High-performance single-atom Ni catalyst loaded graphyne for H2O2 green synthesis in aqueous media

Metal-support spin orders: Crucial effect on electrocatalytic oxygen reduction

Magnetic property (e.g. spin order) of support is of great importance in the rational design of heterogeneous catalysts. Herein, we have taken the Ni-supported ferromagnetic (FM) CrBr3 support (Nix/CrBr3) to thoroughly investigate the effect of spin-order on electrocatalytic oxygen reduction reaction (ORR) via spin-polarized density functional theory calculations. Specifically, Ni loading induces anti-FM coupling in Ni-Cr, leading to a transition from FM-to-ferrimagnetic (FIM) properties, while Ni-Ni metallic bonds create a robust FM direct exchange, benefiting the improvement of the phase transition temperature. Interestingly, with the increase in Ni loading, the easy magnetic axis changes from out-of-plane (2D-Heisenberg) to in-plane (2D-XY). The adsorption properties of Nix/CrBr3, involving O2 adsorption energy and configuration, are not governed by the d-band center but strongly correlate with magnetic anisotropy. It is noteworthy that the applied potential and electrolyte acidity triggers spin-order transition phenomena during the ORR and induces the catalytic pathway change from 4e- ORR to 2e- ORR with the excellent onset potential of 0.93 V/reversible hydrogen electrode, comparable to the existing most excellent noble-metal catalysts. Generally, these findings offer new avenues to understand and design heterogeneous catalysts with magnetic support.

MOF-derived Fe/Ni@C marigold-like nanosheets as heterogeneous electro-Fenton cathode for efficient antibiotic oxytetracycline degradation

The widespread occurrence of organic antibiotic pollution in the environment and the associated harmful effects necessitate effective treatment method. Heterogeneous electro-Fenton (hetero-EF) has been regarded as one of the most promising techniques towards organic pollutant removal. However, the preparation of efficient cathode still remains challenging. Herein, a novel metal-organic framework (MOF)-derived Fe/Ni@C marigold-like nanosheets were fabricated successfully for the degradation of oxytetracycline (OTC) by serving as the hetero-EF cathode. The FeNi3@C (Fe/Ni molar ratio of 1:3) based hetero-EF system exhibited 8.2 times faster OTC removal rate than that of anodic oxidation and possessed many advantages such as excellent OTC degradation efficiency (95.4% within 90 min), broad environmental adaptability (satisfactory treatment performance for multiple antibiotics under various actual water matrixes), good stability and reusability, and significant toxicity reduction. The superior hetero-EF catalytic performance was mainly attributed to: 1) porous carbon and Ni existence were both conducive to the in-situ generation of H2O2 from dissolved O2; 2) the synergistic effects of bimetals together with electron transfer from the cathode promoted the regeneration of ≡ FeII/NiII, thereby accelerating the production of reactive oxygen species; 3) the unique nanosheet structure derived from the precursor two-dimensional Fe-Ni MOFs enhanced the accessibility of active sites. This work presented a promising hetero-EF cathode for the electrocatalytic treatment of antibiotic-containing wastewaters.

Rational design of bimetallic MXene solid solution with High-Performance electrocatalytic N2 reduction

Electrocatalytic N₂ reduction reaction (eNRR) was an effective alternative method for green synthesis of NH₃. By combining the first-principal Density functional theory (DFT) calculations and Monte Carlo (MC) simulation, we systematacially investigated 24 types equal-ratio bimetallic MXene solid solution, involving 88 different catalysts. Our focus was on the catalytic performance of these materials in eNRR. The computational result indicate that MoW(3Mo) has high stability, selectivity (93.8 % against the hydrogen evolution reaction (HER)) and activity (U_L = -0.26 V), which is significantly better than that of monometal Mo₂CO₂ and W₂CO₂. This improvement in catalytic properties is attributed to the unique electronic structure (e.g. d-band center, charge) of bimetallic MXene solid solution. In explicit solvent conditions, the microenvironment of hydrogen bond in aqueous liquid thermodynamically promotes the catalytic property for eNRR and reduce the catalytic property of HER side reaction, but the kinetic barrier is also increased due to the effect of the hydrogen-bond microenvironment on proton migration. Overall, the obtained bimetallic MXene solid solution MoW(3Mo) exhibits excellent catalytic performance in eNRR.

Boosting generation of reactive oxygen and chlorine species on TNT photoanode and Ni/graphite fiber cathode towards efficient oxidation of ammonia wastewater

Photoelectrocatalytic (PEC) process combining the merits of photocatalysis and electrocatalysis is considered as a promising ammonia oxidation technology for water treatment. However, some key issues, such as the limited in situ generation of oxidants on photoanode, slow mass transfer problem and generation of nitrate/nitrite by-products hinder the further application of PEC process in the treatment of ammonia pollutant. In this study, the graphite felt (GF) cathodes modified by different transition metals (Ni, Fe, Mn, Co, Cu) were screened by physicochemical and photoelectrochemical characterizations. The results show that the Ni-GF cathode with more Ni⁰ uniformly distributed on the GF surface had the best electrocatalytic activity to generate H₂O₂. The PEC system composed of 10.0 wt% Ni-GF cathode and optimized titania nanotubes (TNTs) photoanode selectively converted about 96.1% ammonia to N₂ within 90 min. Compared with the single TNTs photoanode system, the ammonia oxidation reaction rate constant of the synergistic PEC oxidation system was increased by about two times, which demonstrated the role of the oxidants simultaneously generated on both anode and cathode. The in situ generated reactive oxygen-based oxidants and chlorine-based oxidants interacted together, and ClO• acted a leading role in the ammonia oxidation which were confirmed by quenching and probe experiments. In addition, the contributions of •OH and ClO• were significantly improved in the synergistic PEC oxidation system, compared with the single TNTs photoanode system. Furthermore, the nitrate by-products generated by the ammonia oxidation were further reduced on the Ni-GF cathode. The large amount of active chlorine and active oxygen generated on the electrode diffused into the bulk, effectively overcoming the mass transfer limitation of direct oxidation. Therefore, the developed TNTs photoanode/Ni-GF cathode system can continuously and efficiently convert ammonia to N₂ without the formation of nitrate/nitrite by-products.

The synergetic effect of an aqua ligand and metal site on the performance of single-atom catalysts in H2O2 synthesis: a density functional theory study

Studying the effect of the coordination field on the catalytic property is critical for the rational design of outstanding electrocatalysts for H₂O₂ synthesis. Herein, via density functional theory (DFT) calculations and ab initio molecular dynamic (AIMD) simulations, we built an effective computational framework to identify the synergetic effect of an aqua ligand and metal ion on the 2e^- ORR catalytic performance under gas condition and aqua solvent. Specifically, the screening results of 29 single-atom catalysts (SACs), TM@C₆N₆ (TM = transition metal), indicated that Cu@C₆N₆ features excellent catalytic property with thermal stability, lowest 2e^- ORR overpotential (0.02 V) and high selectivity of 99.99%. Once an aqua ligand binds with the Cu site, the activity is reduced to the overpotential of 0.42 V and the selectivity decreased slightly (99.98%) due to the reduction of the adsorption strength for the reaction intermediates. A combination of geometric structures and electronic properties revealed that such changes are correlated with the charge of the Cu site. Furthermore, based on molecular orbital theory, the essence of the high catalytic property deeply lies in the effect of the moderate electron back donation bond (d_yz & d_xz→) between Cu and O₂. This work will provide a route to better design high-performance SACs for H₂O₂ synthesis effectively.