Modelling and characterization of supported catalytic centres

Abstract

A series of aluminia and titania promoted, silica- supported chromium (III) acetate catalysts were characterized using X-ray photoelectron spectroscopy (XPS) prior to, and after, activation in oxygen at 780 C. The results indicated that Cr (VI) was formed in each case as a result of the activation process. Increased promoter metal binding energies implied an interaction between the promoter and silica support. It is proposed that there is insertion of aluminium and titanium atoms into the silica network, which leads to formation of surface silicates. A qualitative measure of the metal dispersions has been made using the XPS results. In general, the chromium dispersion fell on activation, but the greatest decline was seen with the lowest chromium loading (0.5% Cr), Promoter metal dispersion was unchanged on activation, except in the case of the highest titanium loading (4.35% Ti), where small titania clusters are formed. Mass spectral analysis of the gases evolved during thermal decomposition in argon led to a mechanism being proposed for the decomposition of the acetate precursor. The first step is dehydration of the silica support, which is followed by decompositon of acetate ligands to form an intermediate, which was thought to be a carbonate, and the final stage is the decomposition of this intermediate to chromium (III) oxide for the unpromoted catalysts. Where a promoter is present a structural and electronic interaction between the chromium complex and the promoter is proposed, which leads to formation of mixed surface oxides of perovskite (M(^II)Ti(^IV)0(_3)) or spinel (M(^II)Al(_2)(^III)0(_4)) structure, where M = Cr. For activation under oxygen the pattern of decomposition was much simpler. Studies of the promoted catalysts showed the oxidation to occur—in two stages. It was not clear which chromium species were present after the first step, but the second step led to the formation of chromium (VI) oxide for all catalysts. Modelling of the adsorption sites on metal surfaces has also been undertaken with a series of triosmium carbonyl complexes containing ligands derived from aniline, phenol, pyrrole, furan, thiophene and benzene. These complexes have been characterized using Fourier Transform Infra Red spectroscopy and their vibrational spectra assigned in full. The usefulness of these complexes as models, and in the assignment of vibrational spectra of adsorbates on metal surfaces, is discussed.