In-situ optical spectroscopy of a single metal nanoparticle in catalytic action
05 / 2007 - 05 / 2012
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)
Catalyst scientists would like to take ?motion pictures? with a camera inside a reactor when heterogeneous catalysts are in action, giving them detailed insight in their working mechanism. On this basis, they want to improve existing catalyst formulations or design completely new ones. Such rational catalyst design approach is in most cases still a dream since the experimental tools currently available for making such ?motion pictures? are unfortunately rather rudimentary. The goal of this research project is to use a ?nano-eye? in the form of a nanometric light source confined at the end of an optical fiber tip. By moving the ?nano-eye? over the catalyst surface ?motion pictures? of a single metal nanoparticle in catalytic action can be taken. More specifically, the project aims to explore the possibilities and limitations of scanning near-field optical microscopy (SNOM) to obtain in-situ fluorescence and Raman spectra of catalytic processes under true reaction conditions. For this purpose, a special in-situ reactor has to be developed. This spectroscopic-reactor cell allows obtaining spatial and time resolved chemical information on the processes taking place during various selective hydrogenation reactions at elevated temperatures. The methodology will be applied to platinum, palladium and silver nanoparticles of about 50 nm in size and both gas-phase and liquid-phase hydrogenation reactions will be studied. The in-situ SNOM method in combination with fluorescence spectroscopy allows to measure in a temporal manner the hydrogenation of a fluorescent olefin and the related intensity bursts in the corresponding emission spectra can directly be translated in turnover frequencies of a single metal nanoparticle. The influence of the metal type, the size of the nanoparticle as well as the presence of a promotor element will be investigated. It is important to note here that at present average turnover frequencies of large ensembles of metal nanoparticles are obtained since the classical in-situ spectroscopic techniques lack the spatial resolution to distinguish between active nano-domains. In other words, new information on the hydrogenation activity of an isolated metal nanoparticle will be obtained. In a second part of the project we will focus on the development of the in-situ SNOM technique in combination with surface enhanced Raman scattering (SERS). SERS effects will result in significant signal-to-noise improvements allowing to measure vibrational spectra of reagent, potential reaction intermediates and reaction products in the vicinity of the supported single metal nanoparticle. If appropriate we will try to further enhance the signal intensity by making use of resonance Raman effects. In addition to its high sensitivity, SERS provides an information-rich vibrational spectrum making it an ideal tool for characterizing reactive intermediates involved in heterogeneous catalyzed reactions. This all will hopefully lead to new or additional mechanistic insights in two sets of catalytic reactions; i.e. (a) the selective hydrogenation of α,β-unsaturated aldehydes, such as acrolein and crotonaldehyde and (b) the enantioselective hydrogenation of α-ketoesters in the presence of a chiral modifier, such as cinchona alkaloids. We expect to study the interaction of both the substrate molecules as well as the chiral modifier with the metal surface in great detail in the absence and presence of hydrogen and at elevated temperatures. It is clear that the envisaged research project is very challenging in the field of in-situ spectroscopy of catalytic solids and will be both inspiring and demanding for the 2 PhD students involved. Furthermore, it fits well within the CW focus area ?Chemistry in relation with Technology/Sustainability? since it provides fundamental insight in the working principle of catalytic materials, which are of paramount importance to make in the future a sustainable society reality.