Research
Electronic structure group
The electronic structure of catalytically active solid materials constitutes the focus of our research. Surface reactions and elementary steps are strongly influenced by the local electronic and geometric properties of the surface. We deal with heterogenous as well as electrocatalysts and therefore we investigate both solid-gas and solid-liquid interfaces.
Our model of heterogeneous catalysis evolved in the last 100 years and now we understand that the catalyst itself does not remain completely unchanged under reaction conditions. In fact, the as-synthesized catalyst can be best seen as only a precursor to the active-phase/active-site ensemble, which is created under operational conditions. Mass and energy transport, local chemical potential, and a catalyst's geometric and electronic structure are all strongly interconnected through the catalyst’s dynamics. This creates feedback loops that make it necessary to study catalytic phenomena under reaction conditions. Based on these principles, our core activities are synchrotron based in-situ/operando X-ray absorption (XAS) and X-ray photoelectron spectroscopy (XPS) experiments, often complemented with DFT calculations, to understand structure-function relations governing catalytic performance.
Our solid sample are sometimes model catalysts but we investigate also high-performance catalysts and therefore we rely on the synthesis expertise anchored in the Catalysis with Oxides and Catalysis on Metal groups. The close collaboration with the Electron Microscopy group enables us to link the electronic structure of the catalysts to their geometric and structural properties.
Please find detailed information of the following projects:
By using pulse voltammetry, operando X-ray absorption and photoelectron spectroscopy (XAS, XPS) measurements together with DFT calculations on iridium oxide we show that the applied bias does not act directly on the reaction coordinate, but affects the electrocatalytically generated current through charge accumulation in the catalyst. We find that the logarithm of the rate of OER linearly correlates with the charge accumulated. The applied potential drives the formation of empty Ir 5d states, ascribed to formally Ir
5+ species, and the concomitant appearance of electron-deficient oxygen surface species (μ1-O and μ2-O) that are responsible for water activation and oxidation.
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The ultimate surface and element sensitivity on the one hand and the possibility to vary the information depth on the other hand adds an extra quality to XPS when operated with a tuneable X-ray source like a synchrotron. In addition, X-ray absorption spectra (XAS) can be obtained at a synchrotron. The HZB
(Helmholtz-Zentrum Berlin für Materialien und Energie) and the FHI/MPI-CEC operate three facilities dedicated to synchrotron based ambient pressure XP spectroscopy (AP-XPS) at the 3
rd generation X-ray source BESSY located in the south east of Berlin with the most recent being “BElChem” (Berlin Joint Lab for ElectroChemical Interfaces) and “EMIL” (Energy Materials In Situ Laboratory Berlin).
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In recent years NiO has come into focus as a low-cost, efficient material in the Oxygen Evolution Reaction (OER) via electrochemical water splitting. However, the detailed mechanism of the electrochemically induced reaction is not yet fully understood. In order to understand the details of OER mechanism of NiO, XPS and XAS techniques for two types of experiments were used. Firstly, as a model catalyst, NiO
x thin films were put in contact with oxygen and subsequently water vapor at 0.5 mbar and elevated temperatures. Secondly, in operando electrochemical experiments were performed on thin NiO
x films. We are able to show that in the reactions oxygen vacancies are formed and seemingly play a crucial role acting as an intermediate state in OER.
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The electronic and geometric factors making copper one of the best catalysts for the electrochemical reduction of CO
2 into valuable hydrocarbons, are investigated by means of in situ X-ray spectroscopies and in situ electron microscopy.
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To probe active electrocatalyst surfaces in a liquid environment, we have developed an XPS/XAS cell in which the catalyst is confined between a proton/anion exchange membrane and graphene. While the membrane supplies a steady flow of electrolyte to the electrode, the X-ray and electron-transparent graphene layer greatly reduces the evaporation of water into the NAP-XPS chamber. With this methodology, electrocatalysts can be studied under operating conditions using surface sensitive soft X-ray XPS and XAS.
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Understanding the catalytic partial oxidation of methanol to formaldehyde is a long standing problem that is poised to face a resurgence in interest. Nowadays, formaldehyde is produced industrially by the reaction of methanol over either metal-oxide or silver based catalysts. The two catalysts require different reaction conditions and show different selectivity. For example, it is believed that formaldehyde is produced through both a partial oxidation and dehydrogenation pathway over silver based catalysts,while only the former seems to be important for the metal-oxide catalyst. The existence ofmultiple pathways on silver is a particularly interesting problem
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While iridium oxide lies at the tip of the volcano plot for the oxygen evolution reaction, its use is limited by reasons of economics and availability, as well as its instability in alkaline conditions. Thus, it is desirable to replace these expensive and rare precious metals that cu electrocatalysts by more abundant first row transition metals such as cobalt.
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The direct partial oxidation of ethylene to ethylene oxide (EO) over Ag highlights the tremendous potential of heterogeneously catalyzed partial oxidation reactions. Ag catalysts can be made to favor ethylene epoxidation by ca. 90% over the thermodynamically favored total combustion, but this performance is not ubiquitous. The Ag catalysts that are so useful for EO production fail dramatically when used to produce another important feedstock epoxide, propylene oxide (PO). This failure may appear surprising owing to the structural similarity between EO and PO, yet extensive studies of Ag-based catalysts in the direct epoxidation of propylene demonstrate Ag favors total combustion. Thus, PO is currently mainly produced by either environmentally unfriendly or costly process.
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