Unraveling the Catalytic Performance of the Nonprecious Metal Single-Atom-Embedded Graphitic s-Triazine-Based C3N4 for CO2 Hydrogenation

Importance of local coordination microenvironment in regulating CO2 electroreduction catalyzed by Cr-corrole-based single-atom catalysts

Single-atom catalysts (SACs) with MN4 active sites are a promising type of electrocatalyst for CO2 reduction reactions (CO2RR). Here, we designed a novel corrole-based CO2RR single-atom catalyst Cr-N4-Cz with a metal center supported by conjugated N4-macrocyclic ligand of corrole, which can serve as an excellent model for regulating the active center microenvironment, thereby achieving the goal of regulating the catalytic activity and selectivity. Density functional theory (DFT) calculations are performed to investigate the stability of Cr-N3X-Cz (X = N, C, O, S, P) and the mechanism of local coordination microenvironment regulating catalytic selectivity. The calculation results show that Cr-N4-Cz demonstrates high electrocatalytic activity for CO2RR with a limiting potential of -0.25 V, and the main product is CO. However, the selectivity of CO2RR is compromised due to the low limiting potential (-0.28 V) of the competitive hydrogen evolution reaction (HER). By substituting one N atom of Cr-N4-Cz with C, O, S and P, the corresponding main products become HCOOH, CO, CO, and CH3OH (or CH4). Moreover, the competing HER reaction is suppressed, thus remarkably increasing the selectivity of electrocatalytic CO2RR. Further mechanism investigation reveals different atomic substitution alters local coordination microenvironment of Cr metal center, resulting in the rising of d-orbital center and stabilizing the key intermediates of the potential determining step (PDS) by enhancing the integrated crystal orbital Hamilton population (ICOHP) between Cr and adsorbed intermediates, thereby regulating the CO2RR process. Especially, P substitution improves charge transfer, thus facilitating hydrogenation CO2 to form CH3OH (or CH4) in CO2RR.

Enhanced conversion of CO2 into C2H4 on single atom Cu-anchored graphitic carbon nitride: Synergistic diatomic active sites interaction

Single atom metal-nitrogen-carbon materials have emerged as remarkably potent catalysts, demonstrating unprecedented potential for the photo-driven reduction of CO2. Herein, a unique Cu@g-C3N5 catalyst obtained by cooperation of single atom Cu and nitrogen-rich g-C3N5 is proposed. The particular CuN diatomic active sites (DAS) in Cu@g-C3N5 contribute to the formation of highly stable CuOCN adsorption, a key configuration for CO2 activation and CC coupling. The synergistic diatomic active sites interaction is found responsible for the efficient photoreduction of CO2 to C2H4 which has been demonstrated in our Gibbs free energy calculation and COHP analysis. The CO2 activation mechanism was studied, the charge density difference and DOS analysis show that the low oxidation state Cu atom significantly affects the electronic structure of g-C3N5 and then enhance the catalytic activity of CO2 hydrogenation.

2D-Mn2C12: An Optimal Electrocatalyst with Nonbonding Multiple Single Centers for CO2-to-CH4 Conversion

The CO2 reduction reaction (CO2RR) is a promising method that can both mitigate the greenhouse effect and generate valuable chemicals. The 2D-M2C12 with high-density transition metal single atoms is a potential catalyst for various catalytic reactions. Using an effective strategy, we screened 1s-Mn2C12 as the most promising electrocatalyst for the CO2RR in the newly reported 2D-M2C12 family. A low applied potential of -0.17 V was reported for the CO2-to-CH4 conversion. The relative weak adsorption of H atom and H2O in the potential range of -0.2 to -0.8 V, ensures the preferential adsorption of CO2 and the following production of CH4. The different loading amounts of Mn atoms on γ-graphyne (GY) were also investigated. The Mn atoms prefer doping in the nonadjacent triangular pores instead of the adjacent ones due to the repulsive forces between d-orbitals when the Mn loading is less than 32.3 wt % (5Mn). As the Mn concentration further increases, adjacent Mn atoms begin to appear, and the Mn@GY becomes metallic or half-metallic. The presence of four adjacent Mn atoms increases the d-band center of Mn@GY, particularly the dz2 center involved in CO2 adsorption, thereby enhancing the adsorption capacity for CO2. These findings indicate that 1s-Mn2C12 with high Mn atomic loadings is an excellent CO2RR electrocatalyst, and it provides new insights for designing efficient CO2RR electrocatalyst.

Review of Theoretical and Computational Studies of Bulk and Single Atom Catalysts for H2 S Catalytic Conversion

Catalytic conversion of hydrogen sulfide (H2 S) plays a vital role in environmental protection and safety production. In this review, recent theoretical advances for catalytic conversion of H2 S are systemically summarized. Firstly, different mechanisms of catalytic conversion of H2 S are elucidated. Secondly, theoretical studies of catalytic conversion of H2 S on surfaces of metals, metal compounds, and single-atom catalysts (SACs) are systematically reviewed. In the meantime, various strategies which have been adopted to improve the catalytic performance of catalysts in the catalytic conversion of H2 S are also reviewed, mainly including facet morphology control, doped heteroatoms, metal deposition, and defective engineering. Finally, new directions of catalytic conversion of H2 S are proposed and potential strategies to further promote conversion of H2 S are also suggested: including SACs, double atom catalysts (DACs), single cluster catalysts (SCCs), frustrated Lewis pairs (FLPs), etc. The present comprehensive review can provide an insight for the future development of new catalysts for the catalytic conversion of H2 S.

Non-Noble-Metal-Doped Carbon Nitride Photocatalysts for Water Splitting Screened Out by Empty Defect States and the d-Band Center

A rational design of water-splitting photocatalysts from the perspective of the electronic structure is highly desirable for optimizing catalytic activities. However, the structure-activity relationship is still unclear, which impedes the development of efficient catalysts. Herein, by comparing systematically the overall water-splitting capability of 20 kinds of metallic elements anchored at three sites (including cavity, carbon vacancy, and nitrogen vacancy) of graphitic carbon nitride (g-C3N4) through density functional theory calculations, we uncover that availability of in-gap empty defect states and the d-band center position are paramount parameters to determine activities of g-C3N4 on photocatalytic water splitting. In-gap empty states play a role in accommodating electrons from H2O to facilitate its splitting. A lower d-band center weakens the interaction between reaction intermediates and g-C3N4, thereby promoting O2 desorption. Metals embedded at carbon vacancies are found to be superior to those at cavities and nitrogen vacancies because the former not only provides ample in-gap empty states but also has a lower d-band center. We also discover a rule that, for a reaction in which the bond order between the metal and intermediate enlarges (reduces), its reaction difficulty increases (decreases) with the increasing atomic number for elements in the same period. After screening, we find that non-noble metals Co, Ni, and Ga anchored at carbon vacancies possess catalytic performances comparable to Pd- and Pt-doped systems, with the rate-determining barriers less than 0.55 eV. Our findings may provide useful information for designing effective photocatalysts.

Enhancing CO2 Hydrogenation Using a Heterogeneous Bimetal NiAl-Deposited Metal-Organic Framework NU-1000: Insights from First-Principles Calculations

The hydrogenation of CO2 to high-value-added liquid fuels is crucial for greenhouse gas emission reduction and optimal utilization of carbon resources. Developing supported heterogeneous catalysts is a key strategy in this context, as they offer well-defined active sites for in-depth mechanistic studies and improved catalyst design. Here, we conducted extensive first-principles calculations to systematically explore the reaction mechanisms for CO2 hydrogenation on a heterogeneous bimetal NiAl-deposited metal-organic framework (MOF) NU-1000 and its catalytic performance as atomically dispersed catalysts for CO2 hydrogenation to formic acid (HCOOH), formaldehyde (H2CO), and methanol (CH3OH). The present results reveal that the presence of the NiAl-oxo cluster deposited on NU-1000 efficiently activates H2, and the facile heterolysis of H2 on Ni and adjacent O sites serves as a precursor to the hydrogenation of CO2 into various C1 products HCOOH, H2CO, and CH3OH. Generally, H2 activation is the rate-determining step in the entire CO2 hydrogenation process, the corresponding relatively low free energy barriers range from 14.5 to 15.9 kcal/mol, and the desorption of products on NiAl-deposited NU-1000 is relatively facile. Although the Al atom does not directly participate in the reaction, its presence provides exposed oxygen sites that facilitate the heterolytic cleavage of H2 and the hydrogenation of C1 intermediates, which plays an important role in enhancing the catalytic activity of the Ni site. The present study demonstrates that the catalytic performance of NU-1000 can be finely tuned by depositing heterometal-oxo clusters, and the porous MOF should be an attractive platform for the construction of atomically dispersed catalysts.

Well-Defined Ultrasmall V-NiP2 Nanoparticles Anchored g-C3N4 Nanosheets as Highly Efficient Visible-Light-Driven Photocatalysts for H2 Evolution

Exploring low-cost and highly active, cost-effective cocatalysts is of great significance to improve the hydrogen evolution performance of semiconductor photocatalysts. Herein, a novel ultrasmall V-doped NiP2 nanoparticle, as an efficient cocatalyst, is reported to largely upgrade the photocatalytic hydrogen evolution reaction (HER) of g-C3N4 nanosheets under visible-light irradiation. Experimental results demonstrate that V-NiP2 cocatalyst can enhance the visible-light absorption ability, facilitate the separation of photo-generated electron-hole pairs and boost the transfer ability of electrons of g-C3N4. Moreover, the V-NiP2/g-C3N4 hybrid exhibits prominent photocatalytic HER activity 17 times higher than the pristine g-C3N4 counterpart, even outperforming the 1 wt.% platinum-loaded g-C3N4. This work displays that noble-metal-free V-NiP2 cocatalyst can serve as a promising and efficient alternative to Pt for high-efficiency photocatalytic H2 evolution.