Catalytically Active Vanadia Species on Silica: Effect of Oxygen and

Article pubs.acs.org/JPCC

Catalytically Active Vanadia Species on Silica: Effect of Oxygen and Water Joachim Sauer,* Marc Pritzsche, and Jens Döbler Institut für Chemie, Humboldt Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany ABSTRACT: Energies and free energies are calculated by density functional theory for the reaction of one and two water molecules with monomeric vanadyl species anchored on silica surface models. Whereas monohydrated species may be present depending on the local surface structure, dihydrated species are disfavored by entropy. Vibrational frequencies are calculated and compared with observed Raman spectra in the range between 900 and 1100 cm−1. Moreover, the oxidation of vanadyl species to vanadium peroxo species and the formation of vanadium oxo peroxo species anchored by one V−O−Si bond to the support (“umbrella model”) is studied. The calculated energies show that the formation of peroxo species is unlikely for surface models.

1. INTRODUCTION For the important class of supported transition-metal oxide catalysts, the characterization of the size and structure of active species is a priority but still a challenge. Here we look at vanadia supported on silica, an industrially relevant solid catalyst.1 The prevailing picture of the surface structure includes active species, ranging from monomeric over oligomeric to polymeric transition-metal oxide clusters anchored on the surface of the supporting oxide.2 Although a clear spectroscopic signature for V−O−V bonds is still missing,3−5 in the low-coverage limit, the presence of monomeric species is consistent with all known experimental facts. Mostly, a pyramidal OV(O−Si)3 structure is assumed with a vanadyl species connected to the surface via three interface oxygen atoms (Figure 1). Hypothetically, such a structure is formed by condensation of OV(OH)3 onto hydroxyl groups on the silica surface. However, the specific geometric structure of the surface may not allow for three interface bonds or the water partial pressure may prevent complete condensation,

hence OV(OH)3−n(O−Si)n sites with only two or one interface bonds and one or two remaining V−OH groups (Figure 1) have also been suggested, for example, refs 6−8. Because these models are interconverted by hydration/ dehydration reactions, their relative stability depends on the water partial pressure in the given environment. Reoxidation of the active sites consumed in the partial oxidation reaction most likely involves pyramidal peroxo species,9 O2V(OSi)3, also shown in Figure 1. The controversial “umbrella” species8,10−12 featuring a vanadyl and a peroxo group and anchored by only one interface V−O−Si bond (Figure 1), OV(O2)(OSi), would require oxidizing and hydrating conditions. The present study uses density functional theory (DFT) in combination with statistical thermodynamics to determine the relative stability of the surface species shown in Figure 1 as a function of the temperature and O2 and H2O partial pressures. To model the different vanadia sites on a generic silica surface, we adopt our well-tested3,13,14 POSS (polyhedral oligomeric silsesquioxane) model15,16 as well as models inspired by the cristobalite surface as previously described.17

2. METHODS 2.1. DFT Calculations. The calculations were performed with TURBOMOLE 5.718,19 and employed the B3LYP-hybrid functional20,21 with TZVP basis sets for all atoms.22 Minimum energy structures were characterized by calculation of their vibrational frequencies based on the analytical second derivatives. The dangling bonds of the POSS models were terminated with deuterium to prevent artificial couplings between the modes of the OV(O−)3 moiety and Si−H bending modes. 2.2. Thermodynamics. For the formation of (partially) hydrated and oxidized models according to Special Issue: John C. Hemminger Festschrift Received: May 24, 2014 Revised: July 8, 2014

Figure 1. Possible vanadium oxo, peroxo, and hydroxo species on the silica surface. © XXXX American Chemical Society

A

dx.doi.org/10.1021/jp505114t | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

model + mH 2O + nO2 → [model, mH 2O, nO2 ]

the Gibbs free energy change is

(1)

23,24

ΔR G(T , pH O , pO ) 2

2

= μ[model, mH O, nO ] − μmodel − mμ(g)H O − nμ(g)O 2

2

2

(2)

2

The chemical potentials for the (ideal) gas phase components H2O and O2 are given by μ(g)(p , T ) = E0 − RT ln(Q vib + Q rot + Q trans) + RT

Figure 2. Polyhedral oligomeric silsesquioxane models for O V(OSi)3 1, OV(OH)(OSi)2 2, OV(OH)2(SiO) 3, O2V(OSi)3 4, and OV(O2)(OSi) sites 5.

(3)

DFT results are used to calculate the partition functions for vibrations, Qvib, and rotations, Qrot, as well as E0

E0 = Eel + EZPV

(4)

3.1. Hydration. The positive ΔG298 values in Table 1 for reactions I and II, 18 and 16 kJ/mol, respectively, show that even for a water partial pressure of 1 bar temperatures below room temperature are needed to keep hydrolyzed Si−O−V bonds on the surface. For silica surface models inspired by the cristobalite surface structure (see ref 17 for details on the models), we have obtained ΔG298 values between 31 and 44 kJ/ mol for reaction with the first water molecule and between −13 and 18 kJ/mol for the second water adsorption. This shows that the first step is the more difficult one, at least for this surface. Tielens and coworkers have studied the hydration of the pyramidal structure for a periodic model of amorphous silica.25 Their OV(OSi)3 structure needs 92 kJ/mol to form from OV(OH)3, and formation of the monohydrated structure is very exothermic, −116 kJ/mol. For a water partial pressure of 1500 Pa (the saturation vapor pressure at 25 °C is 3169 Pa),26 they found that this structure is stable up to 550 K, whereas the dihydrated structure is stable only up to 220 K. The formation energy for the latter (−40 kJ/mol) is close to our value in Table 1 (−44 kJ/mol). For additional local structures, they obtained hydration energies of −6 and −20 kJ/ mol (all energies calculated from the data in table 1 of ref 25). We conclude that vanadium oxo species OV(OH)3−n(O− Si)n sites anchored with only two V−O−Si interphase bonds and one remaining V−OH group may be present on silica supports under ambient conditions, provided that the local surface structure prevents the formation of three stable V−O− Si anchoring bonds. To discuss the effect that hydration may have on the vibrational spectra of monomeric vanadia sites, we compare calculated vibrational wavenumbers of models 2 and 3 with the results for the dehydrated pyramidal model 1. Table 2 shows the V−O bond distances and the wavenumbers of characteristic modes of the OVO3 unit. For model 1, four characteristic modes are considered: the VO stretch vibration, the in-phase vibration, and two out-of-phase vibrations of the three V−O−Si bonds. Note that these are still delocalized normal modes in which the specified patterns dominate. The wavenumbers fall into the ranges that have been derived before from calculations for models spanning a wide range of local structures for silica surfaces:17 1086−1020 cm−1 for V−O−Si in phase modes, 1047−1013 cm−1 for vanadyl modes, and 962−873 for V−O− Si out-of-phase modes.17 These calculations explained the Raman bands observed at 1059, 1034, and 940 cm−1 for vanadia on amorphous silica,27 although the assignment was not the same.27 In multiwavelength Raman experiments the same bands have been observed at 1060, 1032, 920 cm−1,4 but a minority species was also detected characterized by an additional band at

from the total electronic energy, Eel, and the zero point vibrational energy, EZPV. The solid-phase models do not have rotational or translational degrees of freedom, and their volume change is neglected. Their chemical potentials are hence defined as μcluster (T ) = E0 − RT ln Q vib

(5)

With the chemical potential at 0 K as standard μ(g)(0 K) = E0

(6)

the change of the chemical potential becomes Δμ(g)(p , T ) = μ(g) − E0 = −RT ln(Q vib + Q rot + Q trans) + RT

(7)

and the Gibbs free energy for adsorption may be calculated as ΔR G(T , pH O , pO ) = ΔR E0 + Δμ[model, mH O, nO ] − Δμmodel 2

2

2

(g)

− mΔμ

H2O

−

2

nΔμ(g)O 2

(8)

with the energy of adsorption at 0 K ΔR E0 = E0(model, mH 2O, nO2 ) − E0(model) − mE0(H 2O) − nE0(O2 )

(9)

Because only the chemical potential of the gas-phase species depends on pressure Δμ(g)(p , T ) = Δμ(g)(p0 , T ) + RT ln(p /p0 )

(10)

0

we get (p is the standard pressure) ΔR G(T , pH O , pO2 ) = ΔR E0 + ΔR [Δμ(p0 , T )] − mRT 2

ln(pH O /p0 ) − nRT ln(pO /p0 ) 2

2

(11)

3. RESULTS AND DISCUSSION Figure 2 shows the POSS models for which calculations are performed. There are four possible reactions of the pyramidal model 1 with oxygen and water. Chemisorption of one water molecule hydrolyzes one of the three V−O−Si interface bonds, leading to structure 2 with a pair of hydroxyl groups, one on silica and the other on the vanadia species. Hydrolysis of a second V−O−Si interface bond leads to structure 3. Oxidation using half an oxygen molecule forms the peroxo species 4. Finally, the reaction of one water molecule and half an oxygen molecule leads to the hypothetical umbrella structure 5. Table 1 shows the energies and free energies for the reactions. B



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Table 1. Reaction Energies, Enthalpies, and Standard Gibbs Free Energies at 298 and 800 K in kJ/mola I II III IV V a

1 2 1 1 3

ΔREel

ΔRE0

ΔRH298

ΔRG0298

ΔRH800

ΔRG800

−27 −44 136 126 197

−20 −33 139 137 189

−19 −35 138 122 176

18 16 162 199 164

−9 −28 143 139 175

79 103 202 298 116

+ H2O → 2 + H2O → 3 + 1/2O2 → 4 + H2O + 1/2O2 → 5 - H2O + 1/2O2 → 5

B3LYP/TZVP results.

Table 2. Bond Distance (pm) and Vibrational Wavenumbers (cm−1) for Different Models bond distances bond a

VO V−O−Si V−O−Si/Hb V−O−Si/Hc SiOH and VOH bend

a

1 157.7 177.4 177.4 177.4

wavenumbers

2

3

157.8 176.1 176.3 180.0

157.6 177.0 175.6 182.1

1

2 a

1024 1048 899 899

3 a

1025 1033 (s) 917 (as) − 1018 1017

1033a

1104 1009 960

Scaled by 0.934, ref 17. bModel 3. cModels 2 and 3.

1041 cm −1 . It was tentatively assigned to a partially hydroxylated pyramidal structure. The assignment was based on the opinion that the VO stretching frequency of a partially hydrated species is generally lower than that of a dehydrated species, and the authors were calling for further theoretical modeling on this issue. The results for models 2 and 3 in Table 2 provide some insight into the effect of hydration. On formation of VOH groups by hydrolysis of V−O−Si bonds, contrary to the opinion expressed in ref 4, the vanadyl band remains constant or even shifts to higher wavenumbers. For the monohydrated structure, the in-phase and out-of-phase vibrations of the two V−O−Si bonds shift to lower and higher wavenumbers, respectively, which makes it difficult to distinguish between nonhydrated and hydrated vanadyl species. These shifts are due to the change of the f+2c, f−c, f−c splitting pattern for three identical Si−O−V bonds (f = 950, c = 50 cm−1) to an f ′+c′, f ′− c′ pattern for two identical Si−O−V bonds (f ′ = 970, c′ = 63 cm−1) and a change of the f and c parameters due to different couplings with the other modes. The spectra will be further complicated by SiOH bending vibrations at 1017 and 1018 cm−1 for model 2 and delocalized SiOH and VOH bending vibrations around 1104, 1009, and 960 cm−1 for model 3. We also find that the normal modes are very delocalized over the different internal coordinates in this wavenumber range. We conclude that a clear spectroscopic feature that would allow distinguishing between the nonhydrolyzed and (partially hydrolyzed) vanadium oxo species does not exist. Some information is available on the effect that the presence of sites with hydrolyzed V−O−Si bonds will have on the catalytic activity. Calculations for the oxidative dehydrogenation of small alkanes yield only small differences, very few kilojoules per mole, between barriers calculated for the pyramidal O Si(OSi)3 site and the limiting case of a “fully hydrated” OV(OH)3 model,28 but the same may not be true for entropy effects. Also, methanol oxidation has been studied for the OV(OH)3 model.29 3.2. Peroxo Species. According to the data in Table 1, the formation of the originally proposed10 “umbrella” structure 5 with both a vanadium oxo and peroxo group on a partially

hydrated surface, 1 + H2O + 1/2O2 → 5, is very unlikely at any reasonable temperature and oxygen pressure. How was it possible that the authors of ref 10 have been misguided by their model calculations? The reason is use of an ill-designed highly strained model 1′;10 see Figure 3. In the

Figure 3. Models from ref 10 for OV(OSi)3 1′, OV(OH)2(SiO) 3′, and OV(O2)(OSi) sites 5′.

“dry” situation (pyramidal structure), it corresponds to a VO4 tetrahedron sharing a face with one SiO4 tetrahedron of the support, not a corner with three SiO4 tetrahedra. This creates a highly strained totally nonrealistic structure. On hydration, reactions I and II, the strain is released and the energy of reaction I becomes more negative (−373 kJ/mol), almost one order of magnitude. Also, the energy for the reaction with the second water molecule is more negative, −132 compared with −44 kJ/mol. Therefore, it does not come as a surprise that for the formation of the “umbrella” structure according to reaction IV, instead of +126 kJ/mol (model 5), 116 kJ/mol are calculated (model 5′). In agreement with the present calculations Molinari and Wachs have provided evidence, for both hydrated and nonhydrated vanadia-silica catalysts, that the “umbrella” structure is not present.11 They used a vanadium peroxo compound with a chelating ligand as reference, There are many examples of stable peroxo-vanadate compounds; see, for example, ref 30, and a study of surfaceinspired molecular vanadium oxide catalysts has found evidence of peroxide intermediates,31 but on (supported) vanadia surface structures they are very unstable, as the results for reaction III show. This is the ultimate reason for the instability of the umbrella structure. Our results are also in agreement with the findings of a joined experimental-computational study of the interaction of oxygen with a reduced V2O3(0001) surface, C



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which exhibits V(III) sites.32 At low temperatures (90 K) and under UHV conditions, Kuhlenbeck et al. were able to identify peroxo species (O−O vibration at 951 cm−1) on the surface that decomposed into vanadyl species between 170 K and room temperature. The vanadium peroxo species is also present in the V3O8+ gas phase cation (O−O stretching vibration at 991 cm−1),33 whereas the V3O7+ cluster cation is a gas-phase model of the pyramidal OV(−O−)3 site.33 Computational studies9,34 have also shown that peroxo species form on reoxidation of reduced vanadia catalyst. Because they are highly reactive, their stationary concentration will be extremely low. Nevertheless, mechanistic studies have to take them into account. For example, when N2O instead of O2 is used as oxidant, peroxo species do not form,9 and this may explain the observed reactivity differences.

(6) Nguyen, L. D.; Loridant, S.; Launay, H.; Pigamo, A.; Dubois, J. L.; Millet, J. M. M. Study of New Catalysts Based on Vanadium Oxide. J. Catal. 2006, 237, 38−48. (7) Launay, H.; Loridant, S.; Pigamo, A.; Dubois, J. L.; Millet, J. M. M. Vanadium Species in New Catalysts for the Selective Oxidation of Methane to Formaldehyde: Specificity and Molecular Structure Dynamics with Water. J. Catal. 2007, 246, 390−398. (8) Keller, D. E.; Visser, T.; F, S.; Koningsberger, D. C.; Weckhuysen, B. M. Hydration Effects on the Molecular Structure of SilicaSupported Vanadium Oxide Catalysts: A Combined Ir, Raman, UvVis, and Exafs Study. Vib. Spectrosc. 2007, 43, 140−151. (9) Rozanska, X.; Kondratenko, E. V.; Sauer, J. Oxidative Dehydrogenation of Propane: Differences between N2o and O2 in the Reoxidation of Reduced Vanadia Sites and Consequences for Selectivity. J. Catal. 2008, 256, 84−94. (10) van Lingen, J. N. J.; Gijzeman, O. L. J.; Weckhuysen, B. M.; van Lenthe, J. H. On the Umbrella Model for Supported Vanadium Oxide Catalysts. J. Catal. 2006, 239, 34. (11) Molinari, J. E.; Wachs, I. E. Presence of Surface Vanadium Peroxo-Oxo Umbrella Structures in Supported Vanadium Oxide Catalysts: Fact or Fiction? J. Am. Chem. Soc. 2010, 132, 12559−12561. (12) Ding, X.-L.; Xue, W.; Ma, Y.-P.; Zhao, Y.-X.; Wu, X.-N.; He, S.G. Theoretical Investigation of the Selective Oxidation of Methanol to Formaldehyde on Vanadium Oxide Species Supported on Silica: Umbrella Model. J. Phys. Chem. C 2010, 114, 3161−3169. (13) Döbler, J.; Pritzsche, M.; Sauer, J. Oxidation of Methanol to Formaldehyde on Supported Vanadium Oxide Catalysts Compared to Gas Phase Molecules. J. Am. Chem. Soc. 2005, 127, 10861−10868. (14) Rozanska, X.; Fortrie, R.; Sauer, J. Oxidative Dehydrogenation of Propane by Monomeric Vanadium Oxide Sites on Silica. J. Phys. Chem. C 2007, 111, 6041−6050. (15) Sauer, J.; Hill, J.-R. The Acidity of Surface Silanol Groups. A Theoretical Estimate Based on Ab Initio Calculations on a Model Surface. Chem. Phys. Lett. 1994, 218, 333−337. (16) Civalleri, B.; Garrone, E.; Ugliengo, P. Vibrational Modes of Isolated Hydroxyls of Silica Computed Ab Initio in a Cluster Approach. Chem. Phys. Lett. 1998, 294, 103−108. (17) Döbler, J.; Pritzsche, M.; Sauer, J. Vibrations of Silica Supported Vanadia: Variation with Particle Size and Local Surface Structure. J. Phys. Chem. C 2009, 113, 12454−12464. (18) Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Electronic Structure Calculations on Workstation Computers: The Program System Turbomole. Chem. Phys. Lett. 1989, 162, 165−169. (19) Treutler, O.; Ahlrichs, R. Efficient Molecular Numerical Integration Schemes. J. Chem. Phys. 1995, 102, 346−354. (20) Becke, A. D. ″B3LYP″ Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (21) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (22) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829−5835. (23) Hobza, P.; Sauer, J.; Morgeneyer, C.; Hurych, J.; Zahradník, R. Bonding Ability of Surface Sites on Silica and Their Effect on Hydrogen Bonds. A Quantum Chemical and Statistical Thermodynamic Treatment. J. Phys. Chem. 1981, 85, 4061−4067. (24) Hofmann, A.; Sauer, J. The Surface Structure of Hydroxylated and Sulphated Zirconia. A Periodic Density-Functional Study. J. Phys. Chem. B 2004, 108, 14652−14662. (25) Islam, M. M.; Costa, D.; Calatayud, M.; Tielens, F. Characterization of Supported Vanadium Oxide Species on Silica: A Periodic DFT Investigation. J. Phys. Chem. C 2009, 113, 10740− 10746. (26) Lide, D. R. CRC Handbook of Chemistry and Physics, 74th ed.; CRC Press: Boca Raton, FL, 1993. (27) Moisii, C.; Curran, M. D.; van de Burgt, L. J.; Stiegman, A. E. Raman Spectroscopy of Discrete Silica Supported Vanadium Oxide:

4. CONCLUSIONS Vanadium oxo hydroxyl species OV(OH)(O−Si)2 anchored with only two V−O−Si interphase bonds may be present on silica supports under ambient conditions if the local surface structure prevents the formation of three anchoring V−O−Si bonds. Dehydrated structures are disfavored by entropy. Characteristic spectroscopic features that would allow discriminating between nonhydrated and hydrated species could not be found. Specifically, there is no shift of the vanadyl band to lower wavenumbers on hydration. Peroxo groups on a vanadyl site (“umbrella” model) are exceedingly unstable for both energy and entropy reasons. Vanadium peroxo groups are also unstable but may be formed on reoxidation of reduced vanadium oxide sites in the catalytic cycle.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest .

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ACKNOWLEDGMENTS This work has been supported by “Deutsche Forschungsgemeinschaft” (CRC 1109) and the “Fonds der Chemischen Industrie”.

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REFERENCES

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Catalytically Active Vanadia Species on Silica: Effect of Oxygen and

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