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

甲烷选择性氧化制甲醇是化学中理想的反应之一。尽管在过去的研究工作，它的实现在工业规模仍然是不可行的技术。该反应具有三个主要挑战：甲烷活化的困难，避免过度氧化为二氧化碳，以及反应的复杂性。应对这些挑战的一种方法是全面了解反应机制。在这方面，离子-分子反应已经对这种认识做出了很大贡献;特别是对甲烷的C-H键的活化。然而，由于绝大多数研究都是在所谓的单碰撞条件下进行的，因此本项目旨在研究多碰撞条件下的反应，这些条件模拟等温控制反应。通过这种方式，阐明了作为中间体的稳定或通过进一步分子的共吸附的反应性的变化的额外效果。然而，这些研究伴随着实验需求的强烈增加;特别是，如果存在许多不同的反应步骤，例如在共吸附的情况下。对于复杂的反应途径，同时检测几种反应产物是先决条件，过去的研究可能受到这种复杂性的限制。为了克服这一限制，我们已经开发了一种装置，这使得复杂的反应网络的研究在一个有效的方式。在反应网络中的反应的时间演化是由不同的反应方案，这是通过拟合实验数据，以获得反应途径进行评估建模。通过这种方法，金属簇和金属簇氧化物的尺寸高达几十个原子的反应性进行了研究的整体以及部分反应。由于在不同温度下阐明了反应，确定了表观活化能，这为反应机理提供了额外的见解。虽然在这种方法中的微动力学是立即确定的，簇物种的反应性也将提供其属性的间接信息，例如，存在的氧自由基中心与高自旋密度。从共吸附效应或后续反应的调查得到进一步的见解。例如，簇的电子结构的特定变化将通过后续反应步骤的反应途径的变化来反映。在这个项目中，钽和金团簇进行了研究，因为第一个代表了成功的催化反应的前景系统，而后者是已知的表现出明显的尺寸依赖的反应行为。此外，研究还将包括氧化簇和氧化钽簇，以解决与甲烷反应中的氧结合类型。

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.