Supported metals, small particles preparation methods, 61

Gold particles supported on titania are active catalysts for the low temperature oxidation of CO. This phenomenon was originally discovered by Haruta and coworkers in the early 1990s, and has been corroborated by many subsequent studies. - - - - The exact catalytic activity of the Au/TiOj system depends on the method of preparation and the dispersion of the metal on the support, but in general Au particles with sizes between 2 and 4 nm display a catalytic activity for CO oxidation much larger than that of bulk metallic gold. New preparation methods aim for the synthesis of very small Au particles (< 2 nm) with an extremely high catalytic activity. ... 

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

To answer these questions requires some understanding of the properties of small metal particles, both structural and electronic. In this review we shall examine first the evidence relating to metal particles prepared by direct methods, e.g., vapour deposition or condensation in the gas phase. Then we shall consider whether this information can be applied to the case of supported metals where both precursor decomposition and support effects may add to the complexity of the total system. We shall then consider whether further changes in catalytic properties occur after preparation, i.e., during the catalytic reaction. Finally, we shall summarize some of the more recent evidence concerning the nature of structure sensitivity. 

In supported catalysts there is evidence that particle morphology is affected by the nature of the support, and by the methods of preparation and pretreatment. Coalescence and reconstruction of clean particles should be extremely rapid. The fact that in many cases small particles in contact do not combine into a single coherent particle suggests that the surface of supported metal particles may be relatively highly contaminated. When this occurs it must affect catalytic properties and correlations between activity and structure. 

The success of Haruta s early work lay in his choice of preparation method and support. Gold particles of the necessary small size were first obtained by coprecipitation (COPPT) and later by deposition-precipitation (DP) (see Sections 4.2.2 and 4.2.3) classical impregnation with HAuCLj does not work. The choice of support is also critical transition metal oxides such as ferric oxide and titania work well, whereas the more commonly used supports, such as silica and alumina, do not work well or only less efficiently. This strongly suggests that the support is in some manner involved in the reaction. 

The catalytic metal most widely studied by NMR is platinum the hrst observation of oxide-supported Pt was made by Slichter and co-workers 20 years ago. Nearly all of the metal-NMR results in this review are concerned with this nucleus. Some data for Rh will be discussed also. Recently, Tc NMR spectra have been published of small Tc particles (average diameter 2.3 nm, but a rather wide size distribution) on alumina [70]. The spectra were taken between 120 and 400 K. While bulk technetium has the hep structure, these small particles are cubic, and their Tc shift (around 7400 ppm) is about 600 ppm larger than the isotropic part of the bulk shift. The linewidth varies with support material and method of preparation, but remains amazingly small (15-75 ppm). This linewidth/shift ratio of about 0.5% is much less than that found for small particles of rhodium or platinum and is comparable to that found for silver [71]. It is unlikely, however, that the linebroadening mechanisms in small particles of silver and of technetium are the same. 

We have shown that small uniform ruthenium particles can be applied on activated CNFs in a reproducible manner when the HDP method is used with RuN0(N03)3 as catalyst precursor. A very uniform distribution of 1-2 nm sized ruthenium particles at an appreciable loading has been obtained. This high dispersion remained almost unchanged upon heating in inert to 973 K. These results clearly demonstrate the applicability of the HDP technique for the preparation of CNF supported metal catalysts, though no surface compound between precursor and support material can be formed. 

In the preparation of Ni/Hp catalysts by the deposition-precipitation method (DP), nickel hydrosilicates are formed mainly but not exclusively in the external surface of the Hp zeolite. The strong metal-support interaction induced by the DP preparation method prevents the Ni metal particles from sintering during the activation of the catalysts (calcination and reduction) and a homogeneous distribution of small nickel particles is obtained. The catalyst prepared by DP showed better catalytic activity in the hydrogenation of naphthalene than the catalyst prepared by cationic competitive exchange. 

Recent years have witnessed marked progress in the preparation of small metal particles. This has been achieved by the choice of a suitable support, the selection of the appropriate preparation method, or the combination of both. 

Some attempts have been made to measure the electronic properties of small particles by X-ray photoelectron spectroscopy (XPS). The preparation of samples of isolated small metal particles is not easy. The most successful methods are either vapor deposition of noble metals (Pt or Pd) on carbon or silica, or ion exchange used to prepare metals in Y zeolite. For the noble metals and inert supports used, it is assumed that the metal particles are isolated from each other. 

It seems plausible that the catalytic activity of small metal particles would be influenced by the crystal structure. Yacaman et al. (118) have studied pentane hydrogenolysis over Rh -A C, Rh/Si02, Rh/C, Rh/Ti02, and Rh/MgO. The support and preparation method, all for particles of d < 5 nm, determined whether cubooctahedrons or icosahedrons were formed, but the catalytic properties depended more on d than on crystal type. 

Gold deposited on metal oxides has been reported as active catalyst for many reactions [1]. Usually, gold exhibits activity when the size of its nanoparticles is less than 5 nm [2]. The stabilization of small metal particles and their activity strongly depend on preparation method and on support used [1]. Zeolite material possesses adjustable acidic properties and regular molecular-size channels in the crystalline lattice. These features provide inclusion of metal ions into zeolite matrix with subsequent transformation to ultrafine metal particles and clusters [3, 4]. In the present paper, the effect of changing of reduction temperature and concentration on the state of gold in Beta-zeolite were studied. 

A number of different methods for the preparation of supported metal (oxide) catalysts are dealt with in this book. In this chapter we discuss deposition precipitation as a generic method to emplace metals, metal oxides, metal sulfides, or metal hydroxides as small particles onto an existing support material. The deposition of the metal (compound) is brought about by a chemical reaction in the liquid phase. This chemical reaction leads to formation of a metal compound with low solubility in the solvent in question. The precipitation that follows is steered to take place exclusively at the surface of a suspended support material. The precipitation of metal hydroxides from an aqueous solution, such as... 

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