Particle size of metal

Physical adsorption—surface areas of any stable solids, e.g., oxides used as catalyst supports and carbon black Chemisorption—measurements of particle sizes of metal powders, and of supported metals in catalysts... 

Interestingly, when the particle size of metal nanoparticles becomes less than 2 nm, terraces become so small that they carmot anymore support the presence of step-edge site metal atom configurations. This can be observed from Figure 1.15, which shows a cubo-octahedron just large enough to support a step-edge site. 

Hydrogen uptake of reduced catalysts (X) was measured by volumetric method with an AUTOSORB-l-C analyzer (Quantachrome Instruments). Hydrogen adsorption was carried out at 373 K after in situ H2 reduction at 773 K for 6 h in the adsorption cell. The dispersion and particle size of metallic Co were calculated by the following equations, assuming that the stoichiometry for hydrogen adsorption on the metallic site is unity ... 

Heat of combustion, thermal conductivity, surface area and other factors influencing pyrophoricity of aluminium, cobalt, iron, magnesium and nickel powders are discussed [4], The relationship between heat of formation of the metal oxide and particle size of metals in pyrophoric powders is discussed for several metals and alloys including copper [5], Further work on the relationship of surface area and ignition temperature for copper, manganese and silicon [6], and for iron and titanium [7] was reported. The latter also includes a simple calorimetric test to determine ignition temperature. 

The particle size of metallic silicon also has a noticeable effect on the chemical activity of contact mass in case of coarse grinding, the reactivity of powder is lower than required in case of fine grinding the reactivity rises sharply, and the heat has to be intensively diverted from the reaction zone. 

By using dendrimers as templates, dendrimer-metal nanoparticles are synthesized in aqueous and non-aqueous media. Particle sizes of metals are significantly affected by many factors including concentration of dendrimers and generation of dendrimers. In particular, structure of dendrimer-metal nanoparticles is characterized by TEM, SANS, and SAXS. 

The particle size of metal emissions from smelters and metallurgical processes is fikely to vary widely with the type of process and the emission controls used. Table III shows that about 45% of the particles emitted from metallurgical processes is in the fine particle range. Lee et al. (27) studied emissions from an electric arc furnace steel plant equipped with a baghouse control device and found that 57% of the total particulate matter was less than 1 /on in diameter. Little information on the particle size of specific metal emissions is available in the literature. 

The average particle size of metal powders is determined by the Fisher sub-sieve sizer. The method uses air permeability to determine particle size. The method is designed for coarser metal powders having particle sizes in the range of 0.2 to 50 pm. The method should not be used for flakes or fibers. 

The primary reason for preparing metal particles in zeolites is that such particles might be great catalysts. In general it is often believed that the smaller the particle size of metals the greater the surface area and consequently the greater the catalytic activity. 

Hydrogenation of Compounds with C=C Bonds Hirai and Toshima have published several papers on the synthesis of transition metal nanopartides by alcoholic reduction of metal salts in the presence of a polymer such as polyvinyl alcohol (PVA), poly(methyl vinyl ether) (PMVE) or polyvinylpyrrolidone (PVP) in methanol (or ethanol) /water mixtures or in pure alcohols. This simple and reproducible process has been applied for the preparation of rhodium nanopartides from rhodium (III) trichloride [15]. The particles size of metallic fee rhodium is distributed in a narrow range, 3-7nm and the average diameter is 4nm. The PVP-stabilized Rh nanoparticies are more stable. These nanomaterials are effident catalysts for olefin and diene hydrogenation in rruld conditions (30 C PH2=lbar) as shown in Table 11.1. 

Aluminum pigments with a wider particle size distribution (industrial grade) contain mostly enough fine particles to hide well even if the average particle size is coarser. The particle size of metallic pigments is categorized as follows fine 6-15 microns... 

Table 13.19 presents the mean particle sizes of metallic Pt from CO chemisorption and TEM analyses of these samples. The results are in good agreement and in accordance with XRD analyses, suggesting mean particles sizes less than 5 nm. 

Physical adsorption isotherms involve measuring the volume of an inert gas adsorbed on a material s surface as a function of pressure at a constant temperature (an isotherm). Using nitrogen as the inert gas, at a temperature close to its boiling point (near 77K), such isotherms are used to determine the amount of the inert gas needed to form a physisorbed monolayer on a chemically unreactive surface, through use of the Brunauer, Emmett, and Teller equation (BET). If the area occupied by each physisorbed N2 molecule is known (16.2A ), the surface area can then be determined. For reactive clean metals, the area can be determined using chemisorption of H2 at room temperature. Most clean metals adsorb one H atom per surface metal atom at room temperature (except Pd, which forms a bulk hydride), so if the volume of H2 required for chemisorption is measured, the surface area of the metal can be determined if the atomic spacings for the metal is known. The main use of physical adsorption surface area measurement is to determine the surface areas of finely divided solids, such as oxide catalyst supports or carbon black. The main use of chemisorption surface area measurement is to determine the particle sizes of metal powders and supported metals in catalysts. 

One of the major results ofYu Yao was to demonstrate that oxidation of light alkanes was extremely sensitive to the particle size of metals, with turnover frequencies being much higher on big particles (see Table 1.4). This behavior was confirmed by Gololobov et al for oxidation of C1-C6 alkanes over Pt." " Their results are summarized in Table 1.9. 

Table 1.9. Turnover frequency of Pt in oxidation of light alkanes (Cl-C6). Effect of particle size of metal. TOP values are given in 10". HC concen-...

Na2Si03, Na4Si20s, gas phase reduction [11,15,19,21]. Generally, metal chloride salts (H2PtCl6, RuCls, NiCU, etc.) are used as metal precursors. As metal chloride salts may reduce the dispersion of nanoparticles (NPs) on support materials [21], a number of binary and ternary electrocatalysts have also been prepared by using chloride-free precursor salts such as carbonyl, nitrate, sulfite complexes, etc. [15,21]. NaBH4 and polyol reduction methods are repeatedly used [15,21-25]. Recently, the microwave assisted chemical reduction method has also been used to obtain better dispersion and reduce the particle sizes of metal catalysts [24-26]. 

Scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDAX or EDS) is a powerful tool for studying the morphology and chemical composition of corroding surfaces. With proper magnification, the defect size, extended protection distance by conducting polymers on the exposed bare metal surface, and particle size of metal oxides can be precisely determined. 

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