Electrochemical promotion by sodium of the rhodium-catalyzed NO plus CO reaction

XPS shows that the electropumping of Na to a metallic Rh film contacted wit h a solid electrolyte (Na beta " alumina) under potentiostatic conditions r esults in reversible spillover of the alkali from the electrolyte to the ca talytically active surface. As the catalyst potential (V-WR) is decreased o ver a range of 2000 mV, the sodium coverage increases linearly from zero to similar to0.02 monolayers, while the work function (phi) decreases, also l inearly, by 0.65 eV. Over this same regime of catalyst potential, under app roximately stoichiometric conditions, both the activity and N-2 selectivity of the NO + CO reaction are strongly enhanced as Na is pumped to the Rh su rface. The measured Na coverage, electrochemical behavior, and catalytic re sponse of the system are fully reversible with catalyst potential. Our data are understandable in terms of the Na-induced dissociation of adsorbed NO, which is thought to be the rate-limiting step. The nature of electrochemic al promotion, the reaction mechanism, the mode of promoter action, and the inequivalence of Delta phi and DeltaV(WR) are discussed.