- “Indeed, the catalytic activity, for a valid comparison, must be referred to the number of exposed surface atoms of a specified kind. Thus a convenient way to express catalytic activity is by means of a turnover number equal to the number of reactant molecules converted per minute per catalytic site for given reaction conditions.” With these words of Boudart the first definition of what later was called the Turnover Frequency (TOF) entered into the realm of heterogeneous chemistry. It was a term borrowed from enzymatic kinetics, and slowly passed to homogeneous catalysis. Nowadays it is a ubiquitous term, focusing strictly on the catalytic center, as distinct from the classical term “rate of reaction”, which emphasizes the generation of products or the consumption of reactants. Despite its utility and common use, the TOF concept is still not well-defined and leads to confusion. IUPAC’s gold book, the most authoritative source of chemical terminology, has a very concise definition of the turnover frequency: “Commonly called the turnover number, N, and defined, as in enzyme catalysis, as molecules reacting per active site in unit time.” This description of the TOF has two main problems. The first is the difficulty of providing a one-to-one correspondence between name and function, since (as appeared in Boudart’s paragraph) the terms “turnover frequency” (TOF) and “turnover number” (TON) seem to have one and the same meaning. However, in typical catalytic jargon, both expressions have very different connotations. Sometimes also the terms “turnover rate” and “catalytic constant” (kcat) are used interchangeably in the literature with the same meaning. To make matters worse, the TOF is occasionally considered a rate-constant, since the rate of reaction (r = TOF × [Cat]) depends on the catalysts concentration. However, the TOF itself can depend on the concentration of reactants and products even at saturation, and in this sense it is closer to a rate than to a kinetic constant. In spite of this, from a strict terminological stance the TOF is a frequency, with units of time−1. All this debate evidently resembles the biblical story of the Tower of Babel and the confusion of languages. The second problem of IUPAC’s and Boudart’s definitions is a recurring expression of the TOF as a function of the number of reactants consumed, or even of the products generated. In most cases it is indeed an accurate way to derive the TOF, but for instance in bimolecular reactions this is not the case. Moreover, and from a philosophical perspective, when expressing the TOF as a function of produced or consumed molecules, the focus of the measure goes back to those molecules instead of emphasizing the role of the catalyst (see Scheme 1).