Electrochemical promotion and metal-support interactions

Ethylene oxidation on IrO2, which is a metal-type conducting metal oxide, a nd on Pt and Rh was used as a model reaction to compare the rate enhancemen t and kinetic modification induced by (i) electrochemical promotion (non-Fa radic electrochemical modification of catalytic activity effect) via electr ochemical O2- supply from YSZ (Y2O3-stabilized ZrO2) and TiO2 and by (ii) m etal-support interactions obtained by interfacing IrO2 with TiO2 (with and without electrochemical promotion) or by depositing dispersed Rh on TiO2 an d YSZ porous supports. It was found that the addition of TiO2, which is cat alytically 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 soli d electrolyte). Furthermore it was found that the IrO2-TiO2 catalyst mixtur es can only be marginally promoted electrochemically. These observations sh ow conclusively that the mechanism of metal (IrO2)support (TiO2) interactio n in the system IrO2-TiO2 is identical to that of the electrochemically pro moted IrO2-YSZ system (i.e., continuous O2- supply to the IrO2 catalyst sur face). 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 fi lms interfaced with YSZ at various imposed potentials and thus imposed work function values, Phi (i.e., under conditions of electrochemical promotion and also on finely dispersed Rh catalysts deposited on various doped and un doped TiO2 and YSZ porous supports of measured work function Phi). Again it was found that the reaction kinetics are affected in the same way upon var ying 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 disp ersed Rh catalysts. This observation confirms the equivalence of the promot ing mechanism of metal-support interactions and electrochemical promotion ( i.e., O2- migration onto the catalyst surface). The results show conclusive ly that electrochemical promotion is an electrically controlled metal-suppo rt interaction and that at least certain types of metal-support interaction s are induced by reverse spillover of oxygen anions from the carrier onto t he surface of the metal crystallites. (C) 2001 Academic Press.