Structural and spectral features of nano-crystalline copper-stabilized zirconia

Abstract

The structural and spectral characteristics of copper-stabilized zirconia samples with different Cu contents (0.1–30 mol %of Cu), prepared by co-precipitation and sol–gel techniques are reported. The samples are designated as Cu–ZrO2(n), where ‘n’ is the Cu content in mol %. Rietveld analysis of the powder X-ray diffraction (XRD) data reveals that copper stabilizes zirconia both in cubic and tetragonal phases in all the Cu–ZrO2 compositions.With increasing concentration of Cu, zirconia is stabilized more and more in the cubic phase than in the tetragonal phase. A decrease in the crystallite size (from 28 to 8 nm) is also noted. Electron paramagnetic resonance (EPR) and diffuse reflectance UV– visible (DRUV–Vis) spectroscopies reveal that the samples contain at least four different types of paramagnetic Cu species, depending on the Cu content. These are isolated Cu2+ ions substituted in the framework sites of Zr4+ (species I), charge compensating ions located in the interstitial positions (species II), dispersed Cu ions bound to the surface (species III) and CuO-type clusters at the surface (species IV). Species I and II are present in all the Cu–ZrO2 compositons. Species III is present only in the samples containing more than 5 mol % Cu in zirconia. Species IV is found in the samples with still higher Cu content (
20 mol %); this species on treatment at high temperature (1173 K, for example) disintegrates yielding a separate bulk CuO phase. Species I is more difficult to reduce than species II–IV, even at 973 K using dry hydrogen. Electrons trapped in anion vacancies and O radicals are also detected by EPR under the conditions of evacuation, pretreatment and reaction with hydrogen. The spectral studies reinforce the conclusions drawn from XRD that some amount of Cu is incorporated in the zirconia lattice. The copper ions are dispersed up to a Cu concentration of 20 mol % in zirconia. The high catalytic activity of Cu–ZrO2(20) samples in CH4 and CO oxidations reported earlier is attributed to the highly dispersed nature of Cu ions in zirconia, their facile redox property (Cu2+ to Cu+1/0) and oxygen storage capacity of the support oxide.