Ulrike Küst

Surface oxidation of noble metals under electrochemical conditions has a decisive impact on the performance of electrocatalysts for electrolyzers and fuel cells. However, the experimental detection of such oxides on well‐defined model electrodes under operando conditions is a major challenge, limiting our ability to establish clear structure–performance relationships. Using surface‐sensitive total‐reflection X‐ray absorption spectroscopy in real time during cyclic voltammetry, combined with inductively coupled plasma mass spectrometry dissolution measurements, we demonstrate that an ultrathin (~5 Å) hydroxide film forms on the gold (111) surface prior to the onset of the oxygen evolution reaction. This film, composed of Au ³⁺ in similar coordination as in the bulk oxyhydroxide, thus governs its catalytic properties, dissolution, and restructuring. The results highlight the importance of surface oxidation in water splitting catalysis even for Au(111), the most inert surface of the most inert noble metal.

The electro-oxidation of a polycrystalline gold electrode is investigated in situ using two-dimensional surface optical reflectance, electrochemical near ambient pressure X-ray photoelectron spectroscopy, and tomographic surface X-ray diffraction.

This work introduces a sustainable and versatile synthesis platform for nanosized metal–ceria catalysts with tunable composition, directly relevant to renewable energy conversion and CO ₂ utilization.

Ambient pressure X-ray photoelectron spectroscopy (APXPS) has emerged as an important technique for investigating surface and interface chemistry under realistic conditions, overcoming the limitations of conventional XPS restricted to ultrahigh vacuum. This review highlights the capabilities and scientific impact of APXPS at the MAX IV Laboratory, the world’s first fourth-generation synchrotron light source. With the APXPS beamlines SPECIES and HIPPIE, MAX IV offers state-of-the-art instrumentation for in situ and operando studies across a broad pressure range, enabling research in catalysis, corrosion, energy storage, and thin film growth. The high brilliance and small beam size of MAX IV’s synchrotron light are essential for pushing the time-resolution boundaries of APXPS, especially in the soft X-ray regime. We discuss representative studies at MAX IV, including investigations of single-atom catalysts, confined catalysis, time-resolved catalysis, atomic layer deposition, and electrochemical interfaces, showcasing the role of APXPS in advancing material and surface science.

Separating how surface and subsurface species affect catalytic function is a challenging task in heterogeneous catalysis, particularly when deposition and segregation take place at reaction conditions. Here, we report on an operando approach to establish surface/subsurface/function correlations. Using temperature modulations we oscillate carbon deposition and segregation over a Pd catalyst. Catalytic composition and function are monitored during methane oxidation showing that the surface coverage of carbon drives partial oxidation to CO, while subsurface carbon controls the overall methane turnover. Also, we show that a carbon traffic jam in the subsurface leads to a shifting selectivity from H₂ to H₂O formation, highlighting the importance of the catalyst subsurface for the catalytic reaction.

Separating how surface and subsurface species affect catalytic function is a challenging task in heterogeneous catalysis, particularly when deposition and segregation take place at reaction conditions. Here, we report on an operando approach to establish surface/subsurface/function correlations. Using temperature modulations we oscillate carbon deposition and segregation over a Pd catalyst. Catalytic composition and function are monitored during methane oxidation with time- and depth-resolved spectroscopy coupled with Fourier analysis. Hereby, we show that the surface coverage of carbon drives partial oxidation to CO, while the subsurface carbon controls the overall methane turnover. Also, we show that a carbon traffic jam in the subsurface leads to a shifting selectivity from H₂ to H₂O formation.

Catalytic studies traditionally rely on steady-state conditions resulting in time-averaged datasets that do not differentiate between active and spectator species. This limitation can cause misinterpretations of catalytic function, as the signal of short-lived intermediates responsible for producing desired reaction products is often masked by more intense spectator species. Time-resolved ambient pressure X-ray photoelectron spectroscopy (tr-APXPS) mitigates this issue by combining microsecond time resolution under reaction conditions. Using tr-APXPS, we investigate the oxidation of CO over Pt(111) by concurrently tracking reaction products, surface intermediates, and catalyst response. Our findings reveal that chemisorbed oxygen, rather than Pt surface oxide, is the main species reacting with CO to form CO₂, supporting a primary Langmuir-Hinshelwood mechanism. The results shed new light on a heavily-debated reaction in catalysis. Beyond using CO pulses to determine active species, we demonstrate how careful tuning of pulsing parameters can be used for dynamic catalyst operation to enhance CO₂ formation.