Silica-supported Vanadium Oxide Catalysts Research Articles

Investigation of Silica-Supported Vanadium Oxide Catalysts by High-Field 51V Magic-Angle Spinning NMR

Supported V2O5/SiO2 catalysts were studied using solid-state 51V magic-angle spinning NMR at a sample spinning rate of 36 kHz and at a magnetic field of 19.975 T to provide a better understanding of the coordination of the vanadium oxide as a function of environmental conditions. Structural transformations of the supported vanadium oxide species between the catalyst in the dehydrated state and hydrated state under an ambient environment were revisited to examine the degree of oligomerization and the effect of water. The experimental results indicate the existence of a single dehydrated surface vanadium oxide species that resonates at −675 ppm and two vanadium oxide species under ambient conditions that resonate at −566 and −610 ppm. No detectable structural difference was found as a function of vanadium oxide loading on SiO2 (3% V2O5/SiO2 and 8% V2O5/SiO2). Quantum chemistry simulations of the 51V NMR chemical shifts on predicted surface structures were used as an aid in understanding potential surface va...

Potassium-modified silica-supported vanadium oxide catalysts applied for propene epoxidation

The influence of potassium ions on the structure and catalytic properties of silica-supported vanadium oxide catalysts was studied in the epoxidation of propene using N2O as an oxidant. The combination of different techniques (XRD, DR UV–vis, Raman, FTIR, and H2 TPR) for the characterization of potassium-doped V/SBA-15 catalysts reveals the formation of various types of vanadate species, including polyvanadates. Potassium changes the acid–base characteristics of the system and decreases the acidic character of surface vanadia. A decrease in the acidic character accounts for better selectivity in propene epoxidation; however, neutralization of active vanadium forms results in lower propene conversion. Textural, XRD and SEM/TEM, results indicate that the ordered hexagonal mesoporous structure with large pore diameters of the support is retained upon vanadium incorporation, although further modification with potassium ions results in a partial structure damage.

Elucidation of Anchoring and Restructuring Steps during Synthesis of Silica-Supported Vanadium Oxide Catalysts

We investigate the chemical reactions involved during the synthesis of supported vanadium oxide catalysts using chemical grafting of vanadium oxytriisopropoxide (VO(OiPr)3) to thermally pretreated silica under solvent-free conditions. VO(OiPr)3 is found to react with both site-isolated silanol (Si−OH) groups and strained siloxane (≡Si−O−Si≡) bridges at the silica surface. Solid-state 51V and 13C MAS NMR confirms the formation of two slightly different vanadium species associated with the two anchoring mechanisms. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), in situ Raman spectroscopy, and thermogravimetric analysis-differential scanning calorimetry-mass spectrometry (TGA-DSC-MS) were used to study the subsequent calcination, revealing the formation of a transient V—OH intermediate upon the release of propene, followed by the formation of isolated VO4 surface species upon elimination of water. X-ray absorption spectroscopy (XAS) and 51V MAS NMR of the calcined material confirm the conversion of the two original vanadium sites to a species with a single isotropic shift, confirming the formation of isolated, tetrahedral VO4 sites.

Normal mode analysis of silica-supported vanadium oxide catalysts: Comparison of theory with experiment

The vibrational structure of silica-supported vanadium oxide species has been studied by normal mode analysis using polyhedral oligomeric silsesquioxanes (POSSs) to describe the silica support. The analysis reveals that the vanadium oxide-related vibrational bands are characterized by significant contributions of several force constants. Their discussion in terms of single bond-unit designators is therefore not adequate. The consideration of the silica support is shown to be of importance for the vibrational spectrum. The theoretical results are fully consistent with experimental data for a silica-supported vanadium oxide catalyst with a vanadium coverage of 0.7atoms/nm2.

Reply to Comment on “Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts”

Reply to Comment on “Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts”

Hydrogen‐Atom Abstraction from Methane by Stoichiometric Vanadium–Silicon Heteronuclear Oxide Cluster Cations

Vanadium-silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O(2) background. Reactions of the heteronuclear oxide cations with methane in a fast-flow reactor were studied with a time-of-flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V(2)O(5))(n)(SiO(2))(m)](+) (n=1, m=1-4; n=2, m=1), and the estimated first-order rate constants for the reactions were close to that of the homonuclear oxide cluster V(4)O(10) (+) with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal-oxygen-centered radicals (O(t)*) were found in all of the stable isomers. These O(t)* radicals are active sites of the clusters in reaction with CH(4). The O(t)* radicals in [V(2)O(5)(SiO(2))(1-4)](+) clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH(4) activation on silica-supported vanadium oxide catalysts.

Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

The molecular structure of silica-supported vanadium oxide (VOx) catalysts over wide range of surface VOx density (0.0002−8 V/nm2) has been investigated in detail under dehydrated conditions by in situ multiwavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV−vis diffuse reflectance spectroscopy. Resonance Raman scattering is clearly observed using 244 and 325 nm excitations, whereas normal Raman scattering occurs using excitation at the three visible wavelengths. The observation of strong fundamentals, overtones, and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VOx Raman bands (920, 1032, and 1060 cm−1) whose assignments have been controversial. The resonance Raman spectra of dehydrated VOx/SiO2 show a V═O band at a smaller Raman shift than that in visible Raman spectra, an indication of the presence of two different surface VOx species on dehydrated SiO2 even at submonolayer VOx loading. Quantitat...

Characterization of Supported Vanadium Oxide Species on Silica: A Periodic DFT Investigation

The geometry, energetic, and spectroscopic properties of molecular structures of silica-supported vanadium oxide catalysts are studied using periodic density functional calculations. Isolated vanadia units deposed on amorphous silica are modeled at low coverage, 0.44 atoms nm−2. The models are built following the grafting process through the reaction of a vanadium precursor with surface silanols: OV(OH)3 + (Si−OH)n → OV(OH)3−n(O−Si)n + nH2O (with n = 1−3). The most stable grafted structures involve one vanadyl group together with n(V−O−Si) bonds. The predominance of the vanadate groups is analyzed as a function of hydration by means of atomistic thermodynamics. At dehydrated conditions, the trigrafted pyramidal OV(O−Si)3 species are predominant, whereas partial hydration stabilizes digrafted OV(OH)(O−Si)2 and monografted OV(OH)2(O−Si) species. The harmonic vibrational spectra for selected models are compared to recent experimental infrared and Raman data, for representative bands, and vibrational modes. Hydration effects are discussed in terms of thermodynamic stability and vibrational spectra. The results obtained in this study show that the pyramidal OV(O−Si)3, digrafted OV(OH)(O−Si)2, and monografted OV(OH)2(O−Si) models can exist at a support surface, a trend in agreement with recent experimental findings.

SELECTIVE CATALYTIC REDUCTION OF NITRIC OXIDE WITH AMMONIA OVER SILICA-SUPPORTED VANADIUM OXIDE CATALYST

The selective catalytic reduction (SCR) of nitric oxide with excess ammonia in the presence of oxygen on silica-supported vanadium oxide catalyst was studied in a packed-bed reactor, and a mathematical model was proposed for the processes occurring in the reactor. Experimental data were presented for evaluation of the accuracy of the proposed model. Good agreement was observed between the measured and calculated values of the conversion in the outlet of the reactor. Once the validity of the proposed model was verified, it was used to examine the effects of different parameters such as feed temperature, inlet feed composition, and gas hourly space velocity (GHSV) on the conversion of NO over V2O5/SiO2 catalyst for practical application. The results for the employed catalyst showed that high NO conversion occurred at temperatures of 280°–300°C, GHSV less than 2000 h−1 (STP), and O2 concentration greater than 10% v/v. These results clearly demonstrate the high potential for this catalyst to be applied commercially for the control of NOx emissions from flue gases of different sources.

Hydration effects on the molecular structure of silica-supported vanadium oxide catalysts: A combined IR, Raman, UV–vis and EXAFS study

The effect of hydration on the molecular structure of silica-supported vanadium oxide catalysts with loadings of 1–16 wt.% V has been systematically investigated by infrared, Raman, UV–vis and EXAFS spectroscopy. IR and Raman spectra recorded during hydration revealed the formation of V–OH groups, characterized by a band at 3660 cm −1. Hydroxylation was found to start instantaneously upon exposure to traces of water, reflecting a very high sensitivity of the supported vanadium oxide catalysts for H 2O. Further hydration resulted in the appearance of a V–O–V vibration band located around 700 cm −1 pointing to the formation of di- or polymeric species. EXAFS analysis at 77 K indicated structural changes as the oxygen coordination changed from four to five. Moreover, a V⋯V contribution was detected for the hydrated species. The IR, Raman and UV–vis data suggested a pyramidal anchoring of the dehydrated VO x species, whereas, the EXAFS data pointed to the presence of single V–O–Si bonded VO x species. This difference is attributed to water condensation effects at 77 K during EXAFS acquisition, resulting in a partial re-hydroxylation of the dehydrated samples, as confirmed by complementary IR and Raman analysis. Combining the results of this study with data from our previous studies [D.E. Keller, F.M.F. de Groot, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 109 (2005) 10223; D.E. Keller, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 110 (2006) 14313] as well as literature led to a reaction scheme in which a monomeric VO x species anchored by three Si–O–V bonds to the silica support (pyramidal-type structure) is transformed into a monomeric VO x species anchored by one Si–O–V bond (umbrella-type structure) by partial hydration of the catalyst material. This results in the formation of both V–O–H and Si–O–H bonds. At higher water pressures, larger vanadium oxide clusters are formed due to full hydration of the catalyst surface and a de-attachment of the vanadium oxide from the support surface. The results of this study provide evidence, that an umbrella-type structure ( i.e., Si–O–V O(OH) 2) could be present under catalytic conditions where H 2O is a reaction product ( e.g., partial oxidation of methanol to formaldehyde and oxidative dehydrogenation of alkanes). In other words, both the pyramidal ((Si–O) 3–V O) and the umbrella (Si–O–V O(OH) 2) model can exist at a support surface, their relative ratio depending on the hydration degree of the catalyst material. This study also illustrates that a corroborative characterization requires the use of multiple spectroscopic techniques applied at the same samples under almost identical measuring conditions.

Effects of alkali metal cations on the structures, physico-chemical properties and catalytic behaviors of silica-supported vanadium oxide catalysts for the selective oxidation of ethane and the complete oxidation of diesel soot

A comparative study on the effects of alkali metal on the structures, physico-chemical properties and catalytic behaviors of silica-supported vanadium catalysts for the selective oxidation of ethane and the complete oxidation of diesel soot was reported.

On the umbrella model for supported vanadium oxide catalysts

An earlier-proposed model for the molecular structure of VO 4 species on alumina was tested and compared with the pyramid model on a silica support. The model can be described as a chemisorbed O support V O(O 2) species with a similar geometry as on alumina. Results of DFT calculations on this model are consistent with the experimental Raman and EXAFS data collected on low-loaded silica-supported vanadium oxide catalysts. The band observed at 915 cm −1 is assigned to an O O stretch vibration. The thermal motion of the bound O 2 molecule can explain the broadness of this band. These findings, in combination with our previous work, demonstrate that the umbrella model is a viable, internally consistent model for supported vanadium oxide catalysts at least at low loadings.

Dehydrogenation of propane over a silica-supported vanadium oxide catalyst

Dehydrogenation of propane over a silica-supported vanadium oxide catalyst

Modification of photocatalytic center for photo-epoxidation of propylene by rubidium ion addition to V 2O 5/SiO 2

The photo-oxidation of propylene with molecular oxygen was investigated over silica-supported vanadium oxide catalysts (VS) at 323 K. The addition of Rb ions to VS resulted in the great improvement of the formation rate of propylene oxide in the photo-oxidation of propylene (175 μmol g −1 h −1 for Rb-ion-modified VS, V 2O 5 loading 0.5 wt%, Rb/V = 1.5). The photocatalytic activity was significantly enhanced when the Rb/V ratio was greater than 1.0. UV–Vis diffuse reflectance and X-ray absorption spectra of a series of Rb-ion-modified VS catalysts indicated the presence of one-to-one interaction between a Rb ion and isolated VO 4 tetrahedra over silica.

Temperature-Programmed Desorption Study of the Selective Oxidation of Alcohols on Silica-Supported Vanadium Oxide

The partial oxidation of methanol and ethanol on silica-supported vanadium oxide catalysts was studied using temperature-programmed desorption (TPD), Raman spectroscopy, and diffuse reflectance infrared spectroscopy (DRIFTS). Methanol TPD results for V2O5/SiO2 samples as a function of vanadia loading in conjunction with X-ray diffraction data and Raman spectra indicated that dispersed vanadia on silica agglomerates into vanadia crystallites during a CH3OH TPD experiment. For ethanol-dosed samples, agglomeration of the dispersed vanadia was less severe, and it was possible to measure the activation energy for the dehydrogenation of adsorbed ethoxides to produce CH3CHO. Assuming a preexponential factor of 10(13) s(-1), the activation energy for this reaction was estimated to be 132 kJ/mol. The results of this study further demonstrate that there is a relatively weak interaction between vanadia and silica and suggest that adsorbed methoxide species help facilitate agglomeration of dispersed vanadia.

Oxidation of benzene to phenol on supported Pt-VO x and Pd-VO x catalysts

Gas-phase oxidation of benzene using a mixture of oxygen and hydrogen has been carried out on silica-supported vanadium oxide catalysts modified with platinum or palladium. Catalyst activity and phenol selectivity were studied as a function of the precious metal used, the vanadium oxide loading as well as of temperature. The binary catalysts have been characterized by TPR and TEM. Pt-VO x /SiO 2 catalysts were more active than Pd-VO x /SiO 2 catalysts. By using platinum catalysts benzene conversion amounted to 1.0% ( S phenol=97%) at 413 K, whereas palladium catalysts reached a conversion of only 0.2% ( S phenol=86%) for the same contact time and temperature. The most active catalyst for the oxidation of benzene to phenol was a low vanadium loaded 0.5 wt.% Pt–3 wt.% V on silica catalyst. At temperatures above 413 K phenol selectivity decreased strongly because of enhanced total oxidation. Active catalysts need both components: a dispersed transition metal oxide such as VO x as well as small precious metal particles such as platinum. The activity of the catalysts arises from a close interaction between the redox-active compound VO x and the electron mediator and hydrogen activator platinum as was confirmed by correlation of catalytic results and catalyst properties. Highly dispersed platinum particles are exclusively located on the vanadium oxide covered surface as demonstrated by TEM investigations. TPR studies showed and enhanced reducibility of a part of vanadium(V) oxide indication a close neighborhood of VO x and platinum.

Selective photo-assisted catalytic oxidation of methane and ethane to oxygenates using supported vanadium oxide catalysts

Selective photooxidation of light alkanes, mainly methane and ethane, into the corresponding aldehydes was achieved using silica-supported vanadium oxide catalysts under UV irradiation at elevated temperature. Photooxidation of methane using the V2O5/SiO2-IW (incipient wetness) (0.6 mol% V) catalyst at 493 K for 2 h gave 68 µmol of methanal, which corresponds to 76 mol% selectivity and 0.48 mol% one-pass yield. Photooxidation of ethane using V2O5/SiO2 (calcined at 1023 K) -IW (0.6 mol% V) catalyst for 1 h gave 85 µmol of ethanal, which corresponds to 90% selectivity and 1.1% one-pass yield. The catalysts prepared by the sol–gel method also showed activity, especially for the reaction of ethane. Both UV irradiation and a reaction temperature as high as 500 K were essential. The photo-assisted catalytic reactions were very sensitive to the reaction temperature, method of preparation of the catalyst, and addition of water vapour. While the reaction of methane was inhibited by the addition of water vapour, the photooxidation of ethane and propane was promoted in the presence of a controlled amount of water vapour. In addition, the reaction with methane required UV irradiation at a wavelength <310 nm, whereas the reaction with ethane or propane proceeded by irradiation at longer wavelength. According to Raman and UV diffuse reflectance spectroscopic studies and XRD studies, only isolated four-coordinated vanadium oxide surface species were considered to show catalytic activity toward the photooxidation of methane, while accumulated vanadium surface species, not crystalline V2O5, were shown to be active for the photooxidation of ethane and propane.

Partial Oxidation of Methane on Silica-Supported Vanadia Catalysts. The Relevance of Catalyst BET Area and Gas-Phase Activation

Silica-supported vanadium oxide catalysts with different BET areas have been used in partial oxidation of methane to formaldehyde. The BET area of the silica support not only plays an important role in the activation of methane but also in promoting the degradation of reaction intermediates. A reaction scheme is proposed where HCHO and CO2 are primary products, whereas CO is a secondary product from the degradation of formaldehyde and hydrocarbons. A large BET area led to degradation of hydrocarbons to CO, whereas on low-BET-area catalysts, this side reaction is partially inhibited with the simultaneous formation of C2 hydrocarbons. The general trend observed is that the activity decreases with decreasing BET areas. Gas-phase reactivity increases the initial activation of methane, but does not increase the degradation of reaction intermediates as much as surface area does thus providing a selective activation for methane.

On Oxygen Chemisorption for Characterization of Silica-Supported Vanadium Oxide Catalysts

Oxygen chemisorption experiments on prereduced silica-supported vanadia catalysts have been performed in order to determine the optimum conditions for measuring the relative dispersion of vanadium oxide on the silica substrate. By following the extent of vanadia reduction microgravimetrically, it is shown that an appropriate prereduction of the catalyst at 823 K under hydrogen is fundamental in obtaining a reproducible well-defined stoichiometrically reduced vanadium oxide system (V3+). AT 643 K the reduction process is very much slower and requires very long times in order to approach a plateau. Measurement of the binding energies of the V2p3/2core level spectra indicates that high-temperature oxygen chemisorption (643 K) results in a reoxidation of the prereduced vanadium oxide species within crystallites leading to an overestimation of the number of exposed vanadium sites. Chemisorption at low temperatures (298 or 195 K) does not affect the stoichiometrically reduced vanadium oxide species within crystallites, thus allowing the number of exposed surface vanadium sites to be measured.

Comparison of Silica-Supported MoO3and V2O5Catalysts in the Selective Partial Oxidation of Methane

Silica-supported molybdenum oxide and vanadium oxide catalysts have been prepared with a wide range of surface metal loadings (0.3–3.5 metal atom/nm2). Isolated surface metal oxide species are dominant at surface metal loadings on silica below ca. 1 metal atom/nm2while at higher loadings, the metal oxide species tend to aggregate into crystalline MoO3and V2O5particles. Both catalyst series have been tested in the selective oxidation of methane with molecular oxygen at atmospheric pressure. The reactivity for methane conversion appears to be essentially related to dispersed isolated surface metal oxide species, with the surface vanadium oxide species being more reactive than molybdenum oxide species. The higher reactivity of the surface vanadium oxide sites may be related to the different nature of the interaction of the oxides with oxygen. Silica-supported vanadium oxide catalysts, despite their higher activity, result in lower yields to HCHO than silica-supported molybdenum oxide catalysts.