Reduction characteristics of copper oxide in cerium and zirconium oxide systems

Abstract

The reduction of CuO dispersed on fluorite-type oxide catalysts, namely La-doped CeO 2 and Y-doped ZrO 2 was studied in this work. On both supports distinct copper species were identified as a function of copper content by temperature-programmed reduction (TPR) by H 2 and CH 4, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and scanning transmission electron microscopy/energy dispersive X-ray (STEM/EDX) analyses. At low copper loading (<15 at%), the copper phase is present as small clusters, which are reduced at lower temperature than bulk CuO. At higher Cu loading (>15 at%), in addition to clusters, larger CuO particles are present which are reduced at higher temperature close to the reduction temperature of bulk CuO. At copper loading lower than ca. 5 at%, copper is present as highly dispersed clusters or isolated Cu ions, which interact strongly with the fluorite-type oxide, thus requiring higher reduction temperature. However, the latter is still below the bulk CuO reduction temperature. Copper is more stabilized when dispersed in Ce(La)O 2 than in Zr(Y)O 2 matrix, so that reduction of copper oxide species requires lower temperatures on the Zr(Y)O 2-based catalysts. The reducibility of the doped ceria is enhanced by the presence of copper in both H 2- and CH 4-TPR. On the other hand no such interaction is present in CuZr(Y)O 2 system. The activity of various copper species for methane oxidation is discussed.