Reaction Pathways and Properties of Charged Tantalum and Gold Clusters in the Oxidation of Methane under Multi-Collision Conditions

点击翻译按钮获取中文摘要

The selective oxidation of methane to methanol is one of the dream reactions in chemistry. Despite of the research efforts in the past, its realization on an industrial scale is still not technically feasible. The reaction features three major challenges: the difficulty in the activation of methane, the avoidance of over-oxidation to carbon dioxide, and the complexity of the reaction. One way to deal with these challenges is to develop a comprehensive understanding of the reaction mechanisms. In this regard, ion-molecule reactions have already strongly contributed to such insights; particularly to the activation of the C-H bond of methane. However, as the vast majority of studies have been performed under so-called single-collision conditions, this project aims for the investigation of the reactions under multi-collision conditions, which mimic isothermally controlled reactions. This way, additional effects as stabilization of intermediates or changes in the reactivity by co-adsorption of further molecules are elucidated. Such studies, however, are accompanied by a strong increase in the experimental demands; in particular, if many different reaction steps exist, as e.g. in the case of co-adsorption. For complex reaction pathways, the simultaneous detection of several reaction products is prerequisite and investigations in the past may have been limited by this complication. To overcome this restriction, we have developed an apparatus, which enables the study of complex reaction networks in an efficient way. The temporal evolution of the reaction in the reaction network is modelled by different reaction schemes, which are evaluated by being fitted to the experimental data in order to obtain the reaction pathway. By this methodology, the reactivity of metal cluster and metal cluster oxides with sizes up to several tens of atoms is studied for the overall as well as the partial reactions. As the reactions are elucidated at different temperatures, apparent activation energies are determined, which supply additional insights into the reaction mechanisms. While the microkinetics are immediately determined in this approach, the reactivity of the cluster species will also provide indirect information on their properties, as for example the existence of oxygen radical centers with high spin densities. From the investigation of co-adsorption effects or subsequent reactions further insights are gained. For example, specific changes in electronic structure of the clusters will be reflected by a change in the reaction pathway for subsequent reaction steps. In this project tantalum and gold clusters are studied, as the first ones represent prospective systems for a successful catalytic reaction, while the latter ones are known to exhibit pronounced size dependent reaction behaviors. Furthermore, the studies will also include oxidized clusters and tantalum oxide clusters to address the type of oxygen binding in the reactivity with methane.