Theoretical Study on the Degree of CO2 Activation in CO2-Coordinated Ni(0) Complexes

Controlling the Activation at NiII -CO2 2- Moieties through Lewis Acid Interactions in the Second Coordination Sphere

Nickel complexes with a two-electron reduced CO2 ligand (CO2 2- , "carbonite") are investigated with regard to the influence alkali metal (AM) ions have as Lewis acids on the activation of the CO2 entity. For this purpose complexes with NiII (CO2 )AM (AM=Li, Na, K) moieties were accessed via deprotonation of nickel-formate compounds with (AM)N(i Pr)2 . It was found that not only the nature of the AM ions in vicinity to CO2 affect the activation, but also the number and the ligation of a given AM. To this end the effects of added (AM)N(R)2 , THF, open and closed polyethers as well as cryptands were systematically studied. In 14 cases the products were characterized by X-ray diffraction and correlations with the situation in solution were made. The more the AM ions get detached from the carbonite ligand, the lower is the degree of aggregation. At the same time the extent of CO2 activation is decreased as indicated by the structural and spectroscopic analysis and reactivity studies. Accompanying DFT studies showed that the coordinating AM Lewis acidic fragment withdraws only a small amount of charge from the carbonite moiety, but it also affects the internal charge equilibration between the LtBu Ni and carbonite moieties.

Two-dimensional graphitic metal carbides: structure, stability and electronic properties

Via first-principles computational modeling and calculations, we propose a new class of two-dimensional (2D) atomically thin crystals that contain metal-C3(MC3) moieties periodically distributed in a graphenic lattice, which we refer to as 2D graphitic metal carbides (g-MCs). Most g-MCs are dynamically stable as verified by the calculated phonon spectra. Our detailed chemical bonding analyzes reveal that the high stability of g-MCs can be attributed to a unique bonding feature, which manifests as the carbon-backbone-mediated metal-metal interactions. These analyzes provide new insights for understanding the stability of 2D materials. It is found that the calculated electronic band gaps and magnetic moments (per unit cell) of g-MCs can range from 0 to 1.30 eV and 0 to 4.40μB, respectively. Highly tunable electronic properties imply great potential of 2D g-MCs in various applications. As an example, we show that 2D g-MnC can be an excellent electrocatalyst towards CO2reductive reaction for the formation of formic acid with an exceptionally high loading of Mn atoms (∼43 wt%). We expect this work to simulate new experiments for fabrication and applications of g-MCs.

Structure-activity relationship of Cu-based catalysts for the highly efficient CO2 electrochemical reduction reaction

Electrocatalytic carbon dioxide reduction (CO₂RR) is a relatively feasible method to reduce the atmospheric concentration of CO₂. Although a series of metal-based catalysts have gained interest for CO₂RR, understanding the structure-activity relationship for Cu-based catalysts remains a great challenge. Herein, three Cu-based catalysts with different sizes and compositions (Cu@CNTs, Cu₄@CNTs, and CuNi₃@CNTs) were designed to explore this relationship by density functional theory (DFT). The calculation results show a higher degree of CO₂ molecule activation on CuNi₃@CNTs compared to that on Cu@CNTs and Cu₄@CNTs. The methane (CH₄) molecule is produced on both Cu@CNTs and CuNi₃@CNTs, while carbon monoxide (CO) is synthesized on Cu₄@CNTs. The Cu@CNTs showed higher activity for CH₄ production with a low overpotential value of 0.36 V compared to CuNi₃@CNTs (0.60 V), with *CHO formation considered the potential-determining step (PDS). The overpotential value was only 0.02 V for *CO formation on the Cu₄@CNTs, and *COOH formation was the PDS. The limiting potential difference analysis with the hydrogen evolution reaction (HER) indicated that the Cu@CNTs exhibited the highest selectivity of CH₄ among the three catalysts. Therefore, the sizes and compositions of Cu-based catalysts greatly influence CO₂RR activity and selectivity. This study provides an innovative insight into the theoretical explanation of the origin of the size and composition effects to inform the design of highly efficient electrocatalysts.

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

Utilizing CO2 as a sustainable carbon source to form valuable products requires activating it by active sites on catalyst surfaces. These active sites are usually in or below the nanometer scale. Some metals and metal oxides can catalyze the CO2 transformation reactions. On metal oxide-based catalysts, CO2 transformations are promoted significantly in the presence of surface oxygen vacancies or surface defect sites. Electrons transferable to the neutral CO2 molecule can be enriched on oxygen vacancies, which can also act as CO2 adsorption sites. CO2 activation is also possible without necessarily transferring electrons by tailoring catalytic sites that promote interactions at an appropriate energy level alignment of the catalyst and CO2 molecule. This review discusses CO2 activation on various catalysts, particularly the impacts of various structural factors, such as oxygen vacancies, on CO2 activation.