A Kinetic and Spectroscopic Study of the in Situ Electrochemical Promotion by Sodium of the Platinum-Catalyzed Combustion of Propene

The electrochemical promotion (EP) of propene combustion has been studied over a platinum film catalyst supported on sodium β‘ ‘-alumina. Fully reversible promotion and poisoning are observed as a function of catalyst potential (i.e., sodium coverage), the precise behavior being dependent on reactant partial pressures. A model based on a Langmuir−Hinshelwood mechanism and Na-modified chemisorption of the reactants accounts for all the results: Na enhances the chemisorption of oxygen and inhibits the chemisorption of propene. The formation, chemical identity, stability, and electrochemical decomposition of the Na surface compounds produced in the promoted and poisoned regimes were explored using postreaction XPS, AES, and, for the first time, postreaction Na K edge XAFS. These results indicate that thick layers consisting of sodium carbonate are responsible for catalyst poisoning. The promoter phase consists of smaller amounts of sodium carbonate, and much of this material is present as three-dimensional crystallites. These promoter and poisoning phases are stable at reaction temperature, but rapidly destroyed by electropumping Na away from the catalyst surface. Their direct participation in the electrochemically promoted reaction is thereby demonstrated.