The influence of surface structure on the catalytic activity of alumina supported copper oxide catalysts. Oxidation of carbon monoxide and methane

Abstract

X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and electron spin resonance (ESR) have been used to characterize a series of Cu/Al₂O₃ catalysts. The information obtained from surface and bulk characterization has been correlated with CO and CH₄ oxidation activity of the catalysts. For catalysts with Cu/Al atomic ratios ⩽0.051, XPS data indicated that most of the Cu was present as a dispersed surface phase. ESR results showed that the ratio of isolated/interacting copper surface phase decreased with increasing Cu content. For catalysts with Cu/Al atomic ratio ⩾0.077, large CuO crystallites were detected by XRD. XPS results indicated that Cu dispersion decreased with increasing Cu content. The turn over frequency (TOF) for CO oxidation increased with increasing Cu content. This has been attributed to an increase in the amount of crystalline CuO present in the catalysts. The TOF for CH₄ oxidation decreased with increasing Cu content up to Cu/Al = 0.051. This was paralleled by a significant decrease in the relative Cu ESR signal measured for the catalysts. We propose that the isolated Cu surface phase is more active for CH₄ oxidation than the interacting copper surface phase or crystalline CuO. CH₄ oxidation activities were similar for the catalysts with Cu/Al atomic ratio ⩾0.077.