B. Moraweck

New supported palladium-chromium catalysts: characterization and catalytic properties

New silica-supported Pd-Cr catalysts have been prepared either by using mixed inorganic salts of the two metals as precursors or through the deposition of Cr(CO)6 on a monometallic palladium catalyst. Analytic electron microscopy shows that palladium-enriched particles are formed. The EXAFS experiments show that when the reduction temperature reaches 870 K, part of the chromium is reduced and interacts with palladium. In the partial hydrogenation of 1,3-butadiene, these catalysts are less active but 100% selective for the formation of butenes. The explanation, proposed on the basis of hydrogen chemisorption and confirmed by XPS data, is a modification of the electronic structure of palladium, leading to a variation of the adsorption coefficients of the diene and alkenes, compared to the values measured on monometallic palladium catalysts.

Influence of the nature of the platinum precursor on the surface properties and catalytic activity of alumina-supported catalysts

The influence of the nature of the metal precursor — platinum acetylacetonates and chloroplatinic acid — on the surface properties and catalytic activity of Pt/Al2O3 catalysts is reported. The obtained results indicate that the catalysts prepared by the organometallic route present higher metal dispersion and lower acidity compared with those prepared from H2PtCl6. On the other hand, XPS results showed that the state of platinum is essentially Pt0 in the catalysts obtained from Pt(acac)2 while in the solids prepared by impregnation of H2PtCl6 there exists an important contribution of Ptδ+ species which plays a positive role in the hydrogenation of toluene. An additional hydrogen spillover due to the presence of more acidic support is also suggested as an explanation of the observed catalytic results.

Sintering of Pt/Al2O3 reforming catalysts: EXAFS study of the behavior of metal particles under oxidizing atmosphere

The effect of an oxidative atmosphere (300 °C) is studied on fresh and sintered unchlorinated naphtha reforming catalysts containing 0.6–1% Pt. The TPR profiles show that only one species is formed using our experimental conditions, regardless of the mean crystallite size of the metal particles. The structural information supplied by EXAFS compared with cuboctahedral particle modeling, implies that such species is a surface platinum oxide, the structure of which is close to that of PtO2, but largely distorted. This is true whether the catalyst is sintered or not.

Characterization and catalytic activity of Al2O3-supported Pt-Co catalysts

A series of Pt-Co supported on γ-alumina have been prepared from acetylacetonate precursors with 2 wt% as total metal loading and different atomic contents. The catalysts were characterized by hydrogen chemisorption, TPR, TEM, XPS and they were tested in two different reactions: methylcyclohexane dehydrogenation and n-butane hydrogenolysis. The results have shown that platinum is in reduced state and cobalt is mainly in an oxidized state. Additionally, TEM results evidence an increase in the particle size as cobalt content is increased, in agreement with chemisorption results. Monometallic platinum and cobalt supported catalysts showed large differences in the dehydrogenation of methylcyclohexane, while the bimetallic catalysts have activities in the same order of magnitude with respect to the pure platinum catalyst. In n-butane hydrogenolysis all the catalysts show a large decrease in activity compared to other, previously studied Pt-based bimetallic catalysts.

X-ray Absorption Spectroscopy: a powerful tool to investigate intermediate species during sintering-redispersion of metallic catalysts.

Abstract In-situ EXAFS spectroscopy was used for identifying the surface species involved during sintering and redispersion of Pt/Al 2 O 3 catalysts under oxidizing atmospheres. Pt-Pt metallic distances were always detected during the oxidizing treatments. Sintering involved three successive steps. In the first step, a continuous increase of both Pt-O and Pt-Cl coordinations was observed as temperature increased, indicating the progressive formation of hydroxychlorided complexes such as [Pt s (OH) x Cl (6- x ) ] 2− . The formation of Pt(OH) 4 Cl 2 species was detected at the end of the first step. The retransformation of [Pt(OH) 4 Cl 2 ] 2− into a disordered non-stoichiometric oxychlorinated species occurred in the second step. During sintering, the Pt particles were made up of a metal platinum core coated by oxychlorinated species. Redispersion experiments indicated that Pt(OH) 4 Cl 2 2− species were responsible for Pt redispersion. At the end of the redispersion treatment, Pt was present on the support in two different environments: Pt atoms in a raft arrangement containing Cl and O in an octahedral environment and relatively large Pt metallic particles covered by Pt(OH) 4 Cl 2 2− species.