Dynamic study of the oxidation state of copper in the course of carbon monoxide oxidation over powdered CuO and Cu2O

Variations in the oxidation state of copper during CO catalytic oxidation over powdered CuO and Cu2O in a gas mixture of 5% CO/He and air (O-2) were investigated by means of an appropriate combination of evolved gas analysis, thermogravimetry, XRD, and FT-IR in the temperature range of RT to 350 degrees C, and these variations were compared with those observed when both oxides were individually reacted with CO (5% CO/He) and with air, The catalytic rate over CuO was found to almost agree with the bulk reduction rate of CuO --> Cu, while the catalytic rate over Cu2O agreed with the bulk reduction rate of CuO* --> Cu2O, where CuO* denotes a metastable divalent copper oxide produced in the oxidizing process of Cu2O. Adsorption of O-2(-) and/or O- was characteristic of CuO*, in contrast with that of O-2 and/or O of CuO. Variations in copper valence of a catalyst surface were concluded to be II reversible arrow I for CuO and I --> II reversible arrow I for Cu2O, and the catalytic rates were determined by the reducing processes of CuO and CuO* given by O-lc(2-) + O-2 (ads) (or O-ads) + COads --> CO2 + O-2 (ads) (or O-ads) + 2e(-) followed by copper valence changes of II --> 0 and O-lc(2-) + O-2 ads(-) (or O-ads(-)) + 1/2CO --> 1/2O(lc)(2-) + O-2 (-)(ads) (or O-ads(-)) + 1/2CO(2) + e(-) followed by that of II --> I, where O-lc(2-) denotes lattice oxygen. (C) 1999 Academic Press.