Electrochemical promotion by potassium of rhodium-catalyzed Fischer-Tropsch synthesis: XP spectroscopy and reaction studies

The catalytic behavior, chemical state, and morphology of a Rh thin film catalyst electrochemically promoted by alkali have been investigated by reactor studies, XPS, and SEM. At I bar pressure and 473 K, electropumped alkali decreases overall activity toward the production of hydrocarbons from CO and H₂ while increasing selectivity toward higher hydrocarbons, especially alkenes. On the active surface, rhodium is in a partially oxidized state; however, the catalytic activity is assigned to the metallic (Rh^o) component. The promoter phase is an alkali compound other than carbonate, although at high alkali loadings large amounts of crystalline alkali carbonate are deposited on the surface. The heavily carbonated system nevertheless retains most of its catalytic activity, with continued increase in selectivity toward higher hydrocarbons as the alkali loading is increased. Fully reversible behavior is found in every respect, including recovery of the initial state by electrochemical decomposition of the crystalline alkali carbonate deposit. The reaction mechanism is discussed with respect to the identity of the active site and the role of the alkali promoter.