turnover number and turnover frequency
/ TON and TOF /
Suppose you hire one taxi driver and want to know how good a worker she is. You could ask two different questions. First: over her whole career, how many passengers did she carry before retiring? That is a measure of total output, of how durable a worker she is. Second: how many passengers does she carry per hour? That is a measure of speed, of how busy she is right now. Catalysts are judged by exactly these two numbers, and confusing them is a classic mistake.
The turnover number, written TON, is the total number of product molecules that one catalyst site makes before it finally dies (becomes inactive). It is a measure of catalyst lifetime and durability — a high TON means a little catalyst goes a very long way before you have to replace it. Industrial catalysts can reach turnover numbers in the millions or more. The turnover frequency, written TOF, is the number of product molecules made per catalyst site per unit time, usually per second or per hour. It is a measure of how fast the catalyst works at the moment — its activity. A catalyst can have a high TOF (very fast) but a low TON (burns out quickly), or the reverse. Selectivity is a separate, third virtue: of all the products a catalyst could make, what fraction is the one you actually want? A catalyst can be fast and long-lived yet useless if it makes a messy mixture.
These numbers matter because they turn the vague praise good catalyst into hard, comparable engineering. When chemists report a new catalyst, TON and TOF are the headline figures, alongside selectivity and the conditions needed. A homogeneous catalyst often boasts a dazzling TOF and beautiful selectivity but a modest TON and the headache of separating it from the product; a heterogeneous catalyst may be slower per site but robust enough to run for years in a packed reactor. Be careful with the definitions, though: TON and TOF depend on how you count active sites, which is straightforward for a single well-defined metal complex but genuinely hard for a lumpy solid where only some surface atoms are active — so quoted numbers for solids are often per gram or per surface area, not truly per site.
A catalyst with a TON of 1,000,000 and a TOF of 10 per second is durable but fast: a single site makes ten product molecules every second and keeps going until it has made a million — that is roughly a day of nonstop work per site.
TON is total lifetime output; TOF is the rate right now. They answer different questions.
High activity (TOF) and long life (TON) are not the same thing, and neither guarantees selectivity. A truly useful catalyst needs all three — and for solids, the per-site numbers are often estimates because not every surface atom is active.