Nature and Surface Redox Properties of Copper(II)-Promoted Cerium(IV) Oxide CO-Oxidation Catalysts

Abstract

Cu(II)/CeO2 catalyst materials have been prepared by two routes, coprecipitation from aqueous solutions containing Cu2+ and Ce4+ ions and sorption of Cu2+ ions on to ceria gel, and their constitutions after thermal treatment in the temperature range 333−1273 K investigated by nitrogen adsorption, powder X-ray diffraction, EPR, and EXAFS. EXAFS data show that the initial Cu(II) species are polymeric Cu(OH)2 and hexaaqua {Cu(H2O)6}2+ ions sorbed onto the surface of the ceria particles from the coprecipitation or impregnation routes, respectively. The materials are mesoporous except after calcination at 1273 K when they become nonporous and crystallites of CuO are apparent. Promotion of ceria with copper(II) enhances the activity toward the oxidation of carbon monoxide dramatically, 100% conversion occurring at 343 K even for high CO concentrations and stoichiometric CO/O2 compositions. The most active catalyst material is formed by thermal processing of materials obtained by either route at ca. 673 K producing a material comprising small particles of ceria on the surface of which copper(II) is dispersed amorphously. EPR shows that, after calcination at 573 K, the copper(II) is present in both types of material as a mixture of isolated Cu2+ ions and amorphous clusters or aggregates of Cu2+ ions. Additionally Cu2+ dimers are formed at 873 K. In situ redox studies show that the amorphous copper(II) aggregates are reduced most easily by exposure to CO at 473 K, followed by the dimer species at 573 K and finally the isolated Cu2+ monomers at 673 K when signals due to Ce3+ appear (i.e. reduction of the support). Reoxidation of all types of Cu+ can be achieved by exposure of the reduced catalyst material to either O2 or NO, although under the same pressure (200 Torr) oxidation by NO is achieved at a lower temperature than with O2. The mode of action of these catalyst materials appears to be synergistic in nature with the principal role of Cu(II) being mainly in electron transfer, abstracting the negative charge remaining when oxygen vacancies are formed following desorption of CO2. The significant enhancement of catalytic activity is due in large part to the efficiency of the Cu(II)/Cu(I) couple in this process. Catalyst deactivation at low temperatures under reducing conditions is due to depletion of the catalyst surface of active oxygen, but activity is restored by treatment in air at moderate temperatures. Deactivation at high temperatures is irreversible and is due to the phase separation of copper(II) oxide coupled with a dramatic increase in particle size.