Directing Selectivity of Carbon Dioxide Reduction Reaction via Heteroatom Doping

Abstract

The electrochemical CO2 reduction reaction (CRR) presents a promising route to convert CO2 into valuable chemicals and fuels from renewable energy sources. However, this process is complex in nature with multiple reaction pathways and numerous possible intermediates. Due to various C1-C3 products being produced during CO2 electroreduction, directing selectivity to a target product remains a challenging task. Heteroatom doping in catalytic materials provides an achievable method to modify reaction energetics and control selectivity distributions for the CRR. Despite the research efforts in selective CO2 reduction, greater understanding is needed regarding the catalytic mechanisms and key reaction intermediates at the atomic level. Therefore, this Thesis aims to develop some heteroatom doped catalytic materials for directing the CRR selectivity to certain products by using density functional theory (DFT) calculations. In this Thesis, the recent research progress of CRR pathway selection is firstly summarized by identifying the important role of key intermediates in directing selectivity to target products (Chapter 2). This section points out that the development of selective CRR electrocatalysts relies on optimizing reaction energetics of critical elementary steps in the preferred pathway to a specific product. The first aspect of this Thesis focuses on graphitic carbon nitride/doped graphene with and without heteroatom doping in graphene substrate (C3N4/XG) for CO2 reduction by using computational methods (Chapter 3). It is demonstrated that a higher catalytic activity originates from increased interfacial electron transfer among different doping cases. A low overpotential is estimated from a volcano-type CRR activity trend for the selective production of methane on C3N4/XG, indicating the applicability of heteroatom doping to achieve improved CRR activity and selectivity. The second aspect of this Thesis is about copper-based catalytic materials for electrochemical CO2 reduction (Chapters 4 - 6). Copper has shown its unique ability to generate energy dense products during CRR. However, the moderate adsorption strength of key reaction intermediates on Cu lead to its poor CRR selectivity. Therefore, engineering Cu-based catalysts with optimized reaction energetics is important to address the selectivity issue. In the first part (Chapters 4 and 5), Cu-based alloys (M@Cu) are modelled to explore their CRR selectivity trends by using DFT calculations. The heteroatom doping of secondary metals in Cu substrate provides a feasible strategy to tailor multiple active sites and mediate adsorption energies. The different hydrogen and oxygen affinities of the secondary metals in the M@Cu catalysts are found to be effective descriptors in determining CRR selectivity. Furthermore, the reaction kinetics of C-C coupling steps are evaluated on some Cu alloy surfaces by using the Nudged Elastic Band (NEB) method. We discovered that the OC-COH coupling is kinetically more favorable than the OC-CHO coupling to direct CRR selectivity toward multi-carbon products. The linear energy relations for C-C coupling and its reverse dissociation reactions on Cu-based alloy catalysts provides a route to estimate kinetic barriers from reaction energies. In the second part (Chapter 6), heteroatom doping of non-metal elements on Cu surfaces (Cu-X) is applied to modulate C2 products (including ethylene, ethane and ethanol) selectivity during CRR. The thermodynamically derived selectivity amongst competing reaction pathways is demonstrated through the evaluation of adsorption energetics of key post-C2-coupling intermediates on Cu-X model catalysts. The oxygen affinities of the dopant atom site and the Cu site on Cu-X catalysts can serve as useful descriptors for C2 product selectivity. The electron transfer through Bader charge analyses and electronegativity analyses of nonmetal dopant atoms are identified as the underlying electronic properties that impact selectivity through oxygen affinity. From these computational studies, we demonstrated heteroatom doping to electrocatalysts can affect reaction energetics and direct the CRR selectivity to a target product.