Supported metals, small particles distribution

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

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. 

Structure-sensitive reactions are extensively discussed in the catalytic literature, but careful examination of the published work reveals that on the atomic scale the catalytic materials used in these studies are in general poorly characterized with respect to particle size and structure. Extended X-ray absorption fine structure (EXAFS) has been successfully applied to the study of small particles on supportsand small metal molecules in matrices subject to the caveat that samples of these materials consist of a distribution of particle sizes. Information thus obtained is an average over the entire distribution. Supported, monosized clusters have not yet been used in catalytic studies. However, Woste and coworkers demonstrated in the first experiment where monosized clusters were deposited that Ag4 is the critical cluster... 

XRD may be very common in catalysis, but it does have disadvantages. Because XRD is based on interference between reflecting X-rays from lattice planes, the technique requires samples which possess sufficient long range order. Amorphous phases and small particles give either broad and weak diffraction lines or no diffraction at all, with the consequence that if catalysts contain particles with a size distribution, XRD may only detect the larger ones. In unfortunate cases, the diffraction lines from the metal may overlap with those from the support. Finally, the surface region is where catalytic activity resides, but this part of the catalyst is virtually invisible for XRD. 

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. 

Elements which have successfully been applied onto support materials as chelated complexes to produce heterogeneous catalysts with very small metal (oxide) particles and homogeneous distributions. 

Heterogeneous catalysis is increasingly applied in chemical industries to decrease the raw material consumption, pollutants emission and to inqjrove the product selectivity. Catalytically active metals and metal oxides are usually deposited on a carrier or support. The role of the support is to stabilize the active component in highly dispersed small particles and hereby increase their exposed siuface area. The activity and selectivity of the catalyst can be significantly altered by the particle size of the active metaTmetal oxide and the pore size distribution of the support material. 

Historically, GC was first performed using packed columns. Such columns were usually made of coiled metal or glass tubes, with length of 1 to 5 m and internal diameter of a few millimeters. These were packed with small particles of sohd support coated with nonvolatile liquid stationary phases. The solid particles themselves played the role of the stationary phase in GSC. Packed columns were characterized by poor efficiencies related to multiple flow paths among the packing particles (the A term in the van Deemter equation), as well as uneven distribution of the hquid phase within the particles and at the contact points between the particles. The number of theoretical plates in packed columns was several thousand at the most, therefore improvements in resolution were usually achieved by the use of more selective stationary phases. Consequently, hundreds of different stationary phases were available at the peak of packed column development. 

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