Dopant-Induced Electronic States Regulation Boosting Electroreduction of Dilute Nitrate to Ammonium

Electrochemically converting NO₃⁻ into NH₃ offers a promising route for water treatment. Nevertheless, electroreduction of dilute NO₃⁻ is still suffering from low activity and/or selectivity. Herein, B as a modifier was introduced to tune electronic states of Cu and further regulate the performance of electrochemical NO₃⁻ reduction reaction (NO₃RR) with dilute NO₃⁻ concentration (≤100 ppm NO₃⁻−N). Notably, a linear relationship was established by plotting NH₃ yield vs. the oxidation state of Cu, indicating that the increase of Cu⁺ content leads to an enhanced NO₃⁻-to-NH₃ conversion activity. Under a low NO₃⁻−N concentration of 100 ppm, the optimal Cu(B) catalyst displays a 100 % NO₃⁻-to-NH₃ conversion at −0.55 to −0.6 V vs. RHE, and a record-high NH₃ yield of 309 mmol h⁻¹ g_cat⁻¹, which is more than 25 times compared with the pristine Cu nanoparticles (12 mmol h⁻¹ g_cat⁻¹). This research provides an effective method for conversion of dilute NO₃⁻ to NH₃, which has certain guiding significance for the efficient and green conversion of wastewater in the future.     

Relative Local Electron Density Tuning in Metal-Covalent Organic Frameworks for Boosting CO2 Photoreduction

The high local electron density and efficient charge carrier separation are two important factors to affect photocatalytic activity, especially for the CO₂ photoreduction reaction. However, the systematic studies on the structure-functional relationship regarding the above two factors based on precisely structure model are rarely reported. Herein, as a proof-of-concept, we developed a new strategy on the evaluation of local electron density by controlling the relative electron-deficient (ED) and electron-rich (ER) intensity of monomer at a molecular level based on three rational-designed vinylene-linked sp² carbon-covalent organic frameworks (COFs). As expected, the as-prepared vinylene-linked sp² carbon-conjugated metal-covalent organic framework (MCOFs) (VL-MCOF-1) with molecular junction exhibited excellent activities for CO₂-to-HCOOH conversion (283.41 μmol g⁻¹ h⁻¹) and high selectivity of 97.1 %, much higher than the VL-MCOF-2 and g-C₃₄N₆-COF, which is due to the synergistic effect of the multi-electronic metal clusters (Cu₃(PyCA)₃) (PyCA=pyrazolate-4-carboxaldehyde) as strong ER roles and cyanopyridine units as ED roles and active sites, as well as the boosted photo-induced charge separation efficiency of vinyl connection and increased light utilization ability. These results not only provide a strategy for regulating the electron-density distribution of photocatalysts at the molecular level but also offers profound insights for metal clusters-based COFs to effective CO₂ conversion.     

Bromine-Enhanced Generation and Epoxidation of Ethylene in Tandem CO2 Electrolysis Towards Ethylene Oxide

The indirect electro-epoxidation of ethylene (C₂H₄), produced from CO₂ electroreduction (CO₂R), holds immense promise for CO₂ upcycling to valuable ethylene oxide (EO). However, this process currently has a mediocre Faradaic efficiency (FE) due to sluggish formation and rapid dissociation of active species, as well as reductive deactivation of Cu-based electrocatalysts during the conversion of C₂H₄ to EO and CO₂ to C₂H₄, respectively. Herein, we report a bromine-induced dual-enhancement strategy designed to concurrently promote both C₂H₄-to-EO and CO₂-to-C₂H₄ conversions, thereby improving EO generation, using single-atom Pt on N-doped CNTs (Pt₁/NCNT) and Br⁻-bearing porous Cu₂O as anode and cathode electrocatalysts, respectively. Physicochemical characterizations including synchrotron X-ray absorption, operando infrared spectroscopy, and quasi in situ Raman spectroscopy/electron paramagnetic resonance with theoretical calculations reveal that the favorable Br₂/HBrO generation over Pt₁/NCNT with optimal intermediate binding facilitates C₂H₄-to-EO conversion with a high FE of 92.2 %, and concomitantly, the Br⁻ with strong nucleophilicity protects active Cu⁺ species in Cu₂O effectively for improved CO₂-to-C₂H₄ conversion with a FE of 66.9 % at 800 mA cm⁻², superior to those in the traditional chloride-mediated case. Consequently, a single-pass FE as high as 41.1 % for CO₂-to-EO conversion can be achieved in a tandem system.     

Chemoselective Quinoline and Isoquinoline Reduction by Energy Transfer Catalysis Enabled Hydrogen Atom Transfer

(Hetero)arene reduction is one of the key avenues for synthesizing related cyclic alkenes and alkanes. While catalytic hydrogenation and Birch reduction are the two broadly utilized approaches for (hetero)arene reduction across academia and industry over the last century, both methods have encountered significant chemoselectivity challenges. We hereby introduce a highly chemoselective quinoline and isoquinoline reduction protocol operating through selective energy transfer (EnT) catalysis, which enables subsequent hydrogen atom transfer (HAT). The design of this protocol bypasses the conventional metric of reduction reaction, that is, the reductive potential, and instead relies on the triplet energies of the chemical moieties and the kinetic barriers of energy and hydrogen atom transfer events. Many reducing labile functional groups, which were incompatible with previous (hetero)arene reduction reactions, are retained in this reaction. We anticipate that this protocol will trigger the further advancement of chemoselective arene reduction and enable the current arene-rich drug space to escape from flatland.     

Low-Temperature Hydrogenation of CO2 to Methanol in Water on ZnO-Supported CuAu Nanoalloys

Optimizing processes and materials for the valorization of CO₂ to hydrogen carriers or platform chemicals is a key step for mitigating global warming and for the sustainable use of renewables. We report here on the hydrogenation of CO₂ in water on ZnO-supported CuAu nanoalloys, based on ≤7 mol % Au. Cu_xAu_y/ZnO catalysts were characterized using ¹⁹⁷Au Mössbauer, in situ X-ray absorption (Au L_III- and Cu K-edges), and ambient pressure X-ray photoelectron (APXP) spectroscopic methods together with X-ray diffraction and high-resolution electron microscopy. At 200 °C, the conversion of CO₂ showed a significant increase by 34 times (from 0.1 to 3.4 %) upon increasing Cu₉₃Au₇ loading from 1 to 10 wt %, while maintaining methanol selectivity at 100 %. Limited CO selectivity (4–6 %) was observed upon increasing temperature up to 240 °C but associated with a ≈3-fold increase in CO₂ conversion. Based on APXPS during CO₂ hydrogenation in an H₂O-rich mixture, Cu segregates preferentially to the surface in a mainly metallic state, while slightly charged Au submerges deeper into the subsurface region. These results and detailed structural analyses are topics of the present contribution.     

Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio-Inspired Bimetallic MXene Electrocatalyst

Mo- and Fe-containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO₃RR) catalyzed by an Fe-substituted two-dimensional molybdenum carbide of the MXene family, viz., Mo₂CT_x : Fe (T_x are oxo, hydroxy and fluoro surface termination groups). Mo₂CT_x : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo₂CT_x) and features a Faradaic efficiency (FE) and an NH₃ yield rate of 41 % and 3.2 μmol h⁻¹ mg⁻¹, respectively, for the reduction of NO₃⁻ to NH₄⁺ in acidic media and 70 % and 12.9 μmol h⁻¹ mg⁻¹ in neutral media. Regardless of the media, Mo₂CT_x : Fe outperforms monometallic Mo₂CT_x owing to a more facile reductive defunctionalization of T_x groups, as evidenced by in situ X-ray absorption spectroscopy (Mo K-edge). After surface reduction, a T_x vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO₃RR in close analogy to the prevailing mechanism of the natural Mo-based nitrate reductase enzymes.     

Importing Antibonding-Orbital Occupancy through Pd−O−Gd Bridge Promotes Electrocatalytic Oxygen Reduction

The active-site density, intrinsic activity, and durability of Pd−based materials for oxygen reduction reaction (ORR) are critical to their application in industrial energy devices. This work constructs a series of carbon-based rare-earth (RE) oxides (Gd₂O₃, Sm₂O₃, Eu₂O₃, and CeO₂) by using RE metal–organic frameworks to tune the ORR performance of the Pd sites through the Pd−RE_xO_y interface interaction. Taking Pd−Gd₂O₃/C as a representative, it is identified that the strong coupling between Pd and Gd₂O₃ induces the formation of the Pd−O−Gd bridge, which triggers charge redistribution of Pd and Gd₂O₃. The screened Pd−Gd₂O₃/C exhibits impressive ORR performance with high onset potential (0.986 V_RHE), half-wave potential (0.877 V_RHE), and excellent stability. Similar ORR results are also found for Pd−Sm₂O₃/C, Pd−Eu₂O₃/C, and Pd−CeO₂/C catalysts. Theoretical analyses reveal that the coupling between Pd and Gd₂O₃ promotes electron transfer through the Pd−O−Gd bridge, which induces the antibonding-orbital occupancy of Pd−*OH for the optimization of *OH adsorption in the rate-determining step of ORR. The pH-dependent microkinetic modeling shows that Pd−Gd₂O₃ is close to the theoretical optimal activity for ORR, outperforming Pt under the same conditions. By its ascendancy in ORR, the Pd−Gd₂O₃/C exhibits superior performance in Zn-air battery as an air cathode, implying its excellent practicability.     

Carbon Dioxide Reduction on Transition Metal Dichalcogenides with Ni and Cu Edge Doping: A Density-Functional Theory Study

Transition-metal dichalcogenides (TMDs) have promising properties for their use as catalysts of CO₂ reduction to methane via the Sabatier reaction. In this article we use density-functional theory calculations to gain insight into the energetics of this reaction for Mo/W-based and S/Se-based TMDs with non-, Ni- and Cu-doping. We show that sulfur-based TMDs with Ni/Cu doping exhibit better indicators for catalytic performance of the CO₂ reduction reaction than non-doped and doped TMDs without active sites. In addition, the role of the transition metal was found to a much smaller influence in the reaction than the role of the chalcogen and dopant atoms, which influence the bonding strength and type, respectively.     

Continuous Modulation of Electrocatalytic Oxygen Reduction Activities of Single-Atom Catalysts through p-n Junction Rectification

Fine-tuning single-atom catalysts (SACs) to surpass their activity limit remains challenging at their atomic scale. Herein, we exploit p-type semiconducting character of SACs having a metal center coordinated to nitrogen donors (MeN_x) and rectify their local charge density by an n-type semiconductor support. With iron phthalocyanine (FePc) as a model SAC, introducing an n-type gallium monosulfide that features a low work function generates a space-charged region across the junction interface, and causes distortion of the FeN₄ moiety and spin-state transition in the Fe^II center. This catalyst shows an over two-fold higher specific oxygen-reduction activity than that of pristine FePc. We further employ three other n-type metal chalcogenides of varying work function as supports, and discover a linear correlation between the activities of the supported FeN₄ and the rectification degrees, which clearly indicates that SACs can be continuously tuned by this rectification strategy.     

Single-entity Electrochemistry Unveils Dynamic Transformation during Tandem Catalysis of Cu2O and Co3O4 for Converting NO3− to NH3

Electrochemically converting nitrate to ammonia is an essential and sustainable approach to restoring the globally perturbed nitrogen cycle. The rational design of catalysts for the nitrate reduction reaction (NO₃RR) based on a detailed understanding of the reaction mechanism is of high significance. We report a Cu₂O+Co₃O₄ tandem catalyst which enhances the NH₃ production rate by ≈2.7-fold compared to Co₃O₄ and ≈7.5-fold compared with Cu₂O, respectively, however, most importantly, we precisely place single Cu₂O and Co₃O₄ cube-shaped nanoparticles individually and together on carbon nanoelectrodes provide insight into the mechanism of the tandem catalysis. The structural and phase evolution of the individual Cu₂O+Co₃O₄ nanocubes during NO₃RR is unveiled using identical location transmission electron microscopy. Combining single-entity electrochemistry with precise nano-placement sheds light on the dynamic transformation of single catalyst particles during tandem catalysis in a direct way.