Hydrocarbon and ammonia activation by transition metal cations and cluster ions: Spectroscopy and photofragment imaging of reactants and intermediates

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

In this project funded by the Chemical Structure, Dynamics, and Mechanisms program of the Chemistry Division, Professor Ricardo Metz of the University of Massachusetts Amherst is studying how metal ions and metal cluster ions interact with methane and ammonia, and catalyze their conversion to more industrially useful chemicals. Although natural gas is widely used for heating and to generate electricity, it is not typically employed as a transportation fuel. This is because methane (the major component of natural gas) is a gas that cannot be turned into a liquid at room temperature using pressure. Converting methane into a compound that is liquid at room temperature, such as a larger hydrocarbon (e.g., octanes) or methanol, will make it much more useful as a transportation fuel. Professor Metz is also helping to introduce eighth-grade girls to research in STEM fields through the Eureka! program. The broader impacts of this work include potential benefits from improved catalysts as well as the training of graduate and undergraduate students in mass spectrometry and laser spectroscopy and in broader skills such as problem solving, instrument design, data acquisition and analysis and communication of their results. Metal cluster ion-methane/ammonia complexes are produced in a laser ablation source, cooled via supersonic expansion and characterized using vibrational and electronic spectroscopy in a mass spectrometer. Interaction of the methane or ammonia ligand with the metal weakens the carbon (C)-hydrogen (H) or nitrogen (N)-H bonds in the ligand, and this weakening is key to the subsequent catalytic activation of these bonds. The experiments measure bond weakening and polarization by the metal, as they lead to red shifts in vibrational frequencies and enhanced IR absorption. Vibrational spectroscopy also reveals the identity of reaction intermediates and products in cases where multiple isomers could be involved, as in sequential methane activation. By studying different metals and clusters with differing numbers of metal atoms, these studies elucidate the structural and electronic factors that lead to size-dependent reactivity. Complementary velocity map imaging studies study ion dissociation dynamics following electronic excitation, measuring accurate bond strengths and probing the couplings between excited electronic states. The broader impacts of this work include potential benefits from improved catalysts as well as the training of graduate and undergraduate students.