PRIMARY REACTION STEPS AND ACTIVE SURFACE SITES IN THE RHODIUM-CATALYZED PARTIAL OXIDATION OF METHANE TO CO AND H-2

The nature of surface sites responsible for methane activation and COx formation on Rh catalysts for the partial oxidation of methane to syn gas was investigated. The interaction of CH4 with Rh-black after oxida tive and reductive pretreatments was studied applying (a) pulse experi ments at reduced total pressure (10(-4) Pa) and 1013 K in the temporal -analysis-of-product (TAP) reactor and (b) in situ DRIFTS at 973 K. Th e saturation of the metal surface sites with oxygen was found to inhib it methane dissociation. Direct methane oxidation to CO2 on the oxidiz ed surface sites proposed earlier was excluded. Methane is first disso ciated on reduced surface sites; the carbon species formed, then, reac t with surface oxygen to CO2. Rh sites responsible for methane activat ion are neither related to the formation of the Rh2O3 nor Rh-0. Probab ly the partially oxidized species (Rh+) or highly dispersed Rh3+ entit ies act as active surface centers for the dissociation of methane. For supported catalyst, such sites are stabilized by the support, which o n the other side acts as a source of active oxygen involved in the oxi dation of surface carbon and hydrogen.