Jargon in Chemistry Papers

TOF stands for “Turn Over Frequency”. It was originally coined by a guy called Boudart, who was trying to define the efficiency of a catalyst. He said:

“a convenient way to express the 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”

However, the term is still not totally well defined, it usually concentrates on the catalytic centre – ie the particular point in the chemical structure of the catalyst that is thought to encourage a particular reaction. Another text book on chemical terminology says:

“the turnover number N, is defined as in enzyme catalysis, as molecules reacting per active site in unit time.”

Which implies that the Turn Over number TON is the same as the Turn Over Frequency TOF. There are papers which try to clarify the difference, to try to make sure different papers about different catalysts produce numbers which are consistent and directly comparable.

There are also places which reference “turnover rate” and “catalytic constant” (k_cat) and use those interchangeably as well. Sometimes people talk about the TOF as a constant and multiply it by the concentration of the catalyst to get a “rate of reaction” and thus calculate the TOF by dividing the rate or reaction by the concentration of the catalyst. Basically you have to read carefully to see what is meant between different papers. “Reaction rate” is actually defined as the rate of change of the input reactants with respect to time delta[R]/delta[t]

For a Turn Over Frequency, we would always expect it to have the units of 1/time, where time is in seconds, so a dimensionless number, per second. However, Boudart in his original statement has his unit of time as a minute, while the chemistry text book loosely says “unit time”. Some papers I have looked out indicate the time unit is an hour. So maybe that has been one source of confusion?

Another issue is whether to count the number of interim reactions, or whether changes in the catalyst taking part are counted at all. If an overall reaction happens because one of the initial components reacts with a catalyst and changes the catalyst from one chemical to another, then from that second chemical the other initial component reacts to produce the eventual product, but that sends the catalyst back to its original state, is that one reaction or two? It has converted the input chemicals to the required output product, but the catalyst has gone through two reactions in the process.

Usually TOF counts the reactions or individual stages of change, or Turn Overs which happen in a given time, whereas TON can be a “count” of the amount of product produced or a “count” of the amount of reactants used up.

TON has also often been used to indicate how many reactions in total that a catalyst can generate. Catalysts often slowly degrade over time, so knowing how long a catalyst will work for is useful and will be different, possibly independent from the rate at which they make a given reaction happen. This is sometimes also referred to as a TTN Total Turnover Number. However, if some experiments are done using different catalysts, but the same quantities of starting reactants, then the people might quote a TON to indicate how much product was produced by the end of the process, but that would probably be because all the reactants had been used up, rather than because the catalyst had stopped working.

Another confusion is that the reaction rates will change as the concentrations of reactants change, so if you have an experiment where a given amount of stuff is left mixing in the presence of a catalyst the rate of reaction will almost certainly decrease over time as the concentration of the product increases and the concentration of the input chemicals decreases. In most industrial processes things are set up so that you remove the product you want, and replenish the input chemicals, so that the reaction conditions stay pretty much constant. However if you are doing an experiment in a lab, you are probably not doing that. So academic papers sometimes seem to tie themselves in knots trying to work out an instantaneous reaction rate, from working out the slope of the graph of the reactants and products against time, or measuring the time until the concentrations of the input chemicals are down to 90% of the original, and assuming a constant rate of reaction over that time, or some other such way of measuring to give a “number”. Of course there might be an initial reaction which has to happen before the final reaction happens to produce the wanted product. In that case the reaction might seem to do almost nothing for a while, then start to work quickly. In that case going for an average reaction rate for the time until the initial reactants concentration is down to 90% would underestimate the ongoing reaction rate and ongoing TOF.

Yet another problem is that the TOF is supposed to be a rate per “site” in the catalyst, sometimes people work out a TOF per unit mass of a catalyst and call it TOF_mass, or per unit surface area of a catalyst TOF_area.

Ultimately, TOF and TON both give an indication of how good a set of conditions are (catalyst, temperature, pressure), and the higher the number the better for the reaction wanted. We just have to be very careful when comparing TOF and TON numbers quoted in different papers because people might be measuring different things.

There are also some units which may not be familiar from GCSE, O or A level chemistry or science:
Katal (kat) = Mol/s
Mol = a constant number of atoms or molecules of a product which you get if you take the number of grams of the product equal to its atomic (or molecular) weight.