Adsorption and reaction of CO on vanadium oxide Pd(111) inverse model catalysts: an HREELS study

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1 Topics in Catalysis Vol. 14, No. 1-4, Adsorption and reaction of CO on vanadium oxide Pd(111) inverse model catalysts: an HREELS study M. Sock, S. Surnev, M.G. Ramsey and F.P. Netzer Institut für Experimentalphysik, Karl-Franzens-Universität Graz, A-8010 Graz, Austria Dedicated to Gabor A. Somorjai, in honour of his 65th birthday The room temperature adsorption and reaction of CO on Pd(111) surfaces decorated with submonolayer coverages of vanadium oxide so-called inverse model catalysts have been studied by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The HREELS surface phonon spectra of the V oxide phases have been measured and used to monitor the changes in the oxide as a result of the interaction with CO. The intramolecular C O stretching frequency of CO adsorbed on the V- oxide/pd(111) surfaces displays two vibrational loss components as a function of CO coverage as it has been observed on the clean Pd(111) surface. The relative intensities of the two vibrational features as a function of V oxide coverage however suggest that the balance of CO adsorption sites is modified as compared to clean Pd(111) by the presence of the V oxide Pd phase boundary. Preferential population of high coordination adsorption sites by CO in the vicinity of the oxide metal interface is proposed. The analysis of the V oxide phonon spectra indicates that adsorbed CO partially reduces the V oxide at the boundaries of the oxide islands to the Pd metal. The reduction of V oxide by CO is dependent on the oxygen content of the V oxide phase. The reduction of V oxide is confirmed by the XPS V 2p core level shifts. KEY WORDS: CO adsorption; CO reaction; vanadium oxide Pd(111) catalyst; catalyst surface; HREELS; XPS 1. Introduction There has been an enormous effort over the years to develop a detailed atomic picture of chemical interactions at surfaces which are relevant in the field of heterogeneous catalysis [1]. Information about elementary processes in catalytic reactions have been obtained from studies with well-defined single crystal surfaces using techniques from surface physics. However, surfaces of real catalysts are typically inhomogeneous, and model systems which mimic the heterogeneous character are needed to bridge the socalled materials gap, which separates studies on idealised single crystal metal surfaces and on catalysts consisting of oxide supported metal particles. Here we report results from a planar model system, which consists of a metal single crystal surface which is decorated with submonolayer quantities of an oxide phase. This model system contains the important metal oxide interface and oxide metal phase boundaries, and possible active sites associated with them, and yet allows one to use surface science techniques to perform molecular level studies of elementary reaction steps. In view of the complementary character of this oxide-on-metal system as compared to real catalysts, which involve metals on oxide substrates, it may be regarded conceptually as an inverse or inverted catalyst model system. The so-called inverse catalyst surface differs from the frequently used catalyst model systems that are based on metal particles on thin oxide layers, which in turn are grown on metal substrates [2], in one particular aspect: the small metal particles deposited on oxide substrates exhibit a large number of low-coordinated metal sites at steps, kinks, and corners, whereas in the inverted system considered here the metal surface is essentially flat with a low defect density. Thus, the influence of the metal oxide interface on the physico-chemical properties of this heterogeneous surface may be focussed upon, and complementary information to that from real catalyst model systems may be obtained. In this paper we report a study of the adsorption of CO on a vanadium oxide Pd(111) inverse catalyst surface as investigated by vibrational spectroscopy using high-resolution electron energy loss spectroscopy (HREELS). The inverse catalyst surfaces used here consist of vanadium oxide island structures of nanometer dimensions deposited by reactive evaporation of vanadium on to a Pd(111) surface. CO is widely used as a probe molecule for heterogeneous surfaces since it provides a good test case for the modifications of the electron distribution of surface sites. Since CO does not chemisorb on vanadium oxide at room temperature [3], it is well suited as a molecular probe of the Pd sites at the V- oxide/pd surface. The intramolecular C O bond and therefore the ν CO stretching frequency is affected by the adsorption site, with ν CO decreasing with increasing coordination to the metal substrate [4 6], and HREELS is therefore well suited to follow the possible modifications of the CO adsorption site distribution, which may be introduced by the V-oxide phase boundaries, as compared to CO adsorption on the clean Pd(111) surface. Moreover, the oxide phonon spectra are also accessed by HREELS and provide very sensitive fingerprints of the structure and stoichiometry of the various V oxide phases, which are formed on Pd(111) under particular preparation conditions [7]. We have therefore used HREELS to characterise the V oxide modifications, which /00/ $18.00/ Plenum Publishing Corporation

2 16 M. Sock et al. / CO on vanadium oxide Pd(111) surface occur during the reaction with adsorbed CO, via their respective phonon spectra. Complementary results from highresolution X-ray photoelectron spectroscopy (HR-XPS) using synchrotron radiation, confirming and specifying the reaction between CO and V oxide on Pd(111), are included in this paper. The adsorption of CO on V-oxide/Pd(111) surfaces has been investigated previously by our group with synchrotron radiation HR-XPS [3]. We found, monitoring the C 1s and Pd 3d core level spectra as a function of V oxide coverage and CO exposure, that the adsorption of CO is kinetically influenced by the V oxide phase via spill-over of CO molecules from a mobile physisorbed precursor state on the oxide surface on to Pd sites. The analysis of the C 1s binding energies suggested that the distribution of CO adsorption sites may be altered by the presence of the V-oxide/Pd(111) phase boundary, however no definitive conclusions could be drawn from these data, since the small C 1s binding energy shifts were at the limits of the spectral resolution [3]. The present HREELS study was undertaken to shed further light on to the question of the CO adsorption site distribution on Pd(111) in the presence of the V oxide Pd interface and on the interactions of CO with the oxide metal phase boundary. We find that the adsorption site distribution of CO is indeed influenced by the V oxide island structures on Pd(111), but in a rather subtle way, and observe no vibrational signature for strongly modified CO molecules as if bonded to Lewis acid-type sites at the V oxide Pd boundary, as it has been suggested for oxide-decorated Rh surfaces [8,9]. However, evidence for a reduction of the V oxide by CO adsorbed at the perimeters of the oxide islands is reported. 2. Experimental The HREELS experiments were carried out in a customdesigned three-level UHV spectrometer chamber with a base pressure of mbar, which is equipped with a four-grid LEED optics for LEED and Auger electron spectroscopy (AES), and an ErEELS 31 spectrometer for HREELS as described previously [10]. The UHV system contains a preparation chamber in addition to the spectrometer chamber, where surface cleaning and preparation procedures are performed. The samples can be transferred between the two chambers via a magnetically coupled transfer system. The HREELS measurements were performed with a primary energy of 5.5 ev, in a specular reflection geometry θ in = θ out = 60, with a typical resolution of 4.3 mev as measured at the FWHM of the reflected primary peak. The HR-XPS spectra were measured at beamline 22 at the Swedish synchrotron radiation facility MAX I in Lund. The setup contains a modified SX-700 monochromator in conjunction with a large hemispherical electron energy analyser of the Scienta type. The V 2p spectra reported here were recorded with a photon energy hν = 600 ev with a total resolution of 400 mev. The Pd(111) surface has been cleaned by a standard procedure, i.e., Ar ion bombardment and annealing at 750 C; LEED and AES were used to check surface order and cleanness. The V oxide island structures were prepared by reactive evaporation of vanadium from an electron beam evaporator in mbar O 2 onto the Pd(111) substrate, which was heated to 250 C. The evaporation rate employed was 0.2 monolayers V min 1 as measured by a quartz crystal microbalance. Since the true oxide surface coverage depends on the oxide stoichiometry and morphology, which both change with the amount of oxide present at the surface, the oxide coverages cited in this paper are given in monolayer equivalents (MLE) of V; i.e., they correspond to the evaporated amount of V, which has been converted into monolayers based on the atomic density of the Pd(111) surface (1 ML = Vatomscm 2 ). A surface completely covered with V oxide, so that no CO adsorption could be detected, corresponded to a nominal oxide coverage of 0.75 MLE in the present experiments. Note that this coverage is about a factor two less than that cited in our recent XPS paper [3], where 1.4 MLE V oxide on Pd(111) were necessary to suppress CO adsorption. The reason for this discrepancy lies presumably in the different evaporation and microbalance geometries in the two experimental setups. CO was dosed from the system ambient via a leak valve and the exposures are given in Langmuir (1 L = Torr s). 3. Results and discussion 3.1. Characterisation of V-oxide/Pd(111) surfaces by HREELS The growth behaviour of V oxide on Pd(111) is very complicated since structure, morphology, and stoichiometry of the oxide phases vary with the oxide coverage and with annealing conditions after deposition, as reported in a recent STM study [11]. At low coverages, the oxide grows in both random island and step flow modes and forms a twodimensional layer with a porous fractal-type network structure, as shown in the scanning tunneling microscopy (STM) image of figure 1 for θ OXIDE = 0.25 MLE [11]. The oxide phase displays the brighter contrast while the free Pd(111) surface areas are darker, as indicated in the cartoon below the STM image. The branches of this oxide network are ordered on the atomic scale and show a p(2 2) honeycomb structure, which corresponds to a new interface-stabilised surface V-oxide phase with a formal V 2 O 3 stoichiometry, as revealed recently by ab initio density functional (DFT) total energy calculations [11]. This surface-v 2 O 3 layer consists of a hexagonal V layer in direct contact with the Pd(111) surface and of bridging oxygen atoms forming the outer oxide surface. Above a critical coverage of MLE the second layer of oxide becomes populated while the first layer is still incomplete. This surface consists of the surface-v 2 O 3 layer topped by a VO 2 -type phase and free Pd areas [12,13]. After annealing from 250 to 300 C the V-oxide/Pd(111) surfaces in the coverage range MLEdisplayacomplex well-ordered LEED pattern (called the flower pattern), which results from the superposition of the p(2 2)

3 M. Sock et al. / CO on vanadium oxide Pd(111) surface 17 V-oxide Figure 2. HREELS phonon spectra recorded in specular scattering geometry of V-oxide/Pd(111) surfaces: (a) p(2 2) surface-v 2 O 3 monolayer; (b) mixed V-oxide layer (0.5 MLE) containing VO 2 and surface-v 2 O 3 oxide phases; this surface displays a complex flower-type LEED pattern; (c) epitaxial bulk-type V 2 O 3 oxide layer (5 MLE, heated to 350 C) with a ( 3 3)R30 LEED pattern. Pd(111) Figure 1. Constant current topographic STM image of the 0.25 MLE V- oxide/pd(111) inverse catalyst surface. (Tunneling conditions: 50 pa tunneling current, 2.5 V sample bias; Å 2.) The cartoon below the image illustrates the oxide island structures and the free Pd areas. structure and an ordered VO 2 -type phase [13,14]. At higher coverages (>2 MLE) the V oxide layer transforms into a bulk-like V 2 O 3 phase [13]. Heating MLE of V oxide from 300 to 350 C converts the surface with the flower pattern into a surface, where the oxide layer wets the entire Pd surface in a unifom way, covering it with a monolayer of the p(2 2) surface-v 2 O 3 oxide phase [13]. This latter surface has initially been associated with a VO-type oxide phase [13], which however has been identified in our later study as surface-v 2 O 3 [11]. Figure 2 shows HREELS spectra of the phonon region of three representative V-oxide/Pd(111) surfaces as discussed above. Figure 2(a) is from 0.75 MLE V-oxide/Pd(111), deposited at 250 C and subsequently annealed to 350 C, thus forming of a uniform p(2 2) surface-v 2 O 3 monolayer. The phonon spectrum consists of a single peak at 68 mev, which is well understood and due to the Ɣ-point dipole-active optical phonon mode of this structure as shown by DFT calculations [11]. Figure 2(b) displays the HREELS spectrum from the mixed oxide phase containing the surface-v 2 O 3 and VO 2 oxide phases (0.5 MLE V-oxide/Pd(111) heated to 300 C), which shows the flower pattern in LEED. In addition to the 68 mev loss of the p(2 2) structure this spectrum con- tains phonon losses at 46, 107, and 127 mev. The VO 2 part of this surface has not been analysed theoretically yet, but for the present fingerprinting purpose we note that the loss structure at 107 appears to be characteristic of the VO 2 -type oxide phase. The HREELS spectrum of figure 2(c) has been obtained from 5 MLE V-oxide evaporated at 250 Cand heated to 350 C, representing epitaxially grown bulk-like V 2 O 3 island structures with a ( 3 3)R30 LEED pattern [13]. The phonon spectrum is complex with major peaks at 52, 78, 92, and 130 mev. Again, this spectrum will be used in a fingerprinting way to characterise the V 2 O 3 phase with bulk-type structure. Figure 3 shows a series of HREELS phonon spectra recorded after evaporation of V oxide at 250 C as a function of the oxide coverage. A quantitative evaluation of the complex changes of the phonon spectra occurring with increasing oxide coverage is beyond the scope of this paper, but the figure illustrates the evolution of oxide phases with coverage. At low coverages, the surface-v 2 O 3 phase is predominant, then the VO 2 -type oxide is formed for θ OXIDE = MLE, and the V oxide layer transforms then into the bulktype V 2 O 3 phase at higher coverages. We conclude therefore that the phonon signature in HREELS is very sensitive to the particular oxide structures and the evolution of the various V oxide phases with coverage and temperature as seen in previous STM and HR-XPS studies [11,13] is well reflected in figure 3. We will use the phonon fingerprints in this study to follow the changes of the V oxide phases in the course of the reaction with CO.

4 18 M. Sock et al. / CO on vanadium oxide Pd(111) surface 3.2. Adsorption of CO on V-oxide/Pd(111) surfaces Figure 4 compares HREELS spectra of the ν CO stretching frequency region for CO adsorption on the clean Pd(111) surface (a) and on the 0.4 MLE V-oxide/Pd(111) surface (b), normalised intensity s-v 2 O 3 VO 2 b-v O b-v 2 O bulk V 2 O 3 VO 2 surface-v 2 O Energy Loss [mev] V-oxide coverage 2.4 MLE 1.0 MLE 0.75 MLE 0.5 MLE 0.25 MLE Figure 3. HREELS phonon spectra of V oxide layers on Pd(111) as a function of oxide coverage, illustrating the evolution of oxide phases during growth at 250 C. as a function of CO exposure at room temperature. The 0.4 MLE V-oxide/Pd(111) surface contains approximately 50% of free Pd areas, the loss peaks in (b) have therefore been adjusted arbitrarily to display similar size as in (a) to facilitate the comparison. They are however consistently scaled within the series (b). On clean Pd(111) the ν CO loss is observed at 226 mev at low coverages (0.2 L exposure). With increasing CO dose the peak shifts to higher loss energies and broadens asymmetrically towards higher energy. After L CO exposure the ν CO structure consists of two components, one at mev and the other one at 234 mev. At this exposure, the ( 3 3)R30 structure observed in LEED reaches its maximum intensity, indicating a CO surface coverage of 0.33 ML at this point. With increasing CO dose the overall ν CO peak maxima continue to shift to higher loss energies, reaching a value of 238 mev at CO saturation (2 L exposure). The ν CO peak at CO saturation is sharper than at intermediate coverages, but the tailing towards lower loss energies still signals the presence of a second minority peak component. LEED for saturation CO adsorption at room temperature shows a diffuse ( 3 3)R30 pattern with split spots, as has been reported in the literature [15]. The HREELS data of figure 4(a) are in excellent agreement with the earlier IR spectroscopy results on the CO/Pd system of Bradshaw and Hoffmann [15], who have interpreted the vibrational energies and their shifts with CO coverage in terms of localised site adsorption and intermolecular interactions, suggesting the population of threefold hollow sites in the ( 3 3)R30 structure and of bridge sites at the room temperature CO saturation coverage of 0.5 ML. Very recently, Surnev et al. [16] have reinves- Figure 4. HREELS spectra of the C O stretching vibration as a function of CO exposure at room temperature. (a) Clean Pd(111); (b) 0.4 MLE V- oxide/pd(111).

5 M. Sock et al. / CO on vanadium oxide Pd(111) surface 19 tigated the CO/Pd(111) system using HR-XPS, LEED and HREELS, with a view towards solidifying the starting basis for the present CO adsorption experiments on the Pd(111)- based inverse catalysts. They concluded that both hollow and bridge sites become occupied in the dense room temperature CO adlayer on Pd(111), which is poorly ordered and contains many antiphase domain boundaries [16]. Accordingly, the ν CO structure at the lower loss energies, mev, has been associated with CO molecules adsorbed in highly coordinated fcc threefold hollow sites, whereas the higher energy ν CO feature at mev has been ascribed to CO in the antiphase domain boundaries of the split ( 3 3)R30 structure. It has been conjectured that this CO phase contains bridge sites. Note that dipole dipole coupling overestimates the higher frequency components [17] Figure 5. Curve fitting analysis of the ν CO loss peak for CO saturation coverage (2 L) on various V-oxide/Pd(111) surfaces (see text). The insert is a plot of the ratios of the areas of peak 2 : peak 1 as a function of V oxide coverage. such that the relative intensities in HREELS, as seen in figure 4(a), do not reflect the correct site distribution as revealed by the quantitative evaluation of the C 1s and Pd 3d core level spectra [16]. CO adsorption on the V oxide decorated Pd(111) surfaces follows a similar general trend as on clean Pd(111), as illustrated by the HREELS ν CO spectra (figure 4(b)) from the 0.4 MLE V-oxide/Pd(111) surface. Overall, the loss energies are similar, the structures shift to higher loss energies with increasing CO doses, and there is a clear indication of two vibrational loss components at intermediate coverages (see the spectra after 0.8 and 1 L doses). However, closer inspection reveals also significant differences between figures 4 (a) and (b): consider the top spectra of figure 4, obtained for CO saturation coverage, where it is apparent that the tail to lower loss energies is clearly more intense in panel (b) than in panel (a). This suggests that the distribution of CO adsorption sites on the Pd areas may be influenced by the presence of the V oxide phase. It is emphasised, however, that strongly modified CO adspecies, such as expected if attached to Lewis acid centers at the oxide metal boundary, are clearly not observed here on the V-oxide/Pd(111) surfaces. The latter have been proposed by Boffa et al. for a number of transition metal oxides, including V-oxide, on Rh surfaces [8,9]. To gain further insight it is useful to analyse the experimental loss peaks of figure 4 by decomposition into two components. A combination of Lorentzian and Gaussian lineshapes (Voigt functions) has been used as model functions to fit the data, representing the vibrational excitation process by a Lorentzian lineshape and the experimental loss function by a Gaussian profile. Figure 5 displays the results of this curve fitting procedure for CO saturation coverage for V-oxide/Pd(111) surfaces with different oxide coverages; for the convenience of presentation the spectra have been normalised arbitrarily to approximately equal size, but the absolute intensities decrease from bottom to top with increasing V oxide coverage as a result of the decrease of the free Pd area. Using Lorentzian linewidths of 2.6 mev and Gaussian linewidths of mev yields good fits to the data as illustrated by the solid line drawn through the data points. The peak position of the higher energy peak 2 remains within 1 mev constant irrespective of the V oxide coverage, but the position of the lower energy peak 1 varies more, shifting to lower loss energy by 3 mev in going from clean Pd(111) to the 0.4 MLE V oxide covered surface. The insert of figure 5 gives the ratio of the areas of the two vibrational components as a function of V oxide coverage for CO saturation coverage. The ratios of peak 2/peak 1 vary by about a factor of 3, with peak 1 increasing in relative intensity with increasing V oxide surface coverage. This suggests that the fcc hollow CO adsorption sites are increasingly populated in the presence of the V oxide island phase at the expense of CO in antiphase domain bridge sites. A change of the CO adsorption site distribution mediated by the oxide phase, with a reduction of CO molecules in antiphase domain boundaries, has also been tentatively inferred in our recent XPS study of

6 20 M. Sock et al. / CO on vanadium oxide Pd(111) surface Figure 6. Plots of the areas of the two vibrational loss components (peaks 1 and 2), obtained by the curve fitting analysis, as a function of CO exposure. Panel (a): lower energy peak 1; panel (b): higher energy peak 2. the same system to account for the changes of the C 1s core level shifts [3]. Figure 6 shows plots of the areas of the two vibrational loss components, obtained by the curve fitting analysis as described above, as a function of CO exposure for various V oxide coverages. This figure emphasises the absolute intensities of the two vibrational components on the different surfaces. As on clean Pd(111) the fcc hollow sites (peak 1) are also populated first on the V-oxide/Pd(111) surfaces. For CO exposures in the range L, corresponding to the fully developed ( 3 3)R30 structure, the bridge sites corresponding to peak 2 become increasingly occupied and there are indications of some transfer of CO between the sites with increasing CO coverage, both on the clean Pd(111) and the V-oxide/Pd(111) surfaces. This site transfer is quantitatively less prominent than suggested by the intensities of peaks 1 and 2 in HREELS it has to be kept in mind that the vibrational spectroscopy intensities are not always a quantitative measure of the adsorbate coverage due to intermolecular coupling effects but in the present case it has been confirmed by recent XPS analysis [16]. The relatively increased CO population of high coordination Pd sites in the presence of the oxide metal interface may be caused by two effects. First, the oxide metal interface might influence the overall balance of antiphase domains in the CO adlayer on the Pd(111) areas by provid- Figure 7. HREELS spectra of (a) the pristine 0.4 MLE V-oxide/Pd(111) surface, (b) this surface exposed to 4 L CO, and (c) after heating the CO covered surface to 150 C. ing additional boundary lines. Second, the CO molecules adsorbed at or near the oxide metal phase boundary prefer high-coordination sites. As mentioned above it is conjectured that the antiphase domain boundaries contain mostly bridge sites [16]. In view of the relative increase of CO occupation of high coordination sites with increasing oxide metal island boundaries we tend to favour the latter possibility. Whether the preferred high coordination adsorption sites at the oxide metal boundary are the usual Pd(111) fcc hollow sites or particular interface sites is difficult to decide, since the adsorbed CO at the interface seems to exhibit very similar vibrational properties as on fcc hollow sites. It is worthwhile noting, however, that there is a small shift of the corresponding loss feature (peak 1) to lower energies at high V oxide coverages, which might reflect the higher coordination environment at the interface Reduction of V oxide on Pd(111) by CO Figure 7 shows full range HREELS spectra of the pristine 0.4 MLE V-oxide/Pd(111) surface (curve a) as prepared by reactive V evaporation at 250 C, after dosing this surface with 4 L CO (b), and after heating the CO covered surface to 150 C, thus desorbing the adsorbed CO (c). On the

7 M. Sock et al. / CO on vanadium oxide Pd(111) surface 21 pristine surface the intense loss features in the energy range from 40 to 140 mev with peaks at mev, 107 mev, 116 mev, and 128 mev originate from the oxide phonon excitations. The multitude of structures indicates that the oxide deposit is heterogeneous, with a mixture of phases containing surface-v 2 O 3 (loss peak at 68 mev), VO 2 (loss peak at 107 mev), and presumably a precursor phase to bulk- V 2 O 3 (loss peaks at 60, 116, 128 mev); the latter is also seen during the growth of the oxide layer in figure 3. The weak loss structure at 226 mev is due to residual CO adsorption from the system ambient. After exposure to 4 L CO the ν CO stretching vibration at 238 mev appears prominently and there is new loss intensity at mev due to the Pd CO stretching vibration in this range. More interesting, however, are the changes observed in the phonon loss region: the loss peak at 107 mev has decreased in intensity and the losses at 116 and 128 mev have exchanged intensity. On heating the surface to 150 Ctheν CO loss disappears due to CO desorption (apart from a trace due to remnant readsorption of CO) and so does the 107 mev loss, whereas the 68 mev peak shoots up in intensity. We recall that the 107 mev structure is characteristic of VO 2, while the 68 mev loss is due to the dominant optical phonon excitation of the surface-v 2 O 3 layer. The results of figure 7 thus demonstrate unambiguously that the adsorption of CO leads to a partial reduction of the V oxide on Pd. The VO 2 phase appears to be reduced to surface-v 2 O 3, and some subtler changes occur in the bulk-type V 2 O 3 precursor phase. The reaction between CO and V oxide becomes enhanced at elevated temperature as seen in figure 7(c), where the disappearance of CO from the surface is both due to CO and CO 2 desorption, the latter after reduction of the oxide. Curves a and b of figure 8 illustrate the situation for a low-coverage 0.13 MLE V-oxide/Pd(111) surface, whereas curves c and d of figure 8 are from a fully oxide covered Pd(111) surface, with a 2.4 MLE V-oxide layer. The 0.13 MLE V-oxide/Pd(111) surface contains mainly the surface-v 2 O 3 phase as demonstrated by the preponderant 68 mev phonon structure (figure 8(a)). Upon CO adsorption the CO derived vibrations at around 40 mev and 239 mev are observed, but no changes of the 68 mev phonon loss are detectable (figure 8(b)). On heating to 150 C, the CO desorbs from the surface without causing any alteration of the phonon structure (not shown). This indicates that the surface-v 2 O 3 phase is not reduced by CO adsorption. The 2.4 MLE V-oxide/Pd(111) surface contains the bulk-type V 2 O 3 oxide phase as evidenced by its HREELS phonon signature (figure 8(c)). Exposure to 4 L CO causes no changes in the HREELS spectrum (figure 8(d)), thus demonstrating that CO does not chemisorb on V oxide at room temperature. Also, the presence of CO in the gas phase does not lead to the reduction of the V oxide layer (at least not under UHV compatible pressure conditions as employed in this study). Figure 9 presents V 2p 3/2 XPS core level spectra of the pristine 0.5 MLE V-oxide/Pd(111) surface, after exposure of this surface to 5 L CO, and after desorbing CO by heat- Figure 8. HREELS spectra of pristine and CO exposed V-oxide/Pd(111) surfaces. (a) pristine 0.13 MLE V-oxide/Pd(111); (b) surface (a) exposed to 4 L CO; (c) pristine 2.4 MLE V-oxide/Pd(111); (d) surface (c) exposed to 4LCO. ing to 250 C. The V 2p 3/2 XPS structure reveals a broad asymmetric peak shape, which is due to two components corresponding to the surface-v 2 O 3 phase at ev binding energy and to the VO 2 phase at ev binding energy [13]. On exposure to CO the whole V 2p 3/2 structure shifts to lower binding energy by 0.3 ev, and after heating of the CO covered surface to 250 C an additional shift of 0.5 ev to lower binding energy is observed. The shift of the V 2p core level structure is a clear indication of the partial reduction of the oxide: the overall spectral shift is the result of the intensity decrease of the VO 2 component and the intensity increase of the surface-v 2 O 3 core level component. The XPS data thus corroborate the interpretaton of the HREELS results. The messages of figures 7 9 are clear: CO adsorbed on the inverse catalyst surface reacts with the oxide phase and leads to a partial reduction of the oxide. Since no direct reaction of the oxide with CO from the gas phase takes place, and since CO adsorption occurs only on the exposed metal surface, the reduction of the oxide deposit can occur only by reaction of adsorbed CO at the boundaries of the oxide islands. Reduction takes place at the edges of the VO 2 phase and the precursor phase to bulk-type V 2 O 3, whereas the surface-v 2 O 3 phase does not react with CO. (We note parenthetically that the bulk-type V 2 O 3 oxide phase is not formed for oxide coverages <1.5 2 MLE, thus it does not

8 22 M. Sock et al. / CO on vanadium oxide Pd(111) surface Intensity [a.u.] h = 600 ev V2p 3/ o C +5.0LCO as evaporated Binding energy [ev] Figure 9. V 2p 3/2 XPScorelevelspectaof(a)thepristine0.5MLEVoxide/Pd(111) surface, (b) this surface exposed to 5 L CO, and (c) after heating the CO covered surface to 250 C. exist on a surface with bare metal areas, which are essential for CO adsorption.) The reluctance of the boundaries of the surface-v 2 O 3 structure towards reaction with CO may be related to their geometry. The surface-v 2 O 3 oxide forms 2D monolayer islands with hexagonal honeycombs of V atoms, that are in direct contact with the Pd surface, in a p(2 2) arrangement. The honeycomb structures extend to the island edges, which consist mostly of perfect honeycombs with few edge defects [11]. The oxygen atoms take up bridging sites on top of the V atoms. Apparently, these oxygen atoms are not directly accessible for reaction with CO molecules, which are adsorbed on Pd sites adjacent to the oxide island boundaries. On the other hand, the oxide phases such as the VO 2 and the V 2 O 3 bulk precursor phases contain additional layers of V oxygencoordinationwith ragged boundary lines [12,18], which seem to be more apt for CO attack. The present findings, that the less-oxygen containing V oxide phase (the surface-v 2 O 3 structure) cannot be reduced by CO, is in agreement with an earlier report of the Somorjai group on V oxide on Au(111) [19]. Lewis et al. found in oxidation experiments of H 2, ethanol, and ethylene on V-oxide/Au(111) surfaces that oxygen is more difficult to extract as the V oxide overlayers become gradually reduced [19]. 4. Conclusions The interactions of CO with V-oxide/Pd(111) inverse catalyst model surfaces, consisting of V oxide island structures and bare Pd(111) areas, have been investigated by HREELS and complementary XPS measurements with use of synchrotron radiation. The inverse catalyst surfaces have been characterised previously by XPS, LEED, STM, and DFT total energy calculations, and a comprehensive picture of the growth and evolution of V oxide phases has been established [11 14]. In this work, we have measured the phonon spectra of thin layers of V oxides on Pd(111) by HREELS and have used them in a fingerprinting way to follow the changes of the oxide phases as a result of the interaction with CO. Vanadium oxide surfaces do not chemisorb CO at room temperature, and the adsorption of CO has therefore been used in this study to probe the influence of the oxide metal interface on the adsorption behaviour of the Pd sites on these inverse catalyst surfaces. The HREELS measurements of the intramolecular CO stretching vibration frequency as a function of CO exposure indicate that the balance of the occupation of adsorption sites observed for CO adsorption on clean Pd(111) is shifted in the presence of the V oxide phases. It is sugested that CO adsorbs preferentially at high coordination Pd sites in the vicinity of the V oxide phase boundaries, however, strongly modified adsorbed CO as expected if bonded to Lewis acid sites at the metal oxide phase boundary has not been detected. The changes of the oxide phonon spectra observed upon CO adsorption and desorption are interpreted in terms of a partial reduction of the V oxide, taking place at the oxide island edges. The phonon spectra indicate that the reduction by CO is dependent on the oxygen content of the V oxide phase, with higher oxygen content favouring reduction. This is discussed in relation to the geometrical structure of the particular V oxide island boundaries as determined previously by STM [11]. The reduction of V oxide on Pd(111) by CO is further confirmed by the chemical shifts in XPS V 2p core level spectra. Acknowledgement This work has been supported by the Austrian Science Foundation through the Joint Research programme Gas Surface Interactions. We are grateful to F.P. Leisenberger and G. Koller, University of Graz, for contributing to the XPS measurements at MAX-Lab, Lund, Sweden. FPN acknowledges with pleasure the excellent hospitality of Professor G.A. Somorjai, University of California Berkeley, during his stay at Berkeley, where this manuscript has been prepared. References [1] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis (Wiley, New York, 1994). [2] M. Bäumer and H.-J. Freund, Prog. Surf. Sci. 61 (1999) 127.

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