Modelling alkali promotion in heterogeneous catalysis: in situ electrochemical control of catalytic reactions

Electron spectroscopic data and reactor measurements show that electrochemi cal promotion (EP) of thin film catalysts deposited on solid electrolyte su pports is the result of spillover phenomena at the three-phase boundary bet ween the electrolyte, the catalyst and the gas phase. Ions from the electro lyte are discharged at the electrode/electrolyte interface and migrate to c over the catalyst surface whose properties are thereby strongly altered. Th e EP effect and the phenomena that underlie it are illustrated here by refe rence to the Na-promoted catalytic reduction of NO by CO over copper. Elect ro-pumping of Na from a beta "-alumina solid electrolyte to the catalyst su rface results in large improvements in both activity and selectivity of the latter. Under reaction conditions, the alkali promoter is present as submo nolayer amounts of NaNO3 on an oxidised Cu surface. The results indicate th at Cu-0 sites are not of significance and that the catalytically active sur face is dominated by Cu+ and Cu2+ sites. They also show that Cu+ is the cri tically important site for NO adsorption and that EP is due to Na-induced e nhancement of the adsorption and dissociation of NO at Cu+ sites.