THE EFFECT OF MEDIUM-TEMPERATURE PRETREATMENT ON THE ACTIVE-SITE DISTRIBUTION OF PT-GRAPHIMET, PD-GRAPHIMET AND RH-GRAPHIMET

In order to obtain more information about the structure of transition metal-graphite intercalation compounds, we have studied the active sit e distribution of Pt-, Pd- and Rh-graphimet before and after medium-te mperature pretreatment (MTR, 573 K, 1 h). The surface characterization was performed by 1-butene titration in a flow reactor system. The sin gle turnover method enabled us to distinguish between direct and two-s tep hydrogenation sites, and isomerization centres. MTR causes a prono unced effect on the active site distribution of all the samples. For P d- and Rh-graphimet, the decreased number of isomerization and hydroge nation sites observed after MTR is attributed to encapsulation of surf ace metal into graphite or formation of surface carbon deposits. Pt-gr aphimet, on the other hand, displays an increased number of active cen tres following MTR, which make its site distribution similar to that o f supported platinum catalysts. The structural change observed may be due to the joint effect of interlayer platinum migration and recrystal lization of surface platinum particles.