Electrochemical Promotion and Metal–Support Interactions

Abstract

Ethylene oxidation on IrO2, which is a metal-type conducting metal oxide, and on Pt and Rh was used as a model reaction to compare the rate enhancement and kinetic modification induced by (i) electrochemical promotion (non-Faradic electrochemical modification of catalytic activity effect) via electrochemical O2− supply from YSZ (Y2O3-stabilized ZrO2) and TiO2 and by (ii) metal–support interactions obtained by interfacing IrO2 with TiO2 (with and without electrochemical promotion) or by depositing dispersed Rh on TiO2 and YSZ porous supports. It was found that the addition of TiO2, which is catalytically inactive, to IrO2 submicrometer particles enhances the activity of IrO2 for C2H4 oxidation by a factor of 12 and that the same maximum rate enhancement is obtained via electrochemical promotion of the IrO2 catalyst (i.e., via electrochemical O2− supply to the IrO2 catalyst from a YSZ solid electrolyte). Furthermore it was found that the IrO2–TiO2 catalyst mixtures can only be marginally promoted electrochemically. These observations show conclusively that the mechanism of metal (IrO2)–support (TiO2) interaction in the system IrO2–TiO2 is identical to that of the electrochemically promoted IrO2–YSZ system (i.e., continuous O2− supply to the IrO2 catalyst surface). This conclusion is also corroborated by independent kinetic and XPS studies of electrochemical promotion of Pt utilizing TiO2 (instead of YSZ) as the O2− donor. The kinetics of C2H4 oxidation were investigated on Rh films interfaced with YSZ at various imposed potentials and thus imposed work function values, Φ (i.e., under conditions of electrochemical promotion and also on finely dispersed Rh catalysts deposited on various doped and undoped TiO2 and YSZ porous supports of measured work function Φ). Again it was found that the reaction kinetics are affected in the same way upon varying the work function of the Rh film via electrical polarization of the Rh/YSZ interface or upon varying the work function of the support of the dispersed Rh catalysts. This observation confirms the equivalence of the promoting mechanism of metal–support interactions and electrochemical promotion (i.e., O2− migration onto the catalyst surface). The results show conclusively that electrochemical promotion is an electrically controlled metal–support interaction and that at least certain types of metal–support interactions are induced by reverse spillover of oxygen anions from the carrier onto the surface of the metal crystallites.