Siegfried Engels

The state of metallic phase in alumina-supported platinum-chromium catalysts

Abstract The reduction behaviour and the phase composition of platinum-chromium samples (unsupported and on alumina carrier) were investigated with the help of temperature-programmed reduction (TPR), X-ray diffraction, ferromagnetic resonance (FMR) spectroscopy and ethane hydrogenolysis. Co-reduction and the formation of alloys was found to occur when H 2 PtCl 6 and (CrO 3 ) x were used as starting materials. Annealing of the samples above 500°C led to the formation of a ferromagnetic Pt 3 Cr superlattice. Addition of chromium to platinum/alumina attenuates the hydrogenolysis activity; annealing results in an additional strong decrease of this demanding reaction.

Characterization and catalytic activity of unsupported nickel-rhenium catalysts modified by calcium oxide

Abstract Unsupported nickel-rhenium bimetallic catalysts have been prepared from a mixture of NH 4 ReO 4 and nickel oxide containing CaO. It has been established that the total and metal surface areas as measured by X-ray diffraction, by the BET method and by CO chemisorption are enhanced by CaO addition compared to calcium-free samples. In addition, alloy formation is promoted by CaO, as indicated by ferromagnetic resonance (FMR) and X-ray diffraction measurements. X-Ray photoelectron spectroscopy reveals surface enrichment of rhenium and calcium abundance on the surface, particularly in heat treated samples. Sintering is retarded by the presence of CaO. The turnover frequencies for the dehydrogenation of cyclohexane, for the hydrogenation of benzene, and for the hydrogenolysis of ethane increase with increasing addition of Re to Ni up to the composition of 85 at.% Re-15 at.% Ni, followed by a fall for the pure rhenium catalyst. A remarkable effect of calcium addition is a strong suppression in the specific rate of ethane hydrogenolysis. For CO hydrogenation the TOF increases as the amount of nickel decreases and there is a simultaneous change in the olefin and C 2 + selectivity. In the range between pure Ni and 85 at.% Ni-15 at.% Re the formation of higher hydrocarbons is slightly promoted whereas on the Re-rich samples a high methane selectivity can be observed due to the enhanced hydrogenation activity of rhenium.

A model of reduction mechanism of rhenium in Re/Al2O3 catalysts

In Re/Al2O3 there exist a monolayer type of Re and a second one which is inserted in the alumina lattice. The latter is reduced by a two-step mechanism including Re(IV) species.AbstractРений в Re/Al2O3 находится в виде монослоя и в виде частиц, встроенных в решетку. Последний восстанавливается по двухступенчатому механизму, включая Re(IV).