Systemic differences, heterogeneous under operation

This complexity determines that investigations on heterogeneous photo-catalytic processes sometimes report information only on dark adsorption and use this information for discussing the results obtained under irradiation. This extrapolation is not adequate as the characteristics of photocatalyst surface change under irradiation and, moreover, active photoadsorption centers are generated. Nowadays very effective methods allow a soimd characterization of bulk properties of catalysts, and powerful spectroscopies give valuable information on surface properties. Unfortunately information on the photoadsorption extent under real reaction conditions, that is, at the same operative conditions at which the photoreactivity tests are performed, are not available. For the cases in which photoreaction events only occur on the catalyst surface, a critical step to affect the effectiveness of the transformation of a given compound is to understand the adsorption process of that compound on the catalyst surface. The study of the adsorbability of the substrate allows one to predict the mechanism and kinetics that promote its photoreaction and also to correctly compare the performance of different photocatalytic systems. 

We analyze a few of the simplest catalytic systems following. In the systems under consideration, the constituent elementary transformations usually are linear in respect to the concentrations of catalytic intermediates. Generally speaking, the results of this kind of analysis are only applicable when the inter action between the active centers, whether identical or different, of the catalyst can be ignored. This is the case mainly for homogeneous and enzymatic rather than heterogeneous catalysis. However, in some cases, the conclusions can be extended to more complex catalytic systems operating in the steady stationary states even with the lateral interaction between the active centers. 

Gold- and rhodium-catalyzed acylations of ammonia have been reported [112, 132], but similar transformations promoted by homogeneous mthenium catalysts still remain unknown. Such a reaction could only be performed employing a heterogeneous ruthenium-based system under aerobic conditions. However, a completely different mechanism operates in this case (see Sect. 6) [133]. 

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