Thermodynamics of sulfur chemisorption on metals. I. Alumina-supported nickel

Sulfur chemisorption isosteres have been measured for nickel in powdered form and for nickel supported on two different alumina powders. The experiments were conducted in a closed-loop gas recirculation system containing one atmosphere hydrogen. Isotherms were determined by stepwise injection of H2S aliquots into recirculating hydrogen gas and analyzing for the H2S concentration as equilibrium was approached. Isosteres were measured by varying the sample temperature and monitoring the H2S/H2 ratio in the gaseous environment. A gas chromatograph and a photoionization detector was used to measure the H2S concentration to levels below 1 ppb. As monolayer coverage is approached the H2S/H2 ratio attains the equilibrium values reported for the bulk sulfide, Ni3S2. Adsorbed sulfur is very strongly bound to the surface of nickel. The heat of formation of chemisorbed sulfur with respect to 1/2 S2(g) at 800 K is 247 kJ mol−1 more negative than the heat of formation of Ni3S2. The heat of segregation exceeds 190 kJ mol−1. The results demonstrate that the equilibrium sulfur coverage exceeds one half monolayer on Ni/Al2O3 unless the H2S/H2 ratio is less than 1 ppb even at temperatures as high as 940 K.