Cu- and Ag-Modified Cerium Oxide Catalysts for Methane Oxidation

Abstract

The catalytic activity of nanocrystalline doped ceria and Cu- and Ag-modified ceria for the complete oxidation of methane was studied in this work. The catalyst structure was studied by X-ray diffraction (XRD) and related to the availability of low-temperature oxygen species. Selected samples were also analyzed by STEM/EDX, HRTEM, and XPS. Temperature-programmed reduction (TPR) by H2and CH4, as well as oxygen chemisorption, measurements were used to characterize the different oxygen species present on the catalyst. La and Zr were used as dopants to modify the crystal size and reduction properties of ceria. Enhanced activity for the complete oxidation of methane is discussed in terms of ceria reducibility, crystal size, and formation of oxygen defects at the surface (extrinsic oxygen vacancies). Addition of transition metal oxides (CuO) or transition metals (Ag) improves the low-temperature oxidation activity of cerium oxide. The interaction of ceria with Ag and CuO is a strong function of the crystal size of ceria. In the presence of the transition metal or metal oxide, a small crystal size of ceria favors the formation of highly reducible oxygen species and enhances the methane oxidation activity.