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.