The Role Of Deactivation in Decreasing the Surface Heterogeneity of Pt/Sio2 in the Isothermal Hydrogenation of Gaseous Cyclohexene

Abstract

Summary Cyclohexene hydrogenation has been studied at 273–313K over silica-supported Pt. Specific initial rates of cyclohexene hydrogenation are unexpectedly affected by the average Pt particle size derived from monolayer extents of hydrogen chemisorption (estimated at 293K and the zeropressure intercept). One explanation for this would be that hydrogen chemisorption only measures the clean' total metal surface area of Pt/SiO 2 supported catalysts; neglecting the effects of hydrogen spillover and carbonaceous deposition on the catalytic surface during reactions [M.S.W. Vong and P.A. Sermon J. Chem. Soc. Far Trans. (1991)]. This would then highlight the importance of accurately defining the number of active sites when describing turnover numbers of catalysed reactions and structure sensitivity-insensitivity therein. However, isothermal activity decreases with reaction time and at lower temperatures and longer reaction times turnover numbers do not vary with Pt dispersion. This structure-insensitivity is then consistent with earlier results [S.M. Davis and G.A. Somorjai J. Catal. 65 , 78, (1980)] that the reaction is only structure insensitive when the surface is covered by a carbonaceous over layer. In the present work, this occurs at longer reaction times. Thus, deactivation reduces the heterogeneity of Pt/SiO 2 active surfaces and removes the primary structure-sensitivity for cyclohexene hydrogenation seen on the clean surface.