Surface composition and possible rearrangement of disperse Pt and Rh catalysts: does the presence of carbon and oxygen contribute to different catalytic properties ?

The surface composition of Rh and Pt blacks (as determined by XPS) shows ca rbon and oxygen impurities in the untreated state. Oxygen on Pt is present as adsorbed O as well as OH/H2O groups and oxidized carbon. Rh was partly o xidized to Rh2O3, in agreement with UPS showing hardly any Fermi-edge inten sity in untreated Rh as opposed to untreated Pt. High Fermi-edge intensitie s indicated a predominant metallic surface after an in situ treatment with H-2 at 483 K, increasing the purity (XPS) to similar to 90%. This treatment reduced Rh to metal and removed its C impurity. Pt, in turn, retained much carbon after H-2 treatment, present mainly as graphitic carbon. A minor am ount of CO was also detected, some of the O 1s peak belonging to it. The tw o metals were tested in methylcyclopentane reactions. Considering the neces sity of carbon for nondegradative reactions and oxygen enhancing fragmentat ion, a correlation is suggested between the typical impurities of Pt and Rh and their respective catalytic propensities: the high fragmentation activi ty of Rh and the predominant nondegradative reactions to C-6 "ring opening products" on Pt.