Characterizing Potential Energy Surfaces for Hydrocarbon Activation by Transition Metal, Metal Cluster and Metal Oxide Cations via Spectroscopy and Photofragment Imaging

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Through this award, funded by the Chemical Structure, Dynamics, and Mechanisms Program-A of the Division of Chemistry, Prof. Ricardo B. Metz from the University of Massachusetts Amherst, will characterize the structure and bonding of the molecular reactants, products and, especially, reaction intermediates of the reactions of metal, metal cluster and metal oxide cations with hydrocarbons using vibrational and electronic spectroscopy in a dual time-of-flight mass spectrometer. Systems of interest include ions that convert methane to larger hydrocarbons, methane to methanol, and ethylene to benzene. These systems will be studied for several metals and as a function of cluster size, to measure periodic trends in the structure and bonding of intermediates and to correlate them to the measured size-dependent reactivities. Photofragment imaging will also be used to study ions' photodissociation dynamics following electronic excitation, measuring kinetic energy release and fragment anisotropy. This is a detailed probe of excited state dynamics and the couplings between excited electronic states and will also provide accurate bond strengths. These "half-collision" experiments complement full-collision reaction studies by directly probing the potential energy surface for the reaction and by initiating the reaction from an intermediate with well-defined energy, geometry and angular momentum.The U.S. has large production and extensive reserves of natural gas, and it is widely used for heating and to generate electricity. However, it is not typically employed as a transportation fuel. This is because methane (natural gas is 70-90% methane) is a gas that cannot be turned into a liquid at room temperature. To use methane as a transportation fuel, it needs to be converted into a liquid. Catalysts are used to convert one chemical into another, more desirable chemical; new catalysts are needed to directly, selectively and efficiently convert methane into useful liquids such as methanol. Understanding the complex reactions involved in industrial catalysis, and improving catalysts, requires combining fundamental studies, calculations, and studies of active catalysts. Results from this study will contribute to this effort by expanding our fundamental understanding of the detailed ways in which metal atoms and metal clusters react with hydrocarbons by using spectroscopy to reveal the structure and bonding in the intermediates these reactions pass through on the way from reactants to products.