Homogeneous catalysis, single-molecul

Homogeneous catalysis can be classified into single-species and complex catalysis, although the distinction is not always clear-cut. In the former, a single molecule or ion acts as the catalyst in the latter, the catalyst is a system of several species that interconvert into one another and differ in their catalytic properties. A further complication arises if significant fractions of the total catalyst material may be present in the form of reaction intermediates rather than free catalyst. If so, the concentration of the free catalyst is not known and may vary with conversion, and rate equations that instead contain the known, total amount of catalyst material are needed. 

In contrast to single-molecule studies in enzymology, approaches to study reactions in pure chemistry and especially in catalysis were rare for different reasons except for a few examples [14, 26, 27]. Only recently, several applications of SMFS in heterogeneous catalysis and few in homogeneous chemical reactions could... 

Approaching Single-Molecule Studies in Homogeneous Catalysis... 

One should make a distinction between homogeneous catalysis, in which the entire reaction occurs in a single phase, and heterogeneous catalysis, in which the reaction takes place at the interfaces between phases. In heterogeneous catalysis, the reactants and the catalyst are present in different phases. Reactant molecules are adsorbed on the surface of the catalyst, and the reaction takes place on that surface. In the so-called chemical adsorption (or chemisorption), the molecules are held to the surface by bonds that are of approximately equal strength as those in chemical compounds. The adsorbed molecules undergo changes so that some bonds may be stretched and thus weakened, or even broken. 

Today there is ample evidence in catalysis that the assumption of a homogeneous surface is not valid. Thus, surface irregularities exist such as crystallographic dislocations, defects, and planes with different activity adsorbing molecules may interact with the surface and with each other several adsorption states of a species may exist on a single plane. These... 

In the case of basic chemicals the chances for new catalytic processes are small, but they are better for higher value chemicals such as fine and specialty chemicals. Pharmaceuticals and agrochemicals are two areas where homogeneous catalysts have advantages. Complex molecules can often be synthesized in single-step one-pot reactions with the aid of transition metals. This sector has many potential points of overlap with biotechnology, especially enzyme catalysis [5]. 

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