The reactivities of toluene, rn-xylene, phenol and benzene on mono-and multi-layer V/Ti and V/Zr oxides prepared by the gas-phase method have been studied by FTIR spectroscopy. It is found that Lewis-acidic sites (vanadium ion) play an important role in the formation of n-complexes of toluene, rn-xylene and benzene. The methyl group of the n-complex is dehydrogenated by the V -0 species to form benzyl species and benzaldehyde. On the other hand, phenol is adsorbed via its oxygen with its benzene ring oriented perpendicular to the surface. From spectral similarities, it is concluded that benzene is adsorbed to form mono-and/or disubstituted species. The activity of V=O species on the monolayer catalyst is higher than that on the multilayer catalyst.
Molybdenum oxide supported on zirconia has been studied by XRD, FTIR and laser-Raman spectroscopy, together with the oxidation of (Z)-but-2-ene. Mo-Zr catalysts with less than 11 wt % of Mo loadings are mainly composed of amorphous polymolybdates. Mo-Zr catalysts with high MOO, loading exhibit high selectivities to acetaldehyde and acetic acid and both Lewis-and Brsnsted-acidic sites by pyridine adsorption. On the other hand, the catalysts with low MOO, loading exhibit selectivities to furan and buta-1,3-diene and only Lewis sites. ln situ FTIR and Raman spectra show the different character of Mo oxides with MOO, loading. This difference of Mo character leads to the difference in selectivities for butene oxidation.Table 1. Physical properties of Mo-Zr catalyst fraction of surface conc. MOO, surface amorphous of amorphous catalyst (wt %) area/m2 g-' MOO, (%) oxide/pmol mol-' MOZ3.0 3.0 49 100 4.2
The adsorption of NO, NO, and N 2 0 at various temperatures on mono-and multi-layered V20,/Zr0, catalysts prepared by t h e gas-phase method h a s been investigated by IR spectroscopy. On t h e reduced samples of both mono-and multi-layer catalysts, NO is adsorbed giving mono-and di-nitrosylic species. On t h e oxidized form of t h e monolayer catalyst, only nitrite and nitrate species are formed and no nitrosylic species are formed. NO2 adsorbed on reduced multilayer catalysts exhibits IR bands due to nitrate species together with minor amounts of nitrosylic species. The surface nitrosylic species were less readily formed on the monolayer catalyst than on multilayer catalysts. Quantitative analysis of the V=O band in t h e region 1100-900 cm-' indicates that only the surface V=O species in t h e top layer of the catalyst interacts with the adsorbed species. Both NO and NO, reoxidize t h e catalyst surface at higher temperatures and the monolayer catalyst was less readily oxidized than t h e multilayer catalyst.
The adsorption and oxidation of toluene on V-Ti oxide prepared by the gas-phase method (2.1 to 5.6 wt %) has been studied by F.t.i.r. spectroscopy as well as the microcatalytic method. It is found that toluene is bonded as a n-complex to the vanadium ion. The CH, groups of the n-complex are dehydrogenated by the V=O species to form benzyl species at room temperature. The formation of aldehyde-like intermediate before the oxidation of benzyl species to benzoate has been confirmed. The reactivity of surface V=O species of V-Ti oxide prepared by a gas-phase method is greater than that of the impregnation catalyst.
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