(6lz) Probing Hydrogen activation to solve current energy challenges

PI: Prof. Lars Grabow, University of Houston

Keywords: Catalysis, Hydrogen purification, Gold, Density Functional theory, Infrared Spectroscopy, Metal-Support Interface.

Renewable hydrogen (H₂) is touted to be the replacement for carbon based fuels in an effort to reduce greenhouse gas emissions. The current H₂ production route through steam reforming of natural gas followed by water-gas shift treatment, however, indirectly releases CO₂ and accounts for 3-5% of our global energy use. Moreover, the CO content in the effluent H₂ stream from the water-gas shift reactor must be lowered for H₂ applications that are sensitive to CO poisoning, for instance, fuel cells.

My research focuses on purifying hydrogen through preferential oxidation (PROX) of CO with O₂ in hydrogen-rich streams, which is a more energy efficient solution than the currently used CO methanation approach. As catalytic material I am studying supported gold nanoparticles, which is motivated by Haruta’s discovery of their high CO oxidation activity at low temperature. The toolset of my research encompasses density functional theory, FTIR spectroscopy and kinetic experiments to study CO and H₂ oxidation on model Au/TiO₂ catalysts. Water was shown by our group to enhance CO oxidation rates through O₂ activation to active OOH^* intermediate at the metal-support interface (MSI). Moreover, we also suggested that the MSI is the active site for heterolytic H₂ activation over Au/TiO₂. This conclusion is corroborated by DFT calculations and H-D exchange experiments coupled with FTIR spectroscopy. While water promotes O₂activation, we found it to poison the MSI sites and kinetically hinder the H₂ activation. Thus, water plays two distinct roles in enhances the selectivity for CO oxidation during PROX.

One of the applications of hydrogen produced through the steam reforming of methane is to selectively hydrogenate alkynes in polyolefin feedstock in the plastics industry. The alkyne impurities in the feed must be reduced to prevent deactivation of the downstream polymerization catalyst. Au catalysts have great potential as selective hydrogenation catalysts because they do not over-hydrogenate both the alkynes and the alkene feed into alkanes. The selectivity of Au originates from the weak binding of alkenes to Au, thus preventing further hydrogenation to alkanes. Using my Au/TiO₂ computational model, I have explored the active sites for H₂ activation to hydrogenate alkynes. My DFT model predicted that the alkyne hydrogenation requires negatively charged hydrogens which can be produced on the surface of the gold nanoparticle in the form of gold hydrides. In a particle size study we found that the turnover frequencies (TOFs) were identical for different particle sizes when they were based on surface sites for 1-octyne hydrogenation. Overall, computational predictions and kinetic experiments suggest that hydrides on surface gold sites are active for alkyne hydrogenation. We believe that our detailed insights into the hydrogenation kinetics over supported gold catalysts will contribute to further improve selective hydrogenation catalysis in industrially relevant processes.

In the broadest terms, my work will showcase the importance of studying H₂ activation and the application of novel gold catalysts to engineer novel solutions for the current challenges in hydrogen based industrial processes.

Research Experience: As a graduate research assistant, I have used density functional theory (VASP), FTIR spectroscopy and kinetic experiments to study the application of supported gold catalysts for hydrogen purification, selective hydrogenation of alkynes and natural gas upgrading through selective methane oxidation. I have had the opportunity to perform experiments for a year at our collaborator, Prof. Bert Chandler’s lab at Trinity University in San Antonio. I have discovered a novel H₂ activation pathway on Au/TiO₂ and came up with an explanation for enhanced PROX selectivity upon adding water to the feed stream. During my M.Tech. research, I used DFT (Quantum ESPRESSO) to study the effects of doping foreign metals in cerium oxide and the application of Ru-substituted CeO₂ for CO₂ methanation.

Areas of Interest for Postdoc: As a postdoc, I want to learn and apply new theoretical methods and experimental techniques to improve catalysts and materials in clean energy applications. I want to explore the area of machine learning for new material discovery and I want to gain experience in the application of tight binding method for large scale systems. I want to gain hands on experience in synthesizing catalysts and would love to learn new techniques such as mass and NMR spectroscopy. In addition to diversifying my technical skill set, I would love any opportunities to collaborate.

Selected Publications:

1.) Whittaker, T.;† Kumar, K. B. S.;† Peterson, C.; Pollock, M. N.; Grabow, L. C.; Chandler, B. D. “H₂ Oxidation over Supported Au Nanoparticle Catalysts: Evidence for Heterolytic H₂ Activation at the Metal-Support Interface”, J. Am. Chem. Soc, 2018, 140, 16469-16487. († : equal contribution)

2.) Kumar, K. B. S.; Deshpande, P. A. “On identification of labile oxygen in ceria-based solid solutions: Which oxygen leaves the lattice?”, J. Phys. Chem. C, 2015, 119, 8692–8702.

3.) Bruno, J. E.; Kumar, K.B. S.; Dwarica, N. S.; Hüther, A.; Chen, Z.; Guzman IV, C. S.; Hand, E. R.; Moore, W. C.; Rioux, R. M.; Grabow, L. C.; Chandler, B. D. “On the Limited Role of Electronic Support Effects in Selective Alkyne Hydrogenation: A Kinetic Study of Au/MO_x Catalysts Prepared from Oleylamine-Capped Colloidal Nanoparticles”, ChemCatChem, 2019, 11, 1650-1664 (featured in "A Decade of Catalysis brought to you by ChemPubSoc Europe".