Energetics and kinetics of the interaction of acetylene and ethylene with Pd{100} and Ni{100}

The adsorption and reactions of C2H4 and C2H2 on Ni{100} and Pd{100} at room temperature have been investigated by single-crystal adsorption calorimetry (SCAC). On Ni{100}, the initial heats of adsorption are 203 and 264 kJ mol(-1), respectively. From the correlation of the heat changes with the stable species formed (CH and/or CCH), a mean value of approximate to 204 kJ mol(-1) is extracted for the Ni-C bond energy. On Pd{100} acetylene adsorbs molecularly in a rehybridized state with initial heat of 112 kJ mol(-1), corresponding to a Pd-C bond energy of approximate to 177 kJ mol(-1). The kinetics of adsorption are similar for C2H2 on Ni{100} and Pd{100} and C2H4 on Ni{100}, exhibiting precursor-mediated adsorption with high initial sticking probability, while C2H4 adsorbs reversibly on Pd{100} at room temperature. Nickel and palladium surfaces are known to favour the cleavage of C-C bonds, whereas platinum surfaces do not. This is not related to differences in metal-carbon bond energies, and must instead be attributed to a kinetic origin. (C) 2000 Elsevier Science B.V. All rights reserved.