Effect of Preparation Route and Thermal Treatment on the Nature of Copper and Chromium Doubly Promoted Ceria Catalysts

Chromium and copper doubly promoted ceria catalysts have been prepared by three routes: (i) coprecipitation from aqueous solutions containing cerium(IV), copper(II), and chromium(III) ions, (ii) sequential impregnation of CeO 2 by using an aqueous CrO 3 solution followed by an aqueous solution of copper(II) nitrate, and (iii) impregnation of Cu(II)/CeO 2 prepared by coprecipitation by using an aqueous CrO 3 solution. The nature of the chemical transformations occurring during the thermal processing of the catalyst materials obtained by route (i) have been investigated by a combination of FT-IR, powder X-ray diffraction, transmission electron microscopy, X-ray absorption spectroscopy, and electron paramagnetic resonance spectroscopy. Some comparative data for materials obtained by the other two routes is also described. The nature of the transition-metal promoters varies substantially with the calcination processing temperature, the preparative route, and the Cu/Cr content. At low processing temperatures, copper is present as Cu(OH) 2 (polymeric) in the Cu/Cr/CeO 2 -cop material, but a variety of chromium species in oxidation states +III, +V, and +VI are observed. After thermal processing at 573 K, the material comprises small crystallites of ceria-supporting amorphous copper(II) and chromium(III), chromium(V), and chromium(VI) oxide species. Significant physical and chemical changes occur to the material by 873 K, which comprises crystallites of CeO 2 and the spinel CuCr 2 O 4 which is converted at 1273 K to Cu 2 Cr 2 O 4, although a crystalline Cr 2 O 3 phase separates with high chromium loadings. Chromium is present as adsorbed {Cr 2 O 7 2 - } ions in the Cu/Cr/CeO 2 -imp/imp and Cu/Cr/CeO 2 -cop/imp materials at low-thermal treatments, but the dimeric surface anion is largely decomposed at 573 K although chromium(VI) is still present in substantial quantities. No ternary oxide phases are formed in either of these materials on thermal processing at high temperatures, and only phase-separated Cr 2 O 3 and CuO are formed at temperatures from 873 to 1273 K.