Credit: © 2007 ACS

Platinum is a highly active catalyst for some heterogeneous chemical reactions, and because of this it is often found in fuel cells. To maximize its effectiveness, the Pt is usually used in the form of nanoparticles, and for most processes taking place inside a fuel cell, the optimal particle size is roughly 3 nm. The exception to this, however, is the hydrogen oxidation reaction (HOR) — where electrons are removed from the H2 molecule — for which even smaller particles with sizes down to 1 nm or less could prove more effective.

Now, to explore the lower size limit of Pt nanoparticles as viable catalysts for the HOR, Lin Zhuang and co-workers1 from Wuhan University in China have prepared particles of around 1 nm in size and tested their activity. These small nanoparticles were found not to have ordered structures, unlike larger 2.5 nm particles, which are crystalline. Computer simulations performed on these 1 nm platinum clusters confirmed the experimental observations, suggesting that particles of this size do not form stable crystalline structures, but collapse into an amorphous state.

When tested in the HOR, a lower activity was observed for the amorphous platinum particles when compared with their larger crystalline counterparts. Further calculations indicated that the electronic characteristics of the amorphous particles differ from those of the crystalline ones, and this results in stronger bonds being formed between Pt and H atoms, which slows down the HOR. This reduction in activity suggests that 1 nm should now be considered to be the lower size limit for Pt nanoparticles that can be used as HOR catalysts.