Ingenious experiment finally reveals how gold oxidises water
Using a clever experiment, PhD candidate Shengxiang Yang discovered how gold electrodes convert water into oxygen. He is the first to unravel the mechanism of this reaction. Yang published his results in the journal ACS Catalysis.
Precious metal
Gold is a noble metal and therefore inert. This means that gold hardly reacts with other substances, which is one of the reasons it is such a desired metal. For a catalyst - which has to accelerate the reaction between substances - that might not be the best attribute, one would think. And yet, gold is used as a catalyst. Associate professor Dennis Hetterscheid, Yang's supervisor, explains how this works. ‘When you apply a voltage on gold, the outer layer changes from gold into gold oxide, a process similar to the rusting of iron. In contrast to normal gold, this new outer layer reacts with other substances and is able to convert water into oxygen.’
The origin of this reaction lies near Leiden, in the Dutch city Haarlem to be precise. As early as 1789, Paets van Troostwijk, a member of the Dutch Chemists, demonstrated that you can split water into oxygen and hydrogen with gold. Nowadays, this reaction is important for the storage of sustainable energy. Hetterscheid: ‘We didn't understand at all how gold catalyses this reaction. Shengxiang has now changed that.’
Asset for Leiden
Three years ago, Shengxiang Yang came from China to Leiden via a CSC Scholarship. A real asset to the Faculty, according to Hetterscheid. ‘He devised and carried out the research all by himself.’
It all started in literature: Yang looked at earlier research on gold electrodes but saw that several papers contradict each other. Through clever experiments, he discovered where literature went wrong. When a voltage oxides gold, two layers of gold oxide are formed. The prevailing idea was that one was a 'neat' layer and the other an amorphous layer, which is somewhat sloppier in structure. First, the neat layer forms, followed by the messy layer. At least, that is what scientists thought. ‘Yang has now shown that there are two kinds of gold oxides that exist side by side and both do their own thing,’ says Hetterscheid. ‘They also catalyse the oxidation of water in different ways.’
Neutral climate
To discover this, Yang came up with new experiments. He studied the oxidation chemistry of gold very systematically under different conditions. Hetterscheid: ‘Electrochemists normally do experiments with an acidity of pH 0 (extremely acidic) or an acidity of pH 14 (extremely basic). This has certain practical advantages, but it also has its limitations. Shengxiang decided to investigate the reaction of gold with water at a neutral acidity of pH 7. By performing brief spectroscopic measurements during the reaction, he finally managed to uncover the two reaction mechanisms.’
An important fundamental insight, which can also have practical consequences. ‘Because we now have a better understanding of how gold catalyses this reaction, we can look for better catalysts for oxidising water. Especially because this reaction is crucial in the conversion and storage of renewable energy,’ Hetterscheid concludes.
Paper
Shengxiang Yang & Dennis Hetterscheid, Redefinition of the Active Species and the Mechanism of the Oxygen Evolution Reaction on Gold Oxide, ACS Catalysis (2020)
Image Paets van Troostwijk: H.A.M. Snelders, Het Gezelschap der Hollandsche Scheikundigen.
Text: Bryce Benda