TiO2 Catalysts

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Oxidation of Formaldehyde to Formic Acid over V2O5/TiO2 Catalysts: A DFT Analysis of the Molecular Reaction Mechanisms† Vasilii I. Avdeev* and Valentin N. Parmon BoreskoV Institute of Catalysis, Russian Academy of Science, NoVosibirsk 630090, Russian Federation ReceiVed: July 15, 2008; ReVised Manuscript ReceiVed: December 4, 2008

Oxidation of formaldehyde to formic acid on the surface of titania-supported vanadia is studied with density functional theory (DFT) and a dioxo-vanadyl (OdV-O-VdO) catalyst model. Two probable mechanisms (a redox Mars-van Krevelen mechanism and an associative mechanism) are considered and transition states and intermediates along these reaction pathways are investigated. The key intermediate of the redox mechanism is a dioxymethylene complex. Its successive transformation leads to a stable surface formate complex. The subsequent transformation of formate to formic acid can occur only in the presence of dioxygen. The key intermediate of the associative mechanism is a peroxo-oxo-methylene complex. Its successive transformation leads to the adsorbed formic acid. On the basis of the calculated activation barriers, the associative mechanism appears to be more probable. 1. Introduction Catalytic oxidation of organic substances, H-R, by dioxygen over oxide catalysts is usually assumed to occur in accordance with either the associative or redox mechanisms.1 The latter mechanism is widely known as the redox Mars-van Krevelen mechanism2 which assumes that the process proceeds through two major steps:

MnOL + H-R f Mn-2-0 + R-OL-H

(I)

and

Mn-2-0 + 1⁄2O2 f MnOL The first step includes the incorporation of lattice oxygen OL from the oxide catalyst MnOL (where n is the oxidation state of the metal cation, M) into the substrate H-OL-R. This step results in the formation of an oxygen vacancy, Mn-2-0, on the oxide surface, which is accompanied by the change in the oxidation state of the metal in the oxide by 2 (n f n - 2). The subsequent reoxidation of the metal cation by dioxygen regenerates the initial oxidation state of the catalyst surface. The general features of selective oxidation of hydrocarbons via the Mars-van Krevelen mechanism were summarized by Vedrine.3,4 In the associative mechanism, both reactants (the substrate and dioxygen) interact with the catalyst active-site simultaneously. The resulting intermediates include both the lattice oxygen of the catalyst and dioxygen. In this case the oxidation state of the metal cation in the oxide does not change. In other words, the cation reduction and its reoxidation are synchronized to occur in one step. The associative mechanism of the selective oxidation can be represented by the following scheme

MnOL + 1⁄2O2 + H-R f {Mn-O-R-OL-H}{Mn-OLR-O-H} f MnOL + R-OH (II) The possible intermediates are marked in braces. * Corresponding author, [email protected]., fax +7 383-3308056. † Dedicated to Professor Kirill Zamaraev on the occasion of his 70th birthday.

Below we shall discuss supported vanadium-based oxidation catalysts only. A lot of studies of the supported vanadia systems were aimed at elucidating the nature of their catalytic action and the structure of surface vanadium species and their reactivity. The last subject was a particular topic of special issues in refs 5 and 6. Experimentally, the molecular structure of supported vanadia catalysts was studied by a variety of spectral methods, including IR-Raman and UV-vis spectroscopy.7-19 A combination of these data with those obtained by other physicochemical methods has distinguished three forms of the surface species depending on the coverage of the surface with vanadia. At low surface coverage, monomeric forms of vanadia (OdV-On) proved to be the key component of the surface species.17-19 When the coverage increases, the monomeric forms are polymerized with the formation of bridge bonds V-O-V. The samples containing monomeric and polymeric vanadium species are commonly referred to as the “monolayer” samples. It was noted that high activity of vanadium catalysts is observed at selective oxidation of hydrocarbons only for the “monolayer” samples.20-27 The further increase of the active component concentration leads to the formation of the V2O5 crystalline phase on the surface of the support.14-16 Among various selective oxidation reactions catalyzed by supported vanadia catalysts, the oxidation of formaldehyde to formic acid (H2CdO + 1/2O2 f HCOOH) is of special interest.28-33 This reaction is one of the simplest reactions among the partial oxidation of organic substances and thus can serve as a convenient model for selective oxidation studies. Experimentally it has been found that among the series of vanadiabased catalysts, V/SiO2, V/Al2O3, V/ZrO2, V/Nb2O5, V/TiO2, the vanadia-titania catalysts are the most active and selective in the formaldehyde oxidation to formic acid.28 The high activity and selectivity on titania support are assigned to the formation of the monomer and polymeric vanadia species on the support surface. The latter species were not observed for other supports like SiO2, γ-Al2O3, ZrO2, and Nb2O5, which show inferior activity.28 On the basis of this, a new one-step catalytic synthesis of formic acid from formaldehyde over the V/Ti-O catalyst has been suggested.34 Such a one-step process will be attractive

10.1021/jp806231e CCC: $40.75  2009 American Chemical Society Published on Web 01/26/2009

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for industry, because formaldehyde is a cheap and readily available feedstock for the process, whereas formic acid is a midscale chemical commodity, which is currently synthesized by complex multistage methods. The main objective of this communication is to analyze the structure and stability of intermediates formed at the oxidation of formaldehyde to formic acid in order to determine the transition states which link the intermediates into the particular reaction pathways. Another purpose of the present work is to explain the high selectivity of the oxidation with respect to formic acid via qualitative comparison of the energy profiles along the pathways of the formaldehyde transformation to formic acid and CO2. Below, we consider in detail two reaction pathways corresponding to both redox Mars-van Krevelen and associative mechanisms. On the basis of the calculated activation barriers, the associative mechanism appears to be more probable and the reaction pathway leading to deep oxidation of the formaldehyde to CO2 and CO is in fact forbidden. 2. Methods The calculations were performed with the GAUSSIAN98 program package35 using the DFT technique.36 The hybrid exchange functional Becke37,38 combined with the LeeYang-Parr39 correlation functional B3LYP (UB3LYP, for the open-shell singlet and triplet states) was used for all treated structures. This correlation functional provides a good description of the reactions of transition-metal complexes including the reactions of insertion and hydrogen transfer, as well as acidic and basic properties of the oxide surfaces, thermochemical data, etc.40-43 The 6-31G* basis set was used for carbon, oxygen, and hydrogen atoms.44 The core electrons of vanadium and titanium were included into the effective CEP potential with the valence-split CEP-31G basis set.45,46 The CEP-31G/6-31G* basis set has been used previously to study ethylene adsorption47 and the reaction pathways of the 1,2-dichloroethane dechlorination to ethylene48 on the model bimetallic Cu-Pt catalysts, for analysis of the UV-vis, IR, and Raman spectra49,50 of the model catalysts V2O5 on silica and titanium. In all cases the agreement between theory and experiments was found to be satisfactory. We carried out full optimization of the structures of possible intermediates and transition states with the subsequent calculation of frequencies. Correct structures of transition states were defined by the presence only imaginary frequencies at saddle points. Entropy corrections at the formation of stable surface structures on oxide with an oxygen-rich environment can give an appreciable contribution at high temperatures (T ∼ 1000 K).51 Experimentally, the formaldehyde oxidation to formic acid was investigated at lower temperatures and pressure (T < 300 K, P ) 1 atm).28,33 For these conditions entropy corrections are probably small and in our calculations were not included in the estimation of the reaction heats and transition states energies. To assess the accuracy of the 6-31G* basis set here we report the dissociation energy of the triple dioxygen which is ED(O2) ) 122 kcal/mol (120 with the zero-point energy correction), whereas the experimental value is 118 kcal/mol. For the calculations of biradical species, the broken-symmetry approach was used.52 The broken bonds of the oxygen atoms in the TiO2 group were saturated with the hydrogen-like pseudoatoms X with m ) 1000mH and 3-21G basis functions. The structural information in figures was prepared using the ChemCraft program.53 3. Results and Discussion The analysis of the reaction mechanism at the molecular level requires theoretical estimates of the activated complex energies.

Figure 1. Optimized structures of the ASdO (doubly O-bridged) and linear-ASdO (one-O-bridged) oxo-vanadyl complexes simulating the active sites of the supported “monolayer” catalysts V2O5/TiO2. The AS-0 structure is the reduced form ASdO. Here 0 is an oxygen vacancy.

The essence of this problem is, indeed, the electronic structures of the catalyst active sites and intermediates formed on this center. 3.1. The Active-Site Models. The structure and properties of active sites stabilized on the surface of finely dispersed metal oxides supported on other oxides may be very different from those formed on regular oxide surfaces.54-56 In the literature several models of the V/Ti-O catalyst have been analyzed in which small stable gas-phase vanadia clusters are used as key components of the active site of the “monolayer” V/Ti-O samples.57-74 The general principles of the model construction were discussed by Calatayud et al.75 The stability and structures of the gas-phase clusters VnOm were investigated experimentally by Bell et al.76 and theoretically by Vyboishchikov and Sauer.77 In present work we started with a four-square stable gasphase V4O10 isomer. Two dimer isomers are formed by isomorphous replacement of two V atoms in V4O10 by Ti atoms. Figure 1 shows optimized structures of the ASdO (double O-bridge) and linear ASdO (single O-bridge) oxo-vanadyl complexes simulating the active-sites of the supported “monolayer” catalysts V2O5/TiO2. The AS-0 structure is the reduced form ASdO, where 0 denotes an oxygen vacancy. Our calculations have shown that the double O-bridge structure (ASdO model) is more stable by 15 kcal/mol than the one with single O-bridge. The bond distances obtained for the ASdO model are r(VdO) ) 1.59 Å and r(V-O) ) 1.80 Å. These values are in fine agreement with periodic DFT calculations on the V2O5 (001) surface.78 So, the ASdO model was accepted as the base model in our calculations. It was noted above that in the redox mechanism, the gasphase dioxygen does not participate in the formation of intermediates and appears in the reaction only at the final step of the catalytic cycle during the reoxidation of the active site. For our models of ASdO and AS-0 (Figure 1), the calculation of the enthalpy change ∆H gave the following results

AS-0 + 1⁄2O2 f ASdO

(III)

∆H ) -76 kcal ⁄ mol In the associative mechanism, dioxygen interacts with the active site before the reoxidation processes. Our detailed analysis

Catalyst Oxidation of Formaldehyde to Formic Acid

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Figure 2. Optimized structure of the surface peroxo complex AS(O-O) forming at the interaction of dioxygen with the active sites ASdO and AS-0.

Figure3. ThestructuresofintermediatesFRD-ASdOandFRD-AS(O-O) resulted from the formaldehyde adsorption on active sites ASdO and AS(O-O). Only the structures in the vicinity of one vanadium center are shown in Figures 3-8. The coordinates of all atoms in the intermediates and transition states in xyz format are given in Supporting Information.

of the possible oxygenated active-site structures showed that in this case dioxygen is associated to form a peroxide complex

ASdO + 1⁄2O2 f AS(O-O)

(IV)

∆H ) + 20 kcal ⁄ mol The same intermediate AS(O-O) may also be formed via the oxidation of reduced species AS-0 by dioxygen

AS-0 + O2 f AS(O-O)

(V)

∆H ) -56 kcal ⁄ mol Figure 2 presents structural modifications of ASdO and AS-0 upon the contact with molecular oxygen yielding vanadium peroxo complex AS(O-O). According to the redox mechanism, reoxidation step III is preceded by the vanadium reduction step V5+ f V3+ (the dissociation energy of the VdO bond in the complex ASdO, ED(VdO) ) 137 kcal/mol)

ASdO f AS-0 + 1⁄2O2

(VI)

∆H ) + 76 kcal ⁄ mol For the redox mechanism, steps III and VI are separated by an intermediate (or intermediates) on the reaction pathway of substrate oxidation. The energy losses (76 kcal/mol) at the step of the oxygen vacancy formation (VI) are compensated at the reoxidation step (III). In the associative mechanism, instead of steps III and VI, step IV takes place that requires considerably lower energy, ca. 20 kcal/mol. In this case, the energy losses are partially compensated at the step of the organic substrate adsorption. Overall, vanadium does not alter the valent state and remains in the highest oxidation state V5+. An important role of peroxide oxygen in homogeneous catalysis is known for many oxidation reactions with participation of vanadium peroxo complexes.79 In heterogeneous oxidation catalysis, the first reports on their possible participation appeared in the literature only a few years ago. Cheng et al.80 investigated the mechanism of the propane oxidative dehydrogenation on the V4O10 cluster, which simulated the surface of a V2O5 catalyst and noted the key role of the V(O-O) peroxide in the hydrogen abstraction in propane and the following H2O desorption. On the basis of the experimental data and theoretical DFT calculations, Ohler and Bell81,82 as well as Chempath and Bell83 proposed that peroxide oxygen is the active species in the oxidation of methane to formaldehyde on a MoOx/SiO2 catalyst. In this case, the formation of formaldehyde occurs via the reaction of methane with the peroxide species. 3.2. Optimized Structures of Intermediates. The adsorbed forms of formaldehyde (FRD) on centers ASdO and AS(O-O) are the first intermediates in the oxidation reaction. Formaldehyde is bonded to the vanadium cation through the coordination of the lone pair of the H2CdO oxygen. Figure 3 shows the

Figure 4. The structures of the dioxymethylene (DOM) and the peroxooxymethylene (POM) complexes resulted from the transformation of intermediates FRD-ASdO and FRD-AS(O-O).

structures of the corresponding intermediates FRD-ASdO and FRD-AS(O-O). The formaldehyde adsorption energies are ∆H ) -12.4 and ∆H ) -15.5 kcal/mol, respectively. Figure 4 shows two intermediates formed at the incorporation of the terminal oxygen VdO and the peroxide oxygen V(O-O) of the complexes FRD-ASdO and FRD-AS(O-O) into methylene groups. In the former case, a dioxymethylene complex (DOM) is stabilized via the formation of the dioxymethylenevanadium four-member cycle. In the latter case, a peroxooxymethylene complex (POM) is stabilized via the formation of a five-member cycle. The methylene groups (>CH2) in both intermediates are structurally identical, and the carbon atoms exist in the sp3 hybridization states. Another possible set of intermediates is formed by the deprotonation of the methylene group. This leads to the hydrogen shift from the methylene group to oxygen atom of the active site. According to the redox mechanism this process consists of two consecutive steps H-shift-V

H-shift-O

DOM 98 HD 98 SFmk The structures corresponding to these steps are shown in Figure 5. A metastable hydride complex HD which includes bond V-H is formed at the first step. At the second step the hydrogen atom of the bond V-H is shifted to the bridge oxygen V-O-V, thus resulting in the formation of stable symmetric formate SFmk. Vanadium cations in complex SFmk are in the oxidation state 4+, which corresponds formally to a “biradical” form V•-O-V•. The calculated electron spin density of the vanadium cations is Fs ) (1.0. For the associative mechanism, the hydrogen transfer to the oxygen atom of the POM complex occurs also in two steps activation-O-O

H-shift-V-O•

POM 98 BR 98 SFss The structures corresponding to these steps are shown in Figure 5. The first step corresponds to the activation of the


Avdeev and Parmon

Figure 5. The structures of stable formates SFmk and SFss formed by the methylene groups deprotonation of intermediates DOM and POM through metastable biradical complexes HD and BR. Figure 7. Potential energy profiles (in kcal/mol) of the first step for the formaldehyde conversion to formic acid which correspond to the redox (A) and associative (B) reaction mechanisms. The appropriate structures of transition states (TS1mk, TS1as) are shown in the insets in the vicinity of one vanadium center.

Figure 6. The chemisorbed formic acid (ACD) on active sites AS-0 and ASdO is formed as a result of hydrogen transfer from the bridge oxygen V-OH-V of formate SFmk and terminal oxygen V-OH of formate SFss to oxygen of methoxy group O-CH-O-V.

peroxide bond O-O and is completed by formation of complex BR with OsO distance of 2.98 Å. The key component of this intermediate is a biradical form of oxygen, O•sO•, with the spin density on the oxygen atoms Fs ) +0.85 and Fs ) -0.88. At the second step, the functional group V-O•sO• inserts in the C-H bond. It results in the formation of asymmetric formate AF, which transforms into symmetric formate SFss with the hydrogenated terminal bond V-OH (Figure 5). During this step, both the vanadium atoms remain in the highest oxidation state V5+. The formation of coordinated formic acid HCOOH is achieved by protonation of the functional methoxy groups HCOO in complexes SFmk and SFss. The appropriate structures of these intermediates are shown in Figure 6The ACD-AS complex represents chemisorbed formic acid (ACD) on the reduced site AS-0 due to the hydrogen transfer from the bridge oxygen of the moiety V-OH-V to the oxygen atom of methoxy group. The complex ACD-ASdO is formed via hydrogen shift from terminal bond V-OH to oxygen of the group O-CH-O in the SFss formate and represents chemisorbed formic acid on the active site ASdO. Estimations of desorption heats of formic acid give: ∆H(ACD-ASdO) ) 16.2 kcal/mol and ∆H(ACD-AS) ) 31.6 kcal/mol. The intermediates shown in Figures 3-6 can be combined into two groups. The first group of intermediates (FRD-ASdO, DOM, HD, SFmk) contains only oxygen from the catalyst. Intermediates of the second group (FRD-AS(O-O), POM, BR, AF, SFss) contain both oxygen

of the catalyst and oxygen which originates from dioxygen. Other sets of intermediates that can be formed during deeper oxidation of formaldehyde to CO and CO2 are not discussed in the present paper. 3.3. Reaction Pathways. Let us discuss two sequences of intermediates, which correspond to the formaldehyde oxidation to formic acid according to the redox and associative mechanisms 1

⁄2O2 + FRD-ASdO f 1⁄2O2 + DOM f 1⁄2O2 + HD f SFmk + 1⁄2O2 f SFss (VII)

1

⁄2O2 + FRD-ASdO f FRD-AS(OO) f POM f BR f AF f SFss f ACD-ASdO (VIII)

The main difference between these two mechanisms is in the time and manner in which dioxygen enters the reaction pathway. Our calculations predict that in the absence of dioxygen the reaction follows the redox mechanism until the formation of a stable symmetric SFmk complex (sequence VII). But, the inclusion of dioxygen into the reaction during generation of the FRD-ASdO complex (coadsorption of the formaldehyde and dioxygen on ASdO) directs the reaction pathway toward the associative mechanism (sequence VIII). The energy profiles of pathways VII and VIII remain different until the stages leading to the formation of chemisorbed formates SFmk and AF, correspondingly, and pass through three elementary steps. In the redox mechanism, the first step starts with implementation of the terminal oxygen VdO of the active-site ASdO in the formaldehyde methylene group CH2 through transition state TS1mk to form complex DOM (Figure 7A). . In the associative mechanism, the terminal oxygen VdO is modified by dioxygen to form peroxide oxygen V(O-O) in accordance with reaction IV. Peroxide oxygen is embedded in the methylene group CH2 through transition state TS1as to form another complex POM (Figure 7B). The surface complexes DOM and POM are the basic intermediates in this reaction. The following transformations result from the intramolecular structural rearrangements of the surface complexes DOM and



Figure 8. Energy profiles (in kcal/mol) of the methylene group CH2 deprotonation of the intermediates DOM and POM are completed by the formation of the stable symmetric formate SFmk through transition states TS2mk, TS3mk in redox mechanism (A) and asymmetric formate AF through transition states TS2as, TS3as in associative mechanism (B). The structures of the transition states are shown in the insets.

Figure 9. Comparison of the energy diagrams of two reaction pathways of the formaldehyde oxidation to formic acid on the V/Ti-O oxide catalyst according to the redox and associative mechanisms. The energy is counted from the initial reactant energy E(1/2O2 + FRD + ASdO). The energy level E(O + FRD + ASdO) is a start mutual point of two of the reaction pathways. The rectangle shaded frames with arrows show (i) moments of the atomic oxygen inclusion in the reaction for the redox and associative mechanisms and (ii) desorption of formic acid (ACD). Both reaction pathways are completed with the regeneration of the active site ASdO. Here ED is the O2 dissociation energy and ∆H is the heat of reaction.

POM. The most important among them is apparently the hydrogen shift followed by the dissociation of the C-H bond in the methylene group (the deprotonation processes). These processes pass through two elementary steps and include the activation of the C-H bond, sp3 f sp2 rehybridization of the carbon in the methylene group and the actual hydrogen transfer.

Figure 8 illustrates the reaction pathways of the deprotonation processes according to the redox mechanism (Figure 8A) and associative mechanism (Figure 8B). These pathways are completed by the formation of the stable formate complexes. The second step in the redox mechanism leads to the formation of the hydride complex HD through transition states TS2mk. The


Figure 10. The calculated IR spectrum of peroxo complex AS(O-O) obtained with the Lorenz broadening ∆ν ) 5 cm-1 of the theoretical frequencies. All frequencies are scaled by 0.95812 (ref 90).

subsequent transformation of the complex HD (the third step) through TS3mk forms the symmetric formate SFmk. The second step in the associative mechanism through transition states TS2as generates the biradical intermediate BR. The third step through TS3as completes the hydrogen transfer and, as a result, the asymmetric formate AF is formed. Our numerical analysis showed that formate SFmk is the final product of the formaldehyde oxidation, and the subsequent transformation of the formate complex SFmk is possible only with participation of dioxygen. On the contrary, the asymmetric formate AF is subjected to a subsequent transformation through symmetric formate SFss to chemisorbed formic acid. Figure 9 shows full reaction pathways under discussion. The reaction pathways of the redox and associative mechanisms intersect at the point corresponding to the SFss intermediate. The SFss complex can further decompose following two reaction pathways. The first pathway is shown in Figure 9. It leads to the formation of the adsorbed formic acid ACD-ASdO with zero activation energy and the stabilization energy ∆E ) -78 kcal/ mol. The stabilization energy consists of two parts: the formic acid adsorption energy (-16 kcal/mol) and the reaction enthalpy ∆H ) - 62 kcal/mol. The formic acid desorption regenerates active site ASdO, and the consecutive conjoint adsorption of formaldehyde and dioxygen resumes the catalytic cycle. The second pathway leading to the deep oxidation starts by the hydrogen transfer from the HCOO group to the terminal oxygen V-OH: HCOO-V-OH f COO-V-OH2. This reaction results in the formation of chemisorbed species CO2 and H2O. Since this pathway is accompanied by breaking the C-H bond, one can expect that its activation energy will be high. Thus, the energetic parameters of the reaction profiles predict the high selectivity of the oxidation with respect to formic acid because the reaction pathway leading to the deep oxidation of formaldehyde appears in fact to be forbidden. The comparison of the activation energies shows that the associative mechanism step seems to be more probable. 3.4. Comparison with Experimental Data. The calculations have indicated that the energetic properties of the key step of the associative mechanism are determined by the formation energy of the metastable intermediate BR. As this intermediate is formed upon activation of the -O-O- bond in the peroxyoxo-methylene complex POM, an experimental identification of this complex by, e.g., Raman spectroscopy would be decisive proof of the associative mechanism in the studied reaction. We failed to find any reports on such experiments in the literature. However, there are some experimental data that seem to support indirectly the associative mechanism of the formaldehyde oxidation.

Avdeev and Parmon Busca et al.84,85 studied the formaldehyde adsorption and oxidation of methanol on oxide catalysts Al2O3, Fe2O3, MgO, SiO2, TiO2, and ZrO2 by FTIR spectroscopy and identified several intermediates like coordinated H2CdO, dioxymethylene, formate species, etc., whereas formic acid appeared as a minor product of the transformation of the formate species with water. The same intermediates were observed during the oxidation of formaldehyde30-32 over V/Ti-O catalysts but formic acid appeared to be the main product of the formaldehyde oxidation in the presence of dioxygen in the temperature range 100-200 °C.28,29 It was shown that the formic acid formation rate was nearly 3.5 times higher in the oxygen-containing pulses (CH2O/air) compared to the CH2O/He pulses free of oxygen. These results are in agreement with our conclusions. Indeed, the reaction pathway following the redox Mars-van Krevelen mechanism (CH2O/He pulses free of oxygen) does not lead to the desired product, formic acid, since the reaction is stopped after the formation of stable formate structures (scheme VII). However, the inclusion of dioxygen at this step initiates the continuation of the reaction to formic acid. In this case, the following transformations of the formate species to formic acid are evidently coupled to the reoxidation processes V-0 + 1/2O2 f VdO. Another series of experiments was related to the temperatureprogrammed reduction of several V/Ti-O supported oxide catalysts in hydrogen and followed by their oxidation.86-88 It was shown that monovanadates were the most active in the toluene partial oxidation and that the desorption of the oxygenated product (benzaldehyde) was impossible in the absence dioxygen. The monovanadate species were found to be the easiest to reduce but the most difficult to reoxidize. Popova et al.89 came to similar conclusions for the formaldehyde oxidation too. The monovanadate species were the most active in the formaldehyde oxidation to formic acid. Crystalline and amorphous V2O5 phases were shown to oxidize formaldehyde selectively to formic acid but exhibit low activity in this reaction. The observed features of the oxidation find a natural explanation, if to assume the participation of peroxide species in these processes. Indeed, the easier reducibility of monovanadates by H2 promotes the higher concentration of the reduced centers of the surface active sites (in our case active site f V3+-0). Its subsequent reaction with dioxygen results in the formation of peroxide species via the reaction, V3+-0 + O2 f V(O-O), which is energetically more preferable than the reaction V5+dO + 1/2O2 f V(O-O). According to our calculations, the appearance of the peroxide oxygen forces the reaction pathways to follow the associative mechanism. For the crystalline and amorphous forms of V2O5 that are very difficult to reduce, the conditions required for the formation of vanadium peroxo complexes are less favorable. As a result, the reaction follows a more energetically demanding redox mechanism. However, in this case the oxidation product is formed only with the participation of the dioxygen. Then desorption of the final product is coupled to the active-site reoxidation. This final reaction step is the same as in the associative mechanism. In our case, this pathway corresponds to the following sequence: SFmk + O f SFss f ACD-ASdO f ACD + ASdO (Figure 9). Earlier a similar result was obtained at the analysis of the reaction pathways for the oxidative dehydrogenation of propane over the V2O5 surface.80 The propene desorption is induced by binding of the gas-phase O2 to a V3+ site and the formation of a cyclic V(O-O) peroxide. The V(O-O) complex participates in the activation of the C-H bond of the propane and appears to be the key intermediate in the catalytic cycle.80



Figure 11. The catalytic cycle for the formaldehyde oxidation to formic acid on the Vi/Ti-O catalysts according to the associative mechanism (see the text for details). The shown surface structures of the intermediates and transition states are only in the region of the oxo-methylene-vanadia cycle. Process g corresponds to the gas-phase (not catalytic) reaction.

Finally, the third series of experiments is related to the adsorbed oxygen species on the reduced V2O3(0001) surface.90 The authors90 proved experimentally the formation of the vanadium peroxo complexes V(O-O) during the interaction of the gas-phase O2 with the reduced V3+ sites. Two frequencies observed at 951 and 1030 cm-1 in the FTIR spectra were assigned to the peroxo complex V(O-O) and vanadyl oxygen VdO, correspondingly. These conclusions were confirmed by the DFT calculations of the peroxo complex structure. For the active-site model used in the present work, the peroxo complex AS(O-O) forms via the interaction of gas-phase O2 with the reduced site AS-0. Figure 10 displays the calculated IR spectrum for the AS(O-O) model. Our calculations give ν(O-O) ) 960 and ν(VdO) ) 1041 cm-1. The theoretical value of the frequency shift ∆ν ) ν(VdO) - ν(O-O) ) 81 cm-1 agrees well with the experimental shift ∆ν ) 79 cm-1. Thus, the available experimental data combined with the results of our DFT analysis of the possible reaction pathways favor the catalytic cycle for the formaldehyde oxidation to formic acid over supported V/Ti-O catalysts that is shown in Figure 11. The reaction is initiated by coadsorption of formaldehyde and dioxygen on the active-site ASdO. The activation of the terminal bond VdO forms the peroxo complex FRDAS(O-O) (processes a and b). At the first step (reaction 1) the interaction of methylene group CH2 with peroxide species -O-O- leads to the formation of a new bond C-O-O-V via transition state TS1. This results in a stabilization of the POM complex with a five-membered oxo-metal cycle. The further steps are the deprotonation which includes reactions 2 and 3. The activation of the O-O bond forms an intermediate BR with biradical oxygen form O•-O• via transition state TS2. The next transformation completes the hydrogen transfer from the methylene group to the terminal oxygen through transition state TS3 and results in the formation of the single-bonded asymmetric formate AF. The hydrogen shift from the terminal oxygen V-OH to oxygen of the CdO group via symmetric formate SF forms the adsorbed formic acid ACD-ASdO without an activation barrier (processes 4 and 5). The formic acid desorption regenerates the active-site ASdO and thus closes the catalytic cycle (processes c and d).

4. Conclusions The two possible reaction pathways of the partial formaldehyde oxidation to formic acid on the V/Ti-O model catalyst corresponding to either the redox Mars-van Krevelen mechanism or the associative mechanism are analyzed with DFT. The subsequent transformation of the surface intermediates to formic acid on the suggested active site involves three successive steps. These steps correspond to breaking the C-H bond (deprotonation), hydrogen shift followed by the formic acid formation on the active site. The hydrogen migration processes seem to be the rate-determining steps for both mechanisms. Comparison of the activation barriers shows that the associative mechanism is more probable. This study has demonstrated the importance of peroxide species in formaldehyde oxidation to formic acid on the V2O5/TiO2 catalysts. Acknowledgment. The authors thank I. L. Zilberberg, G. Ya. Popova, and T. V. Andrushkevich for very helpful discussions of the results presented in this paper. This work was supported by the Russian Foundation for Basic Research (Grant 06-0308137). Supporting Information Available: Listings of atomic coordinates for the compounds discussed. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Boreskov, G. K. Kinet. Catal. 1973, 14, 7. (2) Mars, P.; van Krevelen, D. W. Chem. Eng. Sci. Suppl. 1954, 3, 41. (3) Vedrine, J. C. Stud. Surf. Sci. Catal. 1997, 110, 61. (4) Vedrine, J. C. Top. Catal. 2002, 21, 97. (5) Bond, G. C.; Vedrine, J. C. Catal. Today 1994, 20, 171. (6) Bond, G. C.; Vedrine, J. C. Catal. Today 2000, 56, 415. (7) Deo, G.; Wachs, I. E.; Haber, J. Crit. ReV. Surf. Chem. 1994, 4, 141. (8) Amiridis, M. D.; Wachs, I. E.; Deo, G.; Jehng, J. M.; Kim, D. S. J. Catal. 1996, 161, 247. (9) Dutoit, D. C. M.; Reiche, M. A.; Baiker, A. Appl. Catal., B 1997, 13, 275. (10) Khodakov, A.; Olthof, B.; Bell, A. T.; Iglesia, E. J. Catal. 1999, 181, 205.

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TiO2 Catalysts

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