Supported Au nanoparticles (NPs) prepared by colloid deposition method were well characterized, and their catalytic performance was tested for chemoselective reduction of a nitro group of substituted nitroaromatics by H 2 . Systematic studies on the effects of NPs size and support show small size of Au NPs, and acid-base sites of supports are required for high activity. The Au/Al 2 O 3 catalyst with Au particle size of 2.5 nm selectively hydrogenates a nitro group in the presence of various other reducible functional groups, and it shows higher intrinsic activity than the state-of-the-art catalyst (Au NPs on TiO 2 ). In situ FTIR studies provide a reaction mechanism, which explains fundamental reasons of the observed structure-activity relationship. Cooperation of the acid-base pair site on Al 2 O 3 and the coordinatively unsaturated Au atoms on the Au NPs are responsible for the H 2 dissociation to yield a H + /Hpair at the metal/support interface. High chemoselectivity could be attributed to a preferential transfer of the H + /Hpair to the polar bonds in the nitro group as well as a preferential adsorption of nitroaromatics on the catalyst through the nitro group.
Advances in scanning transmission electron microscopy (STEM) techniques have enabled us to automatically obtain electron energy-loss (EELS)/energy-dispersive X-ray (EDX) spectral datasets from a specified region of interest (ROI) at an arbitrary step width, called spectral imaging (SI). Instead of manually identifying the potential constituent chemical components from the ROI and determining the chemical state of each spectral component from the SI data stored in a huge three-dimensional matrix, it is more effective and efficient to use a statistical approach for the automatic resolution and extraction of the underlying chemical components. Among many different statistical approaches, we adopt a non-negative matrix factorization (NMF) technique, mainly because of the natural assumption of non-negative values in the spectra and cardinalities of chemical components, which are always positive in actual data. This paper proposes a new NMF model with two penalty terms: (i) an automatic relevance determination (ARD) prior, which optimizes the number of components, and (ii) a soft orthogonal constraint, which clearly resolves each spectrum component. For the factorization, we further propose a fast optimization algorithm based on hierarchical alternating least-squares. Numerical experiments using both phantom and real STEM-EDX/EELS SI datasets demonstrate that the ARD prior successfully identifies the correct number of physically meaningful components. The soft orthogonal constraint is also shown to be effective, particularly for STEM-EELS SI data, where neither the spatial nor spectral entries in the matrices are sparse.
The surface structure of catalytic gold nanoparticles was observed dynamically during CO oxidation using an environmental-cell transmission electron microscope (E-cell TEM) system. In the developed system, the gold catalyst specimen can be set under the reaction gas condition by separating it from the vacuum using ultra-thin carbon films. We have developed a method capable of yielding for carbon films 8-nm thick, capable of withstanding a gas pressure of 2 atm. In situ observations using the above system indicated marked changes in the surface shape of the gold nanoparticle catalyst during the reaction. Copyright c 2008 John Wiley & Sons, Ltd.Keywords: gold nanoparticle catalyst; environmental-cell; transmission electron microscopy; in situ observation; shape change Although bulk gold is chemically inert, gold particles less than 10 nm in diameter were recently shown to exhibit unique catalytic properties when bound tightly to specific metal oxides, such as TiO 2 , etc. [1 -4] This catalyst shows remarkably high activity, e.g. in CO oxidation. However, the mechanism of action of this gold nanoparticle catalyst is not clear. To determine its mechanism of action, it is necessary to investigate the gold surface and interface atomic structures. [5,6] Transmission electron microscopy (TEM) is effective for analyzing structures at the atomic level, but specimens are conventionally set under vacuum conditions. [7,8] This means that it is difficult to discuss the relationship between the observed structures and catalytic behaviors directly. It is important to observe the catalyst under reaction gas conditions. Therefore, we have developed an environmental-cell (E-cell) TEM system. [6,9,10] Here, we introduce our developed E-cell TEM system, and also present results of in situ observation of the gold catalyst during oxidation reaction. Figure 1 shows a schematic diagram of the E-cell TEM system. The base of this system is a conventional 200 kV-TEM (H-8000; Hitachi). Two dedicated devices, a gas control unit and an E-cell specimen holder, were assembled. The former, which consists of vacuum gages, a gas flow-meter, valves, and stainless steel pipes, is connected to the specimen holder and enables introduction of the reaction gas with control of its pressure and flow rate from a tank. The exhaust gas is then evacuated by an oil-sealed rotary pump. The E-cell specimen holder has a small chamber, the so-called E-cell, at the top, to which two pipes are connected to allow gas to pass in and out. The specimen can thus be set under the reaction gas conditions, as shown in the enlarged diagram of the holder on the top right of Fig. 1. The E-cell is separated from the vacuum in the TEM column by two ultra-thin carbon films set at the top and bottom of the E-cell. During observation, an electron beam passes through these films to form images.These films are therefore among the most important components of this system, and they require two conflicting characteristics: toughness to withstand gas pressure between the vacuum and the E...
Environmental transmission electron microscopy and ultra-high resolution electron microscopic observation using aberration correctors have recently emerged as topics of great interest. The former method is an extension of the so-called in situ electron microscopy that has been performed since the 1970s. Current research in this area has been focusing on dynamic observation with atomic resolution under gaseous atmospheres and in liquids. Since 2007, Nagoya University has been developing a new 1-MV high voltage (scanning) transmission electron microscope that can be used to observe nanomaterials under conditions that include the presence of gases, liquids and illuminating lights, and it can be also used to perform mechanical operations to nanometre-sized areas as well as electron tomography and elemental analysis by electron energy loss spectroscopy. The new instrument has been used to image and analyse various types of samples including biological ones.
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