Electrochemical promotion of rhodium-catalyzed NO reduction by CO and by propene in the presence of oxygen

The catalytic performance of a rhodium thin film in contact with the solid electrolyte Na-beta" alumina can be greatly enhanced by the reversible electrochemically controlled transport of sodium from the electrolyte to the metal surface. By this means, the reduction of nitric oxide by carbon monoxide or by propene can be promoted, even in the presence of oxygen. The effect is due to the Na-enhanced dissociation of adsorbed NO, the key reaction-initiating step. XP and Auger spectroscopies show that, under promoted conditions, the alkali-metal surface phase consists of carbonate, nitrate, or both, depending on the gas composition. To a first approximation, the chemical identity of the counterion appears not to play a significant role. With increasing oxygen partial pressure the promotional effects of sodium are progressively decreased, and the markedly different behavior of CO and propane as reductants is due to the opposite effects of coadsorbed alkali metal on the electronegative or electropositive adsorbate, respectively. At the highest oxygen partial pressures and alkali-metal coverages, drastic poisoning by sodium is due to strong alkali-metal inhibition of propene adsorption, excessive formation of Na2O, and oxidation of Rh to Rh2O3.