Thermodynamic properties of surface carbon on metals. 1. Nickel

Because of the role of surface carbon in catalytic hydrocarbon synthesis, the thermodynamic properties of surface carbon on alumina-supported nickel have been determined by measuring the equilibrium 2CO(g)in equilibriumC(a) + CO/sub 2/(g). For this reaction, the isosteric heat varies with surface coverage (theta) from -121 kJ-mol/sup -1/ at theta = 0.5 to -135 kJ-mol/sup -1/ at theta = 0.9. These values correspond to variations in the heat of formation of surface carbon relative to graphite from 52 kJ-mol/sup -1/ at low coverage to 35 kJ-mol/sup -1/ at high coverage. The surface carbon formed at temperatures below 600 K is highly reactive toward hydrogen as compared with a carbon deposit formed at higher temperatures. The experimental data indicate the presence of oxygen adatoms coadsorbed on the nickel surface. Their chemical potential is very low relative to bulk NiO. The heat of formation of O(a) exhibits little dependence on surface coverage. 9 figures, 2 tables.