J . Chem. Soc., Faraday Trans. I, 1982, 78, 171-183 Infrared Spectroscopic, Thermal Desorption and X-ray Photoelectron Spectroscopic Studies of NO, NO + CO and NO + 0, Adsorbed on Palladium Surfaces BY SHINICHI MORIKI, YASUNOBU INOUE,* EIZO MIYAZAKI AND IWAO YASUMORI Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Received 2nd February, 198 1 The adsorbed state of NO on polycrystalline Pd metal surfaces, its variation with heat treatment and the effect of CO and 0, have been studied by infrared spectroscopy (silica-supported), thermal desorption (powder) and X-ray photoelectron spectroscopy (powder, foil). The i.r. spectra of NO adsorbed at 298 K showed that there exist mainly bent or bridged NO (1660-1650, 1580-1570 cm-l) for BNo < 0.5 and linear NO (1750-1730 cm-') for 8 N o > 0.6.Upon heat treatment, these i.r. bands changed in different ways. X.P.S. showed a broad N 1s peak of molecular NO (398.7 eV) at 298 K and another peak due to NO,-like surface species (404.0-404.2 eV) at 453 K. For ON, < 0.6, the t.d. spectra exhibited a main N, peak at ca. 490 K, whereas for ON, > 0.6 additional peaks of NO appeared at 373-473 K. It is proposed that in N, formation the dissociation of NO is rate-determining. Changes in the adsorbed state of NO upon heat treatment are explained by the interaction of NO with 0 atoms remaining on the surface after part of the NO is decomposed. The effect of preadsorbed oxygen supports this view. Preadsorbed CO works as a scavenger for this 0 atom and accelerates the formation of N,.The adsorption of NO on transition-metal surfaces has been extensively studied in recent years, and considerable interest has developed in comparing the adsorbed state of NO with its state in organometallic nitrosyl complexes. However, metal surfaces often differ from the case of complexes in leading to the dissociation of NO to produce surface oxygen and/or oxides which may have considerable influence on the state of adsorbed NO. Among the transition metals, palladium is potentially interesting, since its interaction with NO occurs to a relatively moderate extent' at room temperature and varies as a function of temperature. Kishi and Ikeda showed the presence of molecular NO on evaporated Pd at room temperature by the use of X .P . ~ . ~ Recently, Conrad et al. applied a combined u.P.s., LEED and t.d. technique in order to study the geometric arrangement of molecularly adsorbed NO on the Pd (1 1 1 ) surface and its decomposition at elevated temperat~res.~3 In addition, Matsumoto et al. investigated the effects of sulphur on NO adsorption and decomposition over a polycrystalline Pd surface by means of A.e.s. and u.P.s.~ However, in view of the lack of information concerning interactions between adsorbed molecules, there still remains some ambiguity in describing the multiple structures and stability of NO on Pd metal over a wide temperature range. The present study was undertaken to obtain as much information as possible on interactions in the Pd-NO system, especially by taking into account the admolecule interaction and the reactivity of adsorbed species as a function of temperature with the aid of i.r., t.d.and X.P.S. techniques. In this regard, the effects of preadsorbed chemical species such as oxygen and carbon monoxide were also investigated. In addition, a bond-energy-bond-order (BEBO) calculation6 was applied to ascertain the validity of the mechanism proposed for the desorption and surface reaction of NO. 171172 NO ADSORPTION ON Pd SURFACES EXPERIMENTAL The preparation of silica-supported palladium used for the i.r. study was carried out in a manner similar to that used by Palazov et al.' The Aerosil silica 130, from the Nihon Aerosil Co., was impregnated with a dilute hydrochloric acid solution of palladium chloride, dried at 423 K and then pressed into discs with a thickness of ca.0.2 mm. The concentration of Pd metal loaded was ca. 5 wt %. In another method of preparation, the silica was pressed into discs and dipped into the solution described above for impregnation; there was no significant difference in the NO spectra obtained. Before NO adsorption, the samples were subjected to in situ reduction with 10 Torr of hydrogen at 773 K and were evacuated at that temperature until the residual pressure fell below 2 x Torr (1 Torr = 133.3 Pa).' This pretreatment provided reproducible i.r. spectra of NO. The samples were heated by an infrared lamp (Osram 25) and the temperature was measured on a Pt-Pt/ 13 % Rh thermocouple. Infrared spectra were recorded at room temperature on a Hitachi 285 spectrometer.The absorption due to the silica support was almost compensated by inserting a Pd-free silica disc in the light path of the reference beam. For the thermal desorption study, palladium powder was prepared by reducing a dilute hydrochloric acid solution of palladium chloride with sodium borohydride. The powder was rinsed thoroughly with ion-exchanged water, dried at room temperature and then reduced in the t.d. cell with 10 Torr of H, at 773 K for 2 h. The surface area of the treated Pd was determined to be 0.9 m2 g-l by a B.E.T. measurement at 77 K using Kr gas. The t.d. spectra were obtained on a high-vacuum apparatus equipped with a Pirani gauge (Wakaida Rigaku, model PG-2) and an ion gauge (Wakaida Rigaku, model VG-51) for pressure measurements, and a quadrupole mass spectrometer (Mitsubishi Electronics, model MF-T,M) for gas analysis.The temperature of the samples was raised at constant rates of 10-20 K min-l by an electric furnace and monitored by a Pt-Pt/l3% Rh thermocouple which was brought into direct contact with the Pd samples. X-ray photoelectron spectra were recorded at room temperature on a Hewlett Packard 5950A ESCA spectrometer with monochromatized A1 Ka exciting radiation. A palladium foil of 99.99% purity, obtained from the Johnson Matthey Ltd, was used. After being etched with aqua regia and thoroughly rinsed with ion-exchanged water, the surface was cleaned in the X.P.S. preparation chamber by cycles of prolonged argon-ion bombardment and annealing at 1023 K in a similar manner to that used by Lloyd et In addition, the Pd powder described above was prepared in the form of discs and used as sample for X.P.S. after reduction in the X.P.S.chamber with a few Torr of hydrogen at 773 K. The Au 44 line, 84.0 eV, was taken as reference. Nitric oxide (99.9% purity), CO (99.5%),0, (99.8%) and H, (99.999%) were obtained from the Takachiho Shoji Co. NO was purified by prompt evacuation at 77 K and by passing through a trap cooled to 173 K. The other gases were used without further purification. RESULTS INFRARED SPECTRA The adsorption of NO on the Pd powder surface at 195 K provided 8.7 x 1014 molecule cm-2 as the saturation amount; this value was assumed to correspond to the surface coverage ON, = 1. In the i.r. experiments, the surface coverage of NO on the silica-supported Pd surface was determined according to this definition, since no adsorption occurred at room temperature on the silica support.Fig. 1 shows the i.r. absorption bands of NO adsorbed at 298 K with different values of ONo. In the region of 1350-2500 cm-l there were three characteristic peaks, i.e. a at 1750-1730 cm-l, PI at 1660-1650 cm-l and Pz at 1580-1570 cm-l. The relative intensities of these three peaks were dependent on the adsorption time as well as on ONo. As time elapsed, the intensity of the a peak was gradually attenuated, being accompanied by the enhancement of the P1 and P2 absorption bands. As soon as the spectra showed little change, the peak area of the respective absorption bands was integrated and plottedS. MORIKI, Y.INOUE, E. MIYAZAKI A N D I. YASUMORI 173 10( 9 ( aJ E Y .- E 2 c Y 8C I I I 1 IS00 1700 1600 1500 wavenumber/cm -’ FIG. 1 .-1.r. spectra of NO adsorbed at 298 K on silica-supported palladium: (1) ONO = 0.13, (2) 0.60 and (3) 0.77. as a function of ONo. As is shown in fig. 2, the values for the a, and p2 peaks increased with ON,, passed through a maximum at ON, z 0.6 and then diminished, whereas the value for the a peak enhanced monotonically with a steep rise at ON0 x 0.6. Fig. 3 shows the variation in the i.r. spectra of the adsorbed NO as the evacuation temperature was raised; the a peak drastically decreased and disappeared at 573 K, whereas the p1 peak reached a maximum at ca. 430 K with shifts towards the higher-frequency side. The p2 peak continued to grow up to 573 K, shifting to the lower-frequency side by 30 cm-l.Fig. 4 shows the effects of preadsorbed oxygen upon the adsorbed state of NO. The a peak remained nearly unchanged in peak position but became slightly asymmetric as a consequence of broadening at the higher-frequency side, whereas the /I1 peak appeared at the higher-frequency position by 20 cm-l. The p2 peak shifted by 50-60 cm-l to the lower-frequency side and was close in frequency to the p2 peak observed in spectra (3) and (4) in fig. 3. When this surface was evacuated at elevated temperatures up to 493 K, the intensities of the a and p1 peaks changed in a manner analogous to the above-mentioned behaviour of the corresponding peaks on an oxygen-free Pd surface. On the other hand, no significant variation was observed in the peak position of the p2 absorption band.The adsorption of CO on the silica-supported Pd surface gave rise to three peaks at 2105,1995 and 1820 cm-l at 298 K which were similar to those reported by Palazov et a1.’ The former two peaks shifted to 2065 and 1970 cm-l, respectively, on evacuation at 363 K. Fig. 5 shows the i.r. spectral variations as a function of evacuation174 NO ADSORPTION ON Pd SURFACES 0.2 0.4 0.6 0.8 0 NO FIG. 2.-Variations in peak areas as a function of BNO: 0, a; 0, B, and a, /I2. ' \ 1 I I I I 1800 1700 1600 1500 wavenumber/cm-' FIG. 3 . 4 . r . spectral changes with increasing temperature of evacuation (Bw0 = 0.77): (1) evacuated at 298 K (-), (2) at 428 K (--.--.), (3) at 493 K (. . .) and (4) at 573 K (---).S. MORIKI, Y.INOUE, E. MIYAZAKI A N D I. YASUMORI 175 1 80 I 1 I I I 1800 1700 1600 1500 waven urn be r/c m FIG. 4.-1.r. spectra of NO adsorbed on oxygen-preadsorbed Pd and their changes with evacuation temperature. Amount of oxygen preadsorbed = 2 x lox4 molecules cm-2; PNo = 1 Torr. (1) Evacuated at 298 K (--.--.-), (2) at 373 K (-) and (3) at 493 K (------). t I 1 I I I I I 2100 2000 1900 1800 1700 1600 1500 waven urn ber/cm-' FIG. 5 . 4 . r . spectra of NO and CO coadsorbed on Pd surface. PNo = Pco = 0.5 Torr. (1) Evacuated at 298 K (-), (2) at 403 K (----.-) and (3) at 573 K (. . . . . .). temperature after a clean Pd surface was exposed to an equimolar mixture of NO and CO. In comparison with the adsorption of single species of CO or NO, coadsorption spectra at 298 K were characterized by an appreciable enhancement of the peak relative to the & peak and by the occurrence of a specific peak at ca.1890 cm-l. By evacuation at 523 K, the CO absorption bands in the region 1900-2100 cm-l were readily removed, in contrast to their high stability in the absence of coadsorbed NO.176 NO ADSORPTION ON Pd SURFACES THERMAL DESORPTION SPECTRA Fig. 6 shows the variations in the t.d. spectra of NO adsorbed on a clean Pd surface as a function of 8NO. For 8NO < 0.4 the desorbed gas was only N,, the peak of which was recorded at 483 K with a shoulder on the high-temperature side; at ON, = 0.5 an additional peak of N,O appeared at ca. 490 K. The appearance of another broad NO peak at 373-473 K was delayed until ca.0.7 monolayer coverage was attained. The t.d. spectra from the surface saturated by the adsorption at 195 K exhibited one more broad peak of NO at ca. 333 K. No oxygen was liberated up to 773 K. The detailed quantitative relationships between ONO and the t.d. peak areas of the respective desorbing species are demonstrated in fig. 7. Fig. 8 shows the effects of preadsorbed oxygen upon the t.d. spectra of NO; a clean Pd surface was pre-covered by oxygen with different surface coverages at 273 K and then exposed to NO gas at the same temperature. As the amount of preadsorbed 300 400 500 600 temperature/K FIG. 6.-T.d. spectra of NO as a function of BNO. (a) Total pressure: (1) ONO = 0.04, (2) 0.18, (3) 0.36, (4) 0.54, (5) 0.73, (6) 0.86 and (7) 1.0. (b) Gaseous composition of spectrum (5) (ONO = 0.73).(c) Gaseous composition of spectrum (6) (ONO = 0.86).S. MORIKI, Y. INOUE, E. MIYAZAKI AND I. YASUMORI 177 oxygen increased, the desorption of N, and N,O was depressed, whereas the desorption of NO at 393 K tended to increase. Fig. 9 shows the t.d. spectra after the Pd surface was exposed to NO and then to CO. When the concentration of adsorbed NO was fixed at 3 x 1014 molecule ern-,, the amount of CO changed from 1 x 1014 to 5 x 1014 molecule ern-,; the main spectral feature was the desorption of CO, and N, as very sharp peaks at ca. 443 K, in addition to a broad peak of CO at ca. 383 and 585 K, although a small N, peak appeared at 530 K in the case of low CO coverage. When the preadsorbed NO was increased to 6 x 1014 molecule ern-,, the t.d.spectra after CO adsorption of 6 x 1014 molecule cm-, were much the same except that an extra peak for NO appeared at ca. 390 K. There was no significant change in the t.d. spectra on reversing the gas admission. 1.0 h m + .- I= -E v 0.6 4 4 m .- c W I= U .- x 2 Y w I= U .d 0.2 0 x 2 x 2 0.2 0.4 0.6 0.8 1 .o 0 NO FIG. 7.-Variations in integrated intensity of t.d. peak with ON0: a, N,; 0, N,O and 0, NO. X-RAY PHOTOELECTRON SPECTRA As is shown in fig. lO(a), the X-ray photoelectron spectra of NO adsorbed at 298 K on a Pd foil surface exhibited a broad and asymmetric N 1s peak at 398.7 eV (peak I). This characteristic structure was consistent with that observed previously.2 Evacuation at 453 K caused an additional peak at 404.0 eV (peak 11) together with little change in the position of peak I (398.5 eV) but a slight broadening towards the higher binding-energy side, On evacuation at 533 K, the position of peak I remained unchanged, whereas peak I1 appeared at 404.3 eV with an enhanced intensity.As is shown in fig. lO(b), the Pd powder surface provided almost the same N 1s photoelectron spectra at 298 K and at elevated temperatures; the binding energy of peak I in spectrum (1) was 398.7 eV, whereas those of peaks I and I1 in spectrum (2) were 398.6 and 404.2 eV, respectively. From the 0 1s region we failed to obtain useful information, since a large peak due to the Pd 3pt line interfered with the analysis of the 0 1s peak of adsorbed NO.178 NO ADSORPTION ON Pd SURFACES n e m * .I -2 2 v ?-.c ." Y e .- 'a) 300 400 500 60 0 temperature/K FIG. 8.-T.d. spectra of NO adsorbed on oxygen-preadsorbed Pd surface: (a) N,O, (b) N, and (c) NO. amount of oxygen amount of atom cm-, molecule cm-, preadsorbed/ 1014 ~0/1014 (-) 0.0 (--) 1.2 ( * * * I 0.3 (---) 2.4 (-. .-) 3.6 8.7 8.6 7.4 6.3 5.4 DISCUSSION In the chemistry of transition-metal nitrosylsgv lo there are three types of bonding between the metal and the NO molecule; they are known as the linear, bent and bridge forms. In the linear form, NO is present as nitrosonium ion and gives absorption bands above 1700 cm-l. The bent form is associated with NO- and provides characteristicS. MORIKI, Y. INOUE, E. MIYAZAKI A N D I. YASUMORI 179 400 500 600 temperature/K FIG. 9.-T.d. spectra after exposure of Pd surface to CO and then NO: (1) amount of CO adsorbed = 5 x 1014 molecule cm+, amount of NO adsorbed = 3 x 1014 molecule cm-2 (-); (2) amount of CO adsorbed = 1 x 1014 molecule cm-2, amount of NO adsorbed = 3 x 1014 molecule (---).absorption bands at 1700- 1500 cm-l. The bridged nitrosyl structure, being similar to that of carbonyls, is found in Ru(CO),,(NO), and Os(CO)l,(NO), complexesll to give rise to absorption bands at ca. 1500 cm-l. By analogy with these nitrosyl complexes one can assign the a peak to the N-0 stretching vibration due to NO in the linear form. The /I1 and #I2 peaks may not be unequivocally assigned but they are character- istic of N-0 stretching vibrations ascribable to either the bent or bridged structure. In accordance with these results, the broad but single N 1s photoelectron peak at ca.399 eV revealed that only molecularly adsorbed NO was formed at room temperature, and it is likely that its asymmetric shape involved at least two molecular states, i.e. linearly and bridge-bonded NO, as was proposed for the N 1s spectra of NO adsorbed on the Ru(OO1) plane below 200 K.12 For NO adsorption with ON, < 0.6, the i.r. spectral change with time was a decrease in the a peak and in turn an increase in the /I peaks, leaving the total amount of adsorbed NO almost unchanged. This phenomenon resembles that observed in the i.r. spectra of CO adsorbed on silica-supported Pd,' and is interpreted in terms of the delay in establishing an equilibrium among the adsorbed species ; the energetically more stable state is attained through the surface migration or hopping of NO, for which some activation energy is necessary.The fact that heating 2t 373 K can accelerate the transfer from the a to the /I state supports this view. With increasing NO coverage, the /3 peaks passed through a maximum and decreased, whereas the180 NO ADSORPTION ON Pd SURFACES ------- I I I I I I 412 408 404 400 396 3 92 binding energyleV FIG. 10.-X-ray photoelectron spectra in N 1s region: (a) Pd foil, (b) Pd powder. (1) Exposed to 1.5 Torr of NO for 8 min at 298 K and evacuated at the same temperature; (2) evacuated at 453 K ; (3) evacuated at 533 K. a peak grew markedly, suggesting that part of the former species was converted into the latter. The consolidation of the above-mentioned findings leads to the following description of stoichiometric and structural variations of adsorbed NO.For ON, < 0.6, where there exists little conformational restriction arising from repulsive interactions between the neighbouring admolecules, the adsorbed species certainly undergo interaction with the Pd metal surface; the bridged structure, bound to two surface atoms, seems to be favoured. This is substantiated by the finding that the /? species produced at an initial stage of adsorption gave rise to a lower frequency for the N-0 stretching vibration. For ON, > 0.6, a densely-packed situation will turn the adsorbed NO into a linear form which corresponds to the a state. The fact that the a species was desorbed at lower temperature than the p species, as is described later, gives support to this view.The t.d. spectra provided the three characteristic desorption peaks. From a correspondence between the temperature-dependence and dependence of amount adsorbed observed in the i.r. and t.d. spectra, the aforementioned a, PI and /?, peaks were respectively associated with the adsorbed species desorbing as NO at 273-373 K, N, (partly N,O) at 473 K and N, (partly N,O) at 530 K. A possible mechanism of N, formation is described by the following processes: k , k-1 NO(a) G N(a) + O(a) k* N(a) -+ #N,S. MORIKI, Y. INOUE, E. MIYAZAKI A N D I. YASUMORI 181 where ki is the forward rate constant for the ith step and k-i designates the reverse rate constant. The symbol (a) represents an adsorbed state. The change in surface coverage of N(a), ON, with time during thermal desorption, d6,/dt, and the rate of N, desorption, rd, are respectively given by dB, = k , 6 ~ 0 - k - , ONO0 - 2k2 6 i dt (3) in the low concentration range of NO(a) and rd = k,8& (4) where 6, denotes the surface coverage of O(a).Replacing eqn (3) in eqn (4), we obtain When ON, was changed to low coverages, no significant shift in the maximum of the N, peak was observed, indicating that the rate of desorption is first-order with respect to ON,. While this relationship is satisfied in eqn (4) for several cases, it is more plausible that k,ONo $ k-,ONOO or d6,/dt, i.e. the dissociation of NO(a) is rate-determining. Another possible pathway for N, desorption is : NO(a) + N(a) -+ N, + O(a) (6) which competes with N,O formation.Based on the BEBO calculation,6 we have tried to evaluate the activation energy, E,, for the three forward steps (I), (2) and (6). The variation in energy as a function of the bond order of the N-N and N-0 bonds is shown in fig. 11 ; the values of E, were 146, 121 and 218 kJ mol-l, respectively, for the above three steps, which indicates that step (2) is preferred to step (6) in N, formation. By assuming a pre-exponential term of 1 x 1013 s-l in the first-order desorption, the activation energy of N, desorption can be evaluated to be 134-151 kJ mol-l, which is close to the calculated value for step (1). The slow decomposition of a dimeric nitric oxide which was observed on silica-supported chromia13 might participate in the N, formation, but this possibility was probably excluded in the present case, since no evidence for the species was given in the i.r.spectra. The present t.d. spectra agreed with those obtained by Conrad et aL314 for NO adsorbed on a Pd(ll1) surface in the two peaks of NO below 370 K and in the small amounts of N,O desorbed at ca. 480 K, but they differed in exhibiting N, desorption instead of NO at ca. 480 K. Such a difference is presumably due to the specific morphology of the polycrystalline surface; judging from the structure of catalytic active sites on Pd foils for the hydrogenation of C,H, and C2H4,14 it is likely that surface imperfections and/or high-index planes facilitate the dissociation of NO at high temperatures. At elevated temperatures, 428-573 K, the i.r. spectra showed a shift of the p1 peak to the higher-frequency side and of the b2 peak to the lower-frequency side.In the X-ray photoelectron spectra, peak I1 appeared at the higher binding-energy side, in addition to the broadening of peak I. Since the resulting i.r. spectra were in substantial agreement with those of NO adsorbed on the oxygen-covered Pd surface, and since the t.d. spectra provided the decomposed species in this temperature range, it is evident that the changes in these spectra were ascribable to the effect of surface oxygen produced by NO decomposition. The effects of preadsorbed oxygen on a clean Pd surface were the depression of N,182 NO ADSORPTION ON Pd SURFACES and N,O desorption but the enhancement of NO desorption in the t.d. spectra, whereas they were the shift of the /I1 peak to higher frequency and of /3, to lower frequency in the i.r.spectra. The oxygen atoms on the Pd metal surface become electron acceptors, as is revealed by the increase in the work function upon 0, adsorption,15 and could affect the metal by lessening the back-donation to the 2n* orbital of the NO molecule. The shift in the peak (and presumably the broadening of peak I in the X-ray photoelectron spectra) can be associated with this effect. Thus, the N-0 bond is thought to be strengthened, and it follows that NO desorption is preferred to dissociation. Another effect of the preadsorbed oxygen is to reduce the number of active sites available for NO dissociation. bond order, N - 0 1.6 1.2 0.8 0.4 0 I 1 I 3 1 2 3 bond order, N - N FIG.1 1 .-Interaction energy plotted against bond orders of N-0 and N-N. (1) NO(a) -+ N(a) + O(a), (2) 2N(a) -+ N,(g), (3) NO@) + N(a) -+ N, + O(a>. The shift of the /?, peak to the opposite-frequency side can be attributed to the formation of a surface-bridged structure, i.e. \ 0 N----O I I * * through the direct interaction between the surface oxygen and nitrogen atom of the NO molecule. This assignment is reasonable, since our preliminary results for NO,S. MORIKI, Y. INOUE, E. MIYAZAKI A N D I. YASUMORI 183 adsorption on the Pd surface gave a band at ca. 1535 cm-l, and the region 1500 20 cm-l was characteristic of the N-0 stretching vibration ascribable to the NO, bridge-ligand.16 The appearance of peak I1 to higher binding energy in the X-ray photoelectron spectra also gives support to the presence of this species with the direct interaction of the nitrogen atom with surface oxygen, since a similar N 1s line was observed in NO, adsorption on Ni." Another plausible assignment is a nitrato bidentate species as proposed for Fe and Ni metals,ls but this possibility is excluded since there was no broad peak in the range 1350-1450 cm-l which is characteristic of this bidentate species.The effect of coadsorption of NO and CO on the Pd surface at room temperature was a slight shift in the PI peak to higher frequency with a considerable enhancement of its intensity, as well as a shift in the CO bands to lower frequency. These findings suggest that /I,-NO and CO on the Pd surface can interact with each other, for instance in a manner similar to that observed in the metal complex, [IrCl(NO) (CO) (P(C6H5)3)2]+, in which the CO and NO ligands form a donor-acceptor-like complex through the interaction of a lone pair on the oxygen atom of the bending NO with an empty 2n* orbital on the CO At higher temperatures the coadsorption of CO and NO resulted in the desorption of N, and CO, at almost the same temperature, 440 K.Thus the following reaction pathway can be proposed : NO@) + CO(a) + (*) -+ N(a) + O(a) + CO(a) -+ 4=N2 + CO, + 3(*) (7) where (*) denotes a vacant site. The feature of this pathway is the remarkable increase in the number of vacant sites during the course of the reaction, which was similar to the autocatalytic mechanism proposed for the decomposition of formic and acetic acids adsorbed on the Ni(ll0) surface.2o Therefore, step (7) could lead to the accelerated evolution of N, and CO,, which was well reflected by the remarkably narrow peak width in N, and CO, desorption.J. Kuppers and H. Michel, Surf. Sci., 1979, 85, L201. K. Kishi and S. Ikeda, Bull. Chem. Soc. Jpn, 1974, 47, 2532. H. Conrad, G. Ertl, J. Kuppers and E. E. Latta, Surf. Sci., 1977, 65, 235; 245. * H. Conrad, G. Ertl, J. Kiippers and E. E. Latta, Furuday Discuss. Chem. Soc., 1975, 58, 116. Y. Matsumoto, T. Onishi and K. Tamaru, J. Chem. SOC., Furuday Trans. I, 1980, 76, 1 116. E. Miyazaki, J. Cutul., 1980, 65, 84, and references herein. A. Palazov, C. C. Chang and R. J. Kokes, J. Cutal., 1975, 36, 338. P. Finn and W. L. Jolly, Znorg. Chem., 1972, 11, 893. and G. 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