Doctoral Defense in Chemical Engineering - Sri Harsha Pulumati

Doctoral candidate: Sri Harsha Pulumati

Title of thesis: Modelling CO2 hydrogenation reaction on Pt functionalized UiO-67 Metal-Organic Frameworks

Opponents:
Dr. Mårten Ahlquist Division of Theoretical Chemistry and Biology, KTH Royal Institute of Technology, Sweden. Dr. Manuel Ángel Ortuño Maqueda, Junior Scientist in CIQUS at Universidade de Santiago de Compostela, Spain.

Advisor: Egill Skúlason, Professor at the Faculty of Industrial Engineering, Mechanical Engineering and Computer Science at the University of Iceland.

Also in the doctoral committee:
Dr. Hannes Jónsson, Professor at the Faculty of Physical Sciences, University of Iceland, Dr. Ainara Nova at Hylleraas Centre for Quantum Molecular Science, University of Oslo

Chair of Ceremony: Dr. Helmut Neukirchen, Professor at the Faculty of Industrial Engineering, Mechanical Engineering and Computer Science, University of Iceland.

Abstract

Converting CO2 into value-added products via hydrogenation is of interest due to its potential to reduce and reuse excess atmospheric CO2 and deal with global warming. Metal-organic frameworks (MOFs) are of great interest in this conversion process due to their appealing properties, such as high specific surface area, porosity, and a tailorable structure and function. Platinum functionalized Zr-based UiO-67 MOFs are one such MOF that demonstrate high selectivity towards methanol formation in CO2 reduction reactions (CO2RR), outperforming other well-known metal-support catalysts. However, the CO2RR mechanisms in this catalytic system remain unclear and have been a topic of ongoing exploration. This dissertation investigates the catalytic properties and proposes mechanisms of CO2 hydrogenation on UiO-67 MOFs embedded with Platinum nanoparticles. In the first part of this study, density functional theory (DFT) calculations and micro-kinetic model were used along with experimental collaboration at the University in Oslo to investigate the role of missing linker defects on the Zr-nodes of UiO-67 MOF in the CO2 hydrogenation reaction. We found that increased linker defects on the Zr-node also increase methanol and methane formation rates. We also explored the influence of H2O on the CO2 hydrogenation reaction, where we found that dehydrated Zr-nodes show higher methanol and methane formation rates. Interestingly, water promotes methanol desorption and does not change the steady-state reaction rate but significantly inhibits methane formation. This finding suggests that water can increase the reaction selectivity to methanol. These discoveries provide a new perspective on the dynamic role of the Zr-node and the influence of water on the reaction. It was also shown that methanol is formed at the interface between the Pt NPs and defect Zr nodes via formate species attached to the Zr nodes, which is a novel finding and understanding of the mechanistic separation between the formation of methanol and the formation of co-products of CO gas and methane became necessary. These results were published in the Journal of the American Chemical Society in 2020. In the second part of this study, we employ DFT calculations to elucidate the CO2RR mechanism where free energy barriers were calculated between the most important intermediates from CO2 gas to all the products: methanol, methane, and CO gas. We used five different atomistic models in order to understand the activity of individual parts of the whole system. This showed that unique and different combinations of the Zr-clusters and the Pt NP interfaces are necessary for a selective production of CO and methane on one hand and for methanol on the other hand. Our findings, supported by experiments done at the University in Oslo, suggest that the synergistic interaction resulting at the interface between Zr-clusters and Pt nanoparticle´s edges play a crucial role in the reduction reaction to methanol but not to methane nor CO gas, which rather take place at the interface of Zr-clusters and flat (111) surfaces of the Pt NPs. Furthermore, it highlights the significance of understanding both individual components of the catalytic system and their interfaces in enhancing catalytic activity. The results show that smaller Pt NPs form more methanol, whereas larger Pt NPs form more methane and CO gas. Finally, we hope this research will have potential implications for developing more efficient and selective catalysts for CO2 hydrogenation to methanol. These results were published in ACS Catalysis in 2024.

About the candidate

Sri Harsha Pulumati earned his B.Sc. and M.Sc. in Physics from the Central University of Tamil Nadu in India in 2018. During his master's program, his research focused on the theoretical investigation of two-dimensional heterostructures for photocatalytic water splitting. Subsequently, he pursued a Ph.D. study under the supervision of Dr. Egill Skúlason, Professor in Chemical Engineering at the University of Iceland. Sri Harsha was affiliated with the NordForsk-funded project, the Nordic Consortium for CO2 Conversion, throughout his doctoral studies. Within this framework, he collaborated with research groups at the University of Oslo, Norway. His research involved the application of quantum mechanical simulations to explore reaction mechanisms for CO2 reduction reactions on UiO-67 MOFs.