exposure time is indicated in the figure (NH3 beam flux is -0.01 ML/s).
from Pt{ 11l}, and so its formation appears to be reaction, not desorption, limited. This is contrary to what we observe on Pt{ loo}. However, molecular NO adsorbed at defect sites on Pt{ 11l} will desorb at this higher t e m ~ e r a t u r e .Defect ~~ sites are known to be important for the ammonia oxidation reaction on Pt{ 11l},I3 and so it is possible that this NO is formed at defect sites and that its production in the gas phase is desorption limited. Figure 8b,c shows how the TPRD spectra of the minor products N2 and H20 change as the oxygen precoverage is varied. First we consider H20 (Figure 8c). Thermal desorption spectra of H20 on Pt{ loo} show that there are no adsorption states above 200 K.26 Hence, since H20 is produced at surface temperatures > 200 K in these experiments, its production is reaction, not desorption, limited. H20 is produced up to T, 340 K. The different H2O peaks are probably due to the sequential stripping of hydrogen from NH3. To produce H20, the ammonia molecules must be stripped to NH2 fragments at least:
-
+ O(a) - NH2(a) + OH(a) NH3(a) + OH(a) - NH(a) + H,O(g)
The desorption of N2 is fast because repulsive interactions from neighboring NO(a) and O(a) at high O(a) coverages reduce the N2 desorption temperature. Therefore, the N2 formed in this reaction desorbs at a lower temperature than from the clean surface .27 As oxygen precoverages decrease, the a-N2 and 280 K H20 peaks attenuate and the P-N2 peak at 340 K grows in. This is accompanied by a H20 peak which persists at higher oxygen precoverages when the P-N2 is attenuated. P-N2 may be formed via a mechanism similar to a-N2, but with a higher N2 desorption temperature at lower O(a) coverages. At high oxygen coverages the NH fragments formed in the above reactions react to form NO in preference to /3-N2. NH(a)
(NH(a)
+ 20(a) NO(a)
+ O(a) - N(a) + OH(a)
+ OH(a), occurs at high oxygen coverages)
NH3(a)
It is interesting to compare our spectra to those produced by Mieher et a1.I6 Our NH3 coverage corresponds approximately to their exposure of 10 units. They observe two distinct H20 desorption peaks, dependent on the ammonia precoverage. At low coverages the high-temperature state is formed, and at high coverages the low-temperature state is formed. They attribute this behavior to different NH, fragments residing in different sites and to crowding effects. Similarly, our H20 desorption can be divided into two states, a low-temperature and a hightemperature state. This suggests that the temperature at which H20 is formed may be dependent on the adsorption state of the NH3 molecules. At low coverages NH3 is believed to adsorb N-end down, and it desorbs at a temperature similar to that of H20 desorption observed at low coverages of NH3. At high coverages NH3 is believed to be H-bonded to the low-coverage state, and it desorbs at a temperature similar to that of H20 desorption observed at high coverages of NH3. Now we consider the TPR spectra of N2 (Figure 8b). Four desorption peaks are observed, depending on the oxygen precoverage. The NZpeaks have been assigned as a , /3, y , and 6, in order of increasing peak temperature. Some of these peaks are coincident with H20 desorption. The desorption temperature of N2 is likely to be related to repulsive interactions caused by oxygen atoms (or possibly NO) present on the surface and, hence, to the surface coverage of these species. At the highest oxygen precoverage the a-N2 desorption peak is observed at Tp (peak temperature) = 280 K, and it is
As oxygen precoverage is decreased further, the y-N2 peak appears at 390 K. This peak is not accompanied by H20 formation. It is possible that the y-N2 is formed by a mechanism similar to P-Nz, but that repulsion is less due to low NO and 0 coverage, causing the peak temperature to be higher. It is also possible that y-N2 could be formed from NH(a) fragments alone, without any interaction with adsorbed oxygen. At -460 K the d-N2 peak is observed for oxygen precoverages lower than 0.45 ML. 6-N2 is unaccompanied by H20 formation and is coincident with a recombinative desorption peak of N2 which occurs from the hex-R surface27 and the desorption of low coverage N2 formed during ammonia dissociation on the (1 x 1) surface.2' The 6-N2 peak temperature is also the same as that of N2 desorption due to NO dissociat i ~ n It. ~also ~ appears at lower oxygen coverages; at these low coverages the preadsorbed oxygen is completely consumed in the reaction as the temperature is raised, leaving NO alone on the surface. Hence, we conclude that 6-Nz is formed by the dissociation of NO which is formed in the initial oxidation of NH3* The 6-N2 peak does not appear at higher oxygen coverages, indicating the suppression of NO dissociation. There are two possible explanations for this: site blocking and the influence of repulsive interactions caused by O(a) on the heat of adsorption. It is known that NO dissociation is inhibited at coverages above @NO 0.5, due to the absence of vacant sites, and that coadsorbed 0 atoms may also inhibit dissociation by
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Bradley et al.
17038 J. Phys. Chem., Vol. 99, No. 46, 1995
almost instantaneously. Then it rises more slowly and after site blocking.28 However, the second mechanism is strongly about 20 s starts to decrease, and eventually a pulse of N:! is supported by recent measurements of the calorimetric heat of observed. Figure 1Ob-c shows that the surface temperature adsorption for oxygen on Pt(100) as a function of oxygen determines the relative amounts of NO and N2 produced in the coverage; a very sharp decrease in heat of adsorption occurs reaction. This is also illustrated in Figures 11 and 12. As the with increasing oxygen coverage at a coverage of 0.2 ML.29 surface temperature is decreased from 550 K (Figures 11 and As indicated in Figure 8, we note that, in contrast, the NO 12), the amount of NO produced decreases, while the amount desorption temperature, Tp, and hence the heat of adsorption, of N2 produced increases. For Ts < 400 K, no NO is observed is relatively insensitive to coverage. Thus, as the O(a) coverage to desorb from the crystal; this temperature is below the increases, the thermodynamically stable state of NO switches desorption temperature for NO (Figures 8a and 9). We discuss from N(a) O(a) at low O(a) coverage to NO(a) at high the formation of each of the products in turn. It is noted that coverage. This is a thermodynamic explanation for the inhibiH20 production is very low, so H2 must be a significant product. tion of NO dissociation at high oxygen coverages: simply, the 3.3.1. NO Formution. It was concluded from the TPR results formation of O(a) N(a) from NO(a) is not energetically that the activation energy for NO formation is relatively low favorable at (00 ON)> 0.2. At the lowest O(a) precoverage because NO was formed on the surface at temperatures < 400 (0.07 ML) 6-N2 is not produced, in keeping with the observation K. Over the temperature range studied in the isothermal that no NO is formed: the channel for 6-N2 depends on the experiments, the surface reaction to produce NO is therefore generation of NO(a) in the first place. In their coadsorption experiments on Pt{ 11l} Mieher et ~ 1 . ' ~ fast. At surface temperatures above 400 K there is an initial very rapid increase in the rate of NO production, implying that do not attribute any of their N2 production to NO dissociation. the surface reaction to produce NO is very rapid. The essential NO does not dissociate readily on Pt{ 11I}, except at defect steps in the formation of NO are sites. If Mieher et al. considered NO formation at defects, then their NO production can be considered as desorption limited, and it is possible that some N2 could originate from NO dissociation, as we observe. NH3(a) O(a) NO(a) 3H(a) From the TPR results we thus reach the following conclusions: (3) (1) Molecular NO is formed via a surface reaction at temperatures below the desorption temperature range, Le. below It is observed that, at surface temperatures of 550 and 500 400 K, with a relatively low activation energy. K, as soon as the NH3 beam impinges on the surface, there is (2) The NO desorption energy is almost independent of an initial very rapid ('1 s) rise in the NO production rate, oxygen coverage, decreasing slightly at high oxygen coverages. followed by a slower rise to a maximum rate. The initial rapid increase is consistent with the lifetime t of adsorbed NO at this (3) N2 is formed from the dissociation of NO on the surface temperature. The lifetime can be approximated to Y-' exp(Edl and also from recombination of N adatoms at 460 K, but NO RT), and from the NO TPR peaks (Figure 9), v and Ed are dissociation is suppressed at higher oxygen coverages, due to evaluated as being 1.7 x IOl5 s-l and 157 kJ mol-', respeceither site blocking or a sharp fall in oxygen adsorption energy tively, giving t 1 s at 550 K. This is in good agreement with increasing oxygen coverage. with the NO rise time (Figure 12a). After the initial very rapid (4) At lower temperatures N2 is formed by an altemative route increase, however, the rate of NO production increases more from NH fragments, involving O(a), under conditions where slowly to a maximum. This suggests that a second process for NO dissociation is not possible. NO production is in operation, one that has some factor limiting 3.3. Isothermal Studies of Ammonia Reacting with the NO production rate. It is possible that the formation or the Preadsorbed Oxygen. As shown above, by varying the surface lifetime of an intermediate (such as OH) is limiting the composition of reactants on the hex-R surface at 500 K, a reaction rate at these temperatures. The rate-limiting step is controlled selection can be made-between the production of N:! therefore contained in (2) and could be and NO. A similar sensitivity was also observed on the (1 x 1) surface at 380 K in that N2 production could be suppressed using NH(a) 20(a)-NO(a) OH(a) high percentage 0 2 beams. This effect (coverage-dependent product selectivity) was further investigated in a series of The increasing reaction rate marks the achievement of a more isothermal experiments. Here the molecular beam provided a optimal NH3/O surface coverage for the reaction. Langmuirconstant supply of NH3 to an initially hex-R surface with a finite Hinshelwood kinetics are suggested here with NO formation amount of preadsorbed O(a) (obtained by oxygen exposure of controlled according to the hex-R surface at 573 K). The crystal was cooled to the required temperature before exposing to an ammonia beam at r =R[NHJ[O]~ t = 0. The surface was held at this temperature and product formation was followed with time, which is, in effect, following The subsequent decrease in NO desorption rate is presumably product formation with changing adlayer composition as 00 due to the further depletion of surface oxygen. (surface oxygen coverage) decreases during the reaction. The For a surface temperature of 450 K the rate rises rapidly to results of three sets of experiments are shown in Figures 10, a plateau. The reason for this will be clarified in the quantitative 11, and 12. Within each figure set the coverage of oxygen kinetic analysis.3o For a surface temperature of 400 K very (either saturation or near saturation) at t = 0 and the beam flux little NO desorption is observed, as expected from the TPRs in are the same, but the surface temperature is varied. The NH3 Figures 8 and 9. At 400 K the limiting factor is the rate of NO beam flux is varied between figure sets, as indicated in the figure desorption. captions. It is possible that the mechanism for NO production changes as both surface oxygen coverage and surface temperature Figure 10a shows the results for a surface held at 500 K, decrease; for example Asscher et al. observe two different precovered with 0.63 ML of O(a) and exposed to a beam of NH3 at t = 0. Initially, the rate of NO and H20 formation rises mechanisms for NO prod~ction.'~
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Catalytic Oxidation of Ammonia over Pt{ 100)
J. Phys. Chem., Vol. 99, No. 46, 1995 17039
~rH,O 0.00
, ,
I
I
I
I
I . I ,
)
.
, ,
0 . 0 0 , 1 , .
1 ,
- 1 0 0
-10 0 10 20 30 40 50 60 70 83 90 loo Time I s
I
-
I
I
,
I
I
I
,
I
I
I
1 0 2 0 3 0 4 0 5 0 6 0 Time I s
Ts=400K
Ts=4WOK
-
.
0.04
NO
v1
52
N2
Time I s
c
c
0.02
-J --rs 0.00 -100 1 0 2 0 3 l 4 0 ~ 6 0 7 0 8 3 9 0 1 ~ Time I s .
,
l
l
l
,
l
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.
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l
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.
l
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l
.
3.3.2. N2 Formation. For T, > 350 K, N2 is proguced in the form of a delayed pulse; for T, = 350 K a small amount of N2 is produced before this pulse (Figure llc); for T,= 300 K only prompt N2 is formed (Figure 1Id). The indication is that there are two distinct mechanisms for N2 formation, dependent on surface temperature. This agrees with the conclusions from the TPR results. We consider first the mechanism responsible for the delayed pulse of N2. Figure 13 shows that at 500 K, if the O(a) precoverage is varied and the beam flux is kept constant, then the time between the beam hitting the crystal and the onset of N2 production (referred to as the delay time) varies also. The delay time increases as the oxygen precoverage is increased up to a value of about 0.3 ML. This indicates that the onset of N2 production marks the achievement of a critical surface oxygen coverage; as the oxygen precoverage is increased, more reaction must occur before this critical coverage is reached. For all surface temperatures investigated, increasing the incoming N H 3 flux causes the delay time to decrease, which can be seen by comparing Figures 10, 11, and 12. The beam flux is increased between each experimental set, causing the delay time to decrease from 40 s (Figure 10) to 20 s (Figure 11) to 10 s (Figure 12). The NH3 flux determines the rate of removal of O(a) and thus the point at which a critical O(a) coverage is reached for NZ production to be initiated. These
I
0.00
~
Figure 10. Reaction products NO (mass 30), Nz (mass 28), and H20 (mass 18) monitored as a function of time as an NH3 beam of 0.019 ML/s is incident on an 0-saturated initially Pt{ 100}-hex-R surface. Surface temperature was kept constant during each experiment, and the experiment was performed at (a) 500 K, (b) 450 K, and (c) 400 K.
0.06
-10
d
0
10
20 30 Time I s
40
50
I
0.06
0.02L ,I T,-3XlK
0.04
-I
I
,
r 2 ~N~
0.00
- 1 0 0
1 0 2 0 3 0 4 0 5 0 6 0 Time I s
Figure 11. Reaction products NO (mass 30), N2 (mass 28), and HzO (mass 18) monitored as a function of time as an NH3 beam of 0.054 ML/s is incident on an 0-saturated initially pt{ 100)-hex-R surface. Surface temperature was kept constant during each experiment, and the experiment was performed at (a) 550 K, (b) 400 K, (c) 350 K, and (d) 300 K.
results therefore strongly suggest that a critical surface oxygen coverage must be reached before N2 can be produced. By integrating NO and H20 desorption at 550 K up to the point of N2 production, the critical surface oxygen coverage is estimated to be approximately 0.2 ML. We conclude that NO dissociation is involved in the production of the N2 pulse. As discussed in connection with the TPR results, NO dissociation is controlled by oxygen coverage: it only occurs when it is energetically favorable, at low oxygen coverages (-0.2 ML). We believe that this is the surface oxygen coverage reached before N2 is produced at all surface temperatures above 400 K. The surface oxygen is removed due to the formation of NO, which either
Bradley et al.
17040 J. Phys. Chem., Vol. 99, No. 46, 1995
a
-
0.10
1.2
N2angular distribution
T, SSO K
m
2
0.05
N2
4
o.2{
,
,
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,
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.\
1 I
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~
0
,
-10 Time I s
b
9
H,O
\
. -,
0.00
-
0.4
I
10 r) 50 70 Angle from surface normal (ai)
93
1.2
0.10
H.fJ angular distribution
T,=6OOK
1
-io
o
io
io
40
so
Angle from surface normal (rl,)
Timels
c
Figure 14. (a) Angular distribution of NZ (mass 28) desorbing from the initially clean (1 x 1) surface during steady-state ammonia oxidation at 380 K. (b) Angular distribution of HzO(mass 18) desorbing from the initially clean (1 x 1) surface during steady-state ammonia oxidation at 380 K.
0.10
T, = 4SO K
+
+
0.00 -10
0
10 20 Time18
40
30
Figure 12. Reaction products NO (mass 30), NZ (mass 28), and HzO (mass 18) monitored as a function of time as an NH3 beam of 0.084 MUS is incident on an 0-saturated initially Pt{ 100)-hex-R surface. Surface temperature was kept constant during each experiment, and the experiment was performed at (a) 550 K, (b) 500 K, and (c) 450 K.
,
mn V"."
I
4
50.0
*
1
.
I
7 i 40.0 30.0
p;
20.0 10.0
0.0
0.1
0.2 0.3 0.4 Oxygen Coverage (ML)
0.5
0.6
Figure 13. Delay time before the onset of NZproduction versus oxygen precoverage. T, = 500 K.
desorbs or resides on the surface, depending on the surface temperature. The NZ pulse is also accompanied by H2O production, but there is no accompanying change in the H20 production profile; H20 formation appears to be indifferent to whether or not it is accompanied by NO or N2 formation. This suggests that the mechanism of H a formation does not change throughout the reaction despite the change in product formation and demon-
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+
strates that the secondary reaction, NH3 NO(a) N2(g) HnO(g) H(a), is not occurring. Furthermore, the N2 production cannot be due to recombination of nitrogen adatoms formed by any route other than via NO(a) decomposition. However, at lower surface temperatures (e380 K), a route for NZ formation which does not involve an NO intermediate is opened up. At these temperatures NO dissociation is inhibited; whence, N2 formation cannot be due to NO dissociation itself or to the NH3 NO reaction since it requires NO dissociation. The low-temperature N2 formation is therefore due to N adatom combination after the NH3 molecules have been stripped of hydrogen by O(a). This alternative route at low temperatures is also supported by the TPRD data in Figure 8. It is noted that in each case the fall in NO production is not coincident with the onset of NZ production, and so there are not directly competing processes at a given surface temperature. They are controlled only by the surface oxygen coverage. However, the relative amount of each product formed is dependent on surface temperature, which can be attributed to competition between NO dissociation and NO desorption. The energy required for NO desorption (151 kJ mol-') is greater than that required for NO dissociation (118 kI Hence, NO desorption can only compete effectively with NO dissociation, and therefore N2 production, at higher surface temperatures. 3.4. Dynamics of Product Desorption: Angular Distributions. Angular distribution measurements of the product flux under steady-state conditions were made by lock-in detection, for which the rotating, differentially pumped mass spectrometer was used. Traditionally, the angular distribution of desorbing flux about the surface normal is represented by
+
D(0)= D(O0) COSn 6 where 6 is the angle from the surface normal and D ( 0 ) is the desorbing flux at that angle.3' Figure 14 displays the angular distributions of H20 and NZdesorbing from the (1 x 1) surface
J. Phys. Chem., Vol. 99,No. 46, 1995 17041
Catalytic Oxidation of Ammonia over Pt{ 100) NO angular diitribution under steady state conditions T,=MX)K
800-
0
800A
400-
o.2
1
0 ;
-10
I
1
I
I
,
I
I
I
33 50 m Angle from surface normal (6,)
lo
21 90
Figure 15. Angular distribution of NO (mass 30) desorbing from an initially oxygen-precoveredhex-R surface during steady-state ammonia oxidation at 500 K.
during steady-state reaction at 380 K. For H2O the fit gives n = 1, but N2 has a much more peaked distribution, giving n = 3. The angular distribution of NO desorbing during steadystate conditions from an O(a)-grecovered surface at 500 K is shown in Figure 15. This yields a cos" 19 best fit with n = 0.8. Cosine law desorption is consistent with single-step thermal desorption from adsorbates fully equilibrated to the temperature of the substrate, where the activation energy barrier to adsorption is 0.32 Thus, the formation and desorption of H2O in the ammonia oxidation reaction does not proceed by impulsive recombinative desorption from a step such as 20H(a)
-
H20(a)
+ O(a)
but by desorption of fully equilibrated HzO formed in this way. Similarly, NO is formed in the gas phase by desorption from fully equilibrated adsorbed NO. Here, n = 0.8;a lower value than unity may arise from rapid depletion of NO(a) states above the potential energy zero by desorption. However, the peaked N2 distribution indicates that there is an activation barrier to dissociative N2 adsorption. If only translational energy were required to surmount it, the value n = 3 would be consistent with an adsorption activation energy of -0.2 eV.32 However, in the course of the present work we examined the adsorption of N2 on the Pt{ 100) hex-R and (1 x 1) surfaces and found zero sticking probability for translational energies up to 3 eV.33 Therefore, the NZ desorbed in the ammonia oxidation reaction must be vibrationally excited, as found to be the case by Foner and Hudson for N H 3 decomposition on polycrystalline Pt at lo00 K.34 It is concluded that N2 recombinative desorption proceeds directly through a late banier, without trapping and equilibration to the surface temperature. Thus the step 2N(a)
-
N,(g)
proceeds impulsively to vibrationally excited gaseous N2 without the formation of a stable, equilibrated chemisorbed N2 species. 4. Reaction Mechanism
Roduct selectivity in the ammonia oxidation reaction over
R{100) is controlled by the parameters of surface temperature and surface oxygen coverage. The reaction products, displayed as a function of these parameters, are shown in Figure 16. The general trends observed bear a close similarity to those observed on stepped R{111) by Gland et d.I2Our detailed observations can be explained within a model which considers the competition between NO desorption and NO dissociation as a function
A
0
40 60 8D 100 % 0,in mixed beam (which is a m e a " of steady state surface oxygen coverage) a0
Figure 16. Ammonia oxidation reaction product selectivity displayed as a function of surface temperature and mixed NH3 02 beam
+
composition.
of surface temperature and of surface oxygen coverage. The variation of the heat of NO dissociative adsorption with oxygen adatom coverage plays a critical role in determining the product distribution. The proposed mechanism can be broken down into steps. Steps 1 and 2 describe ammonia and oxygen adsorption: NH,(g) 02(g)
+s
-
NH,(a)
+ 2* -20(a)
(1)
(2)
N H 3 and NO are assumed to occupy similar adsorption sites (on-top), denoted by s. All other species are assumed to occupy adsorption sites equivalent to that of oxygen (presumably hollow sites), denoted as *. The steady-state and TPR measurements indicate that adsorbed oxygen is important for the production of N2. It has also been shown that N H 3 will not dissociate readily on the clean (1x 1) surface and will not dissociate at all on the clean hex-R surface.21 So the following step does not occur to any significant extent in this reaction: NH,(a)
+ 3* -N(a) + 3H(a)
The experimental results show that there are two routes for N2 production. Above 350 K N2 is formed mainly from NO dissociation, and below 350 K, when NO dissociation is not possible, N2 comes from N H 3 molecules that have been stripped of hydrogen by oxygen adatoms. Hence as N H 3 molecules are stripped, they are finally converted to either N adatoms or NO(a):
+ O(a) + * -NH(a) + OH(a) + H(a) NH(a) + O(a) + * - N(a) + OH(a) + s
NH,(a)
NH(a)
+ 20(a)
-
NO(a)
+ OH(a)
(3) (4)
(5)
Higher 0 coverages favor step 5 over step 4. From the TPR measurements we know that NO is formed at low surface temperatures and that its formation is relatively facile. So step 5 has a relatively low activation energy and will occur below 400 K. At surface temperatures above 350 K NO will dissociate when adsorbed on a clean R{100) surface:
+ * -N(a) + O(a) 2N(a) -N2(g) + 2*
NO(a)
(6) (7)
This step has to compete with NO desorption; hence, at higher temperatures NO(g) is formed (step 8)in preference to N2(g)
17042 J. Phys. Chem., Vol. 99, No. 46, 1995 (steps 6 and 7): NO(a)
-
NO(g)
+s
Our experimental results show that N2 production is suppressed at high oxygen coverages; that is, if the surface oxygen coverage is increased, step 6 will not occur. NO dissociation is inhibited due to the low heat of adsorption of oxygen at higher oxygen coverages. The critical oxygen coverage is believed to be 0.2 ML. Current understanding of the ammonia oxidation reaction for surface temperatures < 800 K is that NH3 and 0 2 react to form NO, which, depending on conditions, may take part in a secondary reaction with N H 3 to form N2.7~'~ Using A E S , Gland et al. monitored the surface composition during the ammonia oxidation reaction on a stepped Pt{ 111) surface. They related the surface composition during the reaction at different surface temperatures to product selectivity, with Nz(g) being formed when the surface is covered with an N-containing species and NO(g) being formed when O(a) is the majority surface species.12 Surface temperature therefore dictates the surface coverage of the reactants and also controls the surface coverage of NO(a). With this we are in full agreement with Gland et af. However, they proposed that at low temperatures both NH3 and NO have long enough residence times on the surface to react to form N2, and as the temperature is increased, the surface residence time of NO decreases (as does that of NH3) and this secondary reaction (NH3 NO N2 H20 H) cannot occur. We see no evidence for the occurrence of this secondary reaction; in particular this would have caused a change in the H2O profile in the isothermal experiments. Also, for this secondary reaction to occur NO must dissociate. The details of the NO and NH3 reaction on Pt{ 100) are of particular interest here. Briefly, the reaction produces NZand Hz0 simultaneously. Isotope labeling experiments have shown that the N2 formed can come from NO dissociation solely, NH3 dissociation solely, or from both.35 The occurrence of the reaction is controlled by the NO dissociation reaction. Similarly, we suggest that NO dissociation has control over the ammonia oxidation reaction. The surface coverage dependence has been observed previously. Pignet et al. attributed the suppression of NO production as being due to high NH3 pressures inhibiting NO desorption from the surface. Gland et al. discuss this in terms of competition of reactants for active surface sites. A switch from N2 to NO production in terms of their mechanism implies that NO no longer has a long enough residence time on the surface to react with NH3. This could be due to repulsive interactions at high oxygen coverages causing a decrease in NO surface residence time and hence in the desorption temperature. It has been observed that the NO desorption peak temperature decreases slightly with increasing NO coverage.24 However, the inhibition of NO dissociation offers a clear explanation for the suppression of N2 production at high steady-state surface oxygen coverages. Industrial applications arise from these observations. In current practice the reaction is run at very high temperatures (-1300 K), which results in loss of Pt gauze. Yields of NO are high, though, with 94-98% conversion. We have shown that even higher conversions may be achieved at much lower surface temperatures by the use of high-percentage-oxygen reactant mixtures and surface pretreatment in pure 0 2 . The reaction is very exothermic, and the maintenance of low operation temperatures may cause problems. The reactant mixture used in industry is a 10% NHdair mixture; an especially high percentage oxygen mixture would have to be used for efficient low-temperature operation. Thus, the question is
+
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+
Bradley et al. whether the loss of Pt overrides the cost of thermostatic control and the use of special reactant mixtures.
Acknowledgment. J.M.B. acknowledges a studentship from D E N and a CAST award from ICI. The UK EPSRC is acknowledged for a fellowship to A.H. and an equipment grant. Dr. M. A. Moms is acknowledged for helpful discussions. References and Notes (1) Satterfield, C. N. Heterogeneous Caralysis in Industrial Practice; McGraw-Hill Inc.: New York, 1991; p 312. Scott, W. W.; Leech, W. D. Ind. Eng. Chem. 1927, 19, 170. (2) Stacey, M. H. Catalysis (London)1980, 3, 98. (3) Chinchen, G.; Davies, P.; Sampson, R. J. In Caralysis: Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer Verlag: New York, 1987; Vol. 8, p 1. Hughes, D. Chemsa 1975, 49. (4) Pielasek, J. Plar. Mer. Rev. 1984, 28, 109. Schmidt, L. D. J. Vac. Sci. Technol. 1975, 12, 341. (5) Ostermaier, J. J.; Katzer, J. R.; Manogue, W. H. J. Cafal. 1974, 33, 457. MOKOW,B. A.; Cody, I. A. J. Card 1976, 45, 151. (6) Nutt, C. W.; Kapur, S . W. Nature 1968, 220, 697. Nutt, C. W.; Kapur, S. W. Nature 1969, 224, 169. (7) Pignet, T.; Schmidt, L. D. J . Card 1975, 40, 212. (8) Selwyn, G. S.; Lin, M. C. Langmuir 1985, 1 , 212. (9) Flytzani-Stephanopoulos,M.; Schmidt, L. D.; Caretta, R. J . Caral. 1980, 64, 346. (10) Takoudis, C. G.; Schmidt, L. D. J. Catal. 1983, 84, 235. (11) Sheintuch, M.; Schmidt, J. J . Phys. Chem. 1988, 92, 3404. (12) Gland, J. L.; Korchak, V. N. J . Catal. 1978, 53, 9. (13) Gland, J. L.; Woodward, G. C. J . Caral. 1980, 61, 543. (14) Asscher, M.; Guthrie, W. L.; Lin, T.-H.; Somorjai, G. A. J . Phys. Chem. 1984, 88, 3233. (15) Ya, M.; Fogel, B. T.; Nadykto, V. F.; Rybalko, V. I.; Shvachko, V. I.; Korobchanskaya, I. E. Kinet. Cafal. 1964, 5, 496. (16) Mieher, W. D.; Ho, W. Surf. Sci. 1995, 322, 151. (17) Hopkinson, A,; Guo, X.-C.; Bradley, J. M.; King, D. A. J . Chem. Phys. 1993, 99, 1. (18 ) Guo, X.-C.; Bradley, J. M.; Hopkinson, A.; King, D. A. Surf. Sci. 1994, 310, 163. (19) Guo, X.-C.; Bradley, J. M.; Hopkinson, A.; King, D. A. Surf. Sci. 1992, 278, 278. (20) Griffiths, K.; Jackman, T. E.; Davies, T. A,; Norton, P. R. Surf. Sci. 1984, 138, 113. (21) Bradley, J. M.; Hopkinson, A.; King, D. A. Surf: Sci., in press. (22) Guo, X.-C.; Bradley, J. M.; Hopkinson, A.; King, D. A. Surf. Sci. Left. 1993, 292, L786. (23) Bradley, J. M.; Guo, X.-C.; Hopkinson, A.; King, D. A. J. Chem. Phys., submitted. (24) Lesley, M. W.; Schmidt, L. D. Surf. Sci. 1985, 155, 215. Gohndrone, J. M.; Masel, R. I. Surf. Sci. 1989, 209, 44. Gorte, R. J.; Schmidt, L. D.; Gland, J. L. Surf.Sci. 1981, 109, 367. (25) Gland, J. L.; Sexton, B. A. Surf. Sci. 1980, 94, 355. (26) Ibach, H.; Lehwald, S . Surf. Sci. 1980, 91, 187. (27) Schwaha, K.; Bechtold, E. Surf. Sci. 1977, 66, 383. (28) Fink, T.; Dath, J. P.; Bassett, M. R.; Imbihl, R.; Ertl, G. Surf: Sci. 1991, 245, 96. (29) Yeo, Y. Y.; Wartnaby, C.; King, D. A. In preparation. (30) Ge, Q.; King, D. A. In preparation. (31) Rendulic, K. D.; Winkler, A. Surf: Sci. 1994,299/300,261. Comsa, G.; David, R. Surf. Sci. Rep. 1985, 5 (4). (32) Cosser, R. C.; Bare, S. R.; Francis, S. M.; King, D. A. Vacuum 1981, 31, 503. (33) King, D. A. Faraday Discuss. 1993, 96, 79. (34) Foner, S. N.; Hudson, R. L. J . Chem. Phys. 1984, 80,518. (35) van Tol, M. F. H.; Siera, J.; Cobden, P. D.; Nieuwenhuys, B. E. Surf.Sci. 1992, 274, 63. Lombardo, S. J.; Fink, T.; Imbihl, R. J . Chem. Phys. 1993, 98,5526. Lombardo, S. J.; Esch, F.; Imbihl, R. Surf. Sci. Lett. 1992, 271, L367. JF9523090
