Small metal particles turnover frequency

Application of small metal particles has attracted the attention of the scientists for a long time. As early as in the seventies Turkevich already prepared mono-dispersed gold particles [19], and later, using molecular transition metal carbonyl clusters [20], the importance of small nanoparticles increased considerably. One of the crucial points is whether turnover frequency measured for a given catalytic reaction increases or decreases as the particle size is diminished. 

An interesting question, expressed by Boudart (1985), is the following As particle size grows from that of a small cluster to infinite value for a single macroscopic crystal, how does the value of turnover frequency change for a given reaction on a given metal ... 

Applying the Broensted Polanyi correlations is sometimes useful for describing the dependence of the reaction rate on the size of the catalyti caUy active component. A huge amount of experimental data have been compiled to date regarding the effect of the particle size of the catalyst active components on the specific catalytic activity, SCA, as well as on the turnover frequency, TOP, of the active center. Both parameters do not relate to the total surface area of the catalyticaUy active phase or to the total number of active centers and, therefore, characterize directly the properties of the active center. There are also some experimental data on the size dependence of the adsorption properties of small metal parti cles, as well as on the selectivity of a few catalytic processes. 

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. 

The working state of Ru/CeOj catalyst is assumed to be composed of large Ru particles (hexagonal phase) with surface Ru" species surrounded by small clusters of ceria. The mean Ru metal particle is in the range of 20 to 30 nm. The effect of the dispersion of Ru particle was investigated and it was found that the larger the particle, the larger the turnover frequency the turnover frequency increased by a factor of 5 when the metal dispersion decreased by a factor of 8 e.g. from 57 to 7 %. The rate is expressed by... 

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