Turnover number, particle size sensitivity

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

Nevertheless, it is necessary to return to the main point of interest here, which is the effect of deactivat ion and the accumulation of carbonaceous species on the Pt surface on the structure sensitivity or insensitivity exhibited (as inferred from the dependence of K on the platinum surface area)- Turnover numbers so calculated after 10 min reaction time decrease rapidly with increasing platinum surface area in a hyperbolic manner (and increase almost linearly with the mean platinum particle size) for reactions followed at both 3L3K and 295K, in contrast to the results obtained by Boudart over a narrower range of Pt surface area, Such behaviour very unusual but has been reported for structure sensitivity in cyclopentadiene hydrogenation on supported copper. However, the turnover numhers were almost independent of platinum surface area and... 

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. 

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