Systematic morphology changes of gold nanoparticles supported on CeO2 during CO oxidation.

Gold, the most stable metallic element, shows remarkable catalytic activity for CO oxidation even at room temperature. Unlike platinum and palladium, gold must be supported in the form of nanoparticles on crystalline metal oxides such as TiO2 [1] and CeO2. [3] Despite extensive studies, the mechanism of catalysis by gold nanoparticles (GNPs) is still unclear, in particular in relation to CO oxidation at room temperature. In the present study we observed a real Au/CeO2 catalyst in CO/air mixtures by means of in situ environmental transmission electron microscopy (ETEM). The catalyst was also characterized by catalytic chemical analyses. In real GNP catalysts, the structures of the GNPs are not identical at the atomic scale. Hence, we examined a large number of GNPs in the Au/CeO2 catalyst using ETEM, and found that the majority of the GNPs behaved systematically, depending on the partial pressures of CO and O2 at room temperature. GNPs remained faceted during CO oxidation in CO/air and became rounded, or fluctuating multifaceted with decrease of the partial pressure of CO relative to air. We also examined GNPs supported on a non-oxide crystal (TiC) with ETEM. In contrast to GNPs supported on CeO2, switching the gases did not induce any morphology change of GNPs supported on TiC. These experimental results have provided a clue toward elucidation of the peculiar catalytic mechanism of supported GNPs. The interface between GNPs and CeO2 support most likely plays an important role in the catalytic activity, especially the dissociation of O2 molecules at room temperature. This work thus contributes to improving and developing real catalysts. The Au/CeO2 catalyst was prepared by the deposition precipitation method. The conversion of CO to CO2 reached 100 % at room temperature, and the turnover frequency (TOF) of the catalyst was measured as 0.24 molCO (molAusur) 1 s 1 at 303 K. The catalyst sample was examined in vacuum by conventional transmission electron microscopy before and after the oxidation of CO at atmospheric pressure and at 303 K for 5 h. As shown in Figure S1, it was confirmed that the average size and morphology of the GNPs remained unchanged after the oxidation of CO at atmospheric pressure. A detailed description of the catalyst is given in the Supporting Information. First, we summarize the typical morphology of a GNP supported on CeO2 in various environments at room temperature. During CO oxidation in 1 vol% CO/air gas mixture (1 vol% CO, 21 vol% O2, 78 vol% N2) at 1 mbar pressure, the GNP appeared to be faceted in the form of a stable polyhedron enclosed by the major {111} and {100} facets, as shown by Figure 1a. Unexpectedly, the GNP behaved differently, and became rounded in pure O2 gas. The GNP exhibited major facets in both inactive N2 gas at 1 mbar and in vacuum (Figure 1a). In N2 gas, N2 molecules collided with the surface of the GNP at a rate of 3 10 s 1 nm . By comparison of the GNP in N2 gas and in vacuum (Figure 1a), we consider that the impacts of inactive N2 molecules caused no significant