Hydrogen adsorption and hydrogen-deuterium equilibration on sulfided ruthenium and bulk ruthenium sulfide catalysts

Abstract

The effect of presulfidization on hydrogen adsorption over ruthenium catalysts has been studied using volumetric adsorption and H 2D 2 equilibration. On reduced ruthenium catalysts (101 kPa of H 2 at 673 K for 8 h) 10.2 μmol of H 2/m 2 of ruthenium (measured at 373 K) was irreversibly adsorbed, the activation energy for adsorption was negligible and the initial heat of adsorption was estimated to be between −109 and −126 kJ/mole H 2. On “mildly sulfided” ruthenium catalysts (101 of kPa 10% H 2S H 2 at 673 K for 2 h, S a Ru (s) = 0.25 ) both reversible and irreversible hydrogen adsorption were suppressed at temperatures below 473 K. Results of H 2D 2 equilibration kinetics on these catalysts (508 ≤ T <- 623 K) revealed an activation energy of 71 ± 2 kJ/mole. On “extensively sulfided” ruthenium catalysts (101 kPa of 100% H 2S at 673 K for 2 h, i.e., crystalline RuS 2 at the surface) partial restoration of the H 2 adsorption capacity was observed (6.5 μmol/m 2 at 373 K). H 2D 2 exchange rates at 573 K were ca. two orders of magnitude faster than those over mildly sulfided samples. The activation energy for hydrogen adsorption was 32 ± 2 kJ/mole (on RuS 2-like surfaces) and the heat of adsorption was calculated to be −269 kJ/mole H 2. It is suspected that this large heat of adsorption may result in higher hydrogen coverages at HDS reaction conditions. The above noted differences in hydrogen accommodation provide a unique means for understanding the selectivity toward hydrogenated products which extensively sulfided ruthenium Catalysts possess.