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The adsorption of nitric oxide on Pt(111) and Pt(110) surfaces
Surface Science 57 (1976) 619-631 0 North-Holland Publishing Company
THE ADSORPTION
OF NITRIC OXIDE ON
Pt( 111) AND Pt( 110) SURFACES
C.M. COMRIE *, W.H. WEINBERG ** and R.M. LAMBERT Department of Physical Chemistry, Cambridge CB2 1 EP, England
University of Cambridge,
Received 12 January 1976; manuscript received in final form 2 March 1976
The thermal and electro impact behaviour of NO adsorbed on Pt(ll1) and Pt(l10) have been studied by LEED, Auger spectroscopy, and thermal desorption. NO was found to adsorb nondissociatively and with very similar low coverage adsorption enthalpies on the two surfaces at 300 K. In both cases, heating the adlayer resulted in partial dissociation and led to the appearance of N, and 0, in the desorption spectra. The (111) surface was found to be significantly more active in inducing the thermal dissociation of NO, and on this surface the molecule was also rapidly desorbed and dissociated under electron impact. Cross sections for these processes were obtained, together with the desorption cross section for atomically bound N formed by dissociation of adsorbed NO. Electron impact effects were found to be much less important on the (110) surface. The results are considered in relation to those already obtained by Ertl et al. for NO adsorption on Ni(ll1) and Pd(l1 l), and in particular, the unusual desorption kinetics of N, production are considered explicitly. Where appropriate, comparisons are made with the behaviour of CO on Pt(ll1) and Pt(1 lo), and the adsorption kinetics of NO on the (110) surface have been examined.
1. Introduction
There exists a considerable body of published work on the adsorption of CO by the platinum group metals, but similar studies on NO have been far fewer. Recently however, the results of an investigation of the adsorption of NO on both Ni( 111) and Pd(l1 I), using conventional ultra high vacuum (UHV) techniques, have been reported [l-3] . Significant differences in the interaction of NO with the two metals were found: in particular, adsorption of NO on Pd(ll1) occurs non-dissociatively at room temperature, while on Ni(ll1) dissociative adsorption is observed at low
* Permanent address: Department of Physics, University of Cape Town, Rondebosch 7700, South Africa. ** Permanent address: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 9 1125, USA
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oj’nitric oxide on Pi
coverages. The activation energy to NO dissociation on Ni(ll1) apparently increases with the amount of dissociatively adsorbed NO, resulting in molecular adsorption of NO at higher coverages. Thus at low coverages, only N, and 0, were observed in the thermal desorption spectra, while at higher coverages the desorption of NO was also observed. In a previous paper, we have described the competitive coadsorption of CO and NO on Pt( 111) and Pt( 1 lo), together with the Langmuir-Hinshelwood reaction which was found to occur when the mixed adlayer was heated [4]. We have also described the behaviour of CO on these two surfaces [6,7] of Pt, and in the present paper we report the results of an investigation of NO adsorption on the same two surfaces.
2. Experimental Experiments were carried out in two different stainless steel vacuum chambers in which base pressures of <5 X 10-l 1 torr were routinely obtainable. These chambers contained the usual facilities [4] for specimen preparation and cleaning, and the experimental geometry was such that both specimen faces could be subjected to Ar+ bombardment and LEED-Auger analysis - a fact which significantly increases the reliability of the thermal desorption data. LEED and Auger spectroscopy were carried out using a conventional three grid retarding field analyzer. Three different Pt specimens were used in obtaining the results which are described in the following section. (111) and (110) specimens were obtained as thin wafers by spark erosion machining from a Pt single crystal ingot of 99.999% purity after orienting to within 0.25” of the desired direction. They were then mechanically polished and chemically etched using standard techniques [8] . These specimens were 0.8 mm thick and had total face areas of -2 cm2. The thermal desorption results for Pt(ll1) which are presented in section 3.2 were obtained from a polycrystalline ribbon (6 cm2 X 0.1 mm thick) which had been heated in vacuum at 1400 K for 12 h with intermittent flashing to 1700 K. This resulted in a specimen consisting of crystallites of mean diameter -0.2 mm (scanning electron microscopy) and of (111) orientation in the surface plane (LEED, and X-ray back reflection). Thus although the specimen contained a large number of grain boundaries, the vast majority of surface sites were of (111) orientation. This view is supported by the fact that the CO desorption spectrum obtained from this specimen was identical with that observed for the (111) single crystal wafer, and in particular was entirely different from the desorption spectra of (110) [7] and (100) [9] single crystal specimens (two and three peaks respectively). All three specimens were cleaned by an iterative sequence of heating in oxygen (900 K, lop6 torr) followed by Ar+ bombardment (300 K, 450 eV) until the Auger spectra showed Pt transitions only. A Pt(ll1) clean surface spectrum is shown in fig. la, and the Pt(ll0) clean surface spectra were essentially identical with this. Examination by LEED showed that, after annealing, clean Pt( 110) is characterised by a (1 X 2) reconstructed surface.
CM. Comrie et al./Adsorption of nitric oxide on Pt
621
0.25V RMS
1
cc
dN(E) dE
I
I
1
100
I
I 200
300
LOO Electron Energy W)
500
Fig. 1. Pt(ll1) Auger spectra: primary beam 30 /LA at 2 keV: (A) clean surface, (B) spectrum of adsorbed NO.
3. Results 3.1. Electron impact behaviour Exposure of the Pt(ll1) single crystal to NO at 300 K resulted in no significant change in the (1 X 1) LEED pattern other than a general increase in background intensity. The Auger spectrum of the resulting surface indicated the presence of both oxygen and nitrogen (fig. lB), however the intensity of the oxygen Auger signal (lox) decayed rapidly to a vanishingly small value under irradiation by the incident exciting beam; during the same period, the nitrogen signal intensity (IN) decayed to about l/3 of its initial value. Given that the desorption cross sections for atomic adsorbates on metal surfaces are in general much smaller than those for molecular adsorbates [5], these observations are consistent with the electron impact desorption and dissociative desorption of molecularly adsorbed NO, bonded to the surface through the nitrogen atom. The occurrence of such process has already been demonstrated [6] for CO chemisorbed on Pt(l1 l), and in the present case the analogous reactions for NO may be written thus: Pt-NOcPt+NO(g),
(1)
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1 00 I
0 75
2
b
11111
L-T Pt
0.25 0.50
Fig. 2. Nitrogen
Pt-NO 9 Pt-N
of nitric oxide on Pt
+ 0 (g) ,
deposition
within
beam irradiated
area.
(2)
2
where the desorption products in eqs. (1) and (2) are to be taken as mixtures of ionized and neutral molecules and atoms respectively. Graphic evidence that the dissociation of NO takes place only within the beam irradiated portion of the crystal surface is provided by fig. 2. These data were obtained by exposing the crystal to 30 L NO (1 L = 1 Langmuir = lo@ torr set) at 300 K while continuously irradiating part of the surface with a well-focussed 2 keV electron beam. The NO was then pumped out, and the electron beam tracked across the surface while monitoring IN, with the result shown in fig. 2. The qualitative observations already described show that k, is comparable with k,, and that the electron induced desorption rate for atomically adsorbed N is small compared with both (1) and (2). Under these conditions, and with a continuous supply of gaseous NO, a substantial net accumulation of chemisorbed atomic nitrogen should occur within the irradiated area. The subsequent monitoring sweep of the primary exciting beam would then be expected to show a large increment in IN over the background level within the irradiated area, as is indeed observed. The background ZJ,Jsignal arises from a dilute layer of N due to rapid disruption (i.e. mainly desorption) of the NO saturated surface outside the irradiated area. The behaviour of chemisorbed NO is thus seen to be closely similar [6] to that of CO on Pt(ll1). The data of fig. 2 also provide a convenient measure of electron beam diameter (full width at halfmaximum) which can be used for computing electron flux densities in the measurement of absolute reaction cross sections. Absolute total reaction cross sections for the electron impact desorption and dissociation of chemisorbed NO, and for the desorption of atomically adsorbed N, were determined by a method that we have already described in detail [6]. Briefly, the surface was saturated with NO at 300 K, the system evacuated, and the analyzing beam turned on to follow the time dependence of I,,. Since both desorption and dissociation of NO result in loss of surface oxygen [eqs. (1) and (2)] the total loss rate, as monitored by Z,,, will be determined by the sum of the in-
CM. Comvie et al/Adsorption
20
I
of nitric oxide on Pt
101Tfme imml LO 60 80
I
I
I
623
1cO
I I
Fig. 3. Total surface oxygen loss rate from NO covered surface. Electron flux 2.06 X 10” cmw2 s-l (0). Surface nitrogen loss rate from N covered surface. Electron flux 7.22 X 1016 cme2 s-l (0).
dividua~ rate coefficients (kI + k2). Thus by following the time dependence of I,, a value for (kl t k2) may be obtained by elementary application of the first order rate law. Fig. 3 shows a typical result obtained with a primary flux of 2.06 X 1015 electrons cm-2 s-1 at 58” to the surface normal. The slope of this graph gives (kl t k2) = 2.90 X 10e3 s-l which in turn corresponds to a total cross section for the desorption and dissociation of NO of orJo = 1.4 X lo-l8 cm2. (In this work, cross sections are referenced to primary electron flux. It is to be expected that, as we have shown [6] for Pt-CO, the major contribution to the observed effect is due to the secondary electrons which pass through the adlayer). Analysis of the rate equations for reactions (1) and (2) leads to the relationship: e,(t)
= [k&k1
+ k2)1 e,,(O)
11 - exp[-(kl
+ &$I
1,
(3)
where @N(r) is the surface nitrogen coverage generated by dissociation after irradiating an NO adlayer of initial coverage 0 No(O) for time t. Since (kl + kz) is now known, eq. (3) may be solved for k2 provided that (i) the value of tlNo(0) is known (ii) it is possible to separate out the contributions to IN from atomically adsorbed N and chemisrobed NO, and (iii) IN(t) can be related to absolute nitrogen coverage. No direct LEED evidence.regarding (i) is available from our own work. However, Conrad et al. [3] have observed two different LEED patterns for NO on Ni(l11). 6 L of NO was found to produce a ~(4 X 2) structure, while at saturation coverage in the presence of gaseous NO, a complex structure corresponding to about 0.6 monolayers was observed. Since this latter phase was only stable in an ambient atmosphere of NO, it is likely that the saturation coverage would be somewhat less under UHV conditions. On the other hand, it is possible that Pt(ll1) with its larger unit mesh might be able to accommodate a greater coverage of NO than Ni(l11). In any event, a saturation coverage of between 0.25 and 0.6 monolayers might reasonably be expected in our experiments just prior to irradiation. With regard to (ii), the
of nitric oxide on Pt
C.M. Comrie et aLlAdsorption
624
similar desorption temperatures of N, and NO (see sections 3.2) rendered selective desorption of the latter impossible, thus precluding the approach that was successfully employed in the Pt( 1 1 1)-CO system [6] . Since atomically adsorbed N appeared to be perfectably stable in an atmosphere of CO, an attempt was made to displace [4] selectively the NO remaining after irradiation by a large dose of CO. However, this approach was also unsuccessful because of the low efficiency for displacement [4] of cr NO by CO. Instead, the NO saturated surface was irradiated until all the NO had either been desorbed or dissociated (I,, + 0). In this case (kl + k2) t 2 1, and eq. (3) reduces to Q(t)
= W&Q
+
Ql &q(O).
@a)
The left-hand side of (4a) is given by e,(t)
= ]~~@)lW91
e,(-)
1
(4b)
where IN(=) is the saturation nitrogen Auger signal obtained after prolonged irradiation of the crystal in an ambient atmosphere of NO, and ON(m) is the corresponding coverage of atomically adsorbed N. Combining (4a) and (4b) yields IN(WN(9
= ]k2/(kl
+
k2)1 ~,,~w,w.
(4c)
Since ON(w) is produced by dissociation of NO molecules, it seems reasonable to suppose that it does not correspond to a packing of N which is more dilute than the saturation coverage of NO itself, and its value may even be as high as unity. Application of eq. (4~) to the results of a number of experiments using differing incident electron fluxes, together with the upper and lower bounds on e,,(O) and 0,(m), gives the following cross sections for reactions (1) and (2). of0
= 4(?2)
X IO-l9
cmp2 ,
0:’
= l.O(~kO.2) X lo-‘*
cm2 .
NO adlayers on Pt(ll0) were found to be much more resistant to disruption by electron impact than in the case of Pt(ll1). The effects were small and detailed studies were not carried out. However, the total cross section for destruction of the adlayer was found to be <10-20cm2. In the above treatment of the Pt(ll1) results we have tacitly ignored the desorption of atomic N during irradiation. The importance of such an effect was assessed by explicitly following the time dependence of 1, while irradiating an adlayer of atomically adsorbed N produced as already described. A typical result is shown in fig. 3, and corresponds to a very small cross section for atomic N desorption ‘;;= 8 x 10-22 cm2); this justifies our neglect of the process in the extraction of and 0;’ from the data. 01 3.2. Thermal desorption: Pt(l11) Thermal desorption data for Pt( 111) were obtained using the polycrystalline ribbon. Flashing this specimen after exposure to NO was found to result in partial
C.M. Comrie et al/Adsorption
3t
of nitric oxide on Pt
625
Pt (111)
6 65
2 2 e z
FL
m
1
2
I
I
I
I
I
373
L73
573
673
773
Temperature
I
I
873
973
1K 1
Fig. 4. NO, N,, and 0, thermal desorption spectra following exposure to 1.2 L NO. Heating rate 110 K s-l. bursts at 28, 30, and 32 amu. It is evident that the 28 amu signal could be due either to N, resulting from decomposition of NO, or to CO spuriously adsorbed from the background gas during NO exposure. Unambiguous identification of the desorption products was achieved by using .sotopically labelled IsNO; these experiments showed that there was no interference from CO, and that the only desorbing species were N,, NO, and 0,. Thermal desorption spectra following exposure of the specimen to 1.2 L NO are shown in fig. 4; at higher exposures, a shoulder appeared on the low temperature side of the NO peak. It should be pointed out that although the heating rate was constant (110 K s-l) during NO and N2 desorption, it fell off above -800 K so that the area under the 32 amu curve is only a rough measure of the amount of 0, desorption. In addition, the long high temperature tail of the NO spectrum reflects the reduced system pumping speed and concomitant wall effects for this molecule. This problem was effectively eliminated by using an improved pumping arrangement for the experiments on Pt(ll0) (section 3.3). Investigation of the coverage dependence of the NO, N, and 0, desorption spectra showed that in the case of both N2 and NO the peak temperatures were approximately independent of coverage (Tp - 513 K, -490 K for N2 and NO respectively); the 0, peak temperature decreased rapidly with increasing coverage, indicating-that the desorption process has an apparent reaction order greater than unity. Since the results presented in section 3.1 are consistent with the non-dissociative adsorption of NO at room temperature, the presence of N, and 0, in the desorption spectra strongly suggests that some NO must dissociate during the heating cycle. From an examination of the product distribution, it was not possible to make a precise determination of the extent of NO dissociation due to the significant differences in the effective pumping speeds for NO, N,, and 0,. However, the comparable intensities of the N2 and NO desorption spectra at all coverages indicates that approximately 2/3 of the molecularly adsorbed NO desorbs as N, and 0,. The pressure
626
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Comrie et al. lAdsorption
of nitric oxide on Pt
coverage independence of the NO and N2 peak temperatures indicates an essentially first order desorption process for the production of both species; while this is not unexpected for NO - the relevant reaction presumably being NO(a) -+ NO(g) - it is somewhat surprising for N,, especially in view of the “normal” behaviour of the O2 spectrum, and the implications are discussed below. For the moment, we may employ the equation derived by Redhead [lo] for a simple first order desorption process to estimate the apparent desorption enthalpies of NO and N2. This leads to values of 115 and 119 kJ mol-l for NO and N, respectively, assuming a pre-exponential factor of 1013 s-l. 3.3. Thermal desorp tion: Pt( 1 IO) Exposure of the clean Pt(1 lo)-(1 X 2) reconstructed surface to 10m8 torr NO at room temperature resulted in a noticeable increase in the background intensity of the LEED pattern, together with a significant decrease in the intensity of the fractional order beams after only 0.8 L. By 2.5 L the fractional order beams were essentially extinguished, resulting in a (1 X 1) LEED pattern. No other LEED pat-
Pt(ll0)
F
~~
c
dk
C
.,r\__’
P
E
%k7kdTemperature
(Ki
Fig. 5. NO desorption spectra following exposure to various doses of NO, (A) 0.1 L, (B) 0.3 L. (C) 0.6 L, (D) 0.9 L, (E) 1.2 L, (F) 1.8 L. Heating rate 13 K s-‘.
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Comrie et al. lAdsorption
of nitric oxide on Pt
621
terns were obtained upon further exposure, nor could they be generated by thermal treatment in an atmosphere of NO. A series of flash desorption spectra were taken (13 K s-l) following exposure to varying doses of NO, and the results for the desorption of NO itself are shown in fig. 5. It is at once apparent that these data are not significantly distorted by pumping speed limitations, and are amenable to more detailed analysis than in the case of Pt(ll1). It can be seen that a relatively strongly adsorbed state (0) is populated first, and it is only when the /3 state has been appreciably filled that the more weakly bound state (CX)begins to fill. Assuming a preexponential factor of 1013 s-l as before leads to desorption enthalpies for the 01 and /3 states of 96 and 116 kJ mol-’ respectively. Monitoring the 28 amu partial pressure burst showed that some N2 desorption also occurred, but in this case the amount of NO undergoing decomposition (<20%) was substantially less than in the case of the (111) surface. No 0, desorption was detectable, and this is consistent with (i) the reduced decomposition yield, (ii) the smaller relative sensitivity to 0, due to wall pumping (see fig. 4) and (iii) the greater overall pumping speed of the system in the present case. Once again, the peak temperatures of the N2 desorption spectra were essentially coverage independent, and the results are shown in fig. 6. It
Pt(llO) E
;~
0
0
373
L73 Temperature
573
02
(K 1
Fig. 6. N, desorption spectra following (C) 0.6 L, (D) 1.2 L, (E) 6.0 L.
1.0 20 NO Exposure
OL
06
30 ILatqnwsl
08
1D
ONO/ ON0mxl exposure
to various
NO doses, (A) 0.1 L, (B) 0.3 L,
Fig. 7. (A) NO coverage as a function of exposure. (B) relative sticking of coverage; (-4) experimental data, (-) calculated curve for [ = 4.
probability
as a function
628
C.M. Comrie et al./Adsorption
of nitric oxide on Pt
should be pointed out that in this case it was not necessary to use 15N0 in order to provide unambiguous identification of N2 as a product. The desorption peak temperatures of P-CO and N2 from Pt(ll0) differ by > 30 K so that the two species are readily distinguished. Since the N, yield was always relatively small, the areas under the NO desorption curves of fig. 5 provide a measure of the NO coverage prior to flashing. These data may therefore be used to construct the adsorption curve for NO on Pt(ll0) (coverage as a function of exposure - fig. 7A) from which it can be seen that the system follows precursor state kinetics. The corresponding curve of relative sticking probability (S/So) as a function of coverage is shown in fig. 7B. It is evident that if the saturation coverage (e,,) . o f or d er unity, then the initial sticking probability (So) is also 1s of order unity. Although NO does not form any ordered phases on the (110) surface, comparison with the data [7] for CO (which were also obtained in our laboratory) provides a useful indication of tImax in the case of NO. The (2 X 1) plgl structure formed by CO shows that B,,(CO) IS unity; furthermore, the (1 X 2) + (1 X 1) transformation only went to completion at essentially saturation coverage. Since this transformation is also effected by NO, it is reasonable to conclude that fI,,(NO) must likewise be close to unity. This view is supported by the fact that the signal strengths of the desorption spectra at saturation coverage are very similar for these two molecules, and indeed the spectra themselves bear a close resemblance to each other. We may therefore conclude that S, is large and of order 1.O.
4. Discussion The results presented in section 3.1 are consistent with the non-dissociative adsorption of NO on platinum, the molecule being chemisorbed via the N atom. Further evidence for the molecular adsorption of NO on both Pt( 111) and Pt( 110) is provided by the results of competitive coadsorption experiments reported earlier [4] which indicated rapid and efficient displacement of NO from both surfaces by CO. It seems unlikely that a dissociated adlayer would be so readily displaced in this way; indeed, reaction with gaseous CO would be expected to deplete the surface of 0 by rapid reaction [ 11,121 to form CO,, and this was not found to be the case. Recent UPS studies [ 171 have also demonstrated that NO is non-dissociatively adsorbed on Pt(lO0). The magnitudes of the electron stimulated desorption and dissociation cross sections for NO on Pt(ll1) are similar to those already obtained for CO on the same surface [6] , although dissociation appears to be relatively more important in the case of NO. However, it is not possible to correlate this difference in behaviour in any simple way with the relative CO and NO bond strengths in the electronic ground state of the two adsorbed species. Clearly, the nature of the (unknown) excited states involved in the transition must play a crucial part in determining the dissocia-r tion probability of the adsorbate. Indeed, the complexity of the situation is under-
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Comrie et aLlAdsorption
of nitric oxide on Pt
629
lined by comparing the electron impact properties of CO and NO on the two surfaces. Given [7] that the total cross section for desorption of CO from Pt(ll0) is < IO-l8 cm2, it then appears that the behaviour of a given molecule on different surfaces shows greater differences than that of different molecules on the same surface. The appearance of 02 and N2 in the desorption spectra suggests that some thermal dissociation of NO takes place upon heating the adlayer, and it is of interest to compare the behaviour of Pt(l11) in relation to NO dissociation with that [l-3] of Pd(ll1) and Ni(ll1). In the case of Ni(ll1) dissociation of the molecule occurs at room temperature, whereas the behaviour of Pd(ll1) appears to be very similar to that reported here for Pt(ll1). In this connection it is worth noting that the lattice parameters of Pd and Pt are virtually identical, whereas that of Ni is significantly smaller. Assuming that dissociation proceeds via a mechanism which requires the presence of a vacant neighbouring site, it may be that the activation barrier to NO dissociation is correlated with the spacing between nearest neighbour adsorption sites on these three closely related surfaces. Another possible factor arising out of size effects is the effectiveness of the overlap between metal and adsorbate orbitals which will determine the N-O bond order. Once again, the behaviour of Pt and Pd might be expected to differ from that of Ni. The present work indicates that Pt(ll0) is significantly less active in dissociating NO than is Pt(l1 I), although the adsorption enthalpies on the two surfaces are very similar. It may be that this is due to the substantially reduced density of nearest-neighbour site pairs (at the closest packed spacing of 2.77 A) which exists on the (110) surface. However, such factors as reduced mobility of surface species on the markedly anisotropic (110) plane could also play a part. The very different coverage dependences of the N2 and 0, desorption spectra are noteworthy. The O2 spectra show the expected behaviour for a second order associative desorption, but the apparent first order behaviour of the N2 spectra is worthy of comment. Ertl et al. [3] have observed precisely the same behaviour for the Ni(1 11)-NO system in which the adlayer is fully dissociated even at room temperature, but these authors did not suggest an explanation for the N2 desorption kinetics. The problem at hand has a most interesting relationship with the PI, p2 spectra of the CO-W system [ 131. Goymour and King [ 141 have given a detailed analysis of the latter system in terms of associative desorption from a dissociated adlayer of repulsively interacting particles. In the (C + 0)-W case, the most energetically favoured gaseous product is CO, so that the stoichiometry of the adlayer remains invariant throughout the desorption cycle. Under these circumstances, the desorption rate shows two maxima, and the position of the first of these does not vary very much with coverage. If we now consider the (N + 0)-Ni system with lateral repulsive interactions between the adsorbed atoms, an analogous situation obtains, but with one very important difference. Energetically, the most favoured gaseous product is now N2. Preferential desorption of N2 thus occurs, the surface stoichiometry changes continuously, and the rate of N2 desorption is determined
CM. Comrie et al/Adsorption
630
of nitric oxide on Pt
by the instantaneous concentration of nearest neighbour N adatom pairs. This latter quantity is itself dependent on the total repulsive interaction energy in the adsorbate layer and this includes a large contribution from the 0 atoms which are present. Under such conditions, and subject to the approximations introduced by Goymour and King [ 141 ~it would be possible for essentially all the N, to be desorbed within a single peak, analogous to fll -CO from W, whose temperature was almost coverage independent. Oxygen loss would then occur at a higher temperature, and show a single peak spectrum whose position should be much more coverage dependent. This is just the behaviour observed in the case of Pt; however here the problem is further complicated by the incomplete dissociation of the adlayer at the desorption temperature. Nevertheless, this does not vitiate the model for the Pt case, and it seems possible that the tentative suggestions made here may provide the basis for a correct explanation. One potentially serious difficulty with the above argument is that the behaviour just described should only occur at total coverages greater than 0.5, whereas our own results for Pt(ll0) indicate that the behaviour deos not change at coverages below this value. There would appear to be only two ways around this difficulty (i) at low coverage the adsorbate is distributed as islands on an otherwise bare surface, or (ii) following dissociation there are attractive interactions between N atoms in the adsorbate layer. The kinetics of adsorption of NO on Pt(ll0) indicate that the process occurs via a mobile precursor state, which is a fairly common observation for such systems. Analysis of fig. 7B in terms of the surface mobility parameter developed by Gasser and Smith [ 151 using the relation S/So = (1 - 0s j shows that a best fit is obtained for t = 4. This is a reasonable result for the average number of adsorption sites sampled by the precursor molecule before desorption, and is in fact identical with the result for CO on the same surface [7]. More significant is the finding that, in common with CO, the adsorption rate of NO is not sensitive to the extent of the (1 X 2) + (1 X 1) transformation. This is by no means always the case, as the adsorption kinetics of linear polyatomic molecules on Pt(ll0) can be strongly influenced [ 161 by the condition of the surface periodicity along [OOl] .
Acknowledgement This work was supported
in part by NATO Research Grant No. 898.
References [l] G. Ertl, Surface Sci. 47 (1975) 86. [2] H. Conrad, G. Ertl, J. Kiippers and E.E. Latta, Faraday Discussions. Chem. Sot. 58 (1975) 116. [3] H. Conrad, G. Ertl, J. Kiippers and E.E. Latta, Surface Sci. 50 (1975) 296. [4] R.M. Lambert and C.M. Comrie, Surface Sci. 46 (1974) 61.
C.M. Comrie et al./Adsorption
[S] [6] (71 [S] [9] [lo] [ 1 l] [ 121 [13] [ 141 [ 151 [16] [ 171
of nitric oxide on Pt
T.E. Madey and J.T. Yates, J. Vacuum Sci. Technol. 8 (1971) 525. R.M. Lambert and C.M. Comrie, Surface Sci. 38 (1973) 197. C.M. Comrie and R.M. Lambert, J. Chem. Sot. Faraday Trans. I (1975), submitted. R.M. Lambert, W.H. Weinberg, J.W. Linnett and CM. Comrie, Surface Sci. 27 (1971) A.E. Morgan and G.A. Somorjai, Surface Sci. 12 (1968) 405. P.A. Redhead, Vacuum 12 (1962) 203. H. Heyne and F.C. Tompkins, Proc. Royal Sot. (London) A292 (1966) 460. H.P. Bonzel and R. Ku, Surface Sci. 33 (1972) 91. C.G. Goymour and D.A. King, J.C.S. Faraday I, 69 (1973) 736. C.G. Goymour and D.A. King, J.C.S. Faraday I, 69 (1973) 749. R.P.H. Gasser and E.B. Smith, Chem. Phys. Letters 1 (1967) 457. P.D. Reed and R.M. Lambert, Surface Sci. 57 (1976) 000. H.P. Bonzel and T.E. Fischer, Surface Sci. 5 1 (1975) 213.
631
653.
THE ADSORPTION
OF NITRIC OXIDE ON
Pt( 111) AND Pt( 110) SURFACES
C.M. COMRIE *, W.H. WEINBERG ** and R.M. LAMBERT Department of Physical Chemistry, Cambridge CB2 1 EP, England
University of Cambridge,
Received 12 January 1976; manuscript received in final form 2 March 1976
The thermal and electro impact behaviour of NO adsorbed on Pt(ll1) and Pt(l10) have been studied by LEED, Auger spectroscopy, and thermal desorption. NO was found to adsorb nondissociatively and with very similar low coverage adsorption enthalpies on the two surfaces at 300 K. In both cases, heating the adlayer resulted in partial dissociation and led to the appearance of N, and 0, in the desorption spectra. The (111) surface was found to be significantly more active in inducing the thermal dissociation of NO, and on this surface the molecule was also rapidly desorbed and dissociated under electron impact. Cross sections for these processes were obtained, together with the desorption cross section for atomically bound N formed by dissociation of adsorbed NO. Electron impact effects were found to be much less important on the (110) surface. The results are considered in relation to those already obtained by Ertl et al. for NO adsorption on Ni(ll1) and Pd(l1 l), and in particular, the unusual desorption kinetics of N, production are considered explicitly. Where appropriate, comparisons are made with the behaviour of CO on Pt(ll1) and Pt(1 lo), and the adsorption kinetics of NO on the (110) surface have been examined.
1. Introduction
There exists a considerable body of published work on the adsorption of CO by the platinum group metals, but similar studies on NO have been far fewer. Recently however, the results of an investigation of the adsorption of NO on both Ni( 111) and Pd(l1 I), using conventional ultra high vacuum (UHV) techniques, have been reported [l-3] . Significant differences in the interaction of NO with the two metals were found: in particular, adsorption of NO on Pd(ll1) occurs non-dissociatively at room temperature, while on Ni(ll1) dissociative adsorption is observed at low
* Permanent address: Department of Physics, University of Cape Town, Rondebosch 7700, South Africa. ** Permanent address: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 9 1125, USA
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oj’nitric oxide on Pi
coverages. The activation energy to NO dissociation on Ni(ll1) apparently increases with the amount of dissociatively adsorbed NO, resulting in molecular adsorption of NO at higher coverages. Thus at low coverages, only N, and 0, were observed in the thermal desorption spectra, while at higher coverages the desorption of NO was also observed. In a previous paper, we have described the competitive coadsorption of CO and NO on Pt( 111) and Pt( 1 lo), together with the Langmuir-Hinshelwood reaction which was found to occur when the mixed adlayer was heated [4]. We have also described the behaviour of CO on these two surfaces [6,7] of Pt, and in the present paper we report the results of an investigation of NO adsorption on the same two surfaces.
2. Experimental Experiments were carried out in two different stainless steel vacuum chambers in which base pressures of <5 X 10-l 1 torr were routinely obtainable. These chambers contained the usual facilities [4] for specimen preparation and cleaning, and the experimental geometry was such that both specimen faces could be subjected to Ar+ bombardment and LEED-Auger analysis - a fact which significantly increases the reliability of the thermal desorption data. LEED and Auger spectroscopy were carried out using a conventional three grid retarding field analyzer. Three different Pt specimens were used in obtaining the results which are described in the following section. (111) and (110) specimens were obtained as thin wafers by spark erosion machining from a Pt single crystal ingot of 99.999% purity after orienting to within 0.25” of the desired direction. They were then mechanically polished and chemically etched using standard techniques [8] . These specimens were 0.8 mm thick and had total face areas of -2 cm2. The thermal desorption results for Pt(ll1) which are presented in section 3.2 were obtained from a polycrystalline ribbon (6 cm2 X 0.1 mm thick) which had been heated in vacuum at 1400 K for 12 h with intermittent flashing to 1700 K. This resulted in a specimen consisting of crystallites of mean diameter -0.2 mm (scanning electron microscopy) and of (111) orientation in the surface plane (LEED, and X-ray back reflection). Thus although the specimen contained a large number of grain boundaries, the vast majority of surface sites were of (111) orientation. This view is supported by the fact that the CO desorption spectrum obtained from this specimen was identical with that observed for the (111) single crystal wafer, and in particular was entirely different from the desorption spectra of (110) [7] and (100) [9] single crystal specimens (two and three peaks respectively). All three specimens were cleaned by an iterative sequence of heating in oxygen (900 K, lop6 torr) followed by Ar+ bombardment (300 K, 450 eV) until the Auger spectra showed Pt transitions only. A Pt(ll1) clean surface spectrum is shown in fig. la, and the Pt(ll0) clean surface spectra were essentially identical with this. Examination by LEED showed that, after annealing, clean Pt( 110) is characterised by a (1 X 2) reconstructed surface.
CM. Comrie et al./Adsorption of nitric oxide on Pt
621
0.25V RMS
1
cc
dN(E) dE
I
I
1
100
I
I 200
300
LOO Electron Energy W)
500
Fig. 1. Pt(ll1) Auger spectra: primary beam 30 /LA at 2 keV: (A) clean surface, (B) spectrum of adsorbed NO.
3. Results 3.1. Electron impact behaviour Exposure of the Pt(ll1) single crystal to NO at 300 K resulted in no significant change in the (1 X 1) LEED pattern other than a general increase in background intensity. The Auger spectrum of the resulting surface indicated the presence of both oxygen and nitrogen (fig. lB), however the intensity of the oxygen Auger signal (lox) decayed rapidly to a vanishingly small value under irradiation by the incident exciting beam; during the same period, the nitrogen signal intensity (IN) decayed to about l/3 of its initial value. Given that the desorption cross sections for atomic adsorbates on metal surfaces are in general much smaller than those for molecular adsorbates [5], these observations are consistent with the electron impact desorption and dissociative desorption of molecularly adsorbed NO, bonded to the surface through the nitrogen atom. The occurrence of such process has already been demonstrated [6] for CO chemisorbed on Pt(l1 l), and in the present case the analogous reactions for NO may be written thus: Pt-NOcPt+NO(g),
(1)
CM
622
Comrie et al/Adsorption
1 00 I
0 75
2
b
11111
L-T Pt
0.25 0.50
Fig. 2. Nitrogen
Pt-NO 9 Pt-N
of nitric oxide on Pt
+ 0 (g) ,
deposition
within
beam irradiated
area.
(2)
2
where the desorption products in eqs. (1) and (2) are to be taken as mixtures of ionized and neutral molecules and atoms respectively. Graphic evidence that the dissociation of NO takes place only within the beam irradiated portion of the crystal surface is provided by fig. 2. These data were obtained by exposing the crystal to 30 L NO (1 L = 1 Langmuir = lo@ torr set) at 300 K while continuously irradiating part of the surface with a well-focussed 2 keV electron beam. The NO was then pumped out, and the electron beam tracked across the surface while monitoring IN, with the result shown in fig. 2. The qualitative observations already described show that k, is comparable with k,, and that the electron induced desorption rate for atomically adsorbed N is small compared with both (1) and (2). Under these conditions, and with a continuous supply of gaseous NO, a substantial net accumulation of chemisorbed atomic nitrogen should occur within the irradiated area. The subsequent monitoring sweep of the primary exciting beam would then be expected to show a large increment in IN over the background level within the irradiated area, as is indeed observed. The background ZJ,Jsignal arises from a dilute layer of N due to rapid disruption (i.e. mainly desorption) of the NO saturated surface outside the irradiated area. The behaviour of chemisorbed NO is thus seen to be closely similar [6] to that of CO on Pt(ll1). The data of fig. 2 also provide a convenient measure of electron beam diameter (full width at halfmaximum) which can be used for computing electron flux densities in the measurement of absolute reaction cross sections. Absolute total reaction cross sections for the electron impact desorption and dissociation of chemisorbed NO, and for the desorption of atomically adsorbed N, were determined by a method that we have already described in detail [6]. Briefly, the surface was saturated with NO at 300 K, the system evacuated, and the analyzing beam turned on to follow the time dependence of I,,. Since both desorption and dissociation of NO result in loss of surface oxygen [eqs. (1) and (2)] the total loss rate, as monitored by Z,,, will be determined by the sum of the in-
CM. Comvie et al/Adsorption
20
I
of nitric oxide on Pt
101Tfme imml LO 60 80
I
I
I
623
1cO
I I
Fig. 3. Total surface oxygen loss rate from NO covered surface. Electron flux 2.06 X 10” cmw2 s-l (0). Surface nitrogen loss rate from N covered surface. Electron flux 7.22 X 1016 cme2 s-l (0).
dividua~ rate coefficients (kI + k2). Thus by following the time dependence of I,, a value for (kl t k2) may be obtained by elementary application of the first order rate law. Fig. 3 shows a typical result obtained with a primary flux of 2.06 X 1015 electrons cm-2 s-1 at 58” to the surface normal. The slope of this graph gives (kl t k2) = 2.90 X 10e3 s-l which in turn corresponds to a total cross section for the desorption and dissociation of NO of orJo = 1.4 X lo-l8 cm2. (In this work, cross sections are referenced to primary electron flux. It is to be expected that, as we have shown [6] for Pt-CO, the major contribution to the observed effect is due to the secondary electrons which pass through the adlayer). Analysis of the rate equations for reactions (1) and (2) leads to the relationship: e,(t)
= [k&k1
+ k2)1 e,,(O)
11 - exp[-(kl
+ &$I
1,
(3)
where @N(r) is the surface nitrogen coverage generated by dissociation after irradiating an NO adlayer of initial coverage 0 No(O) for time t. Since (kl + kz) is now known, eq. (3) may be solved for k2 provided that (i) the value of tlNo(0) is known (ii) it is possible to separate out the contributions to IN from atomically adsorbed N and chemisrobed NO, and (iii) IN(t) can be related to absolute nitrogen coverage. No direct LEED evidence.regarding (i) is available from our own work. However, Conrad et al. [3] have observed two different LEED patterns for NO on Ni(l11). 6 L of NO was found to produce a ~(4 X 2) structure, while at saturation coverage in the presence of gaseous NO, a complex structure corresponding to about 0.6 monolayers was observed. Since this latter phase was only stable in an ambient atmosphere of NO, it is likely that the saturation coverage would be somewhat less under UHV conditions. On the other hand, it is possible that Pt(ll1) with its larger unit mesh might be able to accommodate a greater coverage of NO than Ni(l11). In any event, a saturation coverage of between 0.25 and 0.6 monolayers might reasonably be expected in our experiments just prior to irradiation. With regard to (ii), the
of nitric oxide on Pt
C.M. Comrie et aLlAdsorption
624
similar desorption temperatures of N, and NO (see sections 3.2) rendered selective desorption of the latter impossible, thus precluding the approach that was successfully employed in the Pt( 1 1 1)-CO system [6] . Since atomically adsorbed N appeared to be perfectably stable in an atmosphere of CO, an attempt was made to displace [4] selectively the NO remaining after irradiation by a large dose of CO. However, this approach was also unsuccessful because of the low efficiency for displacement [4] of cr NO by CO. Instead, the NO saturated surface was irradiated until all the NO had either been desorbed or dissociated (I,, + 0). In this case (kl + k2) t 2 1, and eq. (3) reduces to Q(t)
= W&Q
+
Ql &q(O).
@a)
The left-hand side of (4a) is given by e,(t)
= ]~~@)lW91
e,(-)
1
(4b)
where IN(=) is the saturation nitrogen Auger signal obtained after prolonged irradiation of the crystal in an ambient atmosphere of NO, and ON(m) is the corresponding coverage of atomically adsorbed N. Combining (4a) and (4b) yields IN(WN(9
= ]k2/(kl
+
k2)1 ~,,~w,w.
(4c)
Since ON(w) is produced by dissociation of NO molecules, it seems reasonable to suppose that it does not correspond to a packing of N which is more dilute than the saturation coverage of NO itself, and its value may even be as high as unity. Application of eq. (4~) to the results of a number of experiments using differing incident electron fluxes, together with the upper and lower bounds on e,,(O) and 0,(m), gives the following cross sections for reactions (1) and (2). of0
= 4(?2)
X IO-l9
cmp2 ,
0:’
= l.O(~kO.2) X lo-‘*
cm2 .
NO adlayers on Pt(ll0) were found to be much more resistant to disruption by electron impact than in the case of Pt(ll1). The effects were small and detailed studies were not carried out. However, the total cross section for destruction of the adlayer was found to be <10-20cm2. In the above treatment of the Pt(ll1) results we have tacitly ignored the desorption of atomic N during irradiation. The importance of such an effect was assessed by explicitly following the time dependence of 1, while irradiating an adlayer of atomically adsorbed N produced as already described. A typical result is shown in fig. 3, and corresponds to a very small cross section for atomic N desorption ‘;;= 8 x 10-22 cm2); this justifies our neglect of the process in the extraction of and 0;’ from the data. 01 3.2. Thermal desorption: Pt(l11) Thermal desorption data for Pt( 111) were obtained using the polycrystalline ribbon. Flashing this specimen after exposure to NO was found to result in partial
C.M. Comrie et al/Adsorption
3t
of nitric oxide on Pt
625
Pt (111)
6 65
2 2 e z
FL
m
1
2
I
I
I
I
I
373
L73
573
673
773
Temperature
I
I
873
973
1K 1
Fig. 4. NO, N,, and 0, thermal desorption spectra following exposure to 1.2 L NO. Heating rate 110 K s-l. bursts at 28, 30, and 32 amu. It is evident that the 28 amu signal could be due either to N, resulting from decomposition of NO, or to CO spuriously adsorbed from the background gas during NO exposure. Unambiguous identification of the desorption products was achieved by using .sotopically labelled IsNO; these experiments showed that there was no interference from CO, and that the only desorbing species were N,, NO, and 0,. Thermal desorption spectra following exposure of the specimen to 1.2 L NO are shown in fig. 4; at higher exposures, a shoulder appeared on the low temperature side of the NO peak. It should be pointed out that although the heating rate was constant (110 K s-l) during NO and N2 desorption, it fell off above -800 K so that the area under the 32 amu curve is only a rough measure of the amount of 0, desorption. In addition, the long high temperature tail of the NO spectrum reflects the reduced system pumping speed and concomitant wall effects for this molecule. This problem was effectively eliminated by using an improved pumping arrangement for the experiments on Pt(ll0) (section 3.3). Investigation of the coverage dependence of the NO, N, and 0, desorption spectra showed that in the case of both N2 and NO the peak temperatures were approximately independent of coverage (Tp - 513 K, -490 K for N2 and NO respectively); the 0, peak temperature decreased rapidly with increasing coverage, indicating-that the desorption process has an apparent reaction order greater than unity. Since the results presented in section 3.1 are consistent with the non-dissociative adsorption of NO at room temperature, the presence of N, and 0, in the desorption spectra strongly suggests that some NO must dissociate during the heating cycle. From an examination of the product distribution, it was not possible to make a precise determination of the extent of NO dissociation due to the significant differences in the effective pumping speeds for NO, N,, and 0,. However, the comparable intensities of the N2 and NO desorption spectra at all coverages indicates that approximately 2/3 of the molecularly adsorbed NO desorbs as N, and 0,. The pressure
626
CM
Comrie et al. lAdsorption
of nitric oxide on Pt
coverage independence of the NO and N2 peak temperatures indicates an essentially first order desorption process for the production of both species; while this is not unexpected for NO - the relevant reaction presumably being NO(a) -+ NO(g) - it is somewhat surprising for N,, especially in view of the “normal” behaviour of the O2 spectrum, and the implications are discussed below. For the moment, we may employ the equation derived by Redhead [lo] for a simple first order desorption process to estimate the apparent desorption enthalpies of NO and N2. This leads to values of 115 and 119 kJ mol-l for NO and N, respectively, assuming a pre-exponential factor of 1013 s-l. 3.3. Thermal desorp tion: Pt( 1 IO) Exposure of the clean Pt(1 lo)-(1 X 2) reconstructed surface to 10m8 torr NO at room temperature resulted in a noticeable increase in the background intensity of the LEED pattern, together with a significant decrease in the intensity of the fractional order beams after only 0.8 L. By 2.5 L the fractional order beams were essentially extinguished, resulting in a (1 X 1) LEED pattern. No other LEED pat-
Pt(ll0)
F
~~
c
dk
C
.,r\__’
P
E
%k7kdTemperature
(Ki
Fig. 5. NO desorption spectra following exposure to various doses of NO, (A) 0.1 L, (B) 0.3 L. (C) 0.6 L, (D) 0.9 L, (E) 1.2 L, (F) 1.8 L. Heating rate 13 K s-‘.
CM
Comrie et al. lAdsorption
of nitric oxide on Pt
621
terns were obtained upon further exposure, nor could they be generated by thermal treatment in an atmosphere of NO. A series of flash desorption spectra were taken (13 K s-l) following exposure to varying doses of NO, and the results for the desorption of NO itself are shown in fig. 5. It is at once apparent that these data are not significantly distorted by pumping speed limitations, and are amenable to more detailed analysis than in the case of Pt(ll1). It can be seen that a relatively strongly adsorbed state (0) is populated first, and it is only when the /3 state has been appreciably filled that the more weakly bound state (CX)begins to fill. Assuming a preexponential factor of 1013 s-l as before leads to desorption enthalpies for the 01 and /3 states of 96 and 116 kJ mol-’ respectively. Monitoring the 28 amu partial pressure burst showed that some N2 desorption also occurred, but in this case the amount of NO undergoing decomposition (<20%) was substantially less than in the case of the (111) surface. No 0, desorption was detectable, and this is consistent with (i) the reduced decomposition yield, (ii) the smaller relative sensitivity to 0, due to wall pumping (see fig. 4) and (iii) the greater overall pumping speed of the system in the present case. Once again, the peak temperatures of the N2 desorption spectra were essentially coverage independent, and the results are shown in fig. 6. It
Pt(llO) E
;~
0
0
373
L73 Temperature
573
02
(K 1
Fig. 6. N, desorption spectra following (C) 0.6 L, (D) 1.2 L, (E) 6.0 L.
1.0 20 NO Exposure
OL
06
30 ILatqnwsl
08
1D
ONO/ ON0mxl exposure
to various
NO doses, (A) 0.1 L, (B) 0.3 L,
Fig. 7. (A) NO coverage as a function of exposure. (B) relative sticking of coverage; (-4) experimental data, (-) calculated curve for [ = 4.
probability
as a function
628
C.M. Comrie et al./Adsorption
of nitric oxide on Pt
should be pointed out that in this case it was not necessary to use 15N0 in order to provide unambiguous identification of N2 as a product. The desorption peak temperatures of P-CO and N2 from Pt(ll0) differ by > 30 K so that the two species are readily distinguished. Since the N, yield was always relatively small, the areas under the NO desorption curves of fig. 5 provide a measure of the NO coverage prior to flashing. These data may therefore be used to construct the adsorption curve for NO on Pt(ll0) (coverage as a function of exposure - fig. 7A) from which it can be seen that the system follows precursor state kinetics. The corresponding curve of relative sticking probability (S/So) as a function of coverage is shown in fig. 7B. It is evident that if the saturation coverage (e,,) . o f or d er unity, then the initial sticking probability (So) is also 1s of order unity. Although NO does not form any ordered phases on the (110) surface, comparison with the data [7] for CO (which were also obtained in our laboratory) provides a useful indication of tImax in the case of NO. The (2 X 1) plgl structure formed by CO shows that B,,(CO) IS unity; furthermore, the (1 X 2) + (1 X 1) transformation only went to completion at essentially saturation coverage. Since this transformation is also effected by NO, it is reasonable to conclude that fI,,(NO) must likewise be close to unity. This view is supported by the fact that the signal strengths of the desorption spectra at saturation coverage are very similar for these two molecules, and indeed the spectra themselves bear a close resemblance to each other. We may therefore conclude that S, is large and of order 1.O.
4. Discussion The results presented in section 3.1 are consistent with the non-dissociative adsorption of NO on platinum, the molecule being chemisorbed via the N atom. Further evidence for the molecular adsorption of NO on both Pt( 111) and Pt( 110) is provided by the results of competitive coadsorption experiments reported earlier [4] which indicated rapid and efficient displacement of NO from both surfaces by CO. It seems unlikely that a dissociated adlayer would be so readily displaced in this way; indeed, reaction with gaseous CO would be expected to deplete the surface of 0 by rapid reaction [ 11,121 to form CO,, and this was not found to be the case. Recent UPS studies [ 171 have also demonstrated that NO is non-dissociatively adsorbed on Pt(lO0). The magnitudes of the electron stimulated desorption and dissociation cross sections for NO on Pt(ll1) are similar to those already obtained for CO on the same surface [6] , although dissociation appears to be relatively more important in the case of NO. However, it is not possible to correlate this difference in behaviour in any simple way with the relative CO and NO bond strengths in the electronic ground state of the two adsorbed species. Clearly, the nature of the (unknown) excited states involved in the transition must play a crucial part in determining the dissocia-r tion probability of the adsorbate. Indeed, the complexity of the situation is under-
CM
Comrie et aLlAdsorption
of nitric oxide on Pt
629
lined by comparing the electron impact properties of CO and NO on the two surfaces. Given [7] that the total cross section for desorption of CO from Pt(ll0) is < IO-l8 cm2, it then appears that the behaviour of a given molecule on different surfaces shows greater differences than that of different molecules on the same surface. The appearance of 02 and N2 in the desorption spectra suggests that some thermal dissociation of NO takes place upon heating the adlayer, and it is of interest to compare the behaviour of Pt(l11) in relation to NO dissociation with that [l-3] of Pd(ll1) and Ni(ll1). In the case of Ni(ll1) dissociation of the molecule occurs at room temperature, whereas the behaviour of Pd(ll1) appears to be very similar to that reported here for Pt(ll1). In this connection it is worth noting that the lattice parameters of Pd and Pt are virtually identical, whereas that of Ni is significantly smaller. Assuming that dissociation proceeds via a mechanism which requires the presence of a vacant neighbouring site, it may be that the activation barrier to NO dissociation is correlated with the spacing between nearest neighbour adsorption sites on these three closely related surfaces. Another possible factor arising out of size effects is the effectiveness of the overlap between metal and adsorbate orbitals which will determine the N-O bond order. Once again, the behaviour of Pt and Pd might be expected to differ from that of Ni. The present work indicates that Pt(ll0) is significantly less active in dissociating NO than is Pt(l1 I), although the adsorption enthalpies on the two surfaces are very similar. It may be that this is due to the substantially reduced density of nearest-neighbour site pairs (at the closest packed spacing of 2.77 A) which exists on the (110) surface. However, such factors as reduced mobility of surface species on the markedly anisotropic (110) plane could also play a part. The very different coverage dependences of the N2 and 0, desorption spectra are noteworthy. The O2 spectra show the expected behaviour for a second order associative desorption, but the apparent first order behaviour of the N2 spectra is worthy of comment. Ertl et al. [3] have observed precisely the same behaviour for the Ni(1 11)-NO system in which the adlayer is fully dissociated even at room temperature, but these authors did not suggest an explanation for the N2 desorption kinetics. The problem at hand has a most interesting relationship with the PI, p2 spectra of the CO-W system [ 131. Goymour and King [ 141 have given a detailed analysis of the latter system in terms of associative desorption from a dissociated adlayer of repulsively interacting particles. In the (C + 0)-W case, the most energetically favoured gaseous product is CO, so that the stoichiometry of the adlayer remains invariant throughout the desorption cycle. Under these circumstances, the desorption rate shows two maxima, and the position of the first of these does not vary very much with coverage. If we now consider the (N + 0)-Ni system with lateral repulsive interactions between the adsorbed atoms, an analogous situation obtains, but with one very important difference. Energetically, the most favoured gaseous product is now N2. Preferential desorption of N2 thus occurs, the surface stoichiometry changes continuously, and the rate of N2 desorption is determined
CM. Comrie et al/Adsorption
630
of nitric oxide on Pt
by the instantaneous concentration of nearest neighbour N adatom pairs. This latter quantity is itself dependent on the total repulsive interaction energy in the adsorbate layer and this includes a large contribution from the 0 atoms which are present. Under such conditions, and subject to the approximations introduced by Goymour and King [ 141 ~it would be possible for essentially all the N, to be desorbed within a single peak, analogous to fll -CO from W, whose temperature was almost coverage independent. Oxygen loss would then occur at a higher temperature, and show a single peak spectrum whose position should be much more coverage dependent. This is just the behaviour observed in the case of Pt; however here the problem is further complicated by the incomplete dissociation of the adlayer at the desorption temperature. Nevertheless, this does not vitiate the model for the Pt case, and it seems possible that the tentative suggestions made here may provide the basis for a correct explanation. One potentially serious difficulty with the above argument is that the behaviour just described should only occur at total coverages greater than 0.5, whereas our own results for Pt(ll0) indicate that the behaviour deos not change at coverages below this value. There would appear to be only two ways around this difficulty (i) at low coverage the adsorbate is distributed as islands on an otherwise bare surface, or (ii) following dissociation there are attractive interactions between N atoms in the adsorbate layer. The kinetics of adsorption of NO on Pt(ll0) indicate that the process occurs via a mobile precursor state, which is a fairly common observation for such systems. Analysis of fig. 7B in terms of the surface mobility parameter developed by Gasser and Smith [ 151 using the relation S/So = (1 - 0s j shows that a best fit is obtained for t = 4. This is a reasonable result for the average number of adsorption sites sampled by the precursor molecule before desorption, and is in fact identical with the result for CO on the same surface [7]. More significant is the finding that, in common with CO, the adsorption rate of NO is not sensitive to the extent of the (1 X 2) + (1 X 1) transformation. This is by no means always the case, as the adsorption kinetics of linear polyatomic molecules on Pt(ll0) can be strongly influenced [ 161 by the condition of the surface periodicity along [OOl] .
Acknowledgement This work was supported
in part by NATO Research Grant No. 898.
References [l] G. Ertl, Surface Sci. 47 (1975) 86. [2] H. Conrad, G. Ertl, J. Kiippers and E.E. Latta, Faraday Discussions. Chem. Sot. 58 (1975) 116. [3] H. Conrad, G. Ertl, J. Kiippers and E.E. Latta, Surface Sci. 50 (1975) 296. [4] R.M. Lambert and C.M. Comrie, Surface Sci. 46 (1974) 61.
C.M. Comrie et al./Adsorption
[S] [6] (71 [S] [9] [lo] [ 1 l] [ 121 [13] [ 141 [ 151 [16] [ 171
of nitric oxide on Pt
T.E. Madey and J.T. Yates, J. Vacuum Sci. Technol. 8 (1971) 525. R.M. Lambert and C.M. Comrie, Surface Sci. 38 (1973) 197. C.M. Comrie and R.M. Lambert, J. Chem. Sot. Faraday Trans. I (1975), submitted. R.M. Lambert, W.H. Weinberg, J.W. Linnett and CM. Comrie, Surface Sci. 27 (1971) A.E. Morgan and G.A. Somorjai, Surface Sci. 12 (1968) 405. P.A. Redhead, Vacuum 12 (1962) 203. H. Heyne and F.C. Tompkins, Proc. Royal Sot. (London) A292 (1966) 460. H.P. Bonzel and R. Ku, Surface Sci. 33 (1972) 91. C.G. Goymour and D.A. King, J.C.S. Faraday I, 69 (1973) 736. C.G. Goymour and D.A. King, J.C.S. Faraday I, 69 (1973) 749. R.P.H. Gasser and E.B. Smith, Chem. Phys. Letters 1 (1967) 457. P.D. Reed and R.M. Lambert, Surface Sci. 57 (1976) 000. H.P. Bonzel and T.E. Fischer, Surface Sci. 5 1 (1975) 213.
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653.