The effect of electrochemical oxygen pumping on the steady-state and oscillatory behavior of CO oxidation on polycrystalline Pt

Abstract

The effect of electrochemically pumping O 2− to or from a porous polycrystalline Pt catalyst film used for CO oxidation at atmospheric pressure and temperatures 250–600 °C was studied. The Pt film served both as a catalyst and as an electrode of the solid electrolyte cell CO, O 2, Pt ZrO 2 ( 8 mole% Y 2 O 3) Pt,O 2 . Under open-circuit conditions the Pt catalyst film operates as a regular CO oxidation catalyst. It was found that electrochemical O 2− pumping has a dramatic non-Faradaic effect on the steady-state and oscillatory behavior of CO oxidation on Pt. The steady-state reaction rate typically increases or decreases by a factor of 2 but a 500% increase in reaction rate is observed under severely reducing conditions. The induced changes in reaction rate are typically two orders of magnitude higher than the rate of O 2− transfer to or from the catalyst and are always accompanied by the appearance of activation overpotential at the catalyst electrode. Reaction rate oscillations can be induced or stopped at will by adjusting the rate of O 2− transfer and consequently the potential of the catalyst-electrode. The frequency of electrochemically induced oscillations is linearly related to the applied O 2− current. The observed phenomena are completely reversible and are due to electrochemically induced changes in the oxidation state and catalytic properties of the platinum surface. These changes appear to result from changes in the work function of the metal due to the interaction of O anions with the Pt surface. The very pronounced reaction rate increase upon O 2− removal under reducing conditions appears to be caused by CO decomposition followed by fast carbon combustion by gaseous O 2.