Oxidation activity and O-18-isotope exchange behavior of Cu-stabilized cubic zirconia

A series of Cu-ZrO2 samples with varying concentrations of Cu from 1 to 33 mol% have been prepared by sol-gel technique and calcined at 873 K. XRD cha racterization of the samples with a copper content of 2 to 20 mol% reveals the stabilization of zirconia into cubic (fluorite) phase. The Cu-ZrO2 samp les with higher Cu content (> 20 mol%) revealed the presence of bulk CuO. A linear decrease in lattice parameter with increase in Cu content up to 20 mol% indicates the possible incorporation of CU2+ , the lattice position of Zr4+ ions, XPS data of these samples provide further evidence for the inco rporation of Cu2+ ions in the ZrO2 lattice up to 5 mol%. At higher concentr ations of Cu, about half of the input of Cu goes into the lattice and the r emaining stays as extra lattice Cu, possibly on the surface or subsurface l ayer of ZrO2. The BET surface area of these samples was in the range of 2 t o 8 m(2) g(-1). The activity of these samples in CH4 and CO oxidation was i nvestigated by O-18-isotope exchange as well as by catalytic reaction studi es in complete oxidation of CH4 and CO. The Cu-ZrO2 with 20 mol% Cu was fou nd to be the most active sample in the series, which has the maximum amount of copper in the substitutional position. For comparison, yttrium-stabiliz ed zirconia samples with and without Cu was also prepared following the sam e procedure. Yttrium-stabilized zirconia without Cu was almost inactive in complete methane oxidation, whereas Cu-containing sample was more active. T his confirms that the presence of Cu species in substitutional positions al ong with oxygen vacancies in zirconia lattice are substantially responsible for the catalytic activity in CH4 and CO oxidation as well as in complete heterogeneous O-18 exchange processes. The light-off temperature for 50% co nversion of CH4 (T-50) decreases with an increase in Cu content up to 20 mo l% and matches well with the results of O-18 exchange measurements. The sha pes of the curves of T-50 and T-exchange follow a similar trend indicating that both CH4 oxidation and O-18 exchange processes occur via a completely heterogeneous mechanism. (C) 2001 Academic Press.