Metal particle catalysts

Amorphous carbon is one of the most important and economical materials in catalysis and is generally used as a support for metallic catalysts. Normally, fine metallic particle catalysts are dispersed on such carbon supports. In the following section we briefly review such carbons, its uses and carbon deposits resulting... 

A variety of industrial catalytic processes employ small metal-particle catalysts on porous inorganic supports. The particle sizes are increasingly in the nanometre size range which gives rise to nanocatalysts. As described in chapter 1, commonly used supports are ceramic oxides, like alumina and silica, or carbon. Metal (or metallic) catalysts in catalytic technologies contain a high dispersion of nanoscopic metal particles on ceramic oxide or carbon supports. This is to maximize the surface area with a minimum amount of metal for catalytic reactions. It is desirable to have all of the metal exposed to reactants. 

Based on TEM studies of supported metal catalysts, several workers have concluded that their catalysts were made of two-dimensional discs or rafts , where virtually all atoms are at the particle surface. However, sample tilting experiments in TEM have shown that great care should be exercised in the interpretation of TEM images of small particles (<2 nm in size), since phase contrast effects may dominate and variations in the particle contrast with specimen orientation can occur as a result of amplitude contrast effects (Treacy and Howie 1980). Sample tilting is therefore necessary to ensure correct interpretations of TEM images of metal-particle catalysts. This will be discussed further in the following sections. 

Although HRTEM is a very powerful technique for the study of small particles, imaging metal-particle catalysts on supports such as alumina, silica or carbon presents challenges. In order to understand the structure and contrast of very small (<5 nm) supported metal particles which are thought to be the active species in catalysis, key computations of supported small metallic catalysts have been reported by Gai et al (1986). Image computations for supported particles, carried... 

In the following example, we examine the role of different gases and metal-support interactions in metal-particle catalysts supported on the so-called nonwetting or irreducible ceramic oxides by dynamic EM. The direct observations provide powerful insights into the role of gas environments in catalytic reactions and in the regeneration of supported metallic catalyst particles. 

In situ ETEM permits direct probing of particle sintering mechanisms and the effect of gas environments on supported metal-particle catalysts under reaction conditions. Here we present some examples of metals supported on non-wetting or irreducible ceramic supports, such as alumina and silica. The experiments are important in understanding metal-support interactions on irreducibe ceramics. 

Overall perspectives of the results from ethene and the higher alkenes have been attempted in Sections VI.B.6 and VI.G. What has become clear, particularly in the context of hydrocarbon adsorption, is that the study of spectra on single-crystal surfaces is of great assistance in finding the correct interpretation of the more complex multispecies spectra obtained from finely divided metal catalysts. This has only become possible by the development of VEELS and RAIRS, the latter allied with the Fourier-transform methods that have also transformed the quality of the spectra from metal-particle catalysts obtained by transmission infrared spectroscopy. The use of RAIRS in turn has emphasized the general significance of the MSSR. 

The evolution of modern vapor phase processes starts with metal catalyzed chemical vapor deposition and ends with laser vaporization (see Table I). Most vapor phase processes require metal particle catalysts some proceed without the addition of metal particles. The growth temperatures range from 100 to 4000°C. The length of silicon nanowires is <10 nm [74] that of carbon nanotubes is <300 jm [76] but they can be potentially endless [81]. 

Cuboid and cycloid niobium monocarbide (NbC) whiskers, 0.1-2.0 pm in diameter and 5-100 pm in length, and having a square-shaped tip, were recently synthesized by heating mixtures of niobium oxide (Nb203) and carbon black at temperatures over 1100°C [38]. Silicon carbide nano-whiskers, 20-50 nm in diameter and 2-5 pm in length, were carbothermally synthesized by reducing ultrafine precipitated silica powders with ultrafine carbon black by microwave heating [29]. These processes proceed without addition of metal particle catalysts, and therefore by a VS phase transformation [14] [29] [38]. 

Silica gel, carbon furnace black and cobalt chloride yield silicon carbide whiskers, or Tokawhiskers [30], in a metal catalyzed process at >1450°C. A process variant [9] yields SiC whiskers >1350 C in a fixed bed percolated by a hydrogen flow. The addition of iron above 1450°C affords submicron whiskers ending with a silicon rich droplet. The iron seems to evaporate and condense below 1450°C leaving behind whiskers with silicon rich tip >1450 C. These processes use the same starting materials as the rice hull processes but they also use a metal particle catalyst. As a result, they are believed to proceed by a VLS phase transformation. 

If this carbothermal process is brought to only partial completion (Equation 11a and 11b), a homogeneous mixture of silicon carbide whiskers and silicon nitride powder [10] is obtained which can be fired directly to yield whisker reinforced ceramics. Silicon carbide reinforced alumina composites and silicon carbide whisker reinforced zirconia composites [31] are also products of the "chemical mixing process". The whisker growth rate in the zirconia process can be accelerated by adding metal particle catalysts such as cobalt chloride, thus potentially facilitating a VLS phase transformation. 

Laser assisted chemical vapor deposition is an evolutionary extension of the metal particle catalyzed chemical vapor deposition, wherein a hot laser focus takes the place of a hot solid or liquid metal particle catalyst (Figure 1). (Conventional chemical vapor deposition has no "hot spot" capable of preferentially focusing the vapor phase deposition. 

A preparation of designed catalyst is one of the interest subjects to understand the catalysis. Efforts have been paid for the development of unique preparation method[1] those are metal cluster catalysts derived from metal carbonyls, tailored metal catalysts through organometallic processor and ultra-fine metal particle catalysts prepared by metal alkoxides, etc. These preparation methods are mainly concentrated to design the active sites on support surfaces. However, the property of support itself is also a dominant factor in order to conduct smoothly the catalytic reaction. It is known that some supports are valuable for the improvement of selectivity. For example, zeolites are often used as catalysts and supports for their regular pore structures which act effectively for the shape selective reaction[2]. In order to understand the property of support, the following factors can be pointed out besides the pore structure structure, shape, surface area, pore size, acidity, defect, etc. Since these are strongly correlated to the preparation procedure, lots of preparation techniques, therefore, have been proposed, too. Studies have been still continued to discover the preparation method of novel materials as well as zeolites[3]. 

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