Jan Knudsen

To selectively detect minority sites and measure their catalytic properties when they are shadowed by a large amount of similar spectator sites is challenging. Inspired by bat's bio sonar system used to hunt insects in darkness we here use temperature modulations combined with time‐resolved ambient pressure x‐ray photoelectron spectroscopy to hunt active CO‐vacancy sites in a predominantly CO‐covered Pd(100) surface. By modulating the catalyst temperature with an infrared laser, we selectively probe the catalyst echo signal by spectral analysis of the C 1s and Pd 3d _5/2 level. We show that dynamic CO minority vacancy islands are formed echoing the external temperature modulations. Synchronized with the vacancy island formation we observe a small but detectable CO ₂ oscillation signal both in the O 1s and C 1s core levels. We thus directly measure the catalyst activity of the dynamic vacancy islands.

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.

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.

Starting from a single layer of NbS ₂ grown on graphene by molecular beam epitaxy, the single unit cell thick 2D materials Nb _5/3 S ₃ ‐2D and Nb ₂ S ₃ ‐2D are created using two different pathways. Either annealing under sulfur‐deficient conditions at progressively higher temperatures or deposition of increasing amounts of Nb at elevated temperature result in phase‐pure Nb _5/3 S ₃ ‐2D followed by Nb ₂ S ₃ ‐2D. The materials are characterized by scanning tunneling microscopy, scanning tunneling spectroscopy, and X‐ray photoemission spectroscopy. The experimental assessment combined with systematic density functional theory calculations reveals their structure. The 2D materials are covalently bound without any van der Waals gap. Their stacking sequence and structure are at variance with expectations based on corresponding bulk materials highlighting the importance of surface and interface effects in structure formation.