One-electron reducibility of isolated copper oxide on alumina for selective NO-CO reaction

The H-2-TPR (temperature-programmed reduction) study was performed for supported copper oxide catalysts with low loading (0.5 wt% as copper). Among the various kinds of support materials (gamma-Al2O3, TiO2, ZrO2, SiO2, ZSM-5), alumina-supported copper oxide indicated a one-electron reduction behavior of Cu2+ into Cu+ ions in the presence of H-2. The reduction of the isolated Cu2+ species in a tetragonally distorted octahedral symmetry into the low coordinated Cu+ ions was identified by means of X-ray absorption spectroscopy (XANES and EXAFS). The isolated Cu+ ions hosted by gamma-Al2O3 surface were prevented from further reduction into metallic Cu-0 state under reducing condition with H-2 at 773 K. Less dispersed supported copper oxide species were easily reduced to Cu-0 metal particles with H-2 at 573 K regardless of the kinds of support materials. It is suggested that the one-electron redox behavior of the isolated copper oxide species over gamma-Al2O3 promotes the catalytic reduction of NO with CO in the presence of oxygen on the basis of redox-type mechanism between Cu2+ and Cu+ in atomically dispersed state. (c) 2005 Elsevier B.V. All rights reserved.