Distinction between Surface and Bulk Oxidation of Cu through N2O Decomposition

Abstract

A Cu metal surface was evaluated by a novel technique combining temperature-programmed reduction (TPR) measurement with N2O oxidation. The technique consists of three steps: the usual TPR measurement, the oxidation of the Cu surface by N2O, and the subsequent TPR measurement. The surface Cu oxidized by N2O was determined as a ratio of the peak area of the second TPR profile to that of the first one. It was found that bulk oxidation gradually proceeds after surface oxidation even at 30°C. After the surface oxidation, the Cu2O produced by N2O oxidation varied with N2O exposure time (t) and had a linear correlation with t12 at temperatures below 100°C. The linear correlation in the parabolic plot proves that bulk oxidation proceeds through the diffusion process, and the Y-intercept corresponds to the surface oxidation. Both the dispersion and the Cu metal surface area of the sample were calculated from the intercept in the parabolic plot for the Cu2O produced by N2O oxidation. In addition, we found that a very large Cu metal surface area, as high as 32 m2 g−1cat, was created on a Cu–MgO catalyst through a citrate process using a molten mixture of copper nitrate, magnesium nitrate, and citric acid.