Heterogeneous catalysis is strongly influenced by surface features of the respective catalysts, such as the presence, amount and nature of defects, the electronic structure and compositional variations. As an example, we found in collaboration with the Interface Science Department that nanoscale composition inhomogeneities of Co$_{2}$FeO$_{4}$ impacts the oxygen evolution performance [1] Here,...
At temperatures of 220-280 °C silver catalyzes the conversion of ethylene and oxygen gas into ethylene oxide (EO), and with the addition of promoters it does so with high epoxide selectivity. Why and how silver has the unique ability to catalyze this reaction with such efficiency has been a subject of studies for decades. Under reaction conditions silver has been shown to be covered by two...
We present a detailed quantum treatment of the optical trap for molecules and its extension to the case when the optical trap is embedded within a uniform electrostatic field. For polar molecules, the resulting electro-optical trap [1] offers significant advantages over trapping by an optical field alone that include increased trap depth, apart from orienting the trapped molecules. The quantum...
Angle-resolved photoemission spectroscopy (ARPES) is the most direct probe of a material’s electronic structure. ARPES has been primarily used for mapping the dis-persion and interactions of electronic states, i.e. to quantify the real and imaginary parts of the electron self-energy. A full characterization of the electronic structure, however, requires access to more subtle properties of the...
Solid oxide cells (SOCs) are an efficient technology for power-to-hydrogen conversion from fluctuating renewable electricity sources. While SOCs are well adapted to intermittent operation, anode degradation limits cell performance and lifetime in particular in electrolysis mode. This degradation goes hand in hand with the oxygen evolution reaction (OER) located at the triple-phase boundary...
The versatility of organic molecules generates a rich design space for functional materials such as organic semiconductors (OSCs), dyes or molecular switches. Offering unparalleled promise for materials discovery, the vastness of this design space also dictates efficient search strategies, however. This concerns the ability of a search algorithm to explore a large variety of possible solutions...
Machine-learning (ML) interatomic potentials trained with first-principles data promise steep advances for the predictive-quality modeling and simulation of molecules and materials. At a computational cost that is significantly reduced compared to direct first-principles calculations, such ML potentials allow to address larger system sizes or perform more extensive dynamical simulations and...
X-ray absorption spectroscopy (XAS) in the near-edge- and extended-edge regime is well established using synchrotron light sources and provides element specific access to the unoccupied electronic structure, symmetry of orbitals and chemical bonding. The technique has been advanced in access the ultrafast time domain by generating ultra-broadband soft x-ray continua driven by phase stable...
The discovery of new functional materials is one of the holy grails of computational chemistry, among other reasons because it has the potential to accelerate the adoption of renewable energy sources and reduce the energy consumption of chemical industry. Unfortunately, computational materials discovery is a daunting task, due to the enormous space of possible candidates and the challenges...
The low dimensional nature of transition metal dichalcogenides (TMDCs) and the resulting reduced screening significantly influence their non-equilibrium optical properties. We investigated the role of different photoexcited quasiparticles on the dynamic response of WS2 monolayers and provide a formalism that allows the quantitative comparison of exciton dynamics in TMDCs for different samples,...
Catalysts have the ability of improving the kinetics of a given chemical reaction by providing alternative reaction paths with lower activation energies, thus affecting the selectivity of a chemical reaction.$^1$ Their activity, selectivity, and lifetime depend on their morphology, substrate, chemical composition, and oxidation state.$^2$ Particularly, bimetallic nanoparticles (NPs) are often...
The insufficient green production of hydrogen is a bottleneck in the development of a sustainable hydrogen network. Electrolytic water splitting in acidic electrolytes addresses this problem by making use of the intermittent nature of renewable energies to produce hydrogen at high pressures. The limiting process is the anodic oxygen evolution reaction (OER), which is plagued by high...
We explore sophisticated spectroscopic schemes in order to overcome previous limitations in the transfer efficiency of enantiomer-specific state transfer (ESST).
The ESST method was recently developed using tailored microwave fields [1]. It allows to populate or depopulate a rotational state of a chosen enantiomer, providing a way of quantum-controlled chiral separation.
One main...
The oxygen reduction reaction (ORR) is a key electrocatalytic process for developing sustainable energy technologies. And yet, many aspects of the underlying reaction mechanism are still poorly understood at the molecular level. Especially at weak-binding electrode surfaces such as gold (Au), product selectivity and even the ability to bind aqueous O2 species as a first mechanistic...
Conventional condensed phase approaches for the analysis of biomolecules, such as nuclear magnetic resonance (NMR) and X-Ray spectroscopy, often require large amounts of high purity samples and are therefore not universally applicable. Mass spectrometry (MS) on the other hand requires only minute amounts of sample and its purity is often not an issue. However, the amount of structural...
Fundamentals behind halide-induced promotion effects in catalysis remain debated and lacking reliable structure-property relationships for reactions including the electrochemical CO$_{2}$ reduction (CO$_{2}$RR) hinder rational catalyst and electrolyte design. Here we employ in situ atomic force microscopy, cyclic voltammetry, and DFT theory to unveil the potential-dependent complex surface...
Photoelectron Circular Dichroism (PECD) is a method of chiral discrimination, which can aid in our fundamental understanding of electron dynamics and holds promise for future analytical techniques of chiral compounds. In PECD, irradiation of a non-racemic sample by circularly polarized light, resulting in the detachment of an electron, leads to a forward-backward asymmetry of the photoelectron...
Cobalt oxides are promising, non-precious catalysts for the anodic oxygen evolution reaction (OER) of alkaline water electrolysis. To promote a rational catalyst design, fundamental knowledge is required and topic of investigations. In this regard, it has been shown that as-prepared structurally-different CoO$_x$(OH)$_Y$ materials unify towards an octahedral CoO$_6$ coordination with di...
The H2O molecule is omnipresent in electrochemistry, yet, beyond its role as solvent, its direct impact on catalyst activity, selectivity and deactivation remain poorly understood. Conversely, interfacial H2O reactivity is one of the focus subjects of the newly founded Interfacial Ionics Group.
In the first part of the poster, we discuss our findings on inner-sphere...
The $^1\text{P}_1 \leftarrow ^1\text{S}_0$ line in atomic cadmium shares many properties with the $^1\Pi \leftarrow ^1\Sigma^+$ laser cooling transition of the AlF molecule, making cadmium a convenient test system for laser cooling and trapping of AlF. In addition, cadmium possesses eight stable (six bosonic, two fermionic) isotopes, and a narrow spin-fobidden $^3\text{P}_1 \leftarrow...
Functional interfaces are of crucial importance for the interaction of catalytically active phases with a support. In two projects, we are contributing to adjust catalytic properties via the design of metal-support interfaces. By systematically introducing promotors through incipient wetness impregnation (IWI) as well as strong electrostatic adsorption (SEA), we aim to modify the...
Transport of energetic electrons through gases and dense media is invariably determined by electron scattering. In photoemission experiments, the influence of scattering can be avoided in gas-phase experiments by choosing a sufficiently dilute target, but inelastic scattering becomes noticeable as soon as atomic or molecular clusters are considered. In solids and liquids, inelastic scattering...
The chemical synthesis of glycoconjugates is very cumbersome, due to the unpredictable stereoselectivity in glycosylation reactions. As a result, today only few structures are accessible for functional studies in biology. Despite years of significant progress in method development, no gold standard technique to construct glycosidic bonds with well-defined stereochemistry has been reported....
Electrocatalytic conversion of CO$_2$ to higher order hydrocarbons has been proposed as one of the tools available to help mitigate the effects of anthropogenic climate change and create a carbon-neutral energy cycle.$^1$ The only pure metal that is capable of reducing CO$_2$ to C$_{2+}$ hydrocarbons is copper (Cu). However, it suffers from low selectivity towards these products.$^2$ Recent...
The physicochemical properties of life-essential molecules such as carbohydrates, amino acids, and DNA arise from a series of complex intramolecular and intermolecular interactions within aqueous cellular environments. Knowledge of the aqueous-phase electronic structure of these important biomolecules is indispensable to understand structure–function relationships, chemical activity, and...
Absorbing UV radiation, ozone (O$_3$) that exists in the stratosphere protects life on Earth. Moreover, it plays a crucial role in Earth's temperature balance. In contrast, tropospheric ozone is considered an important greenhouse gas and air pollutant. However, despite its importance and decades of research, the reaction leading to ozone formation is not fully understood due to its complexity....
The layered double hydroxides (LDH) represent a broad class of inorganic lamellar compounds with a high versatility in composition and large range of technological applications1. We have studied the Ni-Cu-Al LDH where the charge of the trivalent ions (Al$^{3+}$) is compensated by carbonate anions in the interlayer space$^{2}$. Within the coprecipitation method the effects of different...
The unique physical and chemical properties of interfaces are governed by a finite depth that describes the transition from the topmost atomic layer to the properties of the bulk material. Understanding the physical nature of interfaces thus requires detailed insight into the different molecular structures, chemical compositions, and physical processes that form this interfacial region. Such...
Knowledge of the electronic properties of aqueous solutions is crucial to understanding water-based chemistry. Here, a quantity of particular interest is the vertical ionization energy (VIE) of solvents and embedded solutes, especially of valence electrons which are directly related to the chemical reactivity. Liquid-jet photoelectron spectroscopy provides direct access to the electronic...
Electrochemical reduction of CO$_2$ (CO$_2$RR) is an attractive technology to turn undesired CO$_2$ back into the carbon cycle driven by renewable energy and a suitable catalyst. Cu$_2$O nanocubes have been shown to be a promising catalyst by converting CO$_2$ into multiple C$_{2+}$ hydrocarbons.$^1$ Thus, an essential challenge is to control the selectivity toward particular products. Several...
The industrial and societal transformation towards carbon neutrality requires the development of mitigating strategies to significantly reduce the greenhouse gas emissions. In this framework, CO$_{2}$ electrocatalytic reduction (CO$_{2}$RR) is an elegant and promising approach to convert excess electrical energy and store it in the chemical bonds of multicarbon (C$_{2+}$) products such as...
The air-water interface is omnipresent in nature, e.g. in marine and freshwater environments, atmospheric aqueous aerosols and fog/cloud droplets.[1] This interface governs numerous heterogeneous chemical reactions, such as the sequestration of CO$_2$ by the oceans and the uptake and release of trace gases by aqueous aerosols. The reactivity of the air-water interface can vary widely depending...
Chiral molecules are ubiquitous in nature and they are of great importance in many biological and chemical processes. The two non-superimposable mirror images of a chiral molecule are referred to as enantiomers. Even though most physical properties of enantiomers are identical – which makes them intrinsically challenging to separate – their handedness often determines their functionality....
Electrocatalytic CO2 reduction reaction (CO2RR) converts CO2 into value-added chemicals and fuels, and is an attractive pathway for closing the carbon cycle [1]. Copper based nanocatalysts are commonly used for CO2RR due to their unique ability to convert CO2 into more complex hydrocarbons, but the distribution of reaction products is hard to control [2]. A commonly used strategy for tuning...
Heterogeneous catalysts are dynamic and become more complex under reaction conditions compared to their pristine states. Reaction induced solid-state chemistry yields in metastable non-stoichiometric phases that extend from the surface to the bulk [1]. This observation is important as these phases may control the activity. Combining operando transmission electron microscopy (TEM) [2],...
The main challenge in the catalytic partial oxidation of alkanes to valuable products, such as olefins, aldehydes or acids, is to control the reaction pathways to desired and undesired products in complex reaction networks. Propane activation and olefin formation over structurally stable perovskites$^{1}$ and alkali-doped supported vanadium oxide catalysts, which show structural dynamics under...
Copper (Cu) is an efficient catalyst towards CO$_{2}$ electrocatalytic reduction (CO$_{2}$RR). Strikingly, the identification of the responsible surface site(s) leading to the observed activity and/or selectivity still remains debatable. A way to address this is by controlled surface modification of model metallic Cu single crystals (SCs) and compare it with CO$_{2}$RR activity. Two pursued...
Terahertz (THz) electromagnetic fields are highly interesting to probe and even drive fundamental ultrafast modes in spintronic nanostructures. Examples include magnons, phonons and spin-polarized electron transport. Here, we demonstrate how free-space THz electric (rather than magnetic) fields can be used to exert torque on the magnetic order parameter (the Neel vector) in the novel...
Electrochemical energy conversion driven by renewable energy is a cost-effective, environmentally friendly route to convert undesired substances (such as CO2) into valuable chemicals and fuels, but a suitable catalyst is needed. Here, Cu-based catalysts are particularly attractive due to their unique ability to convert CO2 into more complex hydrocarbons [1]. A challenge for the practical...
Surface diffusion and chemical reactions on surfaces are important fundamental processes in catalysis. To fully understand these processes at the atomic scale, real time and real space observations are crucial. Since the first experimental evidence for surface diffusion 100 years ago [1], a variety of methods evolved to study atomic processes on well-defined surfaces. The prominent approach of...
Surface Action Spectroscopy (SAS) is a method derived from the messenger action spectroscopy typically applied to aggregates in the gas phase to elucidate their structure. To this end, weakly bound atoms, or molecules, the ‘messengers’, are attached to the aggregates as indicators of a vibrational excitation. They may desorb when a vibrational mode is excited with infrared light, which usually...
A potential strategy for the re-use of carbon dioxide is to convert it into a platform chemical, from which valuable products can be prepared. In particular, from all the possible products, light olefins (C$_{2}$$^{=}$, C$_{3}$$^{=}$) and higher alcohols are of great interest as important synthesis building blocks, so seeking an appropriate catalyst for this process is a key challenge. In...
The oxygen evolution reaction (OER) plays a decisive role in many alternative energy schemes. Electrocatalysts accelerate the reaction by facilitating both the required electron transfer and the formation as well as rupture of chemical bonds. This involvement in fundamentally different processes results in complex electrochemical kinetics that can be challenging to understand and control,...
Prosperity and lifestyle of our society largely rely on improved or even novel materials that make new products possible for the energy, environment, health, mobility, IT sectors, and more. Materials Science has been producing for decades a huge amount of possibly extremely valuable data, which, however, are seldom re-used or even accessed outside the single project they were produced for....
Thermal decomposition of ammonia to hydrogen has received special attention due to the potential use of ammonia as a carbon-free hydrogen storage medium. Supported Ru catalysts are among the most active catalysts showing a NH$_{3}$ conversion of 41 % at 550°C at a WHSV of 30.000 mlg$_{cat}$$^{-1}$h$^{-1}$. Despite their high activity, the scarcity and high cost of Ru limit the applicability of...
In recent years NiO has come into focus as a low-cost, efficient material in the Oxygen Evolution Reaction (OER) via electrochemical water splitting. However, the detailed mechanism of the electrochemically induced reaction is not yet fully understood.[1] Hence, three types of experiments were performed. Firstly, as a model catalyst, NiO$_{x}$ thin films were put in contact with oxygen and...
Crystalline epitaxial thin films have been used as thin film model catalysts for the oxygen evolution reaction (OER) under alkaline conditions. This approach avoids the complexity of real-world nanoparticle catalysts: thin films have a well-defined flat homogenous surface with a known size, no hidden surfaces and a sufficient electric conductivity even for nominally non conducting catalyst...
Transport of spin angular momentum and the conversion of the resulting spin current to a charge current are fundamental operations required for future spin-electronic devices. To push these operations to the terahertz (THz) frequency range, we use femtosecond laser pulses (of ~10 fs duration) to excite prototypical F|N bilayers consisting of a ferromagnetic metal F and a nonmagnetic metal N...
Scattering-type scanning near-field optical microscopy (s-SNOM) allows optical imaging and spectroscopy far below the diffraction limit. Standard s-SNOM detects light scatter¬ed from the apex of a metallic tip in a tapping-mode atomic-force microscope (AFM), where background-free detection of the near-field signal is achieved by high-harmonic demodulation and interferometry. The spatial...
Against the background of dwindling fossil fuel resources and the threats of climate change, sustainable and environmentally friendly production processes for fuels and chemical feedstock become all the more important. Methanol, a liquid fuel of high energy density, may contribute to pawing the way towards sustainable society [1]. It can be produced from green H2 generated via electrochemical...
The interaction of complex solids with intense light pulses can induce novel emergent phenomena far from equilibrium. Prominent examples include optical enhancement of the critical temperature in certain superconductors and photo-stabilization of metastable states. A common theme in materials featuring exotic out-of-equilibrium behavior is the formation of charge density waves (CDWs), i.e., a...
