When customers compare quotes, the most common line is: "Your platinum content is 0.5%, theirs is 0.3% — aren't you padding it a little?" That question treats the catalyst as a raw material and does the accounting accordingly. To answer it, you have to switch to two different metrics — TON and TOF. These two numbers say more about a catalyst's quality than the platinum content ever will.

1. TON: How Many Turns One Platinum Atom Makes

The definition of turnover number (TON) is crisp: moles of substrate converted ÷ moles of platinum.

It answers the question "how many moles of reaction can one mole of platinum drive." A TON of 100 means each platinum atom turned over 100 times on average before being written off; a TON of 100,000 means each atom did the job 100,000 times.

In the previous article, when we went through the Speier–Karstedt cycle, we saw platinum switching back and forth between oxidative addition and reductive elimination, returning to its original state before and after the reaction. TON measures precisely how many turns that cycle actually completed.

2. Converting ppm into TON: An Arithmetic Exercise Done Daily on the Shop Floor

The definition alone doesn't give you a feel for it, so let's work it through.

Suppose you add 10 ppm of platinum to a 100-kilogram kettle of material:

Platinum used = 100 kg × 10×10⁻⁶ = 1 g

Moles of platinum = 1 g ÷ 195.08 g/mol ≈ 5.1 mmol

Taking the substrate as 1-octene (M ≈ 112 g/mol), 100 kg ≈ 893 mol

TON = 893 mol ÷ 5.1×10⁻³ mol ≈ 1.7×10⁵

In other words, that 10 ppm loading — seemingly trivial — translates into a demand that every single platinum atom complete well over a hundred thousand turns. Put the other way round: whether the platinum content can come down from 10 ppm to 5 ppm depends on whether TON in your system can double.

That is what a proper price comparison looks like: at the same platinum content, whoever converts more thoroughly and faster is the one that is genuinely cheaper.

3. TOF: How Many Turns per Unit Time

Turnover frequency (TOF) = TON ÷ time, usually expressed in h⁻¹ or s⁻¹.

TON tells you "how much work it can do"; TOF tells you "how fast it does it." The engineering implications of the two are entirely different:

TOF determines reaction time, and therefore bears directly on your capacity and on equipment turnover. If TOF is too low, a two-hour reaction drags out to eight, and a full kettle effectively becomes half a kettle.

TON determines catalyst life and residual platinum, and therefore bears directly on yield and on downstream processing cost.

Continuing the worked example above: if that TON of 1.7×10⁵ is achieved within 30 minutes, TOF ≈ 5,800 h⁻¹, roughly 1.6 s⁻¹ — each platinum atom turning over a little more than once per second. That is the order of magnitude commonly seen for the addition reaction under elevated temperature and with the right stoichiometry.

One thing must be stated plainly: the figures above are an illustration based on the loading you specify. Real TON and TOF values depend heavily on substrate structure, temperature, inhibitor loading and impurity levels; quoting numbers divorced from a specific system is meaningless. That is exactly why the TON and TOF entries on the technical service sheets we issue to customers always carry their corresponding test conditions.

4. Why Real Values Often Fall Short of Theoretical Ones

Ideally, every platinum atom is at work. In reality there are three discounts:

Dispersion. In a heterogeneous catalyst only the surface atoms can contact the substrate. For platinum particles a few nanometres across, surface atoms account for roughly thirty to forty percent; those buried inside may as well not be there. Homogeneous catalysts suffer no such discount — every platinum atom is exposed, and that is the fundamental reason for their high activity.

Deactivation. Poisoning, sintering and agglomeration stop the cycle midway. TON is a cumulative figure: the earlier the deactivation, the lower the TON.

Diffusion limitation. In viscous systems or on supports with large pores, the substrate never reaches the active sites, and even the fastest platinum sits idle.

So when you look at a catalyst, don't look at platinum content alone. Ask for three numbers: the nominal TON, the TOF under the corresponding conditions, and the activity decay curve.



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