Selective heating of active sites

Selective heating can occur as selective heating of catalyst particles or in the extreme case as selective heating of active sites. 

Hot spots as a temperature gradient between the metal particles and the support, which cannot be detected and measured because they are close to micro scale, i.e. they possess molecular dimensions, they are closed to selective heating of active sites. 

We should note that the calorimetric results obtained in studies at room temperature did not correlate with the catalytic activity at 573 K. Samples with high activity showed essentially the same values of heat of adsorption of NH3 as did samples that showed considerably lower activity. In other cases, samples with lower activity showed higher heats than the sample with highest activity. On the other hand, more recent studies (158) demonstrated that the differential heat of adsorption of NH3 at 473 K on the same samples displayed the same trend as seen for the catalytic cracking at 523 K. Moreover, the cracking activity agreed better with the acidity determined by adsorption of pyridine at 473 K. Thus, when attempting to correlate the activity for an acid-catalyzed reaction with the surface acidity, it is advisable to study the adsorption process at the temperature used for the catalytic reaction, and the basic molecule used should have a similar size to that of the reactants and should interact selectively with the active sites. 

Adsorption measurements generally reveal that the heat of formation of adsorption complexes varies significandy with surface coverage. Such a variation may serve as a good operational definidon of active sites, the existence of which was first clearly formulated by Taylor in 1925. The consequences of the fact that not all sites at the surface of a solid (or of an enzyme molecule) may be equally active are many-sided. Thus, in certain situations, only very few selected sites may participate in the catalytic sequence this is an extreme case of aedve sites. In other situadons, although most, if not all, of the sites take part in the reacdon, they do so at different levels of activity. 

If microwave heating leads to enhanced reactions rates, it is plausible to assume that the active sites on the surface of the catalyst (micro hot spots) are exposed to selective heating which causes some pathways to predominate. In the case of metal supported catalysts, the metal can be heated without heating of the support due to different dielectric properties of both catalyst components. The nonisothermal nature of the microwave-heated catalyst and the lower reaction temperature affects favorably not only reaction rate but also selectivity of such reactions. 

Various works has pointed out the role of the nanostructure of the catalysts in their design.18-26 There is a general agreement that the nanostructure of the oxide particles is a key to control the reactivity and selectivity. Several papers have discussed the features and properties of nanostructured catalysts and oxides,27-41 but often the concept of nanostructure is not clearly defined. A heterogeneous catalyst should be optimized on a multiscale level, e.g. from the molecular level to the nano, micro- and meso-scale level.42 Therefore, not only the active site itself (molecular level) is relevant, but also the environment around the active site which orients or assist the coordination of the reactants, may induce sterical constrains on the transition state, and affect the short-range transport effects (nano-scale level).42 The catalytic surface process is in series with the transport of the reactants and the back-diffusion of the products which should be concerted with the catalytic transformation. Heat... 

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