Reactivity of Epitaxial Vanadia on TiO2 - American Chemical Society

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J. Phys. Chem. C 2009, 113, 2798–2805

Reactivity of Epitaxial Vanadia on TiO2: Are Support Interactions Required for Reactivity? Min Li* and Eric I. Altman Department of Chemical Engineering, Yale UniVersity, New HaVen, Connecticut 06511 ReceiVed: June 27, 2008; ReVised Manuscript ReceiVed: October 17, 2008

The reactivities of vanadium oxide epitaxial thin films were studied using temperature-programmed desorption (TPD). The vanadia films were grown on rutile TiO2(110) using oxygen-plasma-assisted molecular beam epitaxy and were shown to exhibit the (1 × 1) rutile structure using reflection high-energy electron diffraction, low-energy electron diffraction, and scanning tunneling microscopy. Meanwhile X-ray and ultraviolet photoelectron spectroscopy showed that monolayer films contained V5+, while V4+ predominated in the bulk of multilayer films. Two reaction channels, at 400 K and above 500 K, were detected for submonolayer vanadia coverages for 1-propanol oxidation to propionaldehyde. The reaction channel at 400 K persisted through multilayer films and thus was attributed to the dehydrogenation of intermediates attached to V surface cations; meanwhile, the sensitivity of 500 K reaction channel to vanadium oxidation states as well as a comparison of the branching ratio between these two reaction channels suggested that the reaction above 500 K involves intermediates around the periphery of vanadia islands where surface oxygens are bound to both V5+ and Ti4+. It was also found that the activation energy of the lower temperature channel on the submonolayer film is unaffected by reduction and reoxidaton, while the activation energy starts to increase for the higher temperature channel upon reduction. The persistence of the same lower temperature reaction channel at 400 K throughout multilayer films where a direct chemical interaction with the titania support is absent, indicates that multilayer epitaxial films retain dehydrogenation reactivity, in contrast to prior studies where disordered vanadia films were reported to be unreactive. A comparison of the branching ratio for aldehyde desorption versus unreacted alcohol desorption for submonolayer and multilayer films revealed that a higher fraction of the alcohol dehydrogenates on submonolayer films. Together the results indicate that the TiO2 support increases the vanadia reactivity by stabilizing reactive surface structures and by aiding the initial deprotonation of adsorbed alcohols. I. Introduction It has long been known that vanadia monolayers on TiO2 supports are highly active and selective for a number of reactions including the partial oxidation of o-xylene to phthalic anhydride and NOx reduction by NH3 to N2.1-4 The active form of these catalysts with only a monolayer thick vanadia film indicates a uniquely strong interaction between the vanadia and the titania support. However, the mechanisms by which adding a second transition metal (M2), in this case Ti, to binary oxides affects catalytic properties have been in dispute. One thought is that M1-O-M2 linkages change the electron density around the catalytic transition metal of interest, thereby affecting its reactivity.5-9 Alternatively, it has been suggested that the second metal acts as a “structural promoter”, stabilizing the catalytic metal in a surface geometry not seen on the surfaces of the bulk binary oxide.10-12 To distinguish between structural and electronic effects in multicomponent oxide catalysts, we have been fabricating epitaxial thin film model catalysts in which vanadium oxides are grown on different support materials that have the same surface structure and on different phases and surface orientations of TiO2 where the surface structure is varied but the chemistry of the support interaction is fixed.13,14 Characterizing the reactivity of these epitaxial films where the electron density and geometry around the active cation are varied independently allows the importance of these two effects to be separated. In this paper we compare the reactivity of epitaxial * Corresponding author, [email protected].

vanadia monolayers and multilayers on rutile (110) where the structure is held constant but the direct interaction with the TiO2 support is eliminated in the multilayer films. It will be shown that V-O-Ti linkages to the support are not required for vanadia to display oxidative dehydrogenation activity. We have shown in our previous studies that vanadium oxides tend to grow epitaxially on transition metal oxide supports, even on the anatase polymorph of TiO2 where no similar vanadium oxide structure is known in the bulk.13,14 Tens of nanometers thick epitaxial VO2 layers could be formed on anatase (001). Oxygen adsorption on the surface increased the stoichiometry of the topmost monolayer to V2O5, thereby creating a unique surface structure not seen on bulk vanadia samples. For anatase (101), the first monolayer was found to wet the anatase surface. Once the first layer was completed, however, large threedimensional vanadia crystallites started to form on the (101) terraces. Meanwhile, several groups have reported vanadia film growth on rutile (110) in oxidizing backgrounds. The V2Ox (x ) 3-5) films were grown by depositing vanadium in ambient O2, and x depended on the growth temperature.15 A V2O5 film was synthesized from a VOCl3 precursor that decomposed in water vapor.16 In other work, a V2O5 monolayer was produced by oxidizing vapor-deposited vanadium in 10-3 Torr O2 at 400 K.17 However, the structures of the vanadia films were not characterized in these studies. Recently Sambi et al. reported the growth of a 5 monolayer (ML) thick vanadia film on TiO2(110).18 A (1 × 1) low-energy electron diffraction (LEED) pattern was obtained along with X-ray photoelectron diffraction (XPD) data consistent with an epitaxial rutile VO2 structure.

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Reactivity of Epitaxial Vanadia of TiO2 Work on alumina substrates has yielded similar conclusions. For thin vanadia films on sapphire, epitaxial V2O3 is favored with oxygen adsorption oxidizing the outermost V to 5+.19 The presence of capping oxygens on the V2O3 surface was found to strongly affect water adsorption on the vanadia.20 It should be noted that an important difference between the alumina and titania supports is that oxygen adsorption changes the oxidation state of the surface V on alumina from 3+ to 5+, and so one oxygen per V is required to fully oxidize the surface, while for titania the change in oxidation state is from 4+ to 5+ requiring only half the surface V to be covered by capping oxygens to fully oxidized the surface. Thus, even fully oxidized, epitaxial vanadia surfaces on titania are expected to expose coordinatively unsaturated V sites. The reactivity of vanadia films on TiO2 has been characterized using alcohol TPD by several groups. For monolayer and submonolayer V2O3 films supported on rutile (110), formaldehyde was produced from adsorbed methanol above 600 K, while no reaction products were observed on multilayer V2O3 films.21,22 For the industrially more relevant V2O5 films, it was found that the formaldehyde desorption temperature was lowered to 517 K for monolayer films but disappeared on multilayer V2O5 films.16 One interesting aspect in this study was that the V2O5 films could be reduced to V2O3 after one TPD run and then fully reoxidized by exposure to oxygen at 600 K. The formaldehyde peak then cycled between 517 and 600 K following these treatments. Wong et al. also reported methanol oxidation to form formaldehyde on monolayer V2O5 but at a lower temperature, 485 K.17 Similarly, the formaldehyde peak shifted to 615 K upon reduction and went back to 485 K after oxygen exposure. In this paper, we focus on the reactivity of epitaxial vanadia films on rutile (110) using 1-propanol as the probe molecule. Studies on practical high surface area catalysts suggest that active catalysts contain vanadium in the 5+ oxidation state.3,4,17,23-25 To overcome difficulties in preparing V5+ in ultra-high-vacuum environments, we have used oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) to grow the epitaxial vanadia films. To determine whether structure or a direct support interaction is important for surface chemistry, we prepared epitaxial monolayer and multilayer vanadia films where no V-O-Ti bonds exist. The TPD results disclosed that the dehydrogenation of 1-propanol to form propanal exists on submonolayer and multilayer epitaxial films without a significant change in activation energy. Since prior results have shown that poorly ordered vanadia multilayers on TiO2 as well as bulk V2O5 are not active for this reaction,4,16,24 the results indicate that the TiO2 support enhances the reactivity of the vanadia by stabilizing reactive structures. II. Experimental Section The experiments were primarily conducted using an interconnected three-chamber ultrahigh vacuum (UHV) system consisting of an OPA-MBE growth chamber, an analysis chamber, and a microscopy chamber as described previously.13 Vanadium was deposited from an electron beam evaporator with the flux monitored using a quartz crystal oscillators (QCOs) near the source position. The tooling factor for the source QCO was measured by moving an additional QCO to the sample position. For the purposes of this paper, 1 ML of vanadia corresponds to deposition of enough V to entirely cover the surface with one atomic layer of [110]-oriented rutile-structured VO2. This is equivalent to 1.05×1015 V atoms/cm2 and a film thickness of 0.32 nm based on the (110) interplanar spacing in the rutile

J. Phys. Chem. C, Vol. 113, No. 7, 2009 2799 form of VO2. A microwave electron cyclotron resonance (ECR) oxygen plasma source directed toward the substrate at the growth position supplied activated oxygen during growth. The MBE chamber base pressure was maintained around 1×10-9 Torr. A differentially pumped 15 keV electron gun and a fluorescent screen mounted on the opposite side of the chamber were used to obtain RHEED patterns during oxide growth. The analysis chamber was equipped with LEED optics and a double pass cylindrical mirror analyzer (DPCMA), which was used with an electron gun for Auger electron spectroscopy (AES), with an X-ray source for X-ray photoelectron spectroscopy (XPS) and a He discharge lamp for ultraviolet photoelectron spectroscopy (UPS). The 7.1 mm × 7.1 mm × 0.5 mm rutile crystals were obtained from PI-KEM Limited. The substrate temperature was measured using a K-type thermocouple glued to a small rutile piece next to the substrate using a ceramic adhesive. Samples were heated by conduction from an electron beam heated cap pressed against the back of the sample. After introduction into UHV, the substrate was treated by cycles of sputtering and annealing in UHV until no impurities including carbon and calcium26 were detected by AES. The sample was then annealed under the growth conditions described below. A (1 × 1) RHEED pattern as reported previously27 was observed before vanadia growth. The vanadia films were grown in the oxygen plasma with a microwave power of 250 W and an O2 pressure of 3 × 10-5 Torr, conditions that provide an O atom flux on the order of 1014 atoms/cm2.28 Unless specified, films were usually cooled in the oxygen plasma to room temperature after growth. Growth temperatures were between 660 and 700 K, and the growth rates varied between 0.0015 and 0.004 nm/s. The UPS spectra were obtained using He II radiation at 40.8 eV and were corrected for satellite lines at 48.4 and 51.0 eV in the discharge lamp. Meanwhile, Al KR radiation at 1486.7 eV was used to obtain core level XPS spectra. The XPS spectra were corrected for the Al KR3,R4 satellite lines shifted by +9.8 and +11.8 eV. The O 1s peak at 531.6 eV was used as an internal reference for the spectra. All photoelectron spectra were collected with the sample normal to the DPCMA axis. Ion scattering spectroscopy (ISS) was performed using a separate UHV system equipped with an ion gun, an X-ray source, and a hemispherical energy analyzer (Phoibos 100) from Specs Technologies Co. This system was also equipped with a differentially pumped ECR oxygen plasma source for surface preparation. The ion gun was mounted at a ∼75° angle with respect to the analyzer axis yielding a nominal scattering angle of 105°; on the basis of the O and Ti peak positions for a bare rutile substrate, a scattering angle of 98° was calculated. The sample was tilted ∼20° upward toward the ion gun resulting in a ∼55° incident angle with respect to the surface normal. Spectra were collected using 963 eV He ions. Considering the potential for damaging vanadium oxide surfaces by electron-stimulated desorption (ESD), during LEED measurements the samples were moved to different spots to avoid prolonged exposures that could cause the patterns to change. Following the LEED experiments, the samples were reoxidized. The ESD cross sections fall with increasing beam energy, so this is much less of a factor with RHEED.29 In addition, the RHEED experiments were performed with the oxygen plasma on, so the damage in this case would be healed. With laboratory sources, currents in XPS and UPS are roughly 3 orders of magnitude lower and so electron beam damage in this case would be minimal.

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Figure 1. Series of RHEED patterns obtained during vanadia film growth: (a) the rutile TiO2(110) substrate prior to growth in plasma at 660 K; (b) 0.75 ML (0.24 nm) vanadia film grown at 660 K; (c) 1 nm vanadia film grown at 660 K; (d) 11.7 nm vanadia grown at 700 K, and (e) 16.2 nm vanadia at 700 K.

The TPD experiments were carried out in the MBE chamber. The 1-propanol was purified using freeze-pump-thaw cycles until the gas line pressure did not rise above 5 × 10-4 Torr. The alcohol was dosed using a directed doser close to the sample at the growth position. Samples were exposed to 1-propanol at room temperature. A quadrupole mass spectrometer differentially pumped by a 70 L/s turbo pump was used to detect desorbing species. As described in section III, full-range mass spectra were performed during 1-propanol exposure to identify the masses of the cracking fragments. Up to eight masses were monitored during TPD as detailed in section III. The heating rate was set at 2 K/s. The scanning tunneling microscopy (STM) measurements were performed in a separate UHV system equipped with a custom designed variable-temperature STM,30 a cylindrical mirror analyzer for AES, a microwave ECR oxygen plasma source, and LEED. The STM images were recorded at room temperature. Electrochemically etched tungsten tips for STM were cleaned by electron beam bombardment prior to use. The tunneling current was 0.5 nA during all of the STM measurements. III. Results a. Vanadia Film Growth. The evolution of the surface structure was monitored using RHEED during film growth. Figure 1a shows a RHEED pattern of semicircularly arranged spots with the beam incident along the [11j0] direction on the rutile TiO2(110) substrate prior to growth, in agreement with that previously reported.27 The distinct spots indicate that the starting surface was smooth. After 0.75 ML vanadia deposition at 660 K, the spots were replaced by spots arranged periodically in both the [11j0]and 001 directions. A similar pattern was reported on nickel-cluster-covered TiO2(110) surface as revealed by STM.27 Thus, the three-dimensional (3-D) growth mode dominated for vanadia growth even in the sub-monolayer range. The 3-D growth continued throughout the vanadia growth from 1 nm to 11.7 and 16.2 nm as shown from panels c to d in Figure

1. The RHEED 3-D spots, however, became dimmer as the vanadia coverage increased, suggesting surface disordering during film growth. It should be noted that the thermodynamically favored form of VO2 between 340 and 1810 K is a rutilestructured phase with lattice constants 0.288 and 0.455 nm,31 which closely match the 0.296 and 0.459 nm lattice constants of the TiO2 substrate; therefore, no shifts in the positions of the diffraction features could be detected. b. Surface Characterization with LEED, STM, XPS, and ISS. After film growth, the sample was cooled down to 400 K in the plasma and transferred to the analysis chamber for surface characterization. As shown in Figure 2a, a (1 × 1) LEED pattern, similar to the clean substrate, was resolved on the 0.75 ML vanadia-covered surface with a higher background. The spots started to fade with increasing coverage from 1 to 11.7 nm and could barely be seen with vanadia deposition up to 16.2 nm. For these thicker films, however, sharp LEED patterns could be produced by annealing in O2 as shown in Figure 2e. In this case, the (1 × 1) pattern was seen after annealing at 925 K in 1 × 10-8 Torr O2 for 1 h; depending on the temperature and oxygen pressure, higher order reconstructions were observed that will be described in detail elsewhere. It is noted that annealing at 925 K led to a trace of Ti in the AES spectra which increased with more prolonged annealing. The STM images recorded before and after annealing were consistent with the diffraction data. As illustrated in Figure 3a, prior to annealing the epitaxial vanadia films were characterized by small terraces and a high step density. The measured spacing between the atomically resolved rows on the small terraces was 0.63 nm, within experimental error of the expected 0.64 nm for the rutile-structured (110) surface, suggesting the row orientation in the 001 direction. Meanwhile, the image recorded after annealing revealed large flat terraces separated by steps and with an island (∼20 nm × 10 nm) on the top as shown in Figure 3b. The step height was measured to be 0.3 nm, close to the monatomic step height of rutile VO2.

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Figure 2. Corresponding LEED results for surfaces at the different vanadia coverages in Figure 1: (a) 0.75 ML (0.24 nm); (b) 1 nm; (c) 11.7 nm; (d) 16.2 nm; (e) >20 nm thick film annealed at 925 K in 1 × 10-8 Torr O2.

Figure 4 shows XPS spectra of the V 2p and O 1s region for vanadia films of different thicknesses. The spectrum in Figure 4a was collected for the 16.2 nm vanadia film after it was reannealed in the oxygen plasma at 730 K and the plasma extinguished right after the anneal. The film is dominated by the V4+ oxidation state after this treatment as marked by a dotted line on the V 2p3/2 peak.13 Meanwhile, the spectrum in Figure 4f collected for a 21 nm thick vanadia film deposited onto oxidized Si(001) at room temperature showed a completely oxidized V5+ state as marked by a dashed line.13 Clearly in the sub-monolayer range, 0.75 ML in Figure 4b, the film is completely oxidized to V5+. As the films thickened from 1 to 16.2 nm (3-52.5 ML), the surface became dominated by the V4+ oxidation state as shown from panels b to e of Figure 4. This, combined with the RHEED data in Figure 1, the LEED data in Figure 2, and the STM data in Figure 3, indicates epitaxial growth of rutile-structured VO2. Ion scattering spectroscopy was used to characterize the termination of the epitaxial vanadia films. The ISS spectrum of a 14.2 nm thick VO2 film grown on TiO2(110) substrate was

collected as shown in Figure 5a. The surface was cleaned and oxidized in the plasma at the growth temperature of 660 K and then allowed to cool in the plasma. After this treatment, XPS revealed no evidence of emission from Ti 2p levels, indicating that there was no Ti near the surface; however, traces of a Na impurity were observed accounting for the weak feature at ∼0.625 between the O and V peaks. An ISS spectrum of a clean rutile substrate is also shown in Figure 5a as a reference. The stoichiometric TiO2 surface, as indicated by Ti 2p peak shapes from XPS (not shown here), was prepared by sputtering and annealing in UHV. In comparison to the vanadia film, the metal cation peak is much stronger for the bare substrate. The stronger peak for the rutile substrate can be understood in terms of the blocking effect of adsorbed capping oxygens. As discussed in the Introduction, only 1/2 monolayer of capping O atoms is required to oxidize all of the surface V to 5+. In the structural model in Figure 5b, these capping O atoms (large gray balls) cap half of the 5-fold coordinated V atoms at the corner of the (1 × 1) unit cell (dashed rectangle) creating a (2

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Figure 3. STM images of >20 nm thick vanadia films: (a) before annealing and (b) after annealing in O2. The sample biases were 3 V for image a and 2.75 V for image b.

Figure 5. (a) The ISS spectrum of a stoichiometric TiO2(110) surface and a 14.2 thick VO2 film. (b) Structural model of rutile (110)-(1 × 1) surface; small light balls are V atoms on the (110) plane, and large dark balls are in-plane and bridging O atoms. Large gray balls are 1/2 monolayer adsorbed O on top of 5-fold coordinated V atoms. The (1 × 1) unit cell is highlighted by a dashed rectangle.

Figure 4. Comparison of core level XPS spectra of the V 2p and O 1s region for vanadia films grown on rutile TiO2(110) and reference spectra for (a) VO2 and (f) V2O5. The dashed reference lines mark the V4+ and V5+ oxidation states of the V 2p3/2 peaks as aligned with VO2 and V2O5 reference spectra. The film thicknesses were (b) 0.75 ML (0.24 nm), (c) 1 nm, (d) 11.7 nm, and (e) 16.2 nm.

× 1) periodicity. This leads to an O:V ratio on the surface of 7:1, assuming that V atoms capped by bridging oxygens are not detectable by ISS. Meanwhile, on the stoichiometric rutile TiO2(110) surface, the ratio of O:Ti is 6:2, again neglecting Ti atoms under bridging oxygens. Thus, oxidizing the surface layer of the vanadia film would be expected to cut the metal cation peak intensity in half, while increasing the oxygen peak intensity by 1/6. The ISS measurements (Figure 5a) support this conclusion: the O:V peak integral ratio for the vanadia film is 1.26:1, while the O:Ti peak integral ratio for the bare rutile TiO2(110) is 0.56:1. The relative ratio of (O:V)/(O:Ti) is then 2.25, which is close to the value of 7/3 predicted from structural model (Figure 5b). This analysis assumes that V and Ti have similar scattering cross sections, which is reasonable considering that they are adjacent to each other on the periodic table. C. Temperature-Programmed Desorption Studies on Vanadia Films. To probe the catalytic activity of vanadia films, we chose to investigate the oxidative dehydrogenation of adsorbed 1-propanol using TPD as suggested by previous reactivity studies of TiO2 as well as vanadia films.16,32 To determine the desorption products from the vanadia films, and to characterize the 1-propanol cracking pattern, full range mass spectra were initially repeatedly recorded both during 1-propanol dosing and during the heating ramp. During dosing, the dominant peaks occurred at masses 29 (C2H5) and 31 (CH2OH). Meanwhile, the same fragments were also detected as the most intense signals during desorption. Therefore, to

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Figure 7. Helium II UPS spectra collected on 0.75 ML (0.24 nm) vanadia thin films with different surface preparations. The solid line corresponds to the as- grown film, the dotted line corresponds to the reduced surface by previous TPD run, and the dashed line is on the reoxidized surface after annealing in plasma at 660 K for 120 min. Figure 6. A series of TPD curves recorded after 1-propanol adsorption on 0.75 ML (0.24 nm) vanadia thin films with different surface preparations: (a) propionaldehyde desorption on as-grown, reduced, and reoxidized films; (b) 1-propanol desorption on as-grown, reduced, and reoxidized films. The term “reduced” means that the as-grown film was reduced by a previous TPD run and “reoxidized” after annealing in plasma at 660 K for 120 min and cooling down to 400 K in plasma. The 1-propanol exposure was 2100 langmuirs at room temperature.

clarify the mass fragment assignment for the potential product propionaldehyde,32 propionaldehyde vapor was leaked into the chamber. The most intense fragment was at mass 29, but no increase in the mass 31 signal was detected. Therefore, the contribution of 1-propanol desorption to the mass 29 desorption signal was calculated based on the mass 29 to mass 31 intensity ratio observed during 1-propanol dosing. In the TPD data presented below, the 1-propanol desorption curves represent the desorption signal at mass 31, while the propionaldehyde signal was taken as the desorption signal at mass 29 minus the contribution due to 1-propanol at that mass. The following signals were also monitored during TPD runs: 18, 27, 28, 29, 31, 39, 41, and 58 AMU to check for other potential products, particularly propene (major fragments 39 and 41 AMU) which can be produced by dehydration of 1-propanol; no evidence of other reaction products was observed. In all of the following TPD experiments, the surfaces were exposed to 1-propanol at 7 × 10-6 Torr for 5 min, so the total exposure was 2100 × (1 × 10-6 Torr · s) or 2100 langmuirs, easily enough to saturate the surfaces. Figure 6 shows the TPD curves after a 0.75 ML thick vanadia film prepared under different preparation conditions was exposed to 1-propanol at room temperature. As labeled in Figure 6a, for a freshly prepared sample, labeled “as-grown”, a proprionaldehyde desorption peak at 390 K was detected, with a second half-height lower peak appearing on the right shoulder at 520 K, as marked by an arrow. Repeating the 1-propanol desorption experiment caused the higher temperature propionaldehyde desorption peak to shift from 520 to 600 K and to broaden and lose intensity; meanwhile, the peak at 390 K shifted slightly to 410 K as shown in Figure 6a labeled as “reduced”. The film was reoxidized in the oxygen plasma at 660 K and exposed to 1-propanol after the surface cooled to room temperature. Labeled “reoxidized” in Figure 6a, this caused the second broad and lower peak to shift back toward 550 K from 600 K; again, the peak at 390 K shifted to 410 K. Meanwhile, desorption of molecular 1-propanol led to a single broad TPD peak at 500 K on the as-grown film that shifted to 550 K in the second run

but did not shift back to lower temperatures when the film was reoxidized as labeled in Figure 6b. The initial abrupt sharp peaks near 300 K were observed when the sample was turned away from the mass spectrometer and, therefore, were attributed to desorption from the heated areas surrounding the sample such as the e-beam filament and other parts of the sample holder. We also did not see any propionaldehyde desorption in TPD after dosing 1-propanol on the TiO2 substrate. The 0.75 ML vanadia films following the different treatments outlined above were characterized using UPS prior to TPD runs as shown in Figure 7. The data are reported on a kinetic energy scale because the vanadia growth conditions fully oxidized the rutile support making it insulating and thus subject to charging during photoemission measurements. The solid curve shows a peak due to emission from the filled, primarily O 2p-derived valence band. The flat line from the bottom of the peak toward higher kinetic energies (or lower binding energies) indicates the formation of a completely oxidized V2O5 film on the surface. After the first TPD run, a bump could be resolved along the dotted curve to the right of the main valence band peak, indicating the reduction of the film. Although the film was reoxidized under the growth conditions by annealing in the plasma at 660 K, a small bump could still be seen in the same region along the dashed line, indicating that some reduced vanadium species remained on the surface. A series of 1-propanol TPD spectra were collected on vanadia films as a function of film thickness as shown in Figure 8. The propionaldehyde desorption peak at ∼400 K persisted as the vanadia film thickness increased as indicated by the labeled curves in Figure 8a. Meanwhile the intensity of the second lower broad peak at 520 K, as marked by an arrow, decreased with increasing coverage and disappeared above 1 nm thickness. It is interesting that a molecular 1-propanol desorption peak at ∼400 K appears as the film became thicker as shown from labeled curves from Figure 8b. The second 1-propanol peak at ∼500 K that appeared on the 0.24 nm (0.75 ML) thick film persisted through films up to 16.2 nm thick. The branching ratio for propionaldehyde desorption (mass 29 area in Figure 8a) versus molecular desorption (mass 31 area in Figure 8b) was obtained for each coverage from the peak areas of each spectrum in both columns of Figure 8. The curves were integrated from 300 K to where the second high-temperature peak ends, excluding the small initial peaks (0.24 and 1 nm curves in Figure 8b) due to desorption from areas surrounding the sample. The branching ratio decreased from 7.33 to 0.75, 0.87, and 0.77

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Figure 8. A series of TPD curves of 1-propanol adsorption on vanadia films with different thickness as labeled: (a) propionaldehyde desorption spectra and (b) 1-propanol desorption spectra. The vanadia film was oxidized at growth condition by annealing at 700 K in oxygen plasma and cooling down to 400 K in plasma and then exposed to 2100 langmuirs 1-propanol at room temperature.

corresponding to coverages of 0.24, 1, 11.7, and 16.2 nm, respectively. IV. Discussion Reactions of alcohols on oxide surfaces are generally considered to proceed through two steps: (1) deprotonation of the alcohol to form an alkoxide and either (2a) C-O bond scission and β-H abstraction of the alkoxide to form the alkene or (2b) R-H abstracton of the alkoxide to form an aldehyde or ketone.33,34 The observation of propionaldehyde as the main reaction product following 1-propanol adsorption indicates that the epitaxial vanadia films are active for alkoxide dehydrogenation, forming an aldehyde in this case, in agreement with previous alcohol TPD studies on vanadia thin films.17,35 It is noted that on epitaxial vanadia films, the dominant propionaldehyde desorption peaks are anchored around 400 K. Meanwhile, aldehyde desorption peaks above 550 K have been observed for the bare rutile support following adsorption of primary alcohols including 1-propanol.32 Thus, the epitaxial vanadia films are active for alcohol dehydrogenation, with an activation energy lower than the rutile support. In the monolayer regime, propionaldehyde desorption from V2O5 films, as characterized by XPS (Figure 4b), occurred in two temperature ranges, at ∼400 K and a little over 500 K, suggesting two reaction channels. The lower temperature aldehyde formation can be associated with alkoxide intermediates bonded to V5+ because no similar peak is seen for the bare substrate. Meanwhile, it cannot be excluded that the higher temperature reaction channel involves alkoxide intermediates atop exposed Ti cations, considering that others have observed propionaldehyde to desorb from rutile surfaces at similar temperatures32 and that Ti cations should still be exposed on the surface at the 0.75 ML vanadia coverage studied. On the other hand, the reduction of the vanadia monolayer was found to affect both the position and intensity of this peak. Reduction was observed to shift this peak to higher temperatures, while reoxidation caused the peak to shift back to lower temperature, as has been observed in previous studies of alcohol adsorption on monolayer vanadia films.16,17 In addition, the ratio of the peak area for the lower temperature versus the higher temper-

Li and Altman ature aldehyde desorption peak changed from 0.75 for the asgrown film to 1.87 for the reduced film and back to 0.9 after reoxidation (Figure 6a). Reduction of the rutile support has been shown to lead to dehydration of adsorbed 1-propoxide to propene32 which was not observed. Together, these results indicate that the higher temperature propionaldehyde desorption peak can be associated at least in part to the vanadia film. Interestingly, the persistence of the lower temperature propionaldehyde peak for epitaxial vanadia films over 15 nm thick indicates that direct links between surface V cations and the support are not required for dehydrogenation activity. This finding would appear to be at odds with prior surface science studies of the interactions of methanol and higher alcohols with supported vanadia catalysts where films in excess of 1 ML thick and three-dimensional vanadia clusters were found to be inactive.16,17,36 The difference here though is that the substrate and monolayer structures were maintained as the catalytic vanadia surface was moved away from the TiO2 support. In contrast, in the prior work the thicker vanadia films either adopted the bulk V2O5 structure or were poorly ordered. This suggests that the titania support acts as a structural promoter by stabilizing vanadia films in a reactive environment. This suggestion is consistent with prior work that showed that vanadia monolayers on SiO2 could display alcohol dehydrogenation activity, at least until mild heating caused the vanadia to restructure into inactive bulk V2O5 or three-dimensional clusters.36 It should be noted that at higher pressures bulk-structured V2O5 is not completely unreactive toward alcohols. Rather, bulk V2O5 exhibits a higher activation energy than monolayers supported on TiO2 resulting in very low reaction rates at temperatures where the monolayer catalysts are active, and it displays selectivity toward dehydration pathways to alkenes rather than dehydrogenation to aldehydes and ketones.37-39 Our results suggest that the selectivity toward dehydrogenation can be sustained through vanadia multilayers by maintaining the epitaxial structure. Although the main propionaldehyde desorption peak at ∼400 K was largely unaffected by the thickness of the epitaxial vanadia film, some differences between the interaction of 1-propanol with monolayer and thicker films were observed. First, the higher temperature propionaldehyde desorption peak disappeared as the TiO2 support became completely covered by vanadia. The sensitivity of this peak to the oxidation state of the vanadium suggests that adsorption around the periphery of vanadia islands where surface oxygens are bound to both V and Ti contribute to this peak. Another significant change is the appearance of a 1-propanol desorption peak at 400 K. The desorption temperature is low enough that this peak may be associated with desorption of molecularly adsorbed 1-propanol, consistent with previous infrared studies that revealed molecularly adsorbed alcohols near this temperature.5,36,40 As a result of this desorption peak, the fraction of 1-propanol adsorbed at room temperature that is converted to propionaldehyde drops as the epitaxial vanadia surface is moved away from the TiO2 support. The increase in molecular desorption versus dehydrogenation for thicker vanadia films supports Burcham and Wachs proposal that changes in the equilibrium constant for dissociative adsorption of alcohols into an alkoxide and a hydroxyl play an important role in the observed support sensitivity of monolayer vanadia catalysts.40 In Burcham and Wachs mechanistic model, the rate-determining step for alcohol dehydrogenation is dehydrogenation of an adsorbed alkoxide,40,41 although a strongly adsorbed alcohol may also play a role.40 Assuming a low

Reactivity of Epitaxial Vanadia of TiO2 coverage of the reactive intermediate, the reaction rate becomes first order in alcohol partial pressure with the rate constant proportional to the adsorption equilibrium constant for dissociative adsorption to the alkoxide, or alternatively adsorption of a strongly bound alcohol, as well as the rate constant for the ratedetermining step. Therefore, the increase in desorption of unreacted alcohol for the thicker epitaxial films suggests that these films will be less reactive than monolayer vanadia films under steady-state reaction conditions, even though the activation energy for the rate-determining step is similar. Thus, the results suggest a second role for the TiO2 supportspromotion of alcohol dissociation or formation of a strongly bound intact alcohol. A unique structural feature of the epitaxial rutile-structured vanadia films is that even when all of the surface V are oxidized to 5+, V cations are still accessible for alcohol adsorption as indicated by ISS. In contrast to prior studies either where oxygen obscured the V atoms when the surface was oxidized as in the case of alumina19,20,42 or where the V formed disordered or bulkstructured V2O5 when the vanadia coverage was increased,16,17 maintaining the substrate structure ensures that V cations are always accessible to reacting molecules, regardless of the oxidation state of the surface V or their proximity to the support. V. Summary Epitaxial vanadia films supported on rutile TiO2(110) were grown using OPA-MBE. The films maintained the (1 × 1) rutile structure as revealed by RHEED and LEED. Meanwhile, photoelectron spectroscopy showed that V5+ prevailed on submonolayer films while V4+ predominated inside multilayer films. The TPD studies on sub-monolayer films detected two reaction channels for 1-propanol oxidation to propionaldehyde, at 400 K and above 500 K. A comparison between vanadia coverage and the branching ratio between two reaction channels suggested that the lower temperature channel was associated with the deprotonation of alkoxide intermediates atop surface V5+, while the higher temperature channel involved alkoxides atop Ti4+ and along the edges of vanadia islands. Reduction of the vanadia film caused the peak above 500 K to shift to higher temperatures but did not significantly affect the lower temperature peak. Unlike prior studies in which multilayer films were either poorly ordered or adopted the bulk V2O5 structure, epitaxial films over 15 nm thick remained active for alcohol dehydrogenation, although a higher percentage of the adsorbed alcohol reacted on monolayer thick films. The results indicate that titania increases the reactivity of supported vanadia in two ways: (1) by stabilizing reactive surface structures and (2) by promoting the initial deprotonation of adsorbed alcohols. Acknowledgment. The authors acknowledge the help of Huiqiong Wang, Jarrett A. Moyer, Carlos A. F. Vaz, Victor E. Henrich, Yang Yun, Todd C. Schwendemann, and Boris R. Lukanov in carrying out this work. This project is supported by the Department of Energy through Basic Energy Sciences Grant Number DEFG02-98ER14882. The authors also acknowledge the use of Yale Materials Research Science and Engineering Center facilities through NSF Grant No. DMR-0520495.

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