Metal Dispersion Fraction Exposed

The dispersion. Dm, or fraction exposed of a metal catalyst is the ratio of the number of surface metal atoms to the total number of metal atoms  

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Metals Dispersion (Fraction of Metal Exposed) in the Catalysts... 

The dispersion of the catalytic metal is necessary, because these metals are often expensive. Thereafter, if it is applied in a finely dispersed form as particles on a high surface area support, a large fraction of the metal atoms are exposed to the reactant molecules [26], Then, we save in metal amount and increase the efficiency of the catalyst. 

The degree of dispersion is defined as the fraction (percentage) of the total metal atoms which are surface atoms. (The term fraction exposed (percentage exposed) is synonymous with the degree of dispersion). 

Another way to change concentration of active material is to modify the catalyst loading on an inert support. For example, the number of supported transition metal particles on a microporous support like alumina or silica can easily be varied during catalyst preparation. As discussed in the previous chapter, selective chemisorption of small molecules like dihydrogen, dioxygen, or carbon monoxide can be used to measure the fraction of exposed metal atoms, or dispersion. If the turnover frequency is independent of metal loading on catalysts with identical metal dispersion, then the observed rate is free of artifacts from transport limitations. The metal particles on the support need to be the same size on the different catalysts to ensure that any observed differences in rate are attributable to transport phenomena instead of structure sensitivity of the reaction. 

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

Since heterogeneous catalysis is a phenomenon which is exclusively based on the reactivity of surface atoms, a high fraction of the latter, exposed towards reactants, is desired. This demand can be equated with a high degree of dispersion of the metal or a very small particle size, that is, in the lower nanometer range of approximately 1-5 nm. 

The exact stoichiometry of the Pt-H surface complex is still a matter of debate since it depends on the size of the metal particle. For many supported Pt catalysts, an assumption of 1 H atom adsorbing for every 1 Pt surface atom is often a good one. Results from chemisorption can be used to calculate the dispersion of Pt, or the fraction of exposed metal atoms, according to ... 

As described, the addition of Mg to an aluminum alloy permits rapid oxidation of the alloy at elevated temperatures in an oxygen-containing atmosphere. The product is a composite composed of a matrix of interconnected A1203 with a small fraction of residual A1 alloy dispersed in the matrix as interconnected metal channels. From a thermodynamic or phase diagram viewpoint, three distinct oxides can form when an Al-Mg alloy is exposed to such an atmosphere A1203, MgAl204, or MgO. The specific... 

The rate of reaction expressed as molecules reacted (or formed) per unit time per catalytic site (or per exposed atom of active metal for metal catalysts) is called the turnover frequency. For supported metal catalysts the calculation requires knowledge of the dispersion, i.e., the fraction of the active metal available for adsorption of reactants. Boudart coined the term demanding (structure-sensitive) for catalyzed reactions for which the turnover frequency varies with the dispersion. Related to this is the ensemble effect, where the active site requires a specific multiatom grouping.f ... 

The ratio of the number of surface metal atoms to the total number of metal atoms in catalysts is defined as the exposed fraction of surface metal atoms, and also called as dispersion of metal D). The methods traditionally used by the catalytic community for measuring the exposed fraction of smface metal atoms are based on chemisorption. 

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