XAFS Study of Tin Modification of Supported Palladium Catalyst for 1,3-Butadiene Hydrogenation in the Presence of 1-Butene

Alumina-supported palladium catalyst, which is active for the hydrogenation of 1,3-butadiene, was modified by tin using various tin precursors. The structure of modified catalysts was studied by XAFS and TPR. Before modification, palladium on alumina existed in an electron-deficient hydride state, which was revealed by a shift to a higher PdK-edge energy in XANES and by an increased Pd–Pd distance in EXAFS fitting. However, tin modification caused the palladium edge energy to decrease to the value of zero valent palladium metal and destroyed the Pd ensembles by making Pd–Sn bonds, irrespective of the nature of tin precursor. This was responsible for the increased 1-butene selectivity in the hydrogenation of 1,3-butadiene in the presence of 1-butene. A carbonaceous species was formed in the catalyst modified with an organic tin precursor, while the inorganic tin modification produced a tin oxide-like species in addition to the formation of Sn–Pd bonds.