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 syngas was investigated. The interaction of CH4 with Rh-black after oxidative and reductive pretreatments was studied applying (a) pulse experiments 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. The saturation of the metal surface sites with oxygen was found to inhibit methane dissociation. Direct methane oxidation to CO2 on the oxidized surface sites proposed earlier was excluded. Methane is first dissociated on reduced surface sites; the carbon species formed, then, react with surface oxygen to CO2. Rh sites responsible for methane activation are neither related to the formation of the Rh2O3 nor Rh-0. Probably the partially oxidized species (Rh+) or highly dispersed Rh3+ entities act as active surface centers for the dissociation of methane. For supported catalyst, such sites are stabilized by the support, which on the other side acts as a source of active oxygen involved in the oxidation of surface carbon and hydrogen.