Solid electrolytes aided studies of nitric oxide reduction over noble metals

The present work demonstrates that on Pd interfaced with yttria-stabilized zirconia (YSZ), the application of an external negative potential results in the enhancement of both catalytic rate of reduction of NO by CO and H ₂ and of the selectivity of the reaction toward N₂. The effect is non-Faradaic; that is the modification of the rate is typically more than two orders of magnitude larger than the rate of transport of O2− through the solid electrolyte, and it is related to a change of the work function of the metal. Similarly, Pt interfaced with β″ -alumina exhibited a non-Faradaic effect for the Na-promoted Pt-catalyzed NO reduction by CO. These observations can be rationalized by an enhanced adsorption and dissociation of NO as the electric potential and work function of the catalyst surface is decreased. The similarity of the trends obtained with two different solid electrolytes and therefore different promoting species (Na+ or Oδ−) is one more indication of the validity of the theoretical explanation proposed for the non-Faradaic electrochemical promotion of catalytic activity (NEMCA) phenomenon. The Pd|YSZ catalysts were prepared both in fuel-cell type and single-pellet type configurations. In fuel-cell type configuration, the Pd film was coated onto both inner and outer surfaces of a YSZ tube. The inner film exposed to NO-CO reaction mixture served as working electrode and the outer film exposed to atmosphere served as counter electrode. In the single-pellet type configuration, a Pd film was deposited on the face of a YSZ disc, and the Ag counter and reference electrodes were deposited on the other side of the disc. Both the catalyst film and the Ag counter and reference electrodes were exposed to the reacting NO-CO mixture. The observed increase in the reaction rate based on CO₂ production was typically a factor of 100 higher than the rate of O2− removal from the catalyst. In order to characterize the catalysts, solid electrolyte potentiometry and cyclic voltammetry were performed on the fuel-cell type reactor. The NEMCA system provides direct information about the nature of the catalyst and the reaction mechanism under conditions of elevated pressure. The electrochemical promotion of the catalytic activity, r_CO, and the selectivity, S_N2O, are reversible, and the Pd|YSZ system showed both electrophilicity and electrophobicity depending on the direction of oxygen ion pumping. By decomposing the NO-CO reaction into single reactant reaction with its counter part, it was found that electrophilicity is due to enhanced NO dissociation and electrophobicity is due to enhanced CO oxidation. In a typical experiment, the enhancement of the reaction rate is 100 times greater than the rate of oxygen ion transport. Rate enhancements of ρ_CO2 = 2.2, ρ_N2O = 2.2 and ρ_N2 = 4.2 were measured for V_WR = −1.8 V at 370°C. The electrochemical promotion appeared to be sensitive to inlet partial pressures of NO and CO and showed a maximum when 1 < P_NO/P_CO < 2. The formation of N₂ and N₂O for the NO-CO reaction over Pd|YSZ appeared to occur in parallel and thus NEMCA of S_N2O should be interpreted as a modification of catalytically active sites rather than an increase in site density.