Mechanistic and design insights into CO2 conversion on heterogeneous catalysts

Abstract

Rising atmospheric CO₂ levels from fossil fuel use and industrial activities significantly contribute to climate change, driving the need for sustainable solutions that reduce emissions and valorize carbon. Conversion of CO₂ into fuels and chemicals offers a promising path toward carbon neutrality, yet CO2 is highly stable and often requires catalysts for efficient conversions. Designing next-generation catalysts alludes to a fundamental understanding of reaction mechanisms and structure-activity relationships that control catalytic transformations of CO2. This dissertation explores this problem through the lens of computational catalysis using density functional theory (DFT), kinetic modeling, and molecular dynamics. These techniques were employed to garner insights into the influences of catalysts and solvent environments on the activity and selectivity of CO2 reactions on thermo- and electrocatalytic systems, the two most promising platforms for large-scale CO2 utilization. The theoretical insights complement experimental findings to bring a molecular-level understanding of the CO2 mechanisms and aid rational designs of novel catalysts for more efficient conversions.The first focus is the vapor-phase CO2 hydrogenation to methanol over bifunctional Cu/oxide catalysts. Using Cu/UiO-66 as a model system, we elucidated the active sites and reaction mechanisms to rationalize the promotional effects of oxides on methanol synthesis. Combined DFT and microkinetic simulations yielded good agreements with experiments and suggested that methanol and CO form from different pathways that branch early in the reaction mechanism. The rate-determining step for methanol synthesis was found to be a nucleophilic addition activated by the Lewis acidic oxide support. As such, oxides with increasing Lewis acidity result in lower barriers and enhance rates of methanol synthesis. Overall, the results highlight a design guideline and the significance of bifunctional catalysts in which the cooperation between metal and acid sites strongly governs the sites and reactivity of the complex CO2 hydrogenation and metal-organic frameworks (MOFs) as promising catalytic materials. Next, the dissertation focuses on electrochemical CO2 reduction (CO2RR). We first studied the mechanism of CO2RR to C2+ products on copper electrodes in aqueous solutions using models with explicit solvents and grand-canonical DFT. We examined different hydrogen sources and suggested that most hydrogens originate from solvent water at potentials relevant to CO2RR, or from H* at less cathodic potentials. Solvent water tends to favor O-H bond formations as opposed to C-H bonds being favored by H*, leading to ethylene as the major C2 product. Pathways to acetate in alkaline media were found to happen via heterogeneous or homogeneous routes, depending on applied potentials. Additionally, we explored different C-C coupling steps to form C3 products and suggested that the most relevant route is via the coupling between CO* and surface hydrocarbon precursors of ethylene. Detailed pathways to these products were presented, which helped to understand many experimental observations. Reactivity and selectivity of CO2RR over metal electrodes can be further enhanced by solvent effects, particularly by using polar aprotic co-solvents in mixtures with water. We allied DFT, AIMD, and classical MD to show that different interactions between the co-solvent and water lead to characteristically distinct microenvironments at the metal-liquid interface. These different solvent structures significantly influence the hydrogen evolution reaction (HER), while CO2RR appears to be less affected, thereby giving rise to different selectivity for CO2RR. Mixtures of co-solvent forming strong hydrogen bonds with water are predicted to yield higher CO2RR selectivity than aqueous solution, in agreement with experimental results. Additionally, we proposed that co-solvents that are weak hydrogen-bond acceptors can be used to enhance the rates of CO2 reduction because water forms hydrophilic nanodomains near the metal interface that encapsulate hydrophilic alkali metal cations. This enriches the local concentration of cations and lowers the overpotentials required to adsorb CO2. Overall, the presented insights collectively highlight the critical roles of both surface structure and reaction environment in controlling the catalytic activity of CO₂ conversion. The dissertation advances the fundamental understanding of CO₂ activation on heterogeneous catalysts and provides actionable design principles for improving catalytic performance in thermo- and electrocatalytic systems. This work represents a modest yet critical contribution to the broader global efforts toward CO2 mitigation and a carbon-neutral future.