In situ XANES study of Pt/mordenite during benzene hydrogenation in the presence of thiophene

The influence of the Na+/H+ ratio on the benzene hydrogenation activity and the sulfur tolerance of Pt supported on mordenite (MOR) was studied by in situ XANES. An increase of the Pt white line was observed with increasing c oncentrations of thiophene up to 400 ppm, which indicates that an equilibri um exists between the thiophene concentration in the gas phase and the sulf ur compounds present on the metal surface. Above a certain concentration of thiophene the Pt white line was constant, indicating a saturation of the m etal surface with thiophene. The effect of sulfur poisoning on the metal su rface was more effective for Pt supported on H-MOR compared to the more bas ic Pt/Na-MOR catalysts, which is attributed to the more pronounced decompos ition of thiophene and the formation of H2S on Bronsted acid sites. During benzene hydrogenation, a decrease of the Pt white line resulting from the a dsorption of benzene and other reaction intermediates on the Pt surface was observed. In the presence of sulfur, the increase of the Pt white line int ensity during benzene hydrogenation suggests a higher sulfur coverage of th e metal for Pt/H-MOR compared to Pt/NaH-MOR. An optimum concentration of ac id sites was found with respect to the benzene hydrogenation rate and sulfu r tolerance of the catalysts, which is attributed to the presence of two re action pathways for benzene hydrogenation, i.e., one which is catalyzed by Pt and another in which acid sites close to Pt particles are the catalityca lly active sites.