Chemical Reactivity of In2O3 Surfaces

Indium oxide is a material that combines optical transparency and electrical conductivity, properties that are important for electrodes in devices such as solar cells or liquid crystal displays. Recently, it was found that Indium oxide is also a catalytic active material, i.e., chemical reactions are more efficient when they take place on indium oxide. Moreover, some of these reactions can be accelerated by illuminating the indium-oxide sample with (the ultraviolet part of) sunlight, which is referred to as photo-catalysis. This is where my research at the TU Wien and this project start: The aim of my proposal is to understand the chemical reactivity of well-defined indium oxide samples. Therefore, the surface of these samples, which is where the reactions actually take place, is studied under controlled conditions (ultra-high vacuum), where the reactions can be investigated step-by- step at the atomic scale. The methods I am using are a combination of state-of-the art surface science techniques and special setups designed to study surface chemistry. The methods are able to look at the surface locally providing images with atomic resolution (scanning tunneling microscope and atomic force microscope), and in combination with area-averaging techniques (photoelectron emission, temperature-programmed desorption) insights into the atomic processes are gained. To approach reaction conditions special high-pressure setups that were developed in our labs are used that can be combined with our surface science methods. The samples I am using are high-quality indium-oxide single crystals, and indium- oxide thin films grown in-house at the TU Wien for complementary investigations. The main topics of my proposal are (1) the photo-catalytic conversion of the greenhouse gas carbon dioxide into carbon monoxide via the water-gas shift reaction, which is basically the first step of photosynthesis, and (2) the conversion of acetylene into ethylene, which is important for industrial polyethylene (plastic) production. Both topics require a series of preliminary experiments and studies, including the interaction of various molecules and gases (water, carbon dioxide, carbon monoxide, acetylene, atomic and molecular hydrogen etc.) with the indium-oxide surface, and surface modifications obtained by electron bombardment and illumination with ultraviolet light. This project is unique due to the combination of indium oxide single crystals and thin films as model system with striking surface science methods to investigate hot topics in surface catalysis.

The project "Reactivity of In2O3 Surfaces" conducted fundamental research in the field of surface physics with the aim of better understanding the adsorption and interaction of individual molecules important in chemical reactions with a specific surface of the material indium oxide (In2O3). The experimental research was based on cutting-edge methods in surface physics in ultra-high vacuum (UHV), focusing on imaging techniques (scanning tunneling microscopy, atomic force microscopy) as well as spectroscopy (photoemission spectroscopy, thermal desorption spectroscopy). Three molecules were investigated: (1) water, (2) carbon dioxide, and (3) methanol. (1) The first step of many chemical reactions involves the transfer of a proton, and the ability of an atom to perform this is described by its proton affinity (PA). Until now, it has only been possible to determine the PA averaged over the entire sample surface, rather than for individual atoms. Using atomic force microscopy, we developed a method that allows us to determine the PA of individual surface atoms. For this purpose, individual protons (e.g., through water adsorption) were adsorbed onto the In2O3 surface. Based on the binding strength of these protons, the PA can be determined. This has allowed us to know exactly which atoms on the In2O3 surface are particularly important for the initiation of de/hydrogenation reactions. (2) In applied catalysis research it has been known for several years that indium oxide is a promising catalyst for the conversion of carbon dioxide (CO2) to methanol, but the exact processes and locations on the surface (which atoms on the surface are involved) are not known. In this project, it was investigated how CO2 interacts with the In2O3 surface, what lattice sites the molecules adsorb onto, and whether they react, i.e., chemically change. The experiments were carried out both in UHV and at elevated pressures (1 mbar) with the addition of hydrogen and carbon monoxide to the gas feed, where the first step of the reaction, the formation of formate (HCOO), was observed. With these studies, the first step was taken towards understanding the CO2 conversion to methanol on the In2O3 catalyst at the atomic level. (3) Methanol is a promising energy carrier, and could be produced through carbon dioxide reduction when using a suitable catalyst. This way, an undesirable greenhouse gas is converted into a desirable chemical compound. As part of this project, the end product of the reaction, methanol, and the adsorption of individual molecules on the In2O3 surface were also investigated. Of particular interest was the competition between water (a contaminant) and methanol, with the encouraging result that methanol displaces water from the surface. In conclusion, this project has contributed to the knowledge of atomic properties and processes relevant to chemical reactivity on oxide surfaces.