Atomistic Modeling for CO2 Reduction Using Thermal, Plasma, and Electrocatalysis

Abstract

Carbon dioxide emissions are a defining issue of our time, and how we deal with this problem has far-reaching impacts on our climate and future quality of life. Heterogeneous catalyst research offers promising strategies for converting CO2 into useful chemical feedstocks, providing an economic incentive for CO2 conversion. To advance the effort for cost-effective CO2 reduction, catalysts must be highly active and selective while minimizing precious metal use. Here, single-atom catalysis offers multiple advantages over larger nanoparticles. Single-atom catalysts are often highly active and selective due to their single-site nature and their unique electronic properties. The work presented in this dissertation focuses on the capabilities of single-atom catalysts to promote the CO2 reduction reaction by three different energetic driving forces, namely, thermal catalysis, plasma catalysis, and electrocatalysis. These catalyst systems are modeled using density functional theory (DFT) to accurately describe the atomic structure of each catalyst and calculate the thermodynamic and kinetic properties of the reaction mechanism. Thermal catalytic CO2 reduction primarily proceeds by one of two possible reaction pathways, either producing methane via catalytic methanation or carbon monoxide via the reverse water gas shift reaction (rWGSR). Atomically dispersed Rh1 catalysts on TiO2 show high selectivity toward the rWGSR compared with larger Rh nanoclusters. We report DFT calculations and microkinetic simulations that clarify the Rh1 active sites and rWGSR pathway on anatase TiO2(101), as well as the high rWGSR selectivity of Rh1 compared with supported Rh nanoclusters. Predicted turnover frequencies and apparent activation barriers for Rh1 indicate a preferred reaction involving CO2 dissociation assisted by a support oxygen vacancy nearby the Rh1. The single atom catalyst is selective toward CO rather than CH4 because of the weak adsorption of CO, large barrier for C-O bond dissociation, and the lack of nearby metal sites for H2 dissociation, in contrast to Rh nanoclusters, including Rh2 dimers. Low-temperature plasma catalysis offers various synergistic effects for increased activity, yield, or selectivity compared to conventional thermal catalytic approaches. Using DFT, we study single-atom catalyst systems to understand the importance of plasma-induced surface charging on CO2 activation. We analyze six different metals on three different supports to analyze trends across the periodic table. We find that accumulated surface charge on the single atom increases the CO2 adsorption strength and decreases the CO2 dissociation barrier for all studied single-atom/support combinations. Our work demonstrates that surface charging should be considered in strong electric fields because it can have a large effect on molecule chemisorption and bond-breaking on catalytic surfaces. Electrocatalytic reduction of CO2 is a frequently studied strategy to convert CO2 using renewable sources of electricity. Recent work has demonstrated the capability of the molecular catalyst CoPc to convert CO2 into methanol in a single reaction setup. Modifying the CoPc molecule with axial ligands shows increased effectiveness for CO2 adsorption, often the rate-limiting step of the reaction. We investigate the effect of ligand choice on the CoPc binding characteristics for CO2 and the important CO intermediate. CO2 adsorption results agree closely with prior literature measurements, and the results for CO adsorption show a reverse trend with respect to ligand electron donation strength compared to adsorption of CO2. These findings show that a careful choice of ligand must be made that optimizes for strong CO2 adsorption and a moderate CO adsorption in order to optimize for methanol selectivity.