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Oxidative Dehydrogenation of Methanol to Formaldehyde by a Vanadium Oxide Cluster Supported on Rutile TiO2(110): Which Oxygen is Involved? Hyun You Kim,†,§ Hyuck Mo Lee,† and Horia Metiu*,‡ Department of Materials Science and Engineering, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Korea, and Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106-9510 ReceiVed: April 14, 2010; ReVised Manuscript ReceiVed: May 22, 2010
Isolated vanadia clusters supported on titania catalyze the oxidation of methanol to formaldehyde. We used density functional theory to determine the mechanism of this reaction and found a new pathway for the dissociative adsorption of methanol and the dehydrogenation of the methyl group. In this mechanism, methanol adsorbs dissociatively, by inserting into the double bond of the vanadyl group; the methoxy radical binds to the vanadium atom, whereas the hydrogen binds to the oxygen atom of the vanadyl. The dehydrogenation of the methyl group, which is the rate-limiting step, takes place by moving a H atom from CH3 onto an oxygen atom in the -V-O-Ti- group. The V-O bond is broken, and a HO-Ti group is formed. I. Introduction Vanadia submonolayers or monolayers supported on TiO2 catalyze the oxidative dehydrogenation of methanol to formaldehyde (CH3OH + 1/2O2 f CH2O + H2O).1,2 A large number of experimental and theoretical studies have improved our understanding of this catalytic system.1-15 The following statements are widely accepted: Methanol adsorbs to form a methoxide and a hydroxyl; this is followed by the dehydrogenation of the methyl group of the methoxy radical. The dehydrogenation of the methyl group is the rate limiting step.1,2 Some questions remain regarding this chain of events. Two hydrogen atoms are stripped from methanol: one from the OH group, to form the methoxide, followed by the removal of another H atom from the CH3 group to form the formaldehyde. We still do not know which oxygen atoms are involved in these two hydrogen abstraction reactions. The two candidates are the oxygen from the VdO vanadyl group and the oxygen bridging the V to the cation in the support (V-O-Ti in the present study). Wang and Madix,13 Deo and Wachs,5 and Burcham and Wachs4 have suggested that the bridging oxygen atom is involved in the reaction. Bell and co-workers3,8-10 and Freund and co-workers7,12,16 believe that it is the oxygen in the vanadyl. These papers deal with different supports, and there is no reason to believe (or disbelieve) that the mechanism is the same on all supports. In a previous paper,11 we have shown that density functional theory (DFT) calculations for VO3 clusters supported on TiO2(110) favor the following mechanism: Methanol dissociates to form a methoxy (-OCH3) radical and a hydrogen atom by insertion into the V-O-Ti bond. The methoxide binds to the V atom to form a V-OCH3 group, and the hydrogen atom binds to the bridging oxygen to form a Ti-OH group. The preferred methyl dehydrogenation path occurs with the formation of a H2O-Ti group (the H atom removed from -CH3 binds to the * To whom correspondence should be addressed. E-mail: metiu@ chem.ucsb.edu. Fax: 805-893-4120. Tel: 805-893-2256. † KAIST. ‡ University of California, Santa Barbara. § Current address: Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712-0165.
oxygen atom in the Ti-OH group formed by CH3OH dissociative adsorption). The barrier for this process is 0.88 eV. We considered two other reaction paths, in which the H removed from CH3 binds at different locations, and found that they have much higher barriers. Recent studies in Freund and Sauer groups reported that the vanadyl oxygen is involved in the dissociative adsorption of methanol by vanadia supported on Au7,12 or ceria.16 In particular, they found that the vanadyl IR peak disappears when methanol is adsorbed on a ceria-supported vanadia catalyst.16 Bell and co-workers found that the vanadyl oxygen is involved in methyl dehydrogenation but methanol adsorption involves breaking the bond of the bridging oxygen.3,8-10 In this article, we use DFT and DFT + U calculations to study the role of vanadyl in methanol oxidation by a VO3 cluster supported on rutile TiO2(110). We find that the dissociative adsorption of methanol, by addition to the double bond in VdO, is energetically comparable to the process studied previously11 (in which the dissociative adsorption took place by breaking the V-O bond in V-O-Ti to form V-OCH3 and HO-Ti). The methoxy group binds to V and the hydrogen to the oxygen atom in the vanadyl group, converting VdO + CH3OH to HO-V-OCH3. From this configuration, a hydrogen atom from the methyl group moves to break the bond in a CH3O-VO-Ti- group to form a CH2O-V and HO-Ti group; the reaction creates formaldehyde bonded to V and a hydroxyl bonded to Ti. II. Computational Details We perform spin-polarized Kohn-Sham DFT calculations with the plane-wave VASP code17-20 and the Perdew-Wang functional.21 The ionic cores are described by the PAW method implemented in VASP. The energy cutoff was 300 eV. Tests showed that a higher cutoff does not change our conclusions. We used a rutile TiO2(110) slab having 12 atomic layers and a 4 × 1 surface supercell. An isolated single VO3 cluster was placed on the surface of rutile TiO2(110) (see Figure 1). The computed XPS spectrum of this cluster is very close to that of V2O5.22 The VO3 cluster and the upper six atomic layers of the TiO2(110) slab were relaxed during geometry optimization. The
10.1021/jp103361v 2010 American Chemical Society Published on Web 07/22/2010
Oxidation of CH3OH to CH2O by a VO3 Cluster
J. Phys. Chem. C, Vol. 114, No. 32, 2010 13737 method, TiO2 has a narrow gap, whereas in reality, it is an insulator with a band gap of 3.0 eV).25-28 In addition, calculations using this value of U open a gap in Ti2O3 (according to the GGA method, Ti2O3 is a metal, whereas in reality, it is an insulator). There is no need to use U to prevent the electrons in the d orbitals in V from delocalizing because these atoms are isolated. The DFT + U calculations were performed with the VASP code. III. Results and Discussion
Figure 1. Dissociative adsorption of methanol on a rutile TiO2(110)supported VO3 cluster. (a) The methoxide is bound to the V atom and H breaks the V-O bond in V-O-Ti to form a HO-Ti group. (b) The dissociative adsorption of methanol involves the VdO group. The methoxy binds to V to form V-OCH3, and the H binds to the oxygen in the VdO group.
other atoms in the slab were fixed at the bulk positions. A 5 × 3 × 1 k-points mesh with eight irreducible k-points was used in all calculations. The convergence criteria for the electronic wave function and the geometry were 10-5 and 10-4 eV, respectively. We used the Gaussian smearing method with an initial window size of 0.02 eV, which was gradually decreased down to 0 during geometry optimization, to prevent partial occupancy. Monopole, dipole, and quadrupole moment corrections to the energy were applied in the direction perpendicular to the slab. The nudged elastic band method with at least eight images was applied to find the transition state of different methoxide dehydrogenation paths. We examined only reactions that preserve spin polarization:11,23,24 when we determine the activation energy, we force all states along the reaction coordinate (i.e., the points forming the nudged elastic band) to have the same spin polarization as the reactants. It has become customary recently to use DFT + U when calculating the electronic structure of TiO2. It is believed that adding the on-site repulsion U to the ordinary DFT cures some of the errors introduced by electron self-interaction. Unfortunately, there are many ways of deciding the value of U, and the results depend (sometimes dramatically) on this choice. Often U is chosen to modify some feature of the Kohn-Sham orbitals; one may try to improve the band gap, or to create states in the gap when an oxygen atom is removed, or to reduce the formal charge of Ti when an oxygen vacancy is created, etc. Although one expects the energy of the Kohn-Sham orbitals to have some relationship with the measured one-electron excitations in the system, it is not clear how close these quantities ought to be. Therefore, adjusting U to “improve” the properties of the Kohn-Sham energies is somewhat ill-defined. We prefer to adjust U so that the DFT + U calculation fits some total energy change in the system. We use U ) 3.4 eV, which gives the correct energy of the reaction Ti2O3 + 1/2O2 f 2TiO2 and widens the band gap of TiO2 (according to the GGA
At the top of Figure 1, we show the structure of a VO3 cluster supported on TiO2(110). Figure 1, reaction a, shows the geometry formed by the dissociative adsorption of CH3OH by breaking the V-O bond in the -V-O-Ti- bridging oxygen group. The methoxy radical binds to the V atom, and the hydrogen atom forms a hydroxyl bonded to Ti. Judging by the fact that the VdO bond does not change, the properties of the vanadyl should not be affected by this reaction (the valence of the V atom is not changed by the dissociative adsorption of CH3OH). In Figure 1, reaction b, we show the structure formed by the addition of the CH3O and H to the double bond of the VdO group. The methoxy radical binds to V, and the hydrogen from the OH group in methanol forms a hydroxyl with the oxygen atom that belonged to the VdO group prior to methanol adsorption. Upon methanol dissociative adsorption, the VdO becomes CH3O-V-OH. Both reactions (i.e., a and b) are slightly endothermic, and within the accuracy of DFT, they have the same reaction energy. As far as energy is concerned, the structures (a) and (b) are equally probable. We note that the process in Figure 1, reaction a, is the one we have considered in our previous calculations.11 Because the dehydrogenation of the methyl is rate-limiting, one may assume that the methoxy concentration is approximately that reached in thermodynamic equilibrium. The V-OCH3 bond, formed by the dissociative adsorption of CH3OH (Figure 1, reaction b), is an unusually long, single bond. The length of the VdO bond is 1.62 Å, and that of the V-OCH3 is 1.92 Å. This conversion from a double to a single bond, plus the fact that OCH3 is heavier than O, will lower the vibrational frequency of the V-OCH3 stretch. This may explain why the VdO vibration disappears from the IR spectrum after the surface is exposed to methanol.16 One should keep in mind, however, that the experiments in ref 16 were performed on VOx supported on ceria and that the number of oxygen atoms in the VOx cluster studied by the experiments is not known. Nevertheless, the catalytic performance of VOx/CeO2 is similar to that of VOx/TiO2, and both are reducible oxides so that one may cautiously suggest that the experimental findings are consistent with the present calculations. On the basis of the energy of dissociative adsorption of methanol, we conclude that, at thermodynamic equilibrium, a hydrogen atom that is abstracted from methanol can form a hydroxyl with the vanadyl oxygen or the bridging oxygen, with roughly equal probability. To complement our previous work,11 we examine here the mechanism for the dehydrogenation of the methyl radical for the present case, when methanol adsorbs by addition in the vanadyl double bond. Figure 2 shows two possible dehydrogenation pathways of methoxide in the presence of a hydroxyl on the vanadyl oxygen: A hydrogen atom from HO-V-OCH3 is inserted in the V-O-Ti group to form HO-V-OCH2 and HO-Ti (Figure 2, mechanism a); a hydrogen atom from HO-V-OCH3 moves to form H2O-V-OCH2 (Figure 2, mechanism b). The energies of the products of these two reactions are very close, and
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Kim et al. the addition of CH3OH to the double bond in the vanadyl (to form HO-V-OCH3) and the dehydrogenation of the methyl to form HO-V-OCH2 and HO-Ti. The results are the same whether we use GGA or GGA + U. This is not at all surprising because the chemistry in the new mechanism involves only the VdO group, whereas the addition of U to the theory is meant to cure flaws in the electronic structure of the Ti d bands. It is, however, possible that the situation may change for other choices of U. Acknowledgment. We gratefully acknowledge support from the Air Force Office of Scientific Research (Grant No. FAA955006-1-0167) and the Department of Energy (Grant No. DE-FG0289ER140048). This work was partially supported by the Nano R&D program through the KOSEF funded by the MEST (No. 2009-0082472). Computing resources at UCSB have been supported, in part, by the National Science Foundation under Grant No. CHE 0321368. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. References and Notes
Figure 2. Dehydrogenation of methoxide in the presence of the hydrogen on the vanadyl oxygen. (a) The hydrogen atom abstracted from the -OCH3 group produces a hydroxyl on the bridging oxygen in V-O-Ti. (b) The abstracted hydrogen atom forms a water molecule bonded to the V atom.
TABLE 1: Reaction Energies and Activation Energies of Some of the Steps in the Oxidative Dehydrogenation of Methanol Catalyzed by a Rutile TiO2(110)-Supported VO3 Cluster reaction energy for the dissociative adsorption of methanol (eV)
reaction energy for the dehydrogenation of methoxide (eV)
energy barrier for methoxide dehydrogenation (eV)
methanol dissociative insertion in V-O-Ti 0.1711
water-like dihydroxyl Ti-OH2 -1.2211
methanol dissociative addition to VdO GGA ) 0.20 GGA + U ) 0.15
water-like dihydroxyl on the VdO GGA ) -0.99 GGA + U ) -0.97
GGA ) 1.29 GGA + U ) 1.18
two Ti-OH hydroxyls GGA ) -0.91 GGA + U ) -0.97
GGA ) 0.34 GGA + U ) 0.15
0.8811
therefore, as far as thermodynamics is concerned, they are roughly equally probable (assuming that the entropies of the two reactions are not very different). The energy barrier for the formation of two hydroxyls (mechanism a in Figure 2) is much lower. In addition, the formaldehyde produced by mechanism a (Figure 2) desorbs very easily (its binding energy is 0.07 eV). The results of the DFT + U calculations are very similar: not only is the proposed mechanism the same but also the binding and the activation energies are close. Table 1 gives the numerical values of the binding energies and the activation energies for the mechanism proposed in the previous work11 and for the one proposed here. Two conclusions are worth repeating. We have found a new mechanism for methanol oxidation to formaldehyde involving
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