Selectivity in Methanol Oxidation as Studied on Model Systems Involving Vanadium Oxides
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Oxidation catalysts are modeled by oxide single crystals, thin oxide films, as well as supported oxide nanoparticles. We characterize the surface of those materials using a variety of surface sensitive techniques including scanning tunneling microscopy and spectroscopy, photoelectron spectroscopy, infrared spectroscopy, and thermal desorption spectroscopy. We find temperature dependent structural transformations from V2O5(001) to V2O3(0001) via V6O13(001). V2O3(0001) is found to be vanadyl terminated in an oxygen ambient and it loses the vanadyl termination after electron bombardment. It is shown that the concentration of vanadyl groups controls the selectivity of the methanol oxy-dehydrogenation towards formaldehyde. A proposal for the mechanism is made. The results on single crystalline thin films are compared with similar measurements on deposited vanadia nanoparticles. The experimental results are correlated with theoretical calculations and models.
KeywordsOxide surfaces Oxidation catalysis Density functional calculations Surface science Catalysis Methanol oxidation
Vanadium oxides supported by a second oxide such as TiO2, SiO2, or Al2O3 represent an important class of active catalysts industrially applied to a variety of reactions. These include oxidation as well as reduction reactions, e.g., the oxidation of o-xylene to phthalic anhydride, the oxidation of sulfur dioxide to sulfur trioxide, the ammoxidation of aromatic hydrocarbons, and the selective catalytic reduction of NO x with ammonia [1, 2]. Typically, submonolayer to monolayer quantities of vanadium oxides are dispersed on the substrate using impregnation, grafting, or chemical vapor deposition techniques followed by calcination cycles. Numerous investigations have been dedicated to the role of the support oxide and to the identification of active species. It was found that the choice of the support oxide can influence the activity of a catalyst system by several orders of magnitude [1, 2]. The reducibility of the support oxide and the coupling between the support and vanadia, mediated via V–O support bonds, have been proposed to play a decisive role [3, 4, 5, 6]. However, many questions remain unanswered, such as the role of vanadyl groups or of vanadium oxides in an oxidation state lower than +5 [1, 2].
To obtain a more direct insight into structure–reactivity relations supported vanadia systems have been prepared under ultrahigh vacuum conditions on silica, alumina, and ceria thin film supports [8, 9, 10, 11, 12, 13, 14, 15, 16]. Scanning tunneling microscopy has been used to show that under such conditions the vanadia does not wet the surfaces of silica and alumina even if those surfaces had been exposed to water beforehand. Rather the vanadia particles grow three dimensionally. The vibrational and electronic properties of such model systems have been investigated with infrared spectroscopy and X-ray photoelectron spectroscopy and the analysis has been compared with the equivalent information from real catalysts . It was found, in particular, that the vanadium oxidation state is +3 in the bulk but +5 at the surface where the vanadium is fourfold coordinated to three oxygen atoms in the surface plane and to one oxygen atom via a vanadium–oxygen double bond (vanadyl) perpendicular to the surface plane. It is expected that the vanadyl oxygen is a decisive factor for the surface chemistry.
The role of vanadyl species will be investigated in this paper with respect to alcohol oxidation even though in this case the process used commercially involves silver and ironmolybdate catalysts [17, 18]. In this publication we present detailed catalytic studies on alcohol oxidation for supported vanadia and for ordered V2O3(0001) thin films grown on Au(111). Similar to the case of the supported vanadia particles the ordered films exhibit vanadium in a +3 oxidation state in the bulk and +5 at the surface which is terminated by a layer of vanadyl groups [19, 20].
Also, we address the interconversion of vanadium oxides. Specifically, we will report changes of the surface structure of a V2O5(001) single crystal surface upon heat treatment. The experimental studies are paralleled by theoretical investigations using electronic structure calculations and Monte Carlo simulations .
The experiments have been performed in a number of different ultrahigh vacuum chambers. The setups are described in the literature. The V2O5(001) single crystal surfaces were prepared by cleavage in UHV using adhesive tape .
The preparation of vanadia clusters on silica follows the recipes described in ref. [8, 9, 10, 11] and the synthesis of the thin vanadia films has been detailed in Ref. [16, 19]. The V2O3 films have been either used as prepared or they have been reduced by electron bombardment before exposure to the alcohol. Methanol has been dosed from a capillary doser typically at low temperature (~80 K) before annealing to given temperature.
Calculations are based on spin-DFT and employ a plane-wave basis set as implemented in the Vienna Ab initio Simulation Package (VASP). We use gradient corrected exchange-correlation (GGA) functionals (PW91 or PBE) [22, 23]. The electron–core interaction is described by the projector augmented wave (PAW) method . The clean and defective V2O5(001) surfaces are modeled using a fully relaxed two-layer slab and an energy cutoff of 800 eV. We have considered a wide range of defect concentrations (1/6 ≤ Θ ≤ 1). In order to rationalize the scanning tunneling spectra we also perform GGA + U calculations with U = 3 eV . Furthermore, we employ the Monte Carlo method with pair interaction energies between reduced sites fitted to the DFT results to simulate the reduced V2O5(001) surface at a given temperature and defect concentration. The network consists of 50 double rows of 2 × 150 sites, which corresponds to an area of 57.75 × 53.7 nm2. Ten trajectories, starting from random states, are simulated using the Metropolis algorithm  for a varying fraction of reduced sites (0–100%, every 2.5%).
For the study of the oxidation of methanol to formaldehyde on V2O3(0001) surfaces we employed a four-layer slab with a (2 × 2) periodicity and a 400 eV cutoff; the (V–O3–V) bottom layer is kept fixed at its position in the fully relaxed slab representing the clean surface.
Results and Discussion
There are indications that even defects within the material influence the band gap at the surface, and that this band gap varies across the surface . The band gap is believed to be important in oxidation reactions where redox processes involve electron transfer and intermediate storage . It is therefore a working hypothesis to assume that a combination of the ensemble and the local band gap governs the reactivity [4, 5, 6].
Above 500 K small traces of H2O are observed and a considerable signal at mass 29 which largely stems from formed formaldehyde as judged by the small intensity of the signal at mass 31. After the onset of the formaldehyde signal we see desorption of methane as well as traces of H2O and hydrogen. All of the signals above 500 K are absent for a fully vanadyl covered surface (not shown here).
It is the hydrogen attachment to the vanadyl group that goes hand in hand with a change in the electron count and could be influenced by the ability of the vanadium oxide to accept and release electrons, which in turn is controlled by the vanadium oxide surface band gap.
The mechanism suggested by Scheme 3 is supported by DFT calculations using the PBE functional. The energy for molecular adsorption of methanol on the fully oxidized V2O3(0001) surface is only −16 kJ/mol. This step differs substantially from adsorption on silica supported vanadia shown in Scheme 1, for V2O3 forms a closely packed lattice and insertion of a methoxy group in a V–O bond is not possible. On the reduced surface with a vanadyl defect concentration of 1/4 we find stronger adsorption with energies of −122 and −193 kJ/mol for non-dissociative (Fig. 9a) and dissociative adsorption (Fig. 9b). The latter involves hydrogen transfer to a neighboring vanadyl group. Hydrogen transfer to a three-fold coordinated surface oxygen atom is much less favorable and results in an adsorption energy of only −103 kJ/mol. The large energy difference underlines the importance of having both reduced and oxidized surface ion sites for dissociative methanol adsorption creating a hydroxyl and a methoxy site, respectively. In a second step, the methoxy group formed is oxidized to formaldehyde by hydrogen transfer to a neighboring vanadyl group (Fig. 9c). This redox step is similar to silica supported vanadia (Scheme 2). For this reaction we find a reaction energy of +141 kJ/mol with respect to the dissociative adsorption state. This reaction is only possible if two adjacent vanadyl sites are available on the surface, one for the initial adsorption and a second one for the oxidation step. For higher defect concentrations (1/2), dissociative adsorption is 20–30 kJ/mol less exothermic. However, the main difference is that after adsorption vanadyl groups are no longer available. Hence, in the redox step, hydrogen has to be transferred to a three-fold coordinated surface oxygen atom, which is far more endothermic (244 kJ/mol with respect to the dissociative adsorption state). Hence, the effect of the defect concentration on these reactions energies is sizable. This result is supported by the TDS data presented in Fig. 5b which show a formaldehyde desorption peak at 550 K which grows at the expense of the state at ~515 K when the degree of surface reduction is increased. In the light of the results of the calculation presented above, the peak at 515 K could be attributed to formaldehyde formed using vanadyl oxygen while the peak at higher temperature might be assigned to formaldehyde formed involving three-fold coordinated surface oxygen atoms.
We have shown how vanadium oxide surfaces of different composition may be transformed into each other. The final reduction stage of V2O5 is V2O3 and it is possible to collect solid experimental information on reaction steps and mechanism. The results revealed for the single crystal surfaces may be transferred to the situation of vanadium oxide particles on a supporting oxide, i.e. silica.
The oxidation of methanol to formaldehyde was investigated for V2O3(0001) thin films and deposited vanadia clusters. It is shown that the vanadyl oxygen atoms are involved in the reaction and that they influence its selectivity. Indications were found that the band gap plays a prominent role for the reactivity.
This study indicates that through a combination of experimental work on well defined model systems in combination with theoretical modeling useful insight into catalytic active systems may be gained that are otherwise difficult or impossible to obtain.
This work was supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 546) and the “Fonds der chemischen Industrie”. The calculations were carried out on the IBM pSeries 690 system of the Norddeutscher Verbund für Hoch- und Höchstleistungsrechnen (HLRN).
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