IN-SITU ELECTROCHEMICAL PROMOTION BY SODIUM OF THE PLATINUM-CATALYZEDREDUCTION OF NO BY PROPENE

The Pt-catalyzed reduction of NO by propene exhibits strong electroche mical promotion by spillover Na supplied from a beta ''-alumina solid electrolyte. In the promoted regime, rate increases by an order of mag nitude are achievable. At sufficiently high loadings of Na the system exhibits poisoning, and excursions between the promoted and poisoned r egimes are fully reversible. Reaction kinetic data obtained as a funct ion of catalyst potential, temperature, and gas composition indicate t hat Na increases the strength of NO chemisorption relative to propene. This is accompanied by weakening of the N-O bond, thus facilitating N O dissociation, which is proposed as the critical reaction-initiating step. The dependence of N-2/N2O selectivity on catalyst potential is i n accord with this view: Na pumping to the Pt catalyst favors N-2 prod uction al the expense of N2O. X-ray photoelectron spectroscopic (XPS) data confirm that electrochemical promotion of the Pt film does indeed involve reversible pumping of Na to or from the solid electrolyte. Th ey also show that under reaction conditions the promoter phase consist s of a mixture of sodium nitrite and sodium nitrate and that the promo ted and poisoned conditions of the catalyst correspond to low and very high loadings of these sodium compounds, Under all reaction condition s, a substantial fraction of the promoter phase is present as 3D cryst allites.