Supported Vanadium Oxide Research Articles

ChemInform Abstract: Molecular Engineering of Supported Vanadium Oxide Catalysts Through Support Modification.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A Raman Spectroscopy Study of Alumina-Supported Vanadium Oxide Catalyst during Propane Oxidative Dehydrogenation with Online Activity Measurement

The use of an advanced in situ methodology that combines in situ Raman spectra during catalytic operation and simultaneous online activity measurement is used to study the performance and structure of supported vanadium oxide catalysts during propane oxidative dehydrogenation (ODH). In situ spectroscopy during reaction conditions, with online activity measurement, comprises the simultaneous use of both catalytic and spectroscopic measurement on the same sample. Thus, the possibility of assessing a structure–activity relationship at a molecular level becomes reliable since the structures observed correspond to the working catalyst, as determined online. It is shown that the structure of supported vanadium oxide catalyst under propane ODH is close to that of the system under dehydrated conditions but may show a very moderate extent of reduction. The extent of reduction is strongly dependent on the O2/C3H8 ratio in the reaction feed. Raman spectroscopy during reaction conditions with online activity measurement shows that surface polymeric vanadium oxide species are more reducible than isolated surface polymeric species. The reduction of surface polymeric species decreases moderately the conversion values but has no appreciable effect on the conversion and selectivity values during propane ODH reaction. Therefore, the active site for propane ODH reaction on alumina-supported vanadia must be a single VO4 site, and no special arrangement of V sites appears to be critical. It is also discussed that the V–O–V bond may not be critical for this reaction.

Preparation, characterization and activity testing of vanadia catalysts deposited onto silica and alumina supports by atomic layer deposition

A gas phase preparation technique, atomic layer deposition (ALD), was applied to synthesize supported vanadium oxide catalysts. The growth of vanadia on silica and alumina was achieved by chemisorption of the volatilized vanadium precursor onto surface hydroxyl groups of the support, followed by oxidative treatment in air. The influence of the preparative method on surface structure, redox and acidic properties and catalytic activity was demonstrated by examining also the corresponding conventionally impregnated catalysts. The catalysts were characterized by chemical analysis, BET surface area measurements, X-ray diffraction (XRD), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and 51 V solid state nuclear magnetic resonance ( 51 V NMR). Tetracoordinated surface vanadia species were detected as the principal vanadium species in all the catalysts studied. The ALD catalysts, compared to the analogous impregnated ones, were found to be highly dispersed and more easily reduced, as revealed by temperature programmed reduction (TPR). The results from measurements using ammonia adsorption microcalorimetry and pyridine adsorption infrared (IR) spectroscopy indicated that the number, strength and strength distribution of the surface acid sites were affected by the preparative mode and the support. The same Lewis and Brønsted acid sites were detected for all impregnated and ALD catalysts, but those prepared in the gas phase showed slightly stronger acidic characteristics. In propane dehydrogenation (DH), the rate of propene formation by the ALD catalysts was higher than that achieved by the conventionally impregnated catalysts.

Characterization of Supported Vanadium Oxide Catalysts. Nature of the Vanadium Species in Reduced Catalysts

The nature and redox properties of vanadium species on metal oxide supported vanadium catalysts have been studied. The calcined metal oxidesAl2O3, MgO, Mg−Al−hydrotalcite (Al/Mg = 0.33), and Mg−Al−Spinel (Al/Mg = 3.0)have been employed as supports. TPR and IR spectra of adsorbed CO show a different reducibility of the vanadium species related to the acid−base character of the metal oxide support. This is also confirmed by the EPR results of both calcined and reduced catalysts. A reduction of V5+ to the V3+ species seems to be more favored on acid metal oxides. On basic metal oxide supports, the reducibility seems to be hindered, leading preferentially to the formation of V4+, although V3+ ions are also formed at higher reduction temperatures. A shift in the carbonyl frequency of the V4+−CO and V3+−CO complexes to lower frequencies by increasing the basicity of the support could be related to an electron donor effect of the support on the electron density of the vanadium species. In addition, a higher reoxidation ability of the V4+ and V3+ species is observed on the more acidic supports than on the basic ones.

A combined theoretical and experimental study of supported vanadium oxide catalysts

In order to investigate the nature of VO x species supported on SiO 2, γ-Al 2O 3 and TiO 2, a combination of a theoretical study and an experimental characterization is carried out. A molecular orbital approach of the extended Hückel type is performed to measure the plausibility of different postulated VO x species (monovanadates, divanadates, polyvanadates, etc.). The experimental characterization is performed by TPR, oxygen uptake measurements and FTIR of adsorbed pyridine. Both theoretical and experimental results indicate that the structure of VO x and its interaction with the support are strongly influenced by the nature of the oxide carrier. On TiO 2 and γ-Al 2O 3 dimers are the most abundant species, while on SiO 2 only monomers are formed.

Molecular Engineering of Supported Vanadium Oxide Catalysts Through Support Modification

Highly dispersed, multilayered surface metal oxide catalysts (V2O5/MO x /SiO2, M = Ti(IV), Zr(IV) or Al(III)) were successfully synthesized by taking into account various factors that govern the maximum dispersion of metal oxide species on silica. The characterization results revealed that the molecular structures of the surface vanadium oxide species on the modified supports are a strong function of environmental conditions. The surface vanadium oxide species under dehydrated conditions are predominantly isolated VO4 units, similar to the dehydrated V2O5/SiO2 catalysts. Upon hydration, the surface vanadium oxide species on the modified supports consist of polymerized VO5/VO6 units and/or less polymerized (VO3) n species, which depend on the vanadia content and the specific second metal oxide loading. The surface V cations are found to preferentially interact with the surface metal (Ti, Zr or Al) oxide species on silica. The V(V) cations in the dehydrated state appear to possess both oxygenated ligands of Si(IV)–O− and M–O−. Consequently, the reducibility and catalytic properties of the surface vanadium oxide species are significantly altered. The turnover frequencies of the surface VO4 species on these modified supports for methanol oxidation to redox products (predominantly formaldehyde) increase by more than an order of magnitude relative to the unmodified V2O5/SiO2 catalysts. These reactivity enhancements are associated with the substitution of Si(IV)–O− oxygenated ligands by less electronegative M–O− ligands in the O=V(–O–support)3 structure, which strongly suggests that the bridging V–O–support bonds play a key role in determining the reactivity of the surface vanadium oxide species on oxide supports.

Raman spectroscopy during catalytic operations with on-line activity measurement (operando spectroscopy): a method for understanding the active centres of cations supported on porous materials

Raman spectroscopy with on-line activity measurement (operando Raman spectroscopy) is used to reach a molecular understanding of the structure–activity relationship of supported vanadium oxide catalysts during alkane selective oxidation and ammoxidation reactions. The advantage of the operando methodology is that the molecular structures are recorded during true catalytic operation, thus affording information about the structures relevant to the catalytic performance. It is shown that efficient propane ammoxidation requires both surface mono-oxo vanadium oxide species and SbVO4 phases.

Zinc-modified, alumina-supported vanadium oxides as catalysts for propane oxidative dehydrogenation

Zinc-modified, γ-Al 2O 3 supported vanadium oxides were prepared by dry impregnation and co-precipitation methods. The solids were characterized by X-ray diffraction, UV–VIS, 51 V NMR and X-ray photoelectron spectroscopy and compared as catalysts in propane oxidative dehydrogenation (ODH). It was shown that only true co-precipitation methods lead to interaction of amorphous zinc vanadates with the surface of alumina. Selectivity spectra in propane ODH were used as a fingerprint to characterize superficial vanadates.

Calorimetric measurements of the acidity of supported vanadium oxides prepared by ALE and impregnation

The surface acidic properties of supported vanadium oxide catalysts prepared by atomic layer epitaxy (ALE) and for comparison by a conventional incipient wetness impregnation were investigated by adsorption microcalorimetry, using ammonia as probe molecule. The acidic characteristics were strongly affected by the preparation method and the oxide supports. When using γ-Al 2O 3 and TiO 2 as supports, the acidic character of supported vanadium catalysts always decreased compared to vanadia-free supports, whereas on weakly acidic SiO 2, new acidic centres were created when depositing V 2O 5. The same types of acidic sites, Lewis and Brönsted, were present in all the catalysts, but those prepared by ALE showed stronger acidity. The ALE samples were more easily reduced by H 2 thermo-programmed reduction and were twice as active as the impregnated samples in propane dehydrogenation to propene.

The Oxidation of Methanol to Formaldehyde on TiO2(110)-Supported Vanadia Films

The reaction of methanol on TiO2(110)-supported vanadium oxide was studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). TPD results show that methanol is oxidized to formaldehyde on monolayer and submonolayer vanadia films on TiO2(110), whereas both clean TiO2(110) and multilayer vanadia films supported on TiO2(110) are relatively inactive for this reaction. HREELS results demonstrate that methoxides are the primary surface intermediates in the oxidation of methanol to formaldehyde on the supported vanadium layers. The reactivity trends obtained for the model catalysts used in this study are similar to those observed for high surface area analogues. This suggests that vanadia films supported on single-crystal metal oxide substrates are excellent model systems for studying the relationships between the structure and reactivity of supported oxide catalysts.

MCM-48-Supported Vanadium Oxide Catalysts, Prepared by the Molecular Designed Dispersion of VO(acac)2: A Detailed Study of the Highly Reactive MCM-48 Surface and the Structure and Activity of the Deposited VOx

The molecular designed dispersion of vanadyl acetylacetonate (VO(acac)2) is used to prepare MCM-48-supported vanadium oxide catalysts. The complex is deposited on the MCM-48 surface and subsequently thermally converted into the supported vanadium oxide. A thorough study of the interaction of the VO(acac)2 with the MCM-48 support by ESR spectroscopy and chemical methods reveals that the MCM-48 surface exhibits a remarkably higher chemical reactivity in comparison to amorphous silica, which is attributed to the presence of very reactive strained siloxane bridges on the MCM-48 surface. According to the BET, XRD, and FTIR study of the supported vanadium oxide catalysts the unique structural properties of MCM-48 are maintained. Characterization by FTIR, Raman, and UV–vis diffuse reflectance spectroscopy reveals the presence of different VOx structures (monomers, polymers, and crystals) as a function of the vanadium loading (0–6.5 wt% V). The oxidation of methanol similarly shows increasing conversion and formaldehyde yields with increasing loading. Structural characterization of the catalyst after the reaction reveals that the MCM-48-supported vanadium oxides withstand catalytic reaction conditions.

Dynamic behavior of supported vanadia catalysts in the selective oxidation of ethane: In situ Raman, UV–Vis DRS and reactivity studies

The coordination and oxidation states of surface vanadia species on different oxide supports were studied by in situ UV–Vis DRS and in situ Raman spectroscopy. Surface vanadia species remain essentially oxidized during the steady-state ethane oxidation reaction. Polymeric surface vanadia species are more reducible than isolated ones, but this has only a minor effect on the ethane oxidation reactions. It appears that only one surface V site is involved in the rate-determining step for ethane oxidation. The reducibility of supported vanadium oxide species corresponds with the TOF values, but not with the average oxidation state under steady-state reaction. Ceria- and niobia-supported vanadia catalysts do not follow this trend due to solid-state reaction between the surface vanadia species and the oxide support that decreases the number of exposed vanadia sites. This solid-state reaction does not appear to affect the nature of the active site, which is associated with the V–O–Support bond rather than with the terminal VO bond.

Temperature-programmed desorption study of the oxidation of methanol to formaldehyde on TiO 2(110)-supported vanadia monolayers

The reaction of methanol on TiO 2(110)-supported vanadium oxide was studied using temperature-programmed desorption (TPD). TPD results show that methanol is oxidized to formaldehyde on a TiO 2(110)-supported vanadium oxide monolayer at 660 K, whereas both clean TiO 2(110) and TiO 2(110)-supported vanadia multilayers are inactive for this reaction. This result is similar to the reactivity trends found for high-surface-area catalysts. This suggests that an oxide film supported on a single-crystal metal oxide surface may be an excellent model system for studying the structure–reactivity relationships of supported oxide catalysts.

NO-NH3-O2 reaction catalyzed by V2O5/AC at low temperatures

The effects of SO2, V2O5 loading and reaction temperature on the activity of activated carbon supported vanadium oxide catalyst have been studied for the reduction of NO with NH3 at low temperatures (150—250°C). It is found that SO2 significantly promotes the catalyst activity. Both V2O5 loading and reaction temperature are vital to the promoting effect of SO2. The catalysts with V2O5 loadings of 1—5 weight percent have a positive effect on the promotion of SO2, while the catalysts with V2O5 loadings of above 7 weight percent have not such an effect or show a negative effect. At lower temperatures (<180°C) SO2 poisons the catalyst but at higher temperatures promotes it. The reason of the SO2 promotion was also discussed; it may results from the formation of SO4 2− on the catalyst surface, which increases the surface acidity and hence the catalytic activity.

Investigation of Surface Structures of Supported Vanadium Oxide Catalysts by UV−vis−NIR Diffuse Reflectance Spectroscopy

UV−vis−NIR diffuse reflectance spectroscopy (DRS) was applied to study the local structures of V(V) cations on various oxide supports (Al2O3, ZrO2 TiO2, Nb2O5, CeO2, and SiO2) under hydrated and dehydrated conditions. The edge energy (Eg) of the LMCT transitions of V(V) cations was used to elucidate the local structures of V(V) cations, and a correlation between the edge energy and the number of the covalent V−O−V bonds (CVB) around the central V(V) cations was established based on some V(V) reference compounds/oxides. For TiO2, Nb2O5, and CeO2 supported vanadia catalysts, the strong support absorption in the same region as the V(V) cations prevents a reliable determination of the local structure of the surface vanadium oxide species by either the LMCT band position or the edge energy. For Al2O3, ZrO2, and SiO2 supported vanadia catalysts, the average CVB number derived from the edge energy allows the assignment of the possible structure of the surface vanadium oxide species, which is a strong function of the support, environmental conditions, and vanadia surface density. The DRS results provide reliable information and new insights into the structural characteristics of the surface vanadium oxide species on these oxide supports under different environmental conditions.

NO-NH3-O2 reaction catalyzed by V2O5/AC at low temperatures: effects of SO2, V2O5 loading and reaction temperature

The effects of SO 2 , V2O5 loading and reaction temperature on the activity of activated carbon supported vanadium oxide catalyst have been studied for the reduction of NO with NH 3 at low temperatures (150- 250 ). It is found that SO 2 significantly promotes the catalyst activity. Both V 2 O 5 loading and reaction temperature are vital to the promoting effect of SO 2 . The catalysts with V 2 O 5 loadings of 1- 5 weight percent have a positive effect on the promotion of SO 2 , while the catalysts with V 2 O 5 loadings of above 7 weight percent have not such an effect or show a negative effect. At lower temperatures ( 2 poisons the catalyst but at higher temperatures promotes it. The reason of the SO 2 promotion was also discussed; it may results from the formation of SO 4 2- on the catalyst surface, which increases the surface acidity and hence the catalytic activity.

Synthesis and characterization of alumina-supported vanadium oxide catalysts prepared by the molecular designed dispersion of VO(acac)2 complexes

Alumina-supported vanadium oxide catalysts have been prepared by the molecular designed dispersion method, using the vanadyl acetylacetonate complex (VO(acac)2). The complex has been adsorbed on the support from solution, followed by thermal conversion into the corresponding supported vanadium oxide catalyst. The formation of the VO(acac)2–support material has been studied by both spectroscopic (FTIR, FT-Raman, ESR, UV–VIS–DRS; DRS: diffuse reflectance spectroscopy) and chemical methods. The present study reveals that VO(acac)2 complexes react preferentially with the hydroxy groups of the alumina, and only to a limited extent with the coordinatively unsaturated Al3+ sites. In addition, the deposition of the VO(acac)2 complex on the alumina surface yields different supported vanadium configurations as a function of the surface loading, due to both ligand exchange and hydrogen bond interactions. The thermal conversion of the adsorbed VO(acac)2 complexes into the supported vanadium oxide catalyst has been studied by TGA, GC-FTIR and in situ IR, which provides additional information on the VO(acac)2–support interaction. Characterization of these supported vanadium oxide catalysts by FTIR, FT-Raman and UV–VIS–DRS reveals the formation of highly dispersed VOx functionalities.

In situ IR, Raman, and UV-Vis DRS spectroscopy of supported vanadium oxide catalysts during methanol oxidation

The application of in situ Raman, IR, and UV-Vis DRS spectroscopies during steady-state methanol oxidation has demonstrated that the molecular structures of surface vanadium oxide species supported on metal oxides are very sensitive to the coordination and H-bonding effects of adsorbed methoxy surface species. Specifically, a decrease in the intensity of spectral bands associated with the fully oxidized surface (V5+) vanadia active phase occurred in all three studied spectroscopies during methanol oxidation. The terminal V = O (∼1030 cm−1) and bridging V–O–V (∼900–940 cm−1) vibrational bands also shifted toward lower frequency, while the in situ UV-Vis DRS spectra exhibited shifts in the surface V5+ LMCT band (>25,000 cm−1) to higher edge energies. The magnitude of these distortions correlates with the concentration of adsorbed methoxy intermediates and is most severe at lower temperatures and higher methanol partial pressures, where the surface methoxy concentrations are greatest. Conversely, spectral changes caused by actual reductions in surface vanadia (V5+) species to reduced phases (V3+/V4+) would have been more severe at higher temperatures. Moreover, the catalyst (vanadia/silica) exhibiting the greatest shift in UV-Vis DRS edge energy did not exhibit any bands from reduced V3+/V4+ phases in the d–d transition region (10,000–30,000 cm−1), even though d–d transitions were detected in vanadia/alumina and vanadia/zirconia catalysts. Therefore, V5+ spectral signals are generally not representative of the percent vanadia reduction during the methanol oxidation redox cycle, although estimates made from the high temperature, low methoxy surface coverage IR spectra suggest that the catalyst surfaces remain mostly oxidized during steady-state methanol oxidation (15–25% vanadia reduction). Finally, adsorbed surface methoxy intermediate species were easily detected with in situ IR spectroscopy during methanol oxidation in the C–H stretching region (2800–3000 cm−1) for all studied catalysts, the vibrations occurring at different frequencies depending on the specific metal oxide upon which they chemisorb. However, methoxy bands were only found in a few cases using in situ Raman spectroscopy due to the sensitivity of the Raman scattering cross-sections to the specific substrate onto which the surface methoxy species are adsorbed.

Selective Oxidation of n-Butane and Butenes over Vanadium-Containing Catalysts

The oxidative dehydrogenation (OXDH) of n-butane, 1-butene, and trans-2-butene on different vanadia catalysts has been compared. MgO, alumina, and Mg–Al mixed oxides with Mg/(Al+Mg) ratios of 0.25 and 0.75 were used as supports. The catalytic data indicate that the higher the acid character of catalysts the lower is both the selectivity to C4-olefins from n-butane and the selectivity to butadiene from both 1-butene or trans-2-butene. Thus, OXDH reactions are mainly observed from n-butane and butenes on basic catalysts. The different catalytic performance of both types of catalysts is a consequence of the isomerization of olefins on acid sites, which appears to be a competitive reaction with the selective way, i.e., the oxydehydrogenation process by a redox mechanism. Infrared spectroscopy data of 1-butene adsorbed on supported vanadium oxide catalysts suggest the presence of different adsorbed species. O-containing species (carbonyl and alkoxide species) are observed on catalysts with acid sites while adsorbed butadiene species are observed on catalysts with basic sites. According to these results a reaction network for the oxydehydrogenation of n-butane is proposed with parallel and consecutive reactions.

The Oxygen Isotopic Exchange Reaction on Vanadium Oxide Catalysts

The reactivity of lattice oxygen of vanadium oxide catalysts was studied with the oxygen isotopic exchange reaction. The reactivity of pure V2O5 is compared with the reactivity of Li0.33V2O5, V2O5/TiO2, V2O5/Al2O3, V2O5/SiO2, δ-VOPO4, and (VO)2P2O7. According to their behaviour in the oxygen exchange reaction, two types of vanadium oxide catalysts could be distinguished. The first type of catalysts only showed exchange activity in the R2 exchange mechanism and the second type showed activity in both the R1 and R2 exchange mechanisms (mechanisms in which, respectively, one or two oxygen atoms of the gas phase molecule are exchanged with oxygen atoms of the metal oxide). The catalysts which belong to the first group are bulk V2O5 and δ-VOPO4 and the catalysts which belong to the second group are Li0.33V2O5, V2O5/TiO2, V2O5/Al2O3, V2O5/SiO2, and (VO)2P2O7. If only the R2 mechanism is observed then diffusion of lattice oxygen is probably faster than when both the R1 and R2 mechanisms are observed. The activity of the supported vanadium oxide catalysts in the oxygen exchange reaction is dependent on the support. The reactivity order is V2O5/TiO2>V2O5/Al2O3∼V2O5/SiO2.