Towards Dynamical Control over Gas-Surface Reactivity: Light Alkane and Carbon Dioxide Activation at Catalytic Metal Surfares

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In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Ian Harrison of the University of Virginia is using sophisticated laser and molecular beam techniques, coupled with theoretical modeling, to study the reactions of natural gas molecules and carbon dioxide with metal surfaces. For example, one reaction of interest is that between carbon dioxide (CO2) and hydrogen atoms (H), which can be accelerated when CO2 and H first adsorb on a copper surface (in other words, the metal surface is acting as a catalyst). Prof. Harrison's research aims to reveal how the initial structures formed when molecules and/or atoms collide with a metal surface determine the products of the reaction and the rates of the reaction. Understanding the details of molecule-surface reactions may aid the design of next generation catalysts and catalytic processes. In this way, the research may help advance catalytic technologies to more efficiently make products from natural gas, transformations that currently consume approximately 2% of global energy, and to use carbon dioxide as a chemical feedstock in ways that would reduce net greenhouse gas emissions. Further broader impacts of this research stem from the training of graduate and undergraduate students in the design and construction of advanced experimental instrumentation and complex computer modeling. Outreach activities include giving lessons and lab demonstrations at local elementary and middle schools and providing expanded research opportunities to minority and Federal Work Study undergraduate students. The dynamics of activated chemisorption are probed using a heated effusive molecular beam impinging on a single crystal metal surface to measure dissociative/associative sticking probabilities, S(Tg, Ts; angle), as functions of gas & surface temperatures and angle of incidence. New measurements of product state distributions from pulsed laser assisted thermal associative/dissociative desorption (LAAD/LADD) reactions at surfaces will address activated chemisorption beginning from the surface side of the reaction barrier. Interpreted through the principle of detailed balance, the LAAD/LADD measurements define translational energy- resolved thermal sticking probabilities S(Et, T; angle), directly relevant to thermally-driven industrial catalysis. The laser methods provide unique new opportunities to characterize extremely low activated sticking probabilities and to even discriminate between chemically- specific reaction pathways such as C-C or C-H bond cleavage in ethane dissociative chemisorption. The experimental measurements, coupled with dynamically-biased microcanonical theoretical modeling, are being used to test and quantify the control of dynamics over catalytic reactivity, and to relate this dynamical control to specific transition state properties.