We have first observed the nucleation and growth process of carbon nanotubes (CNTs) from iron carbide (Fe 3C) nanoparticles in chemical vapor deposition with C 2H 2 by in situ environmental transmission electron microscopy. Graphitic networks are formed on the fluctuating iron carbide nanoparticles, and subsequently CNTs are expelled from them. Our atomic scale observations suggest that carbon atoms diffuse through the bulk of iron carbide nanoparticles during the growth of CNTs.

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...

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