It is highly desirable to design efficient and stable hydrogen evolution reaction (HER) and oxygen evolution/reduction reaction (OER/ORR) electrocatalysts for the development of renewable energy technologies. Herein, density functional theory (DFT) calculations were conducted to systematically investigate a series of TMN_xO_4−x-HTT (TM = Fe, Co, Ni, Ru, Rh, Pd, Ir and Pt; HTT = hexahydroxy tetraazanaphthotetraphene) analogs of two-dimensional (2D) conductive metal-organic frameworks (MOFs) as potential electrocatalysts for the HER, OER and ORR. The thermodynamic and electrochemical stability simulations suggest that these designed catalysts are stable. Remarkably, CoO₄-HTT, RhN₃O₁-HTT and IrN₃O₁-HTT are predicted to be the most promising catalysts for the HER, OER and ORR, respectively, surpassing the catalytic activity of corresponding benchmark catalysts. The volcano plots were established based on the scaling relationship of adsorption Gibbs free energy of intermediates. The results reveal that regulating combinations of metal active centers and local coordination environments could effectively balance the interaction strength between intermediates and catalysts, thus achieving optimal catalytic activity. Our findings not only opt for the promising HER/OER/ORR electrocatalysts but also guide the design of efficient electrocatalysts based on 2D MOFs materials.

Developing stable and efficient catalysts for the electroreduction of nitrogen remains a huge challenge and single atom catalysts (SACs) are expected to achieve relatively high ammonia selectivity at low applied potential. Based on density functional theory calculations, the potential application of 27 single transition metal (TM = Sc–Zn, Y–Ag, Hf–Au) atoms supported by N(O)-dual-doped graphene (TM-O₂N₂/G) for the electroreduction of nitrogen is intensively investigated. At low nitrogen coverage, W(Mo, Nb, Ta)-O₂N₂/G are predicted to yield low ammonia selectivity (< 13%) at limiting-potential of −0.58, −0.53, −0.56, and −0.76 V starting from adsorbed nitrogen with side-on mode, respectively. With the increasing N₂ coverage, the TM-O₂N₂/G is reconstructed as TM-(N₂)₂N₂/graphene. The electroreduction of nitrogen proceeds from end-on adsorbed nitrogen molecule with high ammonia selectivity, and the limiting-potentials are theoretically predicted as −0.20, −0.40, −0.29, and −0.21 V on W(Mo, Nb, Ta)-(N₂)₂N₂/G, respectively. It is suggested that utilizing the reorganization of local coordination environments of SACs by high coverage of reactant molecules under reaction condition can not only enhance the activity at lower limiting-potential but also improve the ammonia selectivity.

The electrochemical oxygen evolution reaction (OER) is a half-reaction of water-splitting for hydrogen generation, yet suffers from its sluggish kinetics and large overpotential. It is highly desirable to develop efficient and stable OER electrocatalysts for the advancement of water-splitting. Herein, by means of density functional theory (DFT) calculations, we systematically investigated a series of two-dimensional (2D) dual-atom catalysts (DACs) on a novel synthesized covalent organic framework (COF) material as potential efficient catalysts toward the OER. The designed 6 homonuclear (2TM-COF) and 15 heteronuclear (TM₁TM₂-COF) DACs all exhibit good stability. There is a strong scaling relationship between the adsorption Gibbs free energies of HO* and HOO* intermediates, and the OER overpotential (η^OER) volcano curve can be plotted as a function of ΔG_O* − ΔG_HO*. RhIr-COF shows the best OER catalytic activity with a η^OER value of 0.29 V, followed by CoNi-COF (0.33 V), RuRh-COF (0.34 V), and NiIr-COF (0.37 V). These four OER DACs exhibit lower onset potential and higher current density than that of the IrO₂(110) benchmark catalyst. Aided by the descriptor identification study, the Bader charge that correlated with the Pauling electronegativity of the embedded dual-metal atoms was found to be the most important factor governing the catalytic activity of the OER. Our work highlights a potentially efficient class of 2D COF-based DACs toward the OER.

Modulating the local coordination structure of metal single-atom catalysts (SACs) is extensively employed to tune the catalytic activity, but rarely involved in regulating the reaction pathway which fundamentally determines the product selectivity. Herein, we report that the product selectivity of electrochemical CO₂ reduction (CO₂RR) on the single-atom indium-N_xC_4−x (1 ≤ x ≤ 4) catalysts could be tuned from formate to CO by varying the carbon and nitrogen occupations in the first coordination sphere. Surprisingly, the optimal In SAC showed great promise for CO production with the maximum Faradic efficiency of 97%, greatly different from the reported In-based catalysts where the formate is the dominant product. Combined experimental verifications and theoretical simulations reveal that the selectivity switch from formate to CO on In SACs originates from active sites shift from indium center to the indium-adjacent carbon atom, where the indium site favors formate formation and the indium-adjacent carbon site prefers the CO pathway. The present work suggests the active sites in metal SACs may shift from the widely accepted metal center to surrounding carbon atoms, thereby offering a new implication to revisit the active sites for metal SACs.

Photocatalytic aerobic oxidation by using oxygen molecules (O₂) as green and low-cost oxidants is of great attraction, where the introduction of irradiation has been proved as an efficient strategy to lower reaction temperature as well as promote catalytic performance. Moreover, the oxygen vacancies (OVs) of catalyst are highly active sites to adsorb and activate O₂ during photocatalytic aerobic oxidation. However, OVs are easily blocked by oxygen atoms from active oxygen species during the catalytic process, leading to the deactivation of catalysis. Herein, a promising catalyst toward photocatalytic aerobic oxidation was successfully developed by recovering the OVs through doping Au atoms into Ti₃C₂T_x MXene (Au/Ti₃C₂T_x). Impressively, Au/Ti₃C₂T_x exhibited remarkable activity under full-spectrum irradiation towards photooxidation of methyl phenyl sulfide (MPS) and methylene blue (MB), attaining a conversion of >90% at room temperature. Moreover, Au/Ti ₃C₂T_x also manifested remarkable stability by maintaining >95% initial activity after 10 successive reaction rounds. Further mechanistic studies indicated that the OVs of Au/Ti ₃C₂T_x served as the active centers to efficiently adsorb and activate O₂. More importantly, the doped Au atoms of Au/Ti₃C₂T_x were conducive to the recovery of OVs during photocatalytic process from the results of theoretical and experimental aspects. The recovered OVs of Au/Ti₃C₂T_x continuously and efficiently activated O₂, directly contributing to the remarkable catalytic activity and stability.

The electrochemical reduction of nitrogen to ammonia is a promising way to produce ammonia at mild condition. The design and preparation of an efficient catalyst with high ammonia selectivity is critical for the real applications. In this work, a series of transition metal (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, and Cd) atoms supported by gt-C₃N₄ (TM/gt-C₃N₄) are investigated as electrocatalysts for the nitrogen reduction reaction (NRR) based on density functional calculations. It is found that Mo/gt-C₃N₄ with a limiting potential of -0.82 V is the best catalyst for standing-on adsorbed N₂ cases. While for lying-on adsorbed N₂ cases, V/gt-C₃N₄ with a limiting potential of -0.79 V is better than other materials. It is believed that this work provides several promising candidates for the non-noble metal electrocatalysts for NRR at mild condition.