| Can we get a complete understanding of the working mechanisms of catalysts, at the highest detail possible? Is it possible to study catalysis with submolecular resolution, and "see" how single molecules behave when they are involved in a chemical reaction? Will it become possible to gain control over these chemical reactions at the single molecule level? To answer these intriguing questions is the subject of this ECHO proposal, which is aimed at developing a new line of research at the interface of chemistry and physics. It involves the study of catalysis at the single molecule level with Scanning Tunneling Microscopy (STM) at the chemically relevant interface of a solid and a liquid. As a collaboration between the physics and chemistry departments at the Institute for Molecules and Materials (IMM) we started 5 years ago to design and build scanning tunneling microscopes that can operate at a solid-liquid interface. After succeeding in imaging different kinds of organic molecules at such an interface with submolecular resolution, we began studying dynamic chemical processes, and in particular chemical reactions. It was reasoned that the use of liquid STM as a new analytical tool would provide unique and fundamentally new insights into reaction mechanisms, because it allows the study of single molecules during a chemical reaction, instead of an ensemble of millions of molecules of which the behaviour is averaged when traditional macroscopic analysis techniques such as NMR, UV-vis, etc. are used. As a proof of principle we have recently investigated a catalytic epoxidation reaction, which had already been extensively studied in the bulk in our organic chemistry laboratory, at the single catalyst level in a home-built liquid-cell STM. A monolayer of flat manganese porphyrin catalysts was immobilized at the interface of a Au(111) surface and a n-tetradecane liquid and imaged with STM. The gold surface appeared to activate the manganese porphyrins to react with molecular oxygen, and the resulting oxidized catalysts were capable of converting alkenes, which were added to the liquid-cell, into epoxides. Because the turnover rate of the reaction is relatively slow, its course could be imaged in real-time and -space by monitoring changes in oxidation state of the adsorbed catalysts. An intriguing observation that was made was the fact that each reaction of the catalytic monolayer with a molecule of molecular oxygen yielded two identical, adjacent Mn(IV)=O porphyrin species at the surface. So far, STM has been the only technique that can unambiguously reveal such a detailed mechanistic aspect of a chemical reaction. In order to get a complete understanding of Catalysis at the Nanoscale, it is the aim of this proposal to take the study of single molecule epoxidation catalysis studies to the next level and image not only changes in oxidation state of the catalysts during the reaction, but also the alkene reactant that approaches the catalyst and the epoxide product that is being formed. This is not a trivial goal to achieve, because molecules that are not adsorbed at a solid-liquid interface generally behave very dynamically and hence cannot be easily imaged within the scanning time scale of the STM. For this reason we want to use alkenes with large substituents that provide the reactants and products with slower dynamics, or to use alkenes that adsorb to the catalytic surface. Furthermore, we want to obtain a complete characterization of the heterogenic catalytic surface and try to answer the following questions: (i) What role does the solid-liquid interface play on the organization of the adsorbed catalysts, and, as a result, on their catalytic performance? (ii) Is the interaction between the interface and the porphyrin catalysts just governed by physisorption, or is there a degree of chemisorption involved in the form of a bond between the porphyrin metal core and the metal surface? (iii) And finally, is the ordering and mutual distance of the porphyrin catalysts on the surface directly related to their propensity to homolytically split molecular oxygen and distribute the oxygen atoms over adjacent catalysts? The proposed research is highly interdisciplinary, involving synthetic organic chemistry, heterogenous catalysis, and scanning probe microscopy, and it will be centered at the NanoLab Nijmegen as a collaboration between the Scanning Probe Microscopy group (Speller) and Physical Organic and Supramolecular Chemistry group (Nolte). |
