Reactivity and Structure of heterogeneous catalyst systems

A research project is proposed to investigate the chemical and physical properties of prospective heterogeneous catalysts for methanol dehydrogenation, methanol formation from carbon monoxide or carbon dioxide and hydrogen and methanol steam reforming. These reactions are of importance in advanced fuel cells and novel energy generation concepts. In the proposed project the focus will be on model catalysts studied by modern surface science methods in order to gain a basic understanding of the physico- chemical properties of the model catalyst surfaces. The model catalysts investigated will be a Pd(111) - Zn surface alloy and a Pd(111) surface decorated with thin ZnO layers. These systems will be studied with atomic scale precision using a surface science approach. The reactivity of the surfaces with respect to methanol dehydrogenation and CO and CO 2 hydrogenation will be investigated by reflection absorption infrared spectroscopy (RAIRS), thermal desorption spectroscopy (TDS), temperature programmed reaction spectroscopy (TDR) and kinetic studies under high pressures using an ultra high vacuum chamber connected to an all glass high pressure reaction cell. The structures of the Pd - Zn surface alloy and the ZnO - Pd system will be explored at the atomic level using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), density functional theory (DFT) and by using carbon monoxide as a probe molecule with RAIRS. The combination of the results from the reactivity studies and from the structural investigations will yield a complete picture of the microscopic processes that influence the mechanism of the model reactions. The proposed research aims to explain the kinetics of the reactions under high pressure by establishing a detailed physico- chemical reaction mechanism from basic surface science results. These results will provide the scientific basis for the knowledge - based design of advanced catalyst materials.

In the project `Reactivity and structure of heterogeneous catalyst systems` funded by the FWF we investigated the basic physical and chemical principles behind an important catalytic reaction. Electronic gadgets like mobile phones and laptop computers run today mainly on rechargeable batteries. A new and innovative replacement for the batteries are fuel cells powered with methanol. These devices do not need time to recharge and the capacity is much larger enabling for example continued laptop use for up to 10 hours. However, the fuel cell cannot use the methanol directly. The fuel needs to be converted to hydrogen and carbon dioxide and the hydrogen is then fed to the fuel cell. This can be done by the reaction of methanol with water by using a catalyst to speed up the reaction. A catalyst that can be used for this reaction is copper on zinc oxide. This catalyst system has the disadvantage that it looses both reactivity as well as selectivity over time. Especially the latter is a problem for the fuel cell, because the side product carbon monoxide destroys the fuel cell already at very low concentrations. It was shown previously that a different catalyst based on palladium on zinc oxide has a much better stability over time together with higher reactivity and selectivity. The aim of this project was to understand the physics and chemistry behind the increased stability of the palladium - zinc oxide system. In the literature it was shown that a partial reduction of the zinc oxide leads to the formation of a palladium zinc alloy on the catalyst surface. The properties of this alloy were studied in detail in this project and we were able to show that the surface structure of this alloy plays an important part in the reaction. In fact it might be the main reason for the selectivity of the catalyst. We also looked into the surface of the zinc oxide part of the catalyst and found unusual new zinc oxide structures on a palladium surface. This model system was used to study the reaction of methanol on the catalyst surface under controlled conditions. In this investigation we could show that special point defects in the zinc oxide also play a crucial role in the reaction mechanism in addition to the palladium zinc alloy formation. Based on the results of our research it will be possible in the future to further increase the reactivity and selectivity of this catalyst system. This is an important ingredient to bring methanol powered fuel cells for portable electronics to the market.