Elettra | Materials Science Beamline Cracking the catalyst code: How tiny platinum clusters could power the hydrogen economy As renewable energy from the sun and wind becomes more abundant, the question of how to store and use this energy efficiently becomes ever more pressing. One innovative answer lies in liquid organic hydrogen carriers (LOHCs) — compounds that can store hydrogen chemically and release it when needed. These liquid fuels could enable longdistance hydrogen transport and longterm storage, all within the infrastructure we already have for fossil fuels. The most mature LOHC system today uses dibenzyltoluene (DBT), a molecule which can take up and release hydrogen through catalytic reactions. But there's a catch: to extract hydrogen, DBT must be heated to more than 290 °C — far above the 80–180 °C range in which most fuel cells operate. That temperature mismatch complicates system design and causes significant energy loss. Additionally, the process involves two separate stages: hydrogen has to be extracted in a first step and then converted in a fuel cell in a second one step, making the system complex, expensive, and potentially risky due to the handling of gaseous hydrogen. To overcome these limitations, researchers are exploring a new class of materials known as electrochemically active LOHCs (EC-LOHCs). These compounds can be directly oxidized in a fuel cell, releasing energy without the need to first extract hydrogen gas. Among the most promising candidates is the isopropanol/acetone couple. Both compounds are stable liquids, easy to separate and reuse. Crucially, compared to alternative fuels like methanol, isopropanol generates only minimal amounts of CO₂ in fuel cells—CO₂ that cannot be easily reintegrated into the fuel cycle. Still, there's a trade-off. Isopropanol stores less hydrogen by weight than conventional LOHCs like DBT. To bridge this gap, scientists have proposed a hybrid solution: The idea is to use DBT for large-scale storage, and to transfer hydrogen to isopropanol right before the conversion to electricity. This approach bypasses the temperature mismatch and has already shown a major jump in system efficiency — from below 38% to over 50%. Yet even this elegant system needs one critical piece to work well: a highly efficient catalyst. That’s where bimetallic Pt–Ru (platinum–ruthenium) catalysts come in. These materials are currently the gold standard for isopropanol oxidation, prized for their resistance to poisoning by acetone and their unusually low overpotentials — meaning they can start the reaction with far less energy than other catalysts. But why exactly are Pt–Ru catalysts so good at this job? The answer has remained a mystery. Now, our new research is shedding light on that question. Using a carefully controlled surface science approach, we have discovered that the key lies not in the alloy itself, but in ultrasmall platinum aggregates that form when Pt–Ru alloys undergo specific electrochemical treatments. These tiny, highly active aggregates are not part of a smooth alloyed surface, but instead emerge through a process known as dealloying — triggered by electrochemical oxidation and reduction of the Pt−Ru alloys, reshaping the surface at the nanoscale. To probe these chemical and structural transformations, we used a combination of cutting-edge techniques, including synchrotron radiation photoelectron spectroscopy (SRPES) coupled with an ex-situ emersion electrochemical cell, cyclic voltammetry, scanning tunneling microscopy (STM), as well as in-situ electrochemical infrared reflection absorption spectroscopy (EC-IRRAS). In our study, we followed a surface science approach which involved the preparation of well-defined Pt–Ru surface alloys of different compositions on a Ru(0001) substrate in ultrahigh vacuum. The thickness of the model Pt–Ru surface alloys was just one atomic monolayer. We focused on several alloy compositions, but one in particular — with just 0.125 monolayers of platinum — delivered the most striking results. The corresponding CVs, joint Pt 4f and Ru 3s spectra, STM images and EC-IRRA spectra obtained from this model system are shown in Fig. 1. Prior to the dealloying procedure, the onset of isopropanol oxidation occurs around 0.5 VRHE with the peak maximum at ~0.75 VRHE (Fig. 1 b). During the course of the dealloying, we observed a gradual transformation of the high-potential peak into a lowpotential peak with an onset potential at 0.05 VRHE and a peak maximum at ~0.15 VRHE (Figs. 1 c, d). Interestingly, we also observed a clear shift in the chemical state of platinum after dealloying (Figs. 1 e, f). A shift in the Pt 4f binding energy and a broadening of the metallic Pt contribution pointed to the formation of ultrasmall Pt aggregates. Morphologically, the initially well-ordered Pt–Ru surface remained stable during cycling between 0.0 and 0.9 VRHE (Fig. 1 g), but dealloying induced noticeable surface roughening (Fig. 1 h). Finally, we followed the isopropanol oxidation before and after the dealloying procedure by EC-IRRAS to verify that the observed current densities correspond to the conversion of isopropanol to acetone. The main conversion on Pt– Ru alloy takes place between 0.3 and 0.9 VRHE, i.e. in the potential window assigned to isopropanol oxidation on Pt surfaces. After dealloying, a pronounced shift of the conversion toward lower potentials is observed, indicating a reduced overpotential. In summary, our study shows that by dealloying we can convert the original alloy surface, which shows low activity for isopropanol oxidation into a very active state with low-overpotential. By linking surface structure to function at the atomic scale, this work provides a roadmap for designing more efficient catalysts for EC-LOHC systems — a promising pathway for clean, compact, and efficient energy conversion in the hydrogen economy. Elettra Highlights 2024–2025 114
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