Understanding how molecules can restructure the surfaces of heterogeneous catalysts under reaction conditions requires methods that can visualize atoms in real space and time. We applied a newly developed aberration-corrected environmental transmission electron microscopy to show that adsorbed carbon monoxide (CO) molecules caused the {100} facets of a gold nanoparticle to reconstruct during CO oxidation at room temperature. The CO molecules adsorbed at the on-top sites of gold atoms in the reconstructed surface, and the energetic favorability of this reconstructed structure was confirmed by ab initio calculations and image simulations. This atomic-scale visualizing method can be applied to help elucidate reaction mechanisms in heterogeneous catalysis.
on the occasion of its 100th anniversary Gold, the most stable metallic element, shows remarkable catalytic activity for CO oxidation even at room temperature.[1] Unlike platinum and palladium, [2] gold must be supported in the form of nanoparticles on crystalline metal oxides such as TiO 2[1] and CeO 2 .[3] Despite extensive studies, [4][5][6][7][8][9][10][11][12][13] 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/CeO 2 catalyst in CO/air mixtures by means of in situ environmental transmission electron microscopy (ETEM). [14][15][16][17][18][19][20][21][22][23][24] 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/CeO 2 catalyst using ETEM, and found that the majority of the GNPs behaved systematically, depending on the partial pressures of CO and O 2 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 CeO 2 , 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 CeO 2 support most likely plays an important role in the catalytic activity, especially the dissociation of O 2 molecules at room temperature. This work thus contributes to improving and developing real catalysts.The Au/CeO 2 catalyst was prepared by the deposition precipitation method.[1] The conversion of CO to CO 2 reached 100 % at room temperature, and the turnover frequency (TOF) of the catalyst was measured as 0.24 mol CO (mol Ausur )À1 s À1 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 CeO 2 in various environments at room temperature. During CO oxidation in 1 vol % CO/air gas mixture (1 vol % CO, 21 vol % O 2 , 78 vol % N 2 ) 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 1 a. Unexpectedly, the GNP behaved differently, and became rounded in pure O 2 gas. The GNP exhibited major facets in both inactive N 2 gas at 1 mbar and in vacuum (Figure 1 a). In N 2 gas, N 2 molecules collided with the surface of the GNP at a ra...
Despite the fragility of TiO(2) under electron irradiation, the intrinsic structure of Au/TiO(2) catalysts can be observed by environmental transmission electron microscopy. Under reaction conditions (CO/air 100 Pa), the major {111} and {100} facets of the gold nanoparticles are exposed and the particles display a polygonal interface with the TiO(2) support bounded by sharp edges parallel to the 〈110〉 directions.
Aberration-corrected environmental transmission electron microscopy (ETEM) proved that catalytically active gold nanoparticles (AuNPs) move reversibly and stepwise by approximately 0.09 nm on a cerium oxide (CeO2) support surface at room temperature and in a reaction environment. The lateral displacements and rotations occur back and forth between equivalent sites, indicating that AuNPs are loosely bound to oxygen-terminated CeO2 and may migrate on the surface with low activation energy. The AuNPs are likely anchored to oxygen-deficient sites. Observations indicate that the most probable activation sites in gold nanoparticulate catalysts, which are the perimeter interfaces between an AuNP and a support, are not structurally rigid.
The temperature dependence of the shape of Pt nanoparticles supported on CeO2 for CO oxidation was investigated using environmental transmission electron microscopy. Pt nanoparticles in CO/air become round at room temperature (when catalytic activity is low), while they become partially faceted at elevated temperature (when the catalytic activity is high). Based on a comparison between the shapes of the Pt nanoparticles in vacuum, N2, O2, CO, and CO/air at room temperature, 100, and 200 °C, we propose that the change in shape of the Pt nanoparticles is induced by the adsorption of CO molecules and O atoms.
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