Surface organometallic chemistry on metals. I. Hydrogen and oxygen interaction with silica-supported and alumina-supported rhodium

In order to understand the reactivity of Sn(n-C₄H₉)₄ with supported rhodium, the chemisorption properties of silica-supported and alumina-supported rhodium toward O₂ and (or) H₂ has been investigated. It has been found that chemisorption of O₂ on rhodium is particle size dependent. At low particle size, bulk oxidation to Rh₂O₃ occurs at 300 K. At high particle size, surface oxidation to surface Rh₂O₃ occurs at 300 K and is followed by a slow bulk oxidation to Rh₂O₃. The rate of bulk oxidation can be increased considerably at 473 K. Chemisorption of H₂ on rhodium supported on silica or alumina occurs in two ways. Below 120 K, two forms of adsorbed hydrogen are present; above 260 K, only one form, an irreversibly adsorbed hydrogen, is observed. Both the results of chemisorption and those of thermodesorption suggest that below 120 K these two forms of hydrogen are present in equal amounts on the surface. For each form, the stoichiometry is close to one hydrogen atom per surface rhodium atom. Room temperature titration of the chemisorbed oxygen by hydrogen is possible according to the following equation, where y is the amount of adsorbed hydrogen (this value is pressure dependent): Rh₂O₃/support + (3 + y) H₂ → 3 H₂O/support + 2 (Rh - Hy)/support.