Jiao, YanBai, XiaowanTang, Cheng (Tsinghua University)Chen, Ling2024-05-032024-05-032023https://hdl.handle.net/2440/140684The continuous rise of CO2 concentrations in atmosphere leads to severe environmental problems such as global warming and ocean acidification. Natural processes cannot quickly reverse this trend. Therefore, converting atmospheric CO2 to value-added products is beneficial for reducing our carbon footprint and addressing the environmental crisis. Among carbon capture and utilization strategies, the electrocatalytic carbon dioxide (CO2) reduction reaction (ECRR) is one of the most promising solutions due to its efficiency under mild operating conditions. An example is the use of ECRR technology to produce single-carbon (C1) products like CO and CH4, which has been well established. However, there is growing interest in coupling carbonaceous small molecules to produce multi-carbon (C2+) products like ethylene and ethanol. These products are desired due to their higher energy density and better compatibility with existing infrastructure. In addition, only using CO2 and H2O as reactant limits the scope of potential products. Therefore, adding nitrogen (N) atoms to CO2 electroreduction is promising to achieve the electrocatalytic coupling of carbonaceous and nitrogenous molecules. This creates more valuable organonitrogen compounds like urea, methylamine, and glycine. Nonetheless, many small molecule coupling reactions are still in the infancy stage and face many challenges in activity, selectivity, and stability. To address these challenges, a rational design of catalyst and precise control over reaction pathways based on the understanding of matter at the atomic and electronic-structure level, is indispensable. Computational chemistry, or specifically Kohn-Sham Density Functional Theory (DFT) can significantly help to accelerate this process. DFT employs an empirical ‘first principle’ electronic-structure approach to solve the puzzling many-electron problem, achieving a balance between accuracy and efficiency. When combined with the classical molecular dynamic treatment for the nuclei movement (i.e., Ab Initio Molecular Dynamics), DFT can address many problems existing in the complex microenvironment of reaction interfaces, which are hard to be observed in experiments. Through in-depth studies using DFT, more accurate theoretical guidance and strategic planning can be offered for the development of the next generation of electrocatalysts. Therefore, the thesis aims to provide theoretical insights for electrocatalytic coupling of small molecules by DFT simulation, and accordingly propose various strategies to increase the activity, selectivity and stability, including construction of appropriate surface morphology, doping of noble metals, dispersion of catalytically active sites and surface functionalization. Specifically, Chapter 1 outlines the significance, background, and scope of this thesis. Chapter 2 offers a detailed evaluation of advancements in the electrocatalytic coupling of small molecules, focusing on the coupling among carbonaceous small molecules as well as those between carbonaceous and nitrogenous small molecules. Chapter 3 focuses on the understanding of the optimal catalyst morphologies for C2 production. Five polycrystalline Cu nanopyramids, each with different orientations, shapes, and combinations of exposed facets, were simulated, with the Pyramidal Effect revealed as the main factor leading to an anomalous C−C coupling activity and consequently promoting C2 selectivity. The extended ‘square’ principle was developed as an effective C2 active site screening tool to design CRR electrocatalysts. In Chapter 4, a novel structure, i.e., densely-arrayed Cu nanopyramids (Cu-DAN), was introduced. This unique structure promotes C–C coupling, regulates post-C–C coupling steps and retains both oxygen atoms in an alternative pathway toward ethylene glycol (EG) formation. In Chapter 5, the co-adsorption energy of three *CO intermediates was identified as a descriptor for C3 activity. Based on this, a single-atom alloy catalyst (SAAC) strategy by incorporating Ag into Cu-DAN was developed, leading to a new pathway for the production of long-chain oxygenate compound, 1,2- propanediol. The small molecule coupling of significant interest is not limited to C/H/O-based reactants; it also includes the electrocatalytic coupling of carbonaceous and nitrogenous molecules to synthesize valuable organonitrogen compounds. In Chapter 6, by DFT computations, triple-atom catalyst (TAC) Ni2Zn/C9N4 was proposed and confirmed to enable a concurrent N–C–N coupling mechanism. This is a one-step synthesis of N–C–N bond under ambient condition in which *CO is embedded into the *NO-dimerization resultant H2N*–*NH2. This method enables the direct production of urea from CO2 and NO. Chapter 7 describes a simple method to mitigate the repulsive dipole-dipole interactions, by modifying the original catalyst surface with N-heterocyclic carbene. As the proof-of-concept catalyst, dual Cu atoms doped on the O-vacancy of MXene - MoNO2 (Cu2@v-Mo2NO2) were modelled. Surface carbene functionalization significantly cause the charge to redistribute between the Cu pairs and the reaction intermediates. Such redistribution effectively reduces the repulsive dipole-dipole, leading to a facile C– N coupling step. Chapter 8 presents the conclusions drawn from this thesis regarding the electrocatalytic coupling of small molecules, accompanied by a forward-looking perspective.enelectrocatalytic couplingdensity functional theorysmall moleculesmodelingThesis