Turnover Frequencies in Metal Catalysis Meanings, Functionalities and Relationships

Turnover Frequencies in Metal Catalysis Meanings, Functionalities and Relationships 

It would be highly desirable to be able to determine the intrinsic site activity of a catalyst in order to evaluate its site efficiency relative to other catalysts active for a reaction of interest - unbiased by the amount of catalyst used, by the concentration of active sites per weight or volume of catalyst, or even, in the ideal, by the concentration of reactants or products. This intrinsic activity would thus reflect the catalytic essence of the reaction site. Use of such a site activity would permit discrimination between a poor catalyst candidate and an excellent one for further study that, for whatever reason, might have only a few super active sites. One could work to determine the nature/structure of those very active sites and seek a way to design a catalyst having an increased concentration of such sites. 

Initially, catalyst rates were expressed as rates per weight of catalyst. With the advent of surface area measurement using physisorption (BET), rate per total surface area of the catalyst could be determined, although a comparison of catalysts based on such a measure in general was grossly flawed. Use of chemisorption, especially where one of the reactants of interest was used, offered a more reasonable way to compare rates. In 1968, Michel Boudart [1] proposed the use of turnover frequency (TOFchem), based on chemisorption, as an excellent way for comparing catalysts. This quantity has units of sec and represents, ostensibly, the number of molecules reacting per (chemisorption) site per second. This measure has become a standard in fundamental studies of catalytic reactions, especially for metal catalysts. 

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Goodwin JG, Jr, Kim SY, Rhodes WD, Turnover frequencies in metal catalysis meanings, functionalities and relationships, in Spivey J, Roberts G (eds.). Catalysis Vol. 17, Royal Chemistry Society, London, pp. 320-347, 2003. 

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