The chemical state and Cu+ stability for three-way catalytic performance of Cu-added Al2O3 catalysts

Abstract

Cu-added γ-Al2O3 (CuAl) catalysts having varying Cu loadings (3, 5, 8, and 10 mol%) were prepared at 900 °C. The effects of the copper oxidation states and the stability of these states under oxidizing and reducing atmospheres were examined. The use of these materials as three-way catalysts (TWCs) was also assessed. Analyses by X-ray diffraction and transmission electron microscopy indicated that copper oxide species were well dispersed and formed a solid solution in a surface layer of the γ-Al2O3. The TWC activity was found to increase with the addition of copper, and the 8 mol% CuAl catalyst showed very high performance despite being heat-treated at high temperatures. In the case of the 10 mol% CuAl catalyst, the excess Cu loading resulted in the precipitations of copper species on the γ-Al2O3 surface, which decreased catalytic activity. The oxidation states also differed between these catalysts. X-ray absorption fine structure and temperature-programmed reduction data demonstrated variations in copper oxidation states among the catalysts before and after oxidation and reduction treatments. The 8 mol% CuAl catalyst had a high Cu+/Cu2+ ratio and contained stable Cu+ species during both oxidation and reduction trials. The stability of the copper oxidation states is evidently a factor affecting catalytic activity.