Silica supported vanadia

Magg, N., Immarapom, B., Giorgi, J.B., Schroeder, T., Baumer, M., Dobler, J., Wu, Z., Kondratenko, E., Cherian, M., Baems, M., Stair, P.C., Sauer,. and Freund, H.-J. (2004) Vibrational spectra of alumina- and silica-supported vanadia revisited An experimental and theoretical model catalyst study. Journal of Catalysis, 226 (1), 88-100. 

Figure 9.19 Differential heats of ammonia adsorption over a silica support and silica-supported vanadia catalysts prepared by ALD (filled symbols) and Impregnation (open symbols). VS-A6 and VS-16 on one hand, and VS-A12 and VS-llO on the other hand, have comparable vanadia contents.

Both titania and titania/silica supported vanadia, molybdena, tungsta and chromia have been applied as SCR catalysts. Low-temperature and high temperature catalysts have been developed. The vanadia on titania catalysts have received most attention. 

Wet impregnation is not feasible for all catalyst types. For instance, vanadia on silica cannot be prepared by means of wet impregnation. With homogeneous deposition precipitation (HDP), however, it is possible to prepare vanadia on silica catalysts [21], The principle of this method is to use a lower valence of the metal, which may be produced by cathodic reduction of the respective metal ion. The reason for using lower valence state metal ions is the lower acidity compared to that of the higher valence state and the higher solubility of the metal ions. This technique was used for the preparation of silica-supported vanadia, titania, and molyb-dena catalysts [22, 23]. 

The Kinetics of the Partial Oxidation of Methane to Formaldehyde over a Silica-Supported Vanadia Catalyst... 

A kinetic study has been carried out on the partial oxidation of methane to formaldehyde over a silica-supported vanadia catalyst. The results indicate that oxygen was adsorbed on the catalyst and took part in the reaction in an Eley-Rideal or Mars-van Krevelen maimer. The nature of the interaction with the catalyst was dependent on whether the reaction took place in methane rich (pcH4 = 80 kPa) or lean (po,4 = 4 kPa) conditions. A reaction mechanism for the partial oxidation of methane to formaldehyde is proposed, which is consistent with the data reported here. Methanol oxidation experiments over this catalyst suggested that it was not an intermediate under the conditions employed during this study. 

J. Keranen, C. Guimon, E. Liskola, A. Auroux, L. Niinisto, Atomic layer deposition and surface characterization of highly dispersed titania/silica-supported vanadia catalysts,... 

Magg N., Immarapom, B., Giorgi, J., et al (2004). Vibrational spectra of alumina- and silica-supported vanadia revisited An experimental and theoretical model catalyst study, J. Catal.,... 

Owens, L. and Kung, H. (1992). Effects of Loading and Cesium Modifier on Silica-Supported Vanadia in Oxidative Dehydrogenation of Butane, Preprints-American Chemical Society, Division of Petroleum Chemistry, 37(4), pp. 1194-1200. 

This description is consistent with the results of TPR/TPO-Raman investigations of supported vanadia on silica. 

The group 5-7 supported transition metal oxides (of vanadium, niobium, tantalum, chromium, molybdenum, tungsten, and rhenium) are characterized by terminal oxo bonds (M =0) and bridging oxygen atoms binding the supported oxide to the cation of the support (M -0-MSUpport). The TOF values for ODH of butane or ethane on supported vanadia were found to depend strongly on the specific oxide support, varying by a factor of ca. 50 (titania > ceria > zirconia > niobia > alumina > silica). 

Koranne, M.M., Goodwin, J.G.Jr and Marcelin, G. (1994) Gharacterization of silica- and alumina-supported vanadia catalysts using temperature programmed reduction. Journal of Catalysis, 148 (1), 369-77. 

This behavior can be explained in terms of different oxide surface acidities. Whereas silica (pH 4) exhibits acidic properties, alumina surfaces shows a much more basic character (pH 8) [5] and, therefore, allows further hydrolysis of the alkoxysilanes. The influence of the surface pH on the characteristics of anchored molecular structures has also been reported by Deo and Wachs [6] for the preparation of supported vanadia catalysts via incipient wetness. 

With Raman spectroscopy it was possible to distinguish between vanadia adsorbed on silica or vanadia adsorbed on titania. It was concluded that a very narrow vv=o stretchings at 1040 cm and at 1030cm could be assigned to silica- and titania-supported vanadia, respectively. During preparation vanadium first covers the titania faces followed by interaction with silica. 

Supported vanadia, used to promote the selective oxidation of hydrocarbons. Is another example. Vanadia, with or without promoters, may be supported on silica (naphthalene oxidation [38]), on titania o-xylene oxidation [39]) or on a-alumina (benzene oxidation [ AO]). It was believed that supports should have open porosity (and associated lower surface area) in order to minimise over oxidation to carbon oxides. However, it was shown that reasonably high activities and selectivities could be obtained over vanadia supported on high surface area material and it was suggested that low selectivity was, in fact, primarily associated with high acidity on the support [Al]. In agreement with this, vanadia supported on 7-alumina showed zero selectivity for the production of maleic anhydride from benzene If this is the case, then a high surface area support with minimal acidity would be desired,... 

Titania-supported vanadia catalysts have been widely used in the selective catalytic reduction (SCR) of nitric oxide by ammonia (1, 2). In an attempt to improve the catalytic performance, many researchers in recent years have used different preparation methods to examine the structure-activity relationship in this system. For example, Ozkan et al (3) used different temperature-programmed methods to obtain vanadia particles exposing different crystal planes to study the effect of crystal morphology. Nickl et al (4) deposited vanadia on titania by the vapor deposition of vanadyl alkoxide instead of the conventional impregnation technique. Other workers have focused on the synthesis of titania by alternative methods in attempts to increase the surface area or improve its porosity. Ciambelli et al (5) used laser-activated pyrolysis to produce non-porous titania powders in the anatase phase with high specific surface area and uniform particle size. Solar et al have stabilized titania by depositing it onto silica (6). In fact, the new SCR catalyst developed by W. R. Grace Co.-Conn., SYNOX , is based on a titania/silica support (7). 

Researchers have discussed the need of crystalline titania in stabilizing active surface vanadia species for the SCR reaction (8, 18). Handy et al. (8) showed that titania-silica mixed gels can be prepared to give either "titania-like or "silica-like" behavior. Only on supports that contain crystalline titania domains can vanadia be dispersed to give high SCR activities. Even when silica-supported titania is used as a support, Jehng and Wachs (16) found an enhanced activity when bulk anatase particles are present. Tliese authors suggested that interactions between surface vanadia species and bulk titania... 

With the prehydrolysis of TEOS, we expect a homogeneous distribution of the two oxides in our titania-silica support Coupled with the fact that silica is the major component, this sample should behave as the "silica-like" sample prepared by Handy et al. (8) with a two-stage hydrolysis procedure. In other words, there are no crystalline titania domains in the sample, consistent with the X-ray diffraction and Raman results. This titania-silica mixed oxide aerogel is therefore less effective in stabilizing surface vanadia species (see Figure 3) and less active in SCR (see Table II) than pure titania aerogel as observed. 

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