Preparation, Characterization, and Catalytic Performance of Bismuth−Aluminum Binary-Oxide Layers and Clusters on an Al2O3Surface

Abstract

Bismuth−aluminum binary-oxide layers and clusters on an Al2O3 surface were prepared by supporting bismuth triethoxides on Al2O3, followed by calcination at 673 K. The Bi−Al binary oxides were characterized by EXAFS and XPS, and their catalytic activities for ethanol-selective oxidation were investigated. XPS data revealed that Bi atoms were supported on the Al2O3 surface in a Stranski−Krastanov (SK) growth mode, while EXAFS data demonstrated that Bi atoms made bondings with Al atoms at 0.353−0.361 nm through oxygen atoms (Bi−O, 0.211−0.212 nm) irrespective of Bi loadings and that the coordination numbers (CN) of the Bi−Al bond did not significantly change with Bi loadings (CN, 1.0−1.5). It is likely from these results that below 9 wt % Bi loading, Bi−Al (rich) binary-oxide monolayers are formed at the Al2O3 surface and that above 9 wt % Bi loading, Bi (rich)−Al binary-oxide clusters grow on the surface. In the low-Bi-loading catalysts, Bi−O and Bi−Al bonds were observed at 0.211 and 0.353−0.355 nm, respectively, while no Bi−Bi bond was observed, which suggests that Bi atoms are dispersed in the oxide layers. On the other hand, in the high-Bi-loading catalysts, Bi−Bi and long Bi−O bonds were observed at 0.363−0.369 and 0.276−0.279 nm, respectively, in addition to Bi−O at 0.212 nm and Bi−Al at 0.359−0.361 nm. The distances of the long Bi−O and Bi−Bi bonds were comparable to their distances in γ-Bi2O3, and the Bi−Al distance was also close to that in bismuth binary oxides with the γ-Bi2O3 phase. These results suggest that the Bi (rich)−Al binary-oxide clusters have a γ-Bi2O3-like structure. The catalytic activity of these binary oxides at the Al2O3 surface for ethanol-selective oxidation strongly depended on the structure around bismuth atoms. The initial rate for acetaldehyde formation steeply increased around 9 wt % Bi loading. The activation energy for the 11.0 wt % Bi/Al2O3 catalyst was 53 kJ mol-1, while that for the 7.6 wt % Bi/Al2O3 catalyst was 96 kJ mol-1, reflecting the different local structure of the active Bi sites.