Role of Copper in the Characterization of Copper(II)-Promoted Tin(IV) Oxide Catalysts for the Catalytic Oxidation of Carbon Monoxide

Abstract

Cu(II)/SnO2 catalyst materials with Cu:Sn ratios in the range 0.02−0.30 have been prepared by three routes: coprecipitation from aqueous solutions containing Cu2+ and Sn4+ ions, sorption of Cu2+ ions on to tin(IV) oxide gel, and destabilization of choline-stabilized tin(IV) oxide colloidal sols by the addition of aqueous copper(II) nitrate solution, and their constitution after thermal treatment in the temperature range 333−1273 K has been investigated by powder X-ray diffraction and EXAFS. Materials obtained by coprecipitation or impregnation are similar in nature irrespective of the Cu:Sn ratio with particle sizes <5 nm even after calcination at 673 K. Calcination at 873 K results in larger particles, but at temperatures ≥1073 K sintering to give very large particles occurs. EXAFS data show that the initial Cu(II) species from the coprecipitation or impregnation routes are hexaaqua {Cu(H2O)6}2+ ions sorbed onto the surface of the tin(IV) oxide particles but calcination at temperatures of 873 K and above causes the phase separation of CuO. EPR shows that only surface copper(II) is readily reduced on exposure to carbon monoxide. Copper(II)-promoted tin(IV) oxide catalysts effect the catalytic oxidation of CO for stoichiometric CO/O2 mixtures under at 328 K. The mode of operation of the catalysts appears to be synergistic in nature with the principal role of Cu(II) being mainly in electron transfer, i.e., it abstracts the negative surface charges (formed in the oxygen vacancies following desorption of CO2) to form Cu(I), which is then oxidized back to Cu(II) by reaction with oxygen. The activity of catalysts deactivated by running under highly reducing conditions can be restored completely by heating in a flow of air at 355 K. Irreversible deactivation occurs on processing at very high temperatures due to sintering and the phase separation of copper(II) oxide.