Customers often make the same request: "Could you push the particle size down a bit more? Smaller means more active, right?"

There are two traps in that sentence. First, smaller is not automatically better; second, the word "activity" has two entirely different meanings, and mixing them up guarantees a wrong conclusion. This article draws the curve clearly, using published measured data.

1. Separate the Two Kinds of Activity

• Specific activity — normalized per unit surface area (or per surface atom), answering "how strong is each individual site."
• Mass activity — normalized per unit mass of platinum, answering "how much work can a gram of platinum do." This is the one procurement should actually look at.

The two are linked through electrochemical surface area (ECSA): mass activity ≈ specific activity × ECSA × platinum loading.

As particle size decreases, ECSA necessarily rises (the platinum is spread more widely); but specific activity does not necessarily move in the same direction — it is governed by site type and has a peak of its own. Only the product of the two curves determines where the optimum particle size for mass activity falls.

2. What the Measured Data Show

Geometric estimate: a 2 nm platinum particle contains roughly 55 atoms, a 3 nm particle roughly 200; each doubling of diameter increases atom count cubically.

The conclusion in one line: the optimum particle size differs from reaction to reaction. Thermal hydrogenation/dehydrogenation favours 1.8–2.5 nm; electrocatalytic ORR favours 3–5 nm. Taking the "optimum" from one reaction and applying it to another is the most common selection error there is.

3. Why There Is a Peak: The Site Types Are Changing

What particle size changes is not "the nature of platinum" but the proportion of each type of surface site.

• On small particles, low-coordination atoms (corners, edges, steps) make up a high fraction. Studies of hydrogen evolution / hydrogen oxidation give a direct verdict: edge sites are far more active than flat terraces.
• The B5 sites repeatedly cited in the literature (active ensembles that only a specific atomic arrangement can form) reach their density peak in the 1.8–2.5 nm range — which neatly explains why the TOF maxima of several hydrogenation and dehydrogenation reactions fall there.
• Large particles are dominated by (111) and (100) terrace sites, with moderate adsorption strength; for certain reactions their specific activity is actually higher.

Underneath it all is the same adsorption-strength curve: small particles are coordinatively unsaturated and adsorb too strongly — so strongly that products cannot desorb or impurities block the sites permanently, and activity falls; large particles adsorb too weakly — too weakly to hold the reactant, and activity falls again. The peak lies in between.

4. Three Prices Paid for Small Particle Size

Dissolution and ripening. Electrochemical studies confirm that the smaller the particle, the greater platinum's tendency to dissolve. Small particles dissolve preferentially and redeposit on larger ones — Ostwald ripening, one of the main mechanisms of active-area decay in fuel cells.

Sintering at high temperature. Surface energy rises steeply as particle size falls, making the particles thermodynamically very unstable. Under duty at several hundred degrees Celsius, 2 nm particles are hard to maintain over the long term.

Batch consistency becomes harder. The "tail" of the particle-size distribution is the invisible killer: the mean may conform, yet a handful of 8–10 nm particles is enough to drag overall mass activity down significantly — and such large particles are easily overlooked in routine inspection. When reading micrographs, always look at the distribution, never the mean alone.

5. Selection Quick Reference

6. The One-Sentence Summary

The activity difference between 2 nm and 10 nm is not simply "so many times greater" — it is the product of rising ECSA and a shifting specific activity, and where the optimum lies depends on your reaction.

Fix the reaction type first, then set the particle-size target; at acceptance, look at both the mean particle size and the tail of the distribution, and ask for the decay curve under your actual operating conditions. A specification sheet reporting only "mean particle size 3 nm" does not carry enough information.


Diethylenetetramethyldisiloxane platinum complex (platinum catalyst) MY 8115