Heterogeneous catalysis redox sites

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

The key for effective utilisation of microcalorimetry in heterogeneous catalysis is the judicious choice of gas-phase molecules for study. Although total number of surface active sites and potentially active centres can be estimated by this technique, the obtained values are strongly dependent on the nature and size of the probe molecule. As a general rule, no matter which surface property is examined (acidic, basic or redox) probe molecule should be stable with temperature and with time. Furthermore, as discussed before, the adsorbed probe should be sufficiently mobile to equilibrate on active sites at temperature of the experiment. 

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