METHANE ACTIVATION AND DEHYDROGENATION ON NICKEL AND COBALT - A COMPUTATIONAL STUDY

We have studied the adsorption of CH3 and H on nickel clusters of various size and shape. As a next step we have chosen a one-layer 7-atom cluster and a spherical 13-atom cluster to model the nickel and cobalt surface and we have studied the adsorption of CH3, CH2, CH, C, and H on these clusters. Starting from gas phase CH4, the formation of adsorbed CH3(CH3a) and adsorbed H(H-a) is endothermic on all clusters, but the endothermicity is strongly reduced on the 13-atom clusters (142 kJ/mol on Ni-7, 135 kJ/mol on Co-7, 30 kJ/mol on Ni-13, and 8 kJ/mol on Co-13). The formation of adsorbed CH2(CH2a) and Ha from CH3a is endothermic by 25-40 kJ/mol on all clusters, except on Co-7 (3 kJ/mol exothermic), mainly because of the much stronger adsorption of CH2 on this cluster. The formation of adsorbed CH(CHa) and H-a from CH2a is exothermic on all clusters, but the exothermicity differs a factor two between the 7- and 13-atom clusters (61 kJ/mol on Ni-7, 60 kJ/mol on Co-7, 27 kJ/mol on Ni-13, and 32 kJ/mol on Co-13). Finally, the formation of adsorbed C(C-a) and H-a from CHa is strongly endothermic on the 7 atom clusters, but the endothermicity is again strongly reduced on the 13-atom clusters (92 kJ/mol on Ni-7, 77 kJ/mol on Co-7, 27 kJ/mol on Ni-13, and 14 kJ/mol on Co-13).