Towards rational design in electrochemical denitrification by analyzing pH dependence

Insight into pH-dependent synergistic mechanism of the enhanced photoelectro-chlorine conversion of ammonia (NH4+/NH3) to N2 coupled with H2 evolution on Pt-decorated black TiO2 nanotube arrays

Achieving efficient NH4+-N degradation into harmless N2 and simultaneous harvesting hydrogen energy is of great significance for protecting aquatic ecosystems and alleviating energy shortages. Herein, a photoelectrochemical-chlorine (PEC-Cl) system was developed aiming to selective converting ammonia to N2 with simultaneous H2 evolution in a wide pH range of 3-13. The Pt decorated black TiO2 nanotube arrays (Pt/BTNAs) with sufficient oxygen vacancies defects and the enhanced PEC properties were synthesized as the bifunctional electrode. The parameters optimization illustrated high N2 selectivity (88 %) and H2 yield rate (67.5 μmol cm-2 h-1) at pH 9. The ClO• derived from the cross reactions between active chlorine and HO• plays a predominant role in oxidizing ammonia. More importantly, the Gibbs free energy calculation indicated that Pt decorating can also significantly promote the direct electrochemical ammonia oxidation reaction (AOR) on Pt/BTNAs anode. Under alkaline conditions (pH 9-13), the efficient removal and conversion of ammonia into N2 in the PEC-Cl system was achieved primarily through the direct anodic AOR, with N2 selectivity (89 %-93 %) and H2 yield rate (60.9-68.6 μmol cm-2 h-1) comparable to those of the ClO•-mediated indirect AOR. This study presents a flexible method for treating high and medium-concentration NH4+-N and simultaneously producing H2 through the synergistic PEC-Cl and direct electrochemical AOR processes, across a wide pH range.

Toward improved descriptors by refining the complex reaction network in electrocatalysis

Electrocatalysis is one of the key technologies for developing sustainable and fossil resource free routes to produce fuels and chemicals. The limiting potential (UL), defined by the reaction free energy of the most difficult electrochemical step in a given pathway, is an effective descriptor for establishing the activity trend of a set of electrocatalysts, allowing high throughput screening of new catalysts. However, the reaction network of electrocatalytic processes is rather complex, especially for the reactions with necessary thermochemical steps, e.g., the synthesis of valuable C-N bond-containing chemicals. Thermochemical steps cannot be significantly enhanced by electrode potentials, where kinetics is a non-negligible issue at even high overpotentials. This makes it challenge by using limiting potential to accurately describe activity trends for the reactions with necessary thermochemical steps. To this end, we propose an effective scheme to determine an improved descriptor. We suggest refining the rather complex reaction network at first. In particular, it is suggested to decouple electro- and thermochemical steps and exclude the unfavorable pathways with an excessively high thermochemical barrier. Then, a global comparison among the other pathways can be made, to determine the optimal pathway and the improved descriptor (the reaction free energy of the most difficult step of the optimal pathway, defined as ΔGrRPD-limiting). In addition, the studies on reaction kinetics are also suggested to understand the exception of the best catalysts and provide the direction of experimental optimization. This scheme is of a great compromise between practical efficiency and the accuracy toward the rational design of electrocatalysts.

Enrichment of a heterotrophic nitrifying and aerobic denitrifying bacterial consortium: Microbial community succession and nitrogen removal characteristics and mechanisms

This study cultivated a bacterial consortium (S60) from landfill leachate that exhibited effective heterotrophic nitrification and aerobic denitrification (HN-AD) properties. Under aerobic conditions, the removal of NH4+-N reached 100 % when the S60 consortium utilised NH4+-N either as the sole nitrogen source or in combination with NO2--N and NO3--N. Optimal HN-AD performance was achieved with sodium acetate as a carbon source and a pH of 7.0-8.0, dissolved oxygen concentration of 4.0-5.0 mg/L, and a C/N ratio of 10. Furthermore, the presence of functional genes (amoA, hao, napA, nirK, nirS, nosZ), hydroxylamine oxidase, nitrate reductase, and nitrite reductase was confirmed in the S60 consortium. Drawing from these findings, two HN-AD pathways were delineated: NH4+-N → NH2OH → NO2--N → NO3--N → NO2--N → NO → N2O → N2 and NH4+-N → NH2OH → N2O → N2. Metagenomic binning analysis of the S60 consortium uncovered complete pathways for dissimilatory nitrate reduction and denitrification within Halomonas, Zobellella, Stutzerimonas, Marinobacter, and Pannonibacter. These findings offer new insights into the application of HN-AD bacteria and their collaborative nitrogen removal in environments with varying nitrogen sources.

Exploring Nitrogen Reduction Reaction Mechanisms with Graphyne-Confined Single-Atom Catalysts: A Computational Study Incorporating Electrode Potential and pH

This study reconciles discrepancies between practical electrochemical conditions and theoretical density functional theory (DFT) frameworks, evaluating three graphyne-confined single-atom catalysts (Mo-TEB, Mo@GY, and Mo@GDY). Using both constant charge models in vacuum and constant potential models with continuum implicit solvation, we closely mimic real-world electrochemical environments. Our findings highlight the crucial role of explicitly incorporating electrode potential and pH in the constant potential model, providing enhanced insights into the nitrogen reduction reaction (NRR) mechanisms. Notably, the superior NRR performance of Mo-TEB is attributed to the d-band center's proximity to the Fermi level and enhanced magnetic moments at the atomic center. This research advances our understanding of graphyne-confined single-atom catalysts as effective NRR platforms and underscores the significance of the constant potential model for accurate DFT studies of electrochemical reactions.