A new imaging method shows where catalysts work, in the lab

For decades, measuring a catalyst meant averaging it out. Conventional measurements average the activity of many catalyst particles at once, which hides the local differences that could explain why one spot works and another does not. The team at the College of Design and Engineering at the National University of Singapore built a way to see those differences. The method, called DELINE, tracks individual reaction events across a catalyst's surface while the catalyst is working.

Many catalytic reactions give off no light, so they cannot be watched under a microscope. DELINE gets around this by linking the reaction under study to a much faster follow-on reaction that produces a fluorescent signal. The team applied it to thin gold plates while water split on the metal: the water releases hydrogen that can later form hydrogen gas, and it also produces hydroxide ions. A probe molecule reacts with those hydroxide ions and lights up, so each glowing spot records one event. Control experiments confirmed that the probe did not change the gold's catalytic activity. Across 39 gold plates and 195 surface regions, edges and side surfaces were generally more active than the broad, flat faces, although activity also varied within those faces.

The central finding is a trade-off. By varying the applied voltage, the researchers measured how strongly the reaction sped up in each region, and estimated a baseline rate at equilibrium. Regions that responded more strongly to voltage tended to have a lower baseline rate, a relationship the team calls kinetic compensation. Computer simulations and additional imaging pointed to sodium ions in the surrounding liquid, which gathered unevenly near the gold. That accumulation changes nearby water molecules and makes the reaction more sensitive to voltage, while crowding the surface and slowing the baseline rate. "Knowing which parts of a catalyst are active is only the starting point," said assistant professor Mao Xianwen, who led the research. "DELINE allows us to zoom into these differences."

The same method was run on copper plates during water splitting and nitrate reduction, a reaction that converts nitrate into other nitrogen compounds. On copper, the broad, flat faces were more active than the edges, the reverse of the gold pattern. The team also used its reaction maps to model hollow gold plates with different rim widths: widening the rim could raise or lower activity depending on the voltage. For a hydrogen producer, this matters when output must rise. A catalyst that reacts more strongly to voltage may need a smaller voltage increase to speed up, so its sensitivity matters alongside its baseline activity. For researchers, the gain is a way to choose catalyst shapes suited to the conditions a device will actually run in, and the researchers say the approach could extend to other electrocatalysts and reactions. The work so far is laboratory research on metal plates, and the study gives no timeline for industrial use.