SELECTIVE HYDROGENATION AND H-D EXCHANGE OF UNSATURATED-HYDROCARBONS ON PD(100)-P(1X1)-H(D)

The adsorption and reactions of unsaturated hydrocarbons (ethylene, propene, 1-butene, acetylene, allene, 1,3-butadiene, 1,3,5-hexatriene, and 1,5-hexadiene) have been investigated on the Pd(100)-p(1x1)-H and Pd(100)-p(1x1)-D surfaces using temperature-programmed reaction spectroscopy (TPRS). It is found that 1-alkenes (ethylene, propene, and 1-butene) undergo efficient H-D exchange reactions below 300 K for all C-H bonds, whereas no hydrogenation products (alkanes) are observed. Conjugated polyenes (acetylene, allene, 1,3-butadiene, and 1,3,5-hexatriene) undergo selective hydrogenation to corresponding alkenes below or around 300 K, whereas no exchange reaction is observed to occur for the parent molecules. Nonconjugated polyene (1,5-hexadiene) undergoes both exchange and hydrogenation reactions, with its reactivity in between l-alkenes and conjugated polyenes. The exchange reaction is proposed to occur via reversible hydrogenation to a half-hydrogenated intermediate. The relative quantities and evolution temperatures of successively H-D exchanged products are reproduced by kinetic simulations of competition between desorption and reaction. The absence of alkene hydrogenation may be due to stronger metal-hydrogen bonds on Pd(100) than on other metals such as Pt and Rh. Strong bonding of the half-hydrogenated intermediate to the surface may be the reason for the irreversible hydrogenation of conjugated polyenes to alkenes. Both exchange and hydrogenation reactions occur at all coverages, but at low coverages the reaction products dehydrogenate to C(a) and H(a) without product evolution. (C) 1995 Academic Press, Inc.