Summary of recent discussions

As mentioned previously the term photocatalysis was used by Plotkinow to describe the acceleration of a chemical reaction due to the influence of light. Since this definition applies for any photochemical reaction there is no inventive power in it. Also a restriction of the acceleration by light only to endergonic reactions, as proposed by Balzani[3], does not improve the situation. 

During the past two decades a number of authors have contributed to define what is meant by photocatalysis. Thus, independent on our earlier contributions[4] - [8], Wrighton[9], Moggi[10] and Salomon[ll] have proposed several detailed mechanistic schemes to specify photocatalytic reactions. Some of these pathways have been illustrated by Kutal[12] in summarizing of a number of experimental results preferably with respect to definitions given by Salomon[ll]. Recently, Pelizzetti and Serpone [16] have edited an excellent collection of reviews illustrating the state of art in photocatalysis with fundamental definitive contributions of Kisch [17] and Chanon [18]. 

The aim of the note presented by Kisch and Henning [15] was to describe all kinds of photocatalytic reactions within the frame of two categories. The first category was named photocatalytic reactions. After the initial photogeneration of a catalyst the conversion of substrate molecules does not require further irradiation. It was said that such reactions exhibit quantum yields of product formation higher than 1. Therefore, it was pointed out that they are catalytic in the number of incident photons . If continuous irradiation is required to observe product formation, the quantum yield of product formation can not exceed unity. Reactions of this type belong to the second category photoassisted reactions . Such reactions were characterized to be catalytic in the concentration of the photocatalyst . 

In a reply on this note Mirbach [13] emphasized that the quantum yield of product formation only is not a sufficient criterion to identify whether a reaction belongs to the first category, here named as photoinitiated catalytic reactions. When the quantum yield of the initial photoreaction, forming the catalyst is very low, the overall quantum yield may be less than unity, although the catalyst can enter many cycles of thermal substrate conversion. Furtheron, Mirbach classified such photoreactions which need a photon to start a new cycle into three fundamentally different processes Photoassisted catalytic reactions, catalyzed photoreactions, and sensitized photoreactions. 

In another reply Carassiti [14] argued that the catalytic connotation of photocatalysis is not consistent with Ostwald s idea that a catalyst changes the rate of a chemical reaction without any influence on the position of the equilibrium. If a photon is consumed in each cycle to generate an active species, light plays the role of a reactant. The catalyst may lower the activation barrier but the reaction is different from the thermal one in terms of a changed energy balance. In order to avoid unnecessary confusion Carassiti advised the application of the term photocatalysis only to such reactions which satisfy the conceptual requirements of catalytic processes. 

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