DEACTIVATION OF NICKEL-CATALYSTS BY SULFUR-COMPOUNDS .2. CHEMISORPTION OF HYDROGEN-SULFIDE

The chemisorption of hydrogen sulfide was investigated at 323 K in order to explain the different toxicities of hydrogen sulfide determined on various nickel catalysts in the course of benzene hydrogenation. Two chemical states of hydrogen sulfide are observed: the first one corresponds to polysulfides which develop quickly on superficial nickel sulfide, the second one consists of bulk sulfides which are formed slowly by attack of the deeper layers. As the poisoning of nickel catalysts by H2S/H2 mixtures occurs at the front of the bed and the remaining free surface shows the same behavior toward reactives adsorption as a pure nickel surface, the toxicity of sulfur compounds will depend on the ability of the nickel catalysts to dissociate the poison molecule and to form superficial polysulfides or bulk sulfides. The lowest toxicities for the reaction of benzene hydrogenation appear on nickel catalysts which on the one hand strongly dissociate the sulfur compounds, and on the other quickly develop superficial polysulfides.