Ultra-High Pressure Hydride X-ray Photoelectron Spectroscopy (Contact PD Dr. A. Borgschulte)

There is general consensus that the sun’s energy and its derivatives are by far sufficient to supply world’s energy  demand. However, the large daily and seasonal power variation of renewable energy is a serious complication for a wide spread replacement of fossil energy by renewable energy. Thus the full potential of solar energy as a primary energy source can only be exploited, if we develop an efficient way of storing renewable energy, most preferably as a chemical fuel in the form of hydrogen or related compounds.

In this project, we do not want to contribute to materials science in this research field directly, but rather to develop a novel measurement method being able to unravel the mechanisms driving the hydrogen – matter interaction taking place in hydrogen storage, hydrogen selective membranes, and the catalysis of hydrogenation reactions. Although apparently simple, a direct photoemission measurement of the hydrogen induced changes of the electronic structure, which are the origin of the binding of hydrogen with metals, is possible in a few cases only. The reason for this is purely technical: the electronic structure of hydrogen chemisorbed to surfaces can be measured using standard surface science techniques, because the required hydrogen pressure is compatible with the UHVtechnology. However, processes relevant for energy conversion and storage take place at several atmospheres hydrogen pressure; and thus valuable information on these systems is not accessible by commonly used surface science methods due to their incompatibility with high pressures.

The present project relies on a membrane approach for high pressure XPS under development in our laboratory. The method is based on a new type of specimen holder, which is a metallic, hydrogen permeable membrane fed on one side with a high hydrogen pressure and exposed on the other side to the X-ray beam at UHV-pressures. In first papers, we introduced the fundamental idea and demonstrated the feasibility of the method in some well-studied cases, paving the way for its use on relevant questions in energy storage. In this project, we want to utilize the membrane approach to prepare and measure in-situ various ionic and intermetallic hydrides as a function of the chemical potential of hydrogen, which will deliver insightful knowledge on the electronic structure of hydrides. In a second task, we want to elucidate the mechanism of the enhanced hydrogen desorption from hydrogen membranes upon application of polymer coatings. Finding the physical origin of the remarkable effect may help to functionalize also hydrogen permeable metals other than Pd.

Finally, the catalytic formation/decomposition of the C-H bond is one of the main challenges in synthetic chemistry, due to the strong covalent nature of this bond. The (de-) hydrogenation reactions of hydrocarbons, which take place at several tens of bar, are catalysed by Pd, Ru, or similarly expensive and scarce elements. Experiments on the hydrogenation of unsaturated hydrocarbons using the Pd-membrane mimic the technical process, so that information on the reaction can be drawn. Finally, we want to explore the possibility of combining different spectroscopy methods (X-ray absorption spectroscopy and Raman spectroscopy) with the membrane approach.