RAIRS and TPD study of CO and NO on β-Mo2C

surface science ELSEVIER

Surface Science 374 (1997) 197-207

RAIRS and TPD study of CO and NO on fl-Mo2C J. Wang, M. Castonguay, J. Deng, P.H. McBreen * D~partement de chimie, Universit~Laval, Quebec (PQ), Canada GIK 7P4 Received 6 February 1996; accepted for publication 26 September 1996

Abstract The chemisorption of CO and NO on bulk fl-Mo2C was studied using reflectance absorbance infrared spectroscopy (RAIRS) and thermal desorption measurements. The carbide foil substrate was cleaned in situ prior to each experiment by repeated annealing to 1400 K. Low coverage dissociation of CO yields a weak recombinative thermal desorption feature at approximately 1200 K and molecular desorption of CO occurs at 325 K for the lowest exposures studied. RAIRS results show that carbon monoxide on flMo2C at 100 K is characterized by a single CO vibrational stretching frequency, typical of on-top adsorption, which increases from 2057 to 2072 cm 1 as a function of increasing coverage. Preadsorption of oxygen leads to a broadening to higher frequencies but not to the appearance of any new absorbance bands. The vibrational spectrum for adsorbed NO at 100 K displays a band at 1780cm -1 and a shoulder at 1820cm-a. RAIRS spectra acquired as a function of temperature were used to monitor the decomposition of NO. Decomposition occurs over the 250-450 K range and results in high temperature CO and N2 desorption peaks. The results for the chemisorption of NO and CO on molybdenum carbide are compared with literature data for the adsorption of these probe molecules on molybdenum and ruthenium surfaces. This comparison is used to comment on the physical basis for the correspondence in catalytic properties between early transition metal carbides and noble metal based catalysts. © 1997 Elsevier Science B.V. All rights reserved. Keywords: Carbides; Carbon monoxide; Molybdenum carbide; Nitric oxide; Oxygen;Reflectionadsorption IR spectroscopy; Thermal desorption spectroscopy

1. Introduction Early t r a n s i t i o n metal carbides display catalytic properties competitive with those observed for precious metal catalysts [1,2]. I n the particular case of fl-Mo2C , the catalytic activity for C O h y d r o g e n a t i o n [ 3 - 5 ] a n d alkane hydrogenolysis [3,6] reactions matches that f o u n d for r u t h e n i u m catalysts. C a r b i d e materials such as Mo2C offer the a d d i t i o n a l a d v a n t a g e of high resistance to sulfur p o i s o n i n g a n d effectiveness in h e t e r o a t o m

* Corresponding author. Fax: + 1 418 656 7916; e-mail: [email protected]

removal reactions [7,8]. F u r t h e r m o r e , selectivity towards isomerization reactions can be i n d u c e d t h r o u g h modification by oxygen c h e m i s o r p t i o n [ 9 ] . For example, P h a m - H u u et al. [10,11] observed high selectivity towards isomerisation of n - h e x a n e o n oxygen modified Mo2C. I n the analogous case of oxygen modified WC, the i n d u c e d selectivity towards isomerization was a t t r i b u t e d to the i n t r o d u c t i o n of Lewis acid functionality t h r o u g h the f o r m a t i o n of W O x surface species [ 1 2 ] . Clearly, early t r a n s i t i o n metal carbides are p r o m i s i n g substitutes for G r o u p VIII n o b l e metal catalysts a n d the physical basis for the correspondence between the properties of these two classes

0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0039-6028 (96) 01228-9

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of materials merits study. In the latter context, this paper compares RAIRS and TPD data for the chemisorption of 02, CO and NO on bulk /% MozC with literature data for their adsorption on molybdenum, carbon modified molybdenum and ruthenium single crystals. Carbon modification of the surface of supported metal particles occurs naturally in the course of many catalytic reactions [-6,13]. In particular, an early observation [13] of the enhanced activity of molybdenum carbide for ethane hydrogenolysis resulted from a study of the activity of unsupported molybdenum as a function of reaction time. There have been numerous surface science studies of chemisorption and reaction on carbon modified metal single crystal surfaces [14-18]. Typically, single crystals are modified by depositing up to a monolayer of carbon via unsaturated hydrocarbon decomposition. More recently, Chen et al. have reported a series of studies which specifically address the chemisorption and electronic properties of model thin film vanadium carbide [18-23] and molybdenum carbide [14,24] samples. These thin films, of >10,~ thickness, were formed through hydrocarbon decomposition on single crystal metal samples. The present paper deals with chemisorption of the probe molecules CO and NO on bulk foil samples of/~-MozC, a hexagonal close packed interstitial alloy in which the carbon atoms occupy half of the octahedral sites. The study of bulk /~-MozC using surface science techniques provides a necessary complement to previous studies on carbon modified surfaces, carbide thin films and supported molybdenum carbide catalysts [25].

2. Experimental The molybdenum carbide samples were obtained from Professor S.T. Oyama and coworkers (Virginia Polytechnic Institute and State University). The latter group prepared the samples by carburizing pure metal foils with a 20% CH4/H 2 gas mixture in a quartz reactor [2,26]. The flow of reactant gas was maintained at 300 cm 3 min- 1 while the temperature of the reactor bed was raised linearly to 1373 K at a rate of 8 K min -1. The

reaction was continued at this temperature until complete (approximately 3 h), as verified by X-ray diffraction (XRD). The carbided foils were passivated at room temperature in a flow of 0.5% O2/He for 3 h. XRD analysis [26] of the carbided foil revealed only trace metal-phase features, indicating that the samples are essentially pure bulk fl-MozC. Surface analysis studies of the carbide sample were performed using two separate ultrahigh vacuum (UHV) chambers. One chamber was equipped for X-ray photoelectron spectroscopy (XPS) measurements, and the second was equipped for RAIRS and temperature-programmed desorption (TPD) measurements. The base pressure in both systems was below 2 x 10 lo Torr. RAIRS spectra were recorded using a Mattson (Galaxy 4020) Fourier transform infrared spectrometer. The reported spectra are given in absorbance units and represent the ratio of 1000 scans of the adsorbate covered surface to 1000 scans of the clean in situ activated sample. A liquid nitrogen cooled mercury cadmium telluride (MCT) detector, with a low frequency cut-off at 800 cm -1, was employed in the RAIRS studies. The carbide sample was mechanically mounted between two copper feedthroughs on a liquid nitrogen dewar by curving small tabs of a tantalum support foil around the corners of the sample. Resistive heating of the sample was achieved using two tantalum wires spot-welded onto the back of the support. The sample could be quickly cooled down to 86 K due to its metal-like thermal conductivity. The chromel-alumel thermocouple used for TPD and annealing runs was also spot-welded to the tantalum foil. This method of measuring the temperature introduces an error due to the fact that the thermocouple is not in direct contact with the carbide. As a result, the reported TPD temperatures are somewhat higher than the actual values. Previous work in our laboratory on a Cu(ll0) crystal similarly mounted on a tantalum foil showed that the temperature was overestimated by approximately 3, 20 and 40 K at 300, 800 and 1300 K, respectively. The molybdenum carbide foil was cleaned prior to the chemisorption studies by repeated annealing to 1400 K under UHV. XPS C(ls) measurements [26] showed that this procedure removed graphitic

J. Wang et al./Surface Science 374 (1997) 197-207

deposit from the carbide surface. Only the C(ls) peak characteristic of the carbide carbon (282.9 eV) remained after several annealing cycles. As detailed in the next section, the annealing procedure was not sufficient to completely remove the O(ls) signal. Ar ÷ ion sputtering was not performed to remove the residual oxygen since sputtering leads to significant removal of molybdenum [27]. However, the surface was periodically sputtered to remove segregated sulfur resulting from the annealing process. All of the results given in this paper were obtained for samples subjected to several anneals immediately prior to each chemisorption experiment.

3. Results

XPS data for the chemisorption of oxygen on //-Mo2C at 96 K is shown in Fig. 1. A single O(ls) peak at 529.8 eV, FWHM=2.2 eV, grows in and reaches saturation at an exposure of approximately 0.1 L 02. The high binding energy tail in the spectrum recorded at 0.5 L is possibly due to a small amount of adsorbed CO, as it corresponds to the O(ls) peak at 532.5 eV for molecular CO on Mo(ll0) [28]. No shift of the Mo(3d) peaks was detected. The O(ls) peak area for the annealed sample, prior to exposure to oxygen, is close to 10% of the saturation peak area measured at 0.5 L.

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199

This residual oxygen content reflects the ability of carbides to incorporate oxygen [29] as well as the fact that the freshly prepared foils were passivated in an O2/He flow [26]. The observed binding energy, bandwidth and saturation exposure, closely match those reported by Marinova et al. [30] for the interaction of oxygen with M o ( l l l ) at room temperature. They observed O(ls) peaks at 530.2-530.6 eV and a saturation of oxygen uptake at approximately 2 L. The high reactivity of /% M o z C towards oxygen resembles that observed for several metal-terminated, (111) crystal face, monocarbide surfaces [31,32] as well as that for pure molybdenum surfaces [30,33]. This observation implies that the molybdenum carbide surface is not carbon terminated and that molybdenum atoms are accessible at the surface for interaction with oxygen. TPD data for CO adsorption on Mo/C at 96 K are shown in Fig. 2. Three peaks are observed, but the low temperature peak (at 150 K) mainly results from desorption from the sample holder and support material. Nevertheless, experiments performed using a shielded mass spectrometer indicate that there still is a desorption feature from the sample surface at 150 K. The sum of the areas of the desorption peaks at 325-360 K and 1200 K saturates at a CO exposure of 3-4 L, whereas the 1200 K feature saturates at a surface coverage of approximately 0.1 ML. The molecular desorption peak shifts from 360 to 325 K with increasing exposure. RAIRS data for CO adsorption on clean and oxygen modified CO are shown in Fig. 3. For the clean surface, a single band of FWHM equal to 25cm -1, and displaying a slight asymmetry towards higher frequencies, is observed at 2057 cm-1 at low coverages and at 2072 cm-1 near saturation coverage. Adsorption on oxygen modified Mo2C, prepared by exposure to 1.8 L 02 at 104 K, leads to an even greater asymmetry towards higher frequencies. This indicates that the slight asymmetry in the spectrum of the nominally clean surface may be the result of the presence of some oxygen as indicated by the XPS spectra in Fig. 1. We note that the RAIRS peak for CO on Mo(110), at 2035 cm -1, splits into peaks at 2062, 2025 and 1983 cm -1 on a (2 x 2) O/Mo(ll0) surface [17].

~ Wang et al./Surface Science 374 (1997) 197-207

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Annealing the Mo2C/O surface to 203 K resulted in a narrowing of the linewidth without any loss of integrated intensity. By comparison with the T P D results in Fig. 2, the sequence of RAIRS spectra shown in the inset to Fig. 3 suggests that oxygen modification of the surface leads to a small reduction of the CO desorption energy, with the result that the desorption temperature is lower by approximately 50 K. RAIRS data for N O adsorption are shown in Fig. 4. The spectra acquired on exposing to 3.3 L N O at 104 K display a broad band ( F W H M = 45 cm -1) centered at 1761 cm -1 and a shoulder at

approximately 1806cm -1. The latter feature is more clearly resolved on heating the sample to 246 K. Further annealing causes the second band to shift to lower frequencies and, at 325 K, three poorly resolved bands are observed at 1750, 1725 and 1706 c m - t , respectively. The insert shows the temperature dependence of the sum of the integrated absorbances of the envelope of v(NO) peaks. The v(NO) signal decreases gradually, to zero, over the 250-450 K temperature range. T P D spectra obtained following exposure to N O at l l 0 K reveal high temperature mass 28 desorption features as shown in Fig. 5. There is a

J. Wang et al./Surface Science 374 (1997) 197-207

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broad peak at approximately 1100 K at low exposures (1.2 L). An additional broad peak grows in above 700 K at higher exposures, so that at 20 L there are two strong peaks at approximately 850 and 1050 K, respectively. Mass 14 data show that Ne desorption gives rise to the peak at 850 K and a second peak at approximately l l00K. CO desorption results in the intense peak at approximately 1050 K. These high temperature desorption peaks saturate at approximately 10 L. A weak CO desorption feature is observed at 350 K at low coverages and broadens and shifts down in temperature with increasing NO exposure levels. The

latter peak indicates that atomic oxygen from dissociated NO reacts with the carbide surface to form adsorbed CO at low temperatures. A NO molecular desorption feature appears at 180 K, for exposures above 1.1 L, and no other higher temperature NO desorption feature is observed even at the highest exposures. Low temperature N:O and N2 desorption features are also detected. Possible mechanisms for low temperature non-dissociative NN bond formation from adsorbed NO on oxygen modified tungsten were discussed by Baldwin and Friend 1-34]. They favored a mechanism involving a (NO)a dimer intermediate, and such dimer species

J. Wang et al./Surface Science 374 (1997) 197-207

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Temperature / K Fig. 4. RAIRS spectra of NO on molybdenum carbide as a function of anneal temperature. The initial spectrum, spectrum (a), was obtained by exposure to 3.3 L N O at 104 K. The insert displays the integrated intensity of the sum of all the v(NO) peaks as a function of anneal temperature.

have been detected on NO exposed A g ( l l l ) surfaces [35]. Clearly the surface chemistry behind the low temperature desorption peaks is very complicated and additional work using XPS and TPD is required to resolve it. The present paper deals specifically with the high temperature recombinative N2 and CO desorption peaks. Fig. 5 shows that the recombinative desorption peaks represent, in any case, the majority chemistry of NO on molybdenum carbide foils.

4. Discussion

In the following discussion, a comparison will be made between CO and NO adsorption on fl-

M o 2 C and on carbon modified and pure molybdenum single crystal surfaces, so as to evaluate the effect of carbide formation on the chemisorption properties of molybdenum. In addition, a comparison of CO and NO adsorption on /3-Mo2C and on ruthenium will be made in order to examine the correlation in chemisorption properties between early transition metal carbides and Group VIII noble metals. The high temperature recombinative CO desorption peak (at 1200 K) indicates that some dissociation of CO occurs on fl-Mo2C. This is consistent with the fact that molybdenum carbide is an effective methanation catalyst [3-5], assuming that methanation occurs via the carbide mechanism. Previous TPD studies of carbon modified molyb-

J. Wanget al./Surface Science 374 (1997) 197-207

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Temperature (K) Fig. 5. Temperature programmed desorption data for NO on fl-Mo2C, re~e=28 and 30 data is shown for exposures of 1.2 and 20L. m/e=14 is shown for 20L, and re~e=44 data is shown for an exposure of 4.0 L. denum single crystals [,16,24] have shown that although CO dissociation does not occur on Mo(100)-(lx 1)C some dissociation takes place on Mo(l10)-(4 x 4)C leading to a recombinative CO desorption peak at 1023 K. The effective surface stoichiometry of Mo(100)-(1 x 1)C is MoC and this may account for its inactivity, with respect to/%Mo2C, for CO decomposition. The annealed /~-MozC surface is therefore more closely modeled by the atomically rough Mo( 110)-(4 x 4)C surface• However, it should be kept in mind that the CO recombination peak accounts for at most 10% of the total CO desorption signal. Thus, dissociation at a small concentration of defect sites cannot be ruled out. In contrast, low coverage CO completely decomposes on M o ( l l 0 ) at temperatures above 200 K and molecular desorption only sets in at approximately one quarter of saturation coverage [-36,37]. Desorption of CO from ruthenium single crystal surfaces typically results in two peaks, or a broad feature, in the 400-500 K range [-38-42]. However, very little [40], if any [42], CO dissociation takes place even in TPD studies of CO adsorption on stepped ruthenium single crystals. Thus, the reactivity of molybdenum carbide foil towards CO is closer to that of ruthenium than to that of molybdenum. The CO//%Mo2C molecular desorption peak at

203

330-365 K corresponds to the molecular desorption peaks at 325-350 K reported for Mo(100)/C [,16], M o ( l l 0 ) / C [-24] and for high coverage CO on M o ( l l 0 ) , Mo(l12) and Mo(100) [-28,36,43,16], and probably also corresponds to the peak at 4 0 0 K observed by Lee et al. [-25] for CO on alumina supported molybdenum carbide. More recent work, on alumina-supported molybdenum carbide, by Bussell et al. [44] also reveals a CO TPD peak at 325 K. Thus, similar CO desorption energies apply for a range of carbon modified molybdenum surfaces. In the case of molybdenum single crystals, surface modification arises from low coverage dissociation of CO depositing both oxygen and carbon on the surface. The RAIRS spectrum of CO on Mo2C at 100 K displays a single peak at 2057-2072 cm -1. This single peak RAIRS spectrum contrasts sharply with the multipeak EELS spectra reported for CO on molybdenum single crystals. For example, Zaera et al. [-28] found that adsorption of carbon monoxide on Mo(100) at 80 K produces (CO) loss peaks at 2100, 1235 and 1065cm -1. The two species giving rise to the low frequency losses were identified as precursor states to molecular decomposition above 230 K, and the 2100 cm -1 peak was attributed to on-top bonded CO. Colainni et al. [-36,37] observed four electron energy loss peaks for CO on Mo(ll0), at 1920-2055, 1500, 1345 and l l 3 0 c m -1, respectively. The band at 1130 cm -1, which appeared on annealing to 150 K, was attributed to CO bonded through both atoms to the metal surface. This fiat-lying species was unequivocally identified as a precursor to CO decomposition. The species giving rise to the 1345 cm -~ band was attributed to tilted CO by comparing with data for organometallic complexes. Colainni et al. [-36,37] correlated the onset of molecular desorption with the emergence of the 1920cm -1 loss peak characteristic of vertical bridge bonded CO. The frequency of the RAIRS band for CO on /3MozC (2060-2070cm -1) is clearly in the range typical of on-top adsorption on metal surfaces, and the observed frequency is intermediate between the values of 2033 and 2083 cm 1, reported for on-top adsorption in a RAIRS study of clean and carbon modified M o ( l l 0 ) , respectively 1-17]. It is particularly interesting to note that the observed stretch-

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ing frequency for CO on -Mo2C is identical to that recently measured for CO on alumina-supported molybdenum carbide particles [44]. A previous study [45] of CO on M o ( l l 0 ) showed that RAIRS is not sensitive to side-on bonded CO, hence our RAIRS spectra cannot be used to rule out such an adsorption geometry on fl-MozC. However, the electron energy loss (EELS) results for carbon modified Mo(110) by Fruhberger and Chen [24] show only a very weak peak at 1150 cm -1 due to "side-on" bonded CO, in comparison to that observed for CO on clean M o ( l l 0 ) . Similarly, Chen et al. reported offspecular EELS measurements showing the presence of flat-lying CO on V ( l l 0 ) but only on-top bonded CO on VC/V(110) films [ 19]. It is interesting to compare the CO vibrational data for our samples with those for the Mo(l10)-(4 x 4 ) C surfaces studied by Fruhberger and Chen [24] and He et al. [17]. The latter surfaces differ with respect to the carbon to molybdenum ratio, the sample studied by He et al. displaying an AES C(273 eV)/Mo(186 eV) ratio of 0.25 compared to the ratio of 0.17 for the sample studied by Fruhberger and Chen. The CO stretching frequency, 2062 cm-1, for CO on -Mo2C is closer to the value of 2083 c m - 1 CO for the more carbon rich M o ( I I 0 ) - C surface [-17]. Furthermore, in addition to a feature at 2015 cm -1, Chen et al. reported a band at 1850 cm -1 which they attributed to bridge bonded CO. The latter band is not observed in the present study of fl-Mo2C or for the carbon rich Mo(110)-C sample studied by He et al. These differences evidently relate to the details of sample preparation. For example, Chen, in a recent review article [ 14], emphasizes the fact that the chemisorption properties of carbides are very sensitive to whether the carbon is present on the surface or in the interstices of the metal. The correlation between early transition metal carbides and Group VIII-X metals is usually attributed to an increase in the electron-to-atom ratio in carbides [46]. The interstitial carbon contributes sp-electron density making Mo2C, for example, more electron rich than Mo. This generalization does not imply charge transfer from carbon to the metal. In fact, for example, XPS [48] and NEXAFS [14,21,47] measurements show charge

transfer from metal to the carbon. The ionicity of the metal-carbon bond decreases on going from the Group IVa to the Group Via metal carbides [48,49]. Chen et al. [14,21] determined, on the basis of NEXAFS measurements on metal carbides and oxides, that the oxidation states of V in VC and Mo in a carbide overlayer on M o ( l l 0 ) are 1.2 and 0.2, respectively. The valence band of WC, a carbide which displays platinum-like catalytic activity, has been studied by several groups [50-53]. The general consensus is that the monocarbide WC displays a higher density of filled states near the Fermi level and a broader unfilled valence band than pure tungsten. We note the C(ls) binding energy for WC is close to that for MozC and that the binding energy depends on the ionicity of the metal-carbon bonding [48,49]. Hence, we assume that, qualitatively, a similar valence band structure holds for Mo2C as for WC. The results for CO on fl-MozC show that although CO dissociation takes place at low coverages, a molecular desorption channel exists for all coverages studied. Thus, we assume that molecular desorption of CO is an intrinsic characteristic of molybdenum carbide as it is for ruthenium surfaces. It is well known that transition metals to the left of the periodic table are able to readily dissociate CO. Mehandru and Anderson [54,55] proposed that CO could adopt flat-lying geometries on metals such as Cr, due to the energetic gain of llr, 4e and 5t7 bonding to the less than half full dband. In fact, Fulmer et al. [58], in an ARUPS study of CO on Mo(100), found that low coverage CO is tilted by about 40 ° and bonds to the surface via the 1~ and 5tr orbitals. Flat-lying or tilted configurations enhance overlap with the 2~* orbital thereby facilitating dissociation. For example, Colainni et al. [37] have unequivocally shown that flat-lying CO on M o ( l l 0 ) is a precursor to dissociation. The ~/~ bonding of the type proposed by Mehandru and Anderson may not be favorable for the electron rich (with respect to Mo) Mo2C, and therefore horizontal or tilted states are not populated. The Fermi level of molybdenum lies close to the centroid of the d-band [56]. If we assume that the Fermi level of MozC lies further above its d-band centroid, then flat-lying or tilted CO may not be favored on Mo2C. However, VC

J. Wang et al./Surface Science 374 (1997) 197-207

also does not display flat-lying CO although V is less electron rich than Mo. Hence, the effective oxidation state of the metal may be the more important parameter. In contrast to the partial activity of CO towards decomposition on fl-MozC, NO undergoes extensive decomposition. The molecular dissociation of NO on fl-MozC results in the observation of one or two, depending on the coverage, high temperature recombinative desorption peaks. Fig. 5 shows that these peaks result from the interaction of adsorbed atomic oxygen from NO with the carbide carbon, to form carbon monoxide, and from the recombination of atomic nitrogen. Fulmer et al. [58] have shown that both dissociation and molecular desorption of NO occurs on Mo(100). Similarly, dissociation of NO on Ru(001) occurs at low coverages followed by molecular desorption, at 440 K, at higher coverages [59,60]. The high coverage NO desorption peaks from Mo(100) and Ru(001) are attributed to site blocking by the decomposition products Nads and Oad s [58,59]. Apparently, such site blocking is less effective on Mo2C as only a very low temperature NO desorption peak, at 180 K, is observed. Furthermore, the high temperature desorption peaks, arising from NO dissociation, only saturate at the saturation point for NO uptake. That is, dissociation occurs for all exposures of NO up to the saturation exposure of approximately 10 L. A possible explanation for the absence of a strong site-blocking effect for Mo2C may lie in the ability of carbides to incorporate oxygen [29]. It may also be simply related to the fact that polycrystalline surfaces are typically more reactive than low index single crystal surfaces. In the latter context, we note the report by Freyer et al. [61] that almost complete dissociation of NO occurs on Ru(1010) in contrast to what is observed for Ru(001). It should also be noted that atomic oxygen from NO reacts away some of the surface carbon, as evidenced by the weak CO desorption peak at 250-350 K. The latter process may serve to replenish active sites for NO dissociation. The RAIRS spectrum of NO on M o z C at 104 K displays a broad peak at 1758 cm -1 and a poorly resolved shoulder at approximately 1800cm -1. NO on Ru(001) displays a stretching frequency in

205

the region 1790-1820cm -1 [59,60,62,63]. Such frequencies are characteristic of on-top bonded NO, at least from the perspective of the vibrational spectra of nitrosyl complexes [64]. However, tensor LEED studies have shown that the determination of NO adsorption geometries on the basis of vibrational frequencies is not possible [65,66]. The high frequency feature becomes better resolved as the sample temperature is raised. Its stability to temperatures greater than 246 K shows that it may not be attributed to adsorbed (NO)/, or its product NzO, since N20 desorbs at 180 K. Hence, it is possible that NO adopts at least two bonding configurations on clean /~-Mo2C. If we assume, following the discussion of Ranhotra et al. [3], that the (101) plane predominates then two distinct Mo sites are readily available for NO adsorption. One accessible Mo site is located 0.4 ~, below the outermost Mo atoms on the (101) surface [3]. However, the same should then hold true for CO on/~-Mo2C yet the RAIRS data for CO are consistent only with the existence of one state, unless the asymmetry in the v(CO) peak actually reflects the presence of a second state. The multiple RAIRS peaks which grow in on NO exposed/~-Mo2C are attributed to the influence of coadsorbed atomic nitrogen and oxygen formed through NO decomposition. For example, Chen et al. [67] showed that coadsorbed oxygen on Ni(111) leads to three oxygen induced v(NO) bands in the 18001900 cm- 1 region at 85 K. Similarly, preadsorption of oxygen on Ru(001) leads to a splitting of the RAIRS feature assigned to on-top bonded NO [59]. Hence, it is possible that the shoulder at 1800cm -1 results from some NO dissociation which occurs even at 104 K. The chemisorption bond between NO and Mo2C is evidently a strong one, since NO desorption does not compete with dissociation. The temperature dependent RAIRS spectra in the insert to Fig. 4 show that annealing to 475 K is required to remove all of the NO signal. On the basis of a Redhead analysis, the desorption energy for NO on fl-Mo2C is at least 30 kcal mol-1. Molecular desorption of NO from Ru(001) and Pt(111) takes place at 440 K and in the 300-400 K range, respectively [59,69]. Koel et al. [68,69] argued that the covalent interaction between the LUMO electron

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J. Wang et al./ Surface Science 374 (1997) 197-207

and the metal is chiefly responsible for the strength of the NO-metal bond on P t ( l l l ) . Further, they pointed out that singly occupied metal orbitals are required for the formation of this covalent bond. The present results indicate that the NO chemisorption bond on molybdenum carbide is at least as strong as that for the two noble metal surfaces.

5. Conclusions The study demonstrates that RAIRS is an effective technique for the study of bulk carbides prepared using the temperature programmed reaction method. Vibrational spectroscopy measurements of adsorbed NO and CO were used to probe the chemisorption properties of molybdenum carbide. Spectra consistent with on-top bonding were obtained for CO. Complementary thermal desorption results were used to show that almost all adsorbed NO dissociates on molybdenum whereas at most 10% of adsorbed CO undergoes decomposition. The results imply, but do not prove, that carbide formation restricts the number of chemisorption bonding configurations available to CO. A possible mechanism is that the hybridization involved in carbide formation ties up some of the specific orbitals which would take part in effective n/a bonding and/or 2n* backdonation. Hence, carbide formation renders the metal less flexible in its response to simple n-acceptor adsorbates such as CO. NO forms a strong bond with the fl-Mo2C surface to the extent that desorption does not compete with dissociation. The comparison of our results with data for carbon modified metal surfaces show that the latter systems are adequate models for transition metal carbide catalysts. In particular, the CO stretching frequency measured for molybdenum carbide foil is identical to that measured for CO on alumina-supported molybdenum carbide particles [44].

Acknowledgement This study was supported by grants from NSERC (Canada) and FCAR (Quebec). M.C.

acknowledges the receipt of an NSERC graduate fellowship.

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