Bond activation by iron and iron oxide ions

The project investigates the Bond activation by iron and iron oxide ions. Bond activiation means the conversion of an inert molecule into a reactive species. Many industrial scale chemical transformations depend on these activation processes, which are often initiated by a catalyst, including the formation of ammonia or the chemical processing of natural feedstock. Methane, i.e. natural gas, is so far mostly used as fuel. A catalyst is needed to activate methane. Thus, understanding elementary catalytic reactions is of great interest. The catalysts used today are often based on rare and expensive elemtens like palladium or platinum. These elements belong to the so-called transition metals. An aim in research is to replace those elements by compounds which are based on abundant elements like iron, nickel or copper. Gas phase experiments allow the investigation of intrinsic properites by using model systems which reduce the complexity of reaction under real life conditions. Many experiments have investigated elementary reactions of catalytically active species with various molecules. However, little is known about the dynamics of the reactions. The term dynamics refers to the rearrangement of atoms during a reaction. The project will investigate these rearrangement processes and thus will be able to deduce atomic reaction mechanisms from experimentally obtained information. These new results will complement the existing knowledge and be able to test proposed reaction mechanisms. The project will use crossed beam velocity map imaging to investigate the dynamics. The experiments allow the collision of both reactants under controlled conditions. They will focus on iron and iron oxide cations as acitve transition metal species. Methane and molecular hydrogen will be brought into reaction with both cations. Both ions show a complex reactive behavior with both molecules. The behavior of the total reactivity of methane more closely resembles that of molecular hydrogen than of longer chain hydrocarbon molecules. Investigating the dynamics of reactions with both molecular reactants will allow insight into basic structure reactivity relations. In reactions with the iron ion, the reactivity has been found to depend on the electronic state of the iron ion, i.e. its elctron charge distribution. We want to investigate the effects on the atomic level dynamics and devise general concepts. The direct formation of methanol from methane is a hot topic in catalysis. The investigation of the model system iron oxide has gained a lot of attention showing an effect called Two-State reactivity. This concept involves the switching of spin states during a reaction which is uncommom in synthetic chemistry but has been postulated for the reaction of the iron oxide cation with both methane and molecular hydrogen. We will investigate how this phenomenon influences the dynamics.

In large scale industrial application, inert molecules such as nitrogen, carbon dioxide or methane are brought to reaction by use of catalysts. Common examples are the formation of ammonia or the processing of methane to value-added chemicals. Therefore, the understanding of the first step in the value-added chain, i.e. the activation of the inert chemical bond, is a focus of research. The use of model systems is a common approach as the real-life catalytic system is generally complex. In a series of experiments the model system is brought closer and closer to the real reactive system. Gas phase experiments offer one possible approach as they allow us to investigate the intrinsic reactive center. Numerous gas phase experiments targeted the reactivity of catalytically active metals with as many molecules. However, few directly investigated the dynamics of these reactions, i.e. how atoms themselves rearrange during the reactive encounter and how energy is distributed between products. An experimental set-up for the investigation of reactive collisions between small molecules and metal ions has been optimized. We a technique termed crossed beam velocity map imaging. The molecule and ion collide under controlled conditions and the velocity of the product ion is measured. The final distribution of product ion velocities allows us to deduce the atomic-level processes during the reactive encounter itself. Tantalum ions have been crossed with methane and carbon dioxide. Tantalum belongs to the so-called 5d elements as does platinum. It reacts efficiently with methane in its electronic ground state even though there is an energy barrier hindering the reaction. The ground state couples efficiently to another state and thereby circumvents the barrier. This phenomenon is termed two-state reactivity. In the experiment, we look for signatures for these different paths to product formation. In the course of the project, the experimental set-up and procedures have been optimized. Better focusing and diagnostics for the ion beam as well as a new molecular beam source for methane and carbon dioxide have significantly increased the count rate for reactive scattering. We recorded first reactive scattering of product ions for reactions with methane and carbon dioxide. Now, collision energy dependent experiments looking for electronic state effects can be conducted. Experiments will now be extended to iron for which the activation of methane is not happening at room temperature. The aim is to contribute to the derivation of structure reactivity relations and support the design the design of new catalytic materials.