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Abstract: The ambient-pressure endstation and branchline of the Versatile Soft X-ray (VerSoX) beamline B07 at Diamond Light Source serves a very diverse user community studying heterogeneous catalysts, pharmaceuticals and biomaterials under realistic conditions, liquids and ices, and novel electronic, photonic and battery materials. The instrument facilitates studies of the near-surface chemical composition, electronic and geometric structure of a variety of samples using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy in the photon energy range from 170 eV to 2800 eV. The beamline provides a resolving power hν/Δ(hν) > 5000 at a photon flux > 1010 photons s−1 over most of its energy range. By operating the optical elements in a low-pressure oxygen atmosphere, carbon contamination can be almost completely eliminated, which makes the beamline particularly suitable for carbon K-edge NEXAFS. The endstation can be operated at pressures up to 100 mbar, whereby XPS can be routinely performed up to 30 mbar. A selection of typical data demonstrates the capability of the instrument to analyse details of the surface composition of solid samples under ambient-pressure conditions using XPS and NEXAFS. In addition, it offers a convenient way of analysing the gas phase through X-ray absorption spectroscopy. Short XPS spectra can be measured at a time scale of tens of seconds. The shortest data acquisition times for NEXAFS are around 0.5 s per data point.

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Abstract: Hydrotalcite-derived Ni and Fe-promoted hydrotalcite-derived Ni catalysts were found to outperform industrial catalysts in the CO2 methanation reaction, however the origin of the improved activity and selectivity of these catalysts is not clear. Here, we report a study of these systems by means of in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption spectroscopy elucidating the chemical nature of the catalysts` surface under reaction conditions and revealing the mechanism by which Fe promotes activity and selectivity towards methane. We show that the increase of the conversion leads to hydroxylation of the Ni surface following the formation of water during the reaction. This excessive Ni surface hydroxylation has however a detrimental effect as shown by a controlled study. A dominant metallic Ni surface exists in conditions of higher selectivity towards methane whereas if an increase of the Ni surface hydroxylation occurs, a higher selectivity towards carbon monoxide is observed. The electronic structure analysis of the Fe species under reaction conditions reveals the existence of predominantly Fe(III) species at the surface, whereas a mixture of Fe(II)/Fe(III) species is present underneath the surface. Our results highlight that Fe(II) exerts a beneficial effect on maintaining Ni in a metallic state, whereas the extension of the Fe oxidation front from the surface towards the bulk is accompanied by a more extended Ni surface hydroxylation with a negative impact on the selectivity towards methane.

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Abstract: Solar fuels are made from common substances, such as water and carbon dioxide, using solar energy. Producing fuels using artificial photosynthesis is one of the most crucial challenges in sustainable energy. Photocatalytic materials can be used in photo-electrochemical cells to perform water splitting or carbon dioxide reduction for fuels and chemicals. For example, hydrogen production is a key process in the transition towards sustainable energy generation technologies. However, the current generation of photo(electro) catalysts suffer from several limitations related to charge carrier recombination phenomena and inefficient light harvesting. To overcome these problems we need to develop more sustainable and efficient materials. We also need to understand the charge transfer dynamics and identify which species are involved in the process at different timescales. Researchers from the Instituto de Ciencia de Materiales and the IMDEA Energy Institute in Madrid investigated the potential for metal- organic frameworks (MOFs) to act as photo(electro)catalysts. They used Diamond Light Source's Versatile Soft X-ray (VerSoX) beamline (B07), which can perform core-hole clock experiments at the sulphur K edge that provide information on the charge dynamics processes between linkers and metals. These experiments were critical to determining the delocalisation rates of charges in the initial steps of the process. The results demonstrated the organic linker’s key role during the photo(electro)catalytic process with the MOF catalyst. They helped identify intermediate species generated during the reaction and improve our understanding of the charge dynamic process.

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Abstract: Solar fuels production is a cornerstone in the development of emerging sustainable energy conversion and storage technologies. Light-induced H2 production from water represents one of the most crucial challenges to produce renewable fuel. Metal-organic frameworks (MOFs) are being investigated, due to the ability to assemble new structures with the use of suitable photoactive building blocks. However, the identification of the reaction intermediates remains elusive, which negatively impacts in the design of more efficient materials. Here, we report the synthesis and characterization of a new MOF prepared with the use of bismuth and dithieno[3,2-b:2',3'-d]thiophene-2,6-dicarboxylic acid (DTTDC), an electron-rich linker with hole transport ability. By combining theoretical studies and time-resolved spectroscopies, such as core hole clock and laser flash photolysis measurements, we have completed a comprehensive study at different time scales (fs to-ms) to determine the effect of competitive reactions on the overall H2 production. We found that an intermediate radical anion is formed upon reaction of photogenerated holes with an electron donor, which play a key role in the photoelectrocatalytic processes. These results shed new light on the use of MOFs for solar fuel production.

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Abstract: Methanol is a promising chemical for the safe and efficient storage of hydrogen, where methanol conversion reactions can generate a hydrogen-containing gas mixture. Understanding the chemical state of the catalyst over which these reactions occur and the interplay with the adsorbed species present is key to the design of improved catalysts and process conditions. Here we study polycrystalline Cu foils using ambient pressure X-ray spectroscopies to reveal the Cu oxidation state and identify the adsorbed species during partial oxidation (CH3OH + O2), steam reforming (CH3OH + H2O), and autothermal reforming (CH3OH + O2 + H2O) of methanol at 200 °C surface temperature and in the mbar pressure range. We find that the Cu surface remains highly metallic throughout partial oxidation and steam reforming reactions, even for oxygen-rich conditions. However, for autothermal reforming the Cu surface shows significant oxidation towards Cu2O. We rationalise this behaviour on the basis of the shift in equilibrium of the CH3OH* + O* ⇌ CH3O* + OH* caused by the addition of H2O.