Modification of the catalytic properties of platinum by the adsorption of mercaptans

The investigation of the effects of adsorbed propanethiol and hexanethiol on the catalytic properties of platinum has shown that one adsorbed RS group inhibits the adsorption of one hydrogen atom, regardless of its size. The toxicities of these two thiols in the hydrogenation of maleic acid were measured, and compared to that of hydrogen sulphide. At a high concentration of maleic acid, the catalytic activity is proportional to the number of non-poisoned surface platinum atoms, for the three sulphur compounds (H2S, CH3CH2CH2S and CH3 (CH2) 5S). This result can be explained by the effect of the poison on the hydrogen adsorption according to the equation v=KΘH. At a low alkene concentration, the variation of the toxicity with the nature of sulphur compounds is complex and cannot be explained by an ensemble effect only. Indeed, the variation of the free energy of adsorption of maleic acid as a function of the degree of coverage in poison suggests that an electronic effect also occurs, increasing in the order H2S < CH3CH2CH2SH < CH3, (CH2)5SH. So the toxicities can be explained by the combination of an ensemble effect and an electronic effect of sulphur compounds on the platinum surface.