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 ≈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 ≈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.