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

Abstract

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.