Conventional catalytic measurements typically combine the behavior of many particles, producing an average that can conceal substantial differences from one surface region to another. Local variations may hold important clues for materials design because a catalyst’s edges, side surfaces and broad faces can behave differently even when they share the same chemical composition. DELINE was developed to preserve this spatial information by recording where individual catalytic events occur while the catalyst is working, rather than reporting only an aggregate activity value. Assistant Professor Mao Xianwen, who led the research in the Department of Materials Science and Engineering at the NUS College of Design and Engineering, said the method can show which parts of a catalyst are active and how their behavior changes under different operating conditions.

The imaging approach links a catalytic reaction that does not itself emit light to a much faster follow-on reaction that produces fluorescence. A probe molecule reacts with a product of the original reaction, and the locations of the resulting fluorescent signals reveal the relative activity of different regions on the catalyst surface. This arrangement allows the researchers to sample individual events across a working material without requiring the reaction of interest to be fluorescent. Control experiments showed that the probe did not alter the catalytic activity of the gold, supporting the interpretation that the recorded signals reflected the underlying reaction rather than an effect introduced by the measurement system.

To demonstrate the method, the team used thin catalytic gold plates to examine water dissociation, a key step in producing hydrogen from water. During this process, water splits at the metal surface, leaving attached hydrogen that can subsequently form hydrogen gas. The reaction also generates hydroxide ions, described in the study as negatively charged groups containing oxygen and hydrogen. A probe molecule reacts with these hydroxide ions to create the fluorescent events detected by DELINE. The resulting reaction maps show not only whether a region is active but also how individual events are distributed across the material. By resolving individual reaction events, the method provides a way to examine catalytic heterogeneity while the electrocatalyst is under working conditions.

Across 39 gold plates, the researchers compared 195 surface regions. Edges and side surfaces were generally more active than the broad, flat faces of the plates, although activity also varied within the flatter areas. Such differences could disappear in conventional measurements that average the output of multiple particles. The observed variation prompted the team to vary the applied voltage and determine how strongly the water-dissociation reaction accelerated in each mapped region. They also estimated a baseline reaction rate at equilibrium, when forward and reverse reactions balance, to distinguish the underlying speed of a region from its response to electrical driving. The gold experiment was therefore designed to connect spatial maps of reaction activity with both equilibrium-rate estimates and voltage-dependent behavior.

The analysis exposed a trade-off: regions with a stronger response to voltage tended to have a lower baseline rate. These two effects can offset one another in a relationship known as kinetic compensation. The result means that a surface region that performs well under one set of electrical conditions may lose its advantage when those conditions change. For hydrogen production, this distinction matters because a catalyst with greater voltage sensitivity may require a smaller voltage increase to accelerate the reaction, making that sensitivity important alongside baseline activity. A design that emphasizes one property is therefore not necessarily the best option for every electrochemical device. This voltage-response and baseline-rate relationship was measured for water dissociation on gold; the copper tests demonstrated differences in which surface regions were active, rather than establishing the same trade-off there.

Computer simulations and additional surface imaging helped the researchers investigate the physical processes behind the gold trade-off. Sodium ions in the surrounding liquid accumulated unevenly near the gold surface, changing the environment around nearby water molecules and making their reaction more sensitive to voltage. The same ion accumulation could crowd the surface, slowing the movement and rearrangement of water and thereby reducing the baseline reaction rate. The experiments therefore connect a measurable pattern in catalyst activity with the local ionic environment and molecular behavior at the interface. The findings do not establish that the same mechanism applies equally to every catalyst, electrolyte or electrochemical reaction, and the authors presented DELINE as a method whose application to other systems would require further investigation.

The reaction maps were then used to calculate how hollow gold plates with different rim widths would perform. The calculations showed that widening the rim could either improve or reduce overall activity, depending on the applied voltage and the distribution of reactions across the surface. This result illustrates a practical way in which spatial imaging could guide material design: candidate geometries can be evaluated against the local reaction pattern of the material and the electrical conditions expected in a device. The study did not demonstrate a finished hydrogen-production device or establish a universally optimal shape. Instead, it showed how catalyst shapes could be selected with operating conditions and site-specific activity in mind, and how a feature that increases performance at one voltage can become less advantageous when conditions change.

Tests on copper showed that the most useful surface features can differ between materials and reactions. During water dissociation on copper plates, the broad, flat faces were more active than the edges, reversing the general pattern observed on gold. The researchers also applied DELINE to nitrate reduction, a process that converts nitrate into other nitrogen compounds. This reversal supports the need to identify active structures for each catalyst and reaction rather than assuming that edges, corners or flat surfaces will always be preferable. It also demonstrates that the imaging method is not restricted to the single reaction used for its initial gold demonstration. The experiments do not imply that flat surfaces are generally superior to edges: the observed ordering changed with the material, underscoring the need to characterize activity in the specific system under investigation.

DELINE can analyze approximately 100 particles within a single microscope field during a recording lasting about three minutes, allowing multiple particles to be compared under the same conditions. This throughput makes it possible to examine heterogeneous surfaces at a scale that would be difficult to resolve with an aggregate measurement alone. The team is now exploring new probe designs to extend the approach to additional electrically driven reactions, including nitrogen fixation, in which nitrogen gas is converted into chemically useful compounds. The researchers also aim to visualize catalytic processes inside operating batteries. Mao said the broader goal is to connect events on catalyst surfaces with practical choices about material design, while establishing which design principles transfer across systems and which must be tailored to a particular process. The study thus combines a single-event optical measurement advance with a design lesson: average catalyst activity alone may not identify the most useful surface structure for a specific operating environment.

Subject of Research: Single-turnover optical imaging of electrocatalytic water dissociation and kinetic compensation

Article Title: NUS CDE researchers map individual reactions to guide better catalyst design

Article References: NUS CDE researchers map individual reactions to guide better catalyst design. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: DELINE, electrocatalysis, water dissociation, kinetic compensation, gold catalyst, copper catalyst, single-turnover imaging, operando characterization, surface reactions, hydrogen production, researchers, individual