Thermodynamics of sulfur chemisorption on metals. III. Iron and cobalt

Isosteric heats of chemisorption of sulfur on iron and cobalt have been evaluated from measurements of the equilibrium ratio of H2S/H2 gas mixtures in contact with the powdered metals over a temperature range from 677 to 855 K. At low surface coverage of sulfur on cobalt, the heat of chemisorption with respect to 1/2 S2(g) is -236 +- 15 kJ mol-1. With increasing coverage it decreases to -200 +- 10 kJ mol-1. The heat of sulfur chemisorption on iron is -197 +- 15 kJ mol-1. For equivalent coverage, the thermodynamic activity of sulfur chemisorbed on cobalt exceeds that of iron by approximately one order of magnitude. At low surface coverage (<0.5 saturation coverage), the bond strength of sulfur on the various metals is found to vary in the order: Ni>Co>Ru>Fe