Constructing Gold Single-Atom Catalysts on Hierarchical Nitrogen-Doped Carbon Nanocages for Carbon Dioxide Electroreduction to Syngas.

Precious-metal single-atom catalysts (SACs), featured by high metal utilization and unique coordination structure for catalysis, demonstrate distinctive performances in the fields of heterogeneous and electrochemical catalysis. Herein, gold SACs are constructed on hierarchical nitrogen-doped carbon nanocages (hNCNC) via a simple impregnation-drying process and first exploited for electrocatalytic carbon dioxide reduction reaction (CO2RR) to produce syngas. The as-constructed Au SAC exhibits the high mass activity of 3319 A g-1 Au at -1.0 V (vs reversible hydrogen electrode, RHE), much superior to the Au nanoparticles supported on hNCNC. The ratio of H2/CO can be conveniently regulated in the range of 0.4-2.2 by changing the applied potential. Theoretical study indicates such a potential-dependent H2/CO ratio is attributed to the different responses of HER and CO2RR on Au single-atom sites coordinating with one N atom at the edges of micropores across the nanocage shells. The catalytic mechanism of the Au active sites is associated with the smooth switch between twofold and fourfold coordination during CO2RR, which much decreases the free energy changes of the rate-determining steps and promotes the reaction activity.

Membraneless Electrochemical Synthesis Strategy toward Nitrate-to-Ammonia Conversion.

Electroreduction of nitrate (NO3RR) to ammonia in membraneless electrolyzers is of great significance for reducing the cost and saving energy consumption. However, severe chemical crossover with side reactions makes it challenging to achieve ideal electrolysis. Herein, we propose a general strategy for efficient membraneless ammonia synthesis by screening NO3RR catalysts with inferior oxygen reduction activity and matching the counter electrode (CE) with good oxygen evolution activity while blocking anodic ammonia oxidation. Consequently, screening the available Co-Co system, the membraneless NO3--to-NH3 conversion performance was significantly higher than H-type cells using costly proton-exchange membranes. At 200 mA cm-2, the full-cell voltage of the membraneless system (∼2.5 V) is 4 V lower than that of the membrane system (∼6.5 V), and the savings are 61.4 kW h (or 56.9%) per 1 kg NH3 produced. A well-designed pulse process, inducing reversible surface reconstruction that in situ generates and restores the active Co(III) species at the working electrode and forms favorable Co3O4/CoOOH at the CE, further significantly improves NO3--to-NH3 conversion and blocks side reactions. A maximum NH3 yield rate of 1500.9 µmol cm-2 h-1 was achieved at -0.9 V (Faraday efficiency 92.6%). This pulse-coupled membraneless strategy provides new insights into design complex electrochemical synthesis.

Matched Kinetics Process Over Fe2O3-Co/NiO Heterostructure Enables Highly Efficient Nitrate Electroreduction to Ammonia.

Tandem nitrate electroreduction reaction (NO3 -RR) is a promising method for green ammonia (NH3) synthesis. However, the mismatched kinetics processes between NO3 --to-NO2 - and NO2 --to-NH3 results in poor selectivity for NH3 and excess NO2 - evolution in electrolyte solution. Herein, a Ni2+ substitution strategy for developing oxide heterostructure in Co/Fe layered double oxides (LDOs) was designed and employed as tandem electrocataltysts for NO3 -RR. (Co0.83Ni0.16)2Fe exhibited a high NH3 yield rate of 50.4 mg ⋅ cm-2 ⋅ h-1 with a Faradaic efficiency of 97.8 % at -0.42 V vs. reversible hydrogen electrode (RHE) in a pulsed electrolysis test. By combining with in situ/operando characterization technologies and theoretical calculations, we observed the strong selectivity of NH3 evolution over (Co0.83Ni0.16)2Fe, with Ni playing a dual role in NO3 -RR by i) modifying the electronic behavior of Co, and ii) serving as complementary site for active hydrogen (*H) supply. Therefore, the adsorption capacity of *NO2 and its subsequent hydrogenation on the Co sites became more thermodynamically feasible. This study shows that Ni substitution promotes the kinetics of the NO3 -RR and provides insights into the design of tandem electrocatalysts for NH3 evolution.

Synergistic Al-Al Dual-Atomic Site for Efficient Artificial Nitrogen Fixation.

Synthesis of ammonia by electrochemical nitrogen reduction reaction (NRR) is a promising alternative to the Haber-Bosch process. However, it is commonly obstructed by the high activation energy. Here, we report the design and synthesis of an Al-Al bonded dual atomic catalyst stabilized within an amorphous nitrogen-doped porous carbon matrix (Al2NC) with high NRR performance. The dual atomic Al2-sites act synergistically to catalyze the complex multiple steps of NRR through adsorption and activation, enhancing the proton-coupled electron transfer. This Al2NC catalyst exhibits a high Faradaic efficiency of 16.56±0.3 % with a yield rate of 29.22±1.2 µg h-1 mgcat -1. The dual atomic Al2NC catalyst shows long-term repeatable, and stable NRR performance. This work presents an insight into the identification of synergistic dual atomic catalytic site and mechanistic pathway for the electrochemical conversion of N2 to NH3.

Sabatier Phenomenon in Hydrogenation Reactions Induced by Single-Atom Density.

The Sabatier principle is a fundamental concept in heterogeneous catalysis that provides guidance for designing optimal catalysts with the highest activities. For the first time, we here report a new Sabatier phenomenon in hydrogenation reactions induced by single-atom density at the atomic scale. We produce a series of Ir single-atom catalysts (SACs) with a predominantly Ir1-P4 coordination structure with densities ranging from 0.1 to 1.7 atoms/nm2 through a P-coordination strategy. When used as the catalysts for hydrogenation, a volcano-type relationship between Ir single-atom density and hydrogenation activity emerges, with a summit at a moderate density of 0.7 atoms/nm2. Mechanistic studies show that the balance between adsorption and desorption strength of the activated H* on Ir single atoms is found to be a key factor for the Sabatier phenomenon. The transferred Bader charge on these Ir SACs is proposed as a descriptor to interpret the structure-activity relationship. In addition, the maximum activity and selectivity can be simultaneously achieved in chemoselective hydrogenation reactions with the optimized catalyst due to the uniform geometric and electronic structures of single sites in SACs. The present study reveals the Sabatier principle as an insightful guidance for the rational design of more efficient and practicable SACs for hydrogenation reactions.

Coupling of Dissolved Organic Matter Molecular Fractionation with Iron and Sulfur Transformations during Sulfidation-Reoxidation Cycling.

Iron (oxyhydr)oxides and organic matter (OM) are intimately associated in natural environments, and their fate might be linked to sulfur during sulfidation-reoxidation cycling. However, the coupling of DOM molecular fractionation with Fe and S transformations following a full sulfidation-reoxidation cycle remains poorly understood. Here, we reacted Fh and Fh-OM associations with S(-II) anaerobically and then exposed the sulfidic systems to air. S(-II) preferentially reacted with Fh to form inorganic S (e.g., mackinawite, S0, and S22-) over being incorporated into OM as organic S and therefore indirectly affected OM fate by altering Fe speciation. Fh sulfidation was inhibited by associated OM, and the main secondary Fe species were mackinawite, Fe(II)-OM compounds, and lepidocrocite. Concomitantly, organic molecules high in unsaturation, aromaticity, and molecular weight were detached from solid-phase Fe species due to their lower affinities for secondary Fe species than for Fh. During the reoxidation stage, the previously formed Fe(II) species were reoxidized to Fh with a stronger aggregation, which recaptured formerly released OM with higher selectivity. Additionally, •OH was generated from Fe(II) oxygenation and degraded a portion of the DOM molecules. Overall, these results have significant implications for Fe, C, and S cycling in S-rich environments characterized by oscillating redox conditions.

Changes in molybdenum bioaccessibility in four spiked soils with respect to soil pH and organic matter.

Investigation of the inherent relationship between soil physicochemical properties and pollutant's bioaccessibility (BAc) by analyzing different soil types may produce erroneous results or bias, owing to the complexity of natural soil characteristics. However, use of single factor analysis (e.g., soil pH, organic matter) facilitates evaluation of the transition. In this study, the inherent relationship between soil properties and the BAc of molybdenum (Mo) was evaluated in two typical variable-charge soils (Ferralosol and Ferrosol) and constant-charge soils (Alfisol and Inceptisol) spiked with Mo after adjusting their pH and organic carbon content. The Unified Bioaccessibility Research Group of Europe (BARGE) Method (UBM) was applied to evaluate the BAc of Mo in the gastric and intestinal phase (GP and IP, respectively). Isothermal adsorption experiment, Tessier sequential extraction, and field emission scanning electron microscope-energy dispersive spectroscopy (FESEM-EDS) analysis were conducted on these spiked soils. The results indicated that the BAc of Mo in IP (27.42-80.41%) was significantly higher than that in GP (2.52-28.53%). A significantly lower level of BAc of Mo was found in the variable-charge soils, when compared with that in the constant-charge soils. Furthermore, significant negative correlations were identified between the BAc and adsorption of Mo, which decreased with soil pH. These negative correlations can be attributed to the increase in soil negative charge density and enhancement of Mo desorption by hydroxyl, which reinforce the repulsion between Mo and soil particles with increasing soil pH; this was further confirmed by the decrease in Mo adsorption with Alfisol pH. The Mo fractions and FESEM-EDS patterns confirmed that the BAc of Mo in GP was negatively correlated with soil organic carbon (SOC) content, possibly owing to an increase in Mo retention by SOC. These findings indicated that the health risk of Mo contamination in low pH and SOC-rich variable-charge soil is relatively low, thus providing references for rationalizing risk assessment and remediating Mo-polluted soil.

Modeling the Interaction and Uptake of Cd-As(V) Mixture to Wheat Roots Affected by Humic Acids, in Terms of root cell Membrane Surface Potential (ψ0).

The joint toxicological effects of Cd2+ and As(V) mixture on wheat root as affected by environmental factors, such as pH, coexisting cations, and humic acids etc., were investigated using hydroponic experiments. The interaction and toxicological mechanisms of co-existing Cd2+ and As(V) at the interface of solution and roots in presence of humic acid were further explored by incorporating root cell membrane surface potential ψ0 into a mechanistic model of combined biotic ligand model (BLM)-based Gouy-Chapman-Stern (GCS) model and NICA-DONNAN model. Besides, molecular dynamics (MD) simulations of lipid bilayer equilibrated with solution containing Cd2+ and H2AsO4- further revealed the molecular distribution of heavy metal(loid) ions under different membrane surface potentials. H2AsO4- and Cd2+ can be adsorbed on the surface of the membrane alone or as complexes, which consolidate the limitation of the macroscopic physical models.

Atomically Dispersed Manganese on Biochar Derived from a Hyperaccumulator for Photocatalysis in Organic Pollution Remediation.

Phytoremediation is a potentially cost-effective and environmentally friendly remediation method for environmental pollution. However, the safe treatment and resource utilization of harvested biomass has become a limitation in practical applications. To address this, a novel manganese-carbon-based single-atom catalyst (SAC) method has been developed based on the pyrolysis of a manganese hyperaccumulator, Phytolacca americana. In this method, manganese atoms are dispersed atomically in the carbon matrix and coordinate with N atoms to form a Mn-N4 structure. The SAC developed exhibited a high photooxidation efficiency and excellent stability during the degradation of a common organic pollutant, rhodamine B. The Mn-N4 site was the active center in the transformation of photoelectrons via the transfer of photoelectrons between adsorbed O2 and Mn to produce reactive oxygen species, identified by in situ X-ray absorption fine structure spectroscopy and density functional theory calculations. This work demonstrates an approach that increases potential utilization of biomass during phytoremediation and provides a promising design strategy to synthesize cost-effective SACs for environmental applications.

Atomic-Level Modulation of the Interface Chemistry of Platinum-Nickel Oxide toward Enhanced Hydrogen Electrocatalysis Kinetics.

Precise manipulation of the interactions between different components represents the frontier of heterostructured electrocatalysts and is crucial to understanding the structure-function relationship. Current studies, however, are quite limited. Here, we report targeted modulation of the atomic-level interface chemistry of Pt/NiO heterostructure via an annealing treatment, which results in substantially enhanced hydrogen electrocatalysis kinetics in alkaline media. Specifically, the optimized Pt/NiO heterostructure delivers by far the highest specific exchange current density of 8.1 mA cmPt-2 for hydrogen oxidation reaction. X-ray spectroscopy results suggest Pt-Ni interfacial bonds are formed after annealing, inducing more significant electron transfer from NiO to Pt. Also, the regulated interface chemistry, as proven by theoretical calculations, optimizes the binding behaviors of hydrogen and hydroxyl species. These findings emphasize the importance of interface engineering at the atomic level and inspire further explorations of heterostructured electrocatalysts.

Formation of Cr-based layered double hydroxide: effect of the amendments.

Chromium is one of the eight most popular inorganic soil pollutants in China, and its bioavailability is determined by the chemical states. Amendments, which are able to change the chemical forms of chromium and decrease its bioavailability, have received considerable attention in recent years. In this work, the formation of Cr-based layered double hydroxides (LDHs) and the immobilization of Cr in solution and soil were systemically investigated. The formation of Cr-based LDHs is strongly depended on the layer charges, aging temperatures and reaction time, as identified by X-ray diffraction (XRD), transmission electron microscope (TEM) and X-ray absorption fine structure (XAFS) spectrum. According to the pot experiment results, the concentration of Cr in the overground part of Brassica Chinensis L. was significantly decreased to 1.50-2.03 µg kg- 1 in the present of amendments. In total, the finding of LDHs formation on amendments and the thermodynamic stability of LDHs provides a new insight into the remediation of Cr-polluted soils.

Facet-Dependent Photoinduced Transformation of Cadmium Sulfide (CdS) Nanoparticles.

Microbial-mediated transformation of anthropogenic Cd2+ controls its distribution, bioavailability, and potential risks. However, the processes readily form CdS nanoparticles (CdS-NPs), which exhibit dissolution behavior different from that of larger sized particles. Here, we investigated the effects of morphologies and facets of CdS-NPs on their photoinduced dissolution. Three CdS-NPs, CdS-sphere, CdS-rod, and CdS-sheet, and one nanosized biogenic CdS (Bio-CdS) were synthesized with different dominant facets of {101}, {100}, {001}, and {111} and thus distinct surface chemistry. As explored by HRTEM, EPR, and DFT calculations, photogenerated e-/h+ pairs were more likely to generate on CdS-sheet surfaces due to higher surface energies and a narrower band gap, facilitating the formation of •OH and thereby faster dissolution (kobs = 6.126-6.261 × 10-2 h-1). The wider band gaps of CdS-sphere and CdS-rod caused less formation of O2•- and •OH, leading to slower oxidative dissolutions (kobs = 0.090-0.123 and 2.174-3.038 × 10-2 h-1, respectively). Given the similar surface energy as that of CdS-sheet, the dissolution rate of Bio-CdS was close to that of CdS-rod and CdS-sheet, which was 1.6-3.5 times faster than that of larger sized CdS, posing higher environmental risks than thought. Altogether, this work revealed the facet effects on the dissolution of CdS-NPs, manifesting a deeper understanding of metal sulfides' environmental behaviors.

Active Iron Phases Regulate the Abiotic Transformation of Organic Carbon during Redox Fluctuation Cycles of Paddy Soil.

Iron (Fe) phases are tightly linked to the preservation rather than the loss of organic carbon (OC) in soil; however, during redox fluctuations, OC may be lost due to Fe phase-mediated abiotic processes. This study examined the role of Fe phases in driving hydroxyl radical (•OH) formation and OC transformation during redox cycles in paddy soils. Chemical probes, sequential extraction, and Mössbauer analyses showed that the active Fe species, such as exchangeable and surface-bound Fe and Fe in low-crystalline minerals (e.g., green rust-like Fe phases), predominantly regulated •OH formation during redox cycles. The •OH oxidation strongly induced the oxidative transformation of OC, which accounted for 15.1-30.8% of CO2 production during oxygenation. Microbial processes contributed 7.3-12.1% of CO2 production, as estimated by chemical quenching and γ-irradiation experiments. After five redox cycles, 30.1-71.9% of the OC associated with active Fe species was released, whereas 5.2-7.1% was stabilized by high-crystalline Fe phases due to the irreversible transformation of these active Fe species during redox cycles. Collectively, our findings might unveil the under-appreciated role of active Fe phases in driving more loss than conservation of OC in soil redox fluctuation events.

Analysis of the Cd(II) Adsorption Performance and Mechanisms by Soybean Root Biochar: Effect of Pyrolysis Temperatures.

As one of the most harmful environmental pollutants, cadmium (Cd) has arisen much interest, and many researches have been carried out to study the adsorption of heavy metals by biochar, but the mechanisms were poorly explored and the roles components in biochar played are still indistinct. In this study, we evaluated the adsorption capacities and mechanisms of soybean root biochar pyrolyzed at four different temperatures. The results indicate the biochar properties are significantly determined by pyrolysis temperature, which affects the removal mechanisms of Cd(II) consequently. Microstructure characteristics and mechanism analysis further suggest that Cd(II)-π interactions and sulfur-containing functional groups are the main mechanisms of Cd(II) adsorption. This work shows a new perspective to explain the adsorption mechanisms onto biochar adsorbents and has a benefit for the exploitation of economical and effective adsorbents for Cd(II) removal based on biochars.

UV-Irradiation Facilitating Pb Release from Recycled PVC Microplastics.

Microplastics (MPs) are ubiquitously in ecosystem and have evoked wide attention. The potential risk of MPs to the ecosystems is associated with MPs and the additives such as Pb, which serves as a traditional stabilizer. However, the release of Pb from MPs remains largely unknown. In this study, we evaluated the release of Pb from recycled polyvinyl chloride (PVC) under UV-irradiation. The release process was dominated by two processes: H+ facilitated dissolution of Pb, and light-induced hydroxyl radical (·OH) caused C-H bond cleavage from PVC with the generation of alkyl radical. The effects of pH and coexisting low molecular weight organic acids (LMWOAs) were also evaluated. Lower pH speeds up the Pb release from MPs. The LMWOAs act as a filter of UV to restrain the Pb release. Overall, this study shows the release of Pb from recycled PVC MPs and indicates the potential risk of Pb to the environment.

Porous Carbon Membrane-Supported Atomically Dispersed Pyrrole-Type FeN4 as Active Sites for Electrochemical Hydrazine Oxidation Reaction.

The rational design of catalytically active sites in porous materials is essential in electrocatalysis. Herein, atomically dispersed Fe-Nx sites supported by hierarchically porous carbon membranes are designed to electrocatalyze the hydrazine oxidation reaction (HzOR), one of the key techniques in electrochemical nitrogen transformation. The high intrinsic catalytic activity of the Fe-Nx single-atom catalyst together with the uniquely mixed micro-/macroporous membrane support positions such an electrode among the best-known heteroatom-based carbon anodes for hydrazine fuel cells. Combined with advanced characterization techniques, electrochemical probe experiments, and density functional theory calculation, the pyrrole-type FeN4 structure is identified as the real catalytic site in HzOR.

Copper(I) Promotes Silver Sulfide Dissolution and Increases Silver Phytoavailability.

Metal sulfides, including acanthite (Ag2S), are persistent in the environment. In colloidal form, however, they can serve as a "Trojan horse", facilitating the mobility of trace metal contaminants. The natural processes that lead to the in situ dissolution of colloidal metal sulfides in soil are largely unknown. In this study, the dissolution of colloidal Ag2S in topsoil and Ag phytoavailability to wheat were examined in Ag2S-Cu(II)-thiosulfate systems. Cu(II) and thiosulfate strongly increased silver release (up to 83% of total Ag) from Ag2S in the dark. Electron paramagnetic resonance, X-ray photoelectron spectroscopy, and Cu K-edge X-ray absorption spectroscopy identified Cu(I) as the driving force of Ag2S dissolution. Density functional theory calculations further demonstrated the ability of Cu(I) to substitute for surface Ag on Ag2S in an energetically favorable manner. However, excess Cu(II) could enhance the formation of precipitates containing Cu(I), Ag, and S. Our results indicate that at ambient temperature and in the dark, Cu(I) can promote the dissolution of Ag2S and act as a precipitating agent. These findings reveal previously unrecognized biogeochemical processes of colloidal Ag2S and their importance in determining the fate of metal sulfides in the environment and probably also in vivo.

Role of Reduced Sulfur in the Transformation of Cd(II) Immobilized by δ-MnO2.

Mn oxides are the major sinks for Cd(II) in the aquatic environment. At the redox interface, reduced sulfur might affect the fate of sorbed Cd(II) by either reducing Mn oxides or forming strong complexes with Cd(II). Here, we investigated the fate of Cd(II) immobilized on δ-MnO2 affected by reduced sulfur (S2- and cysteine). A low concentration of S2- led to Cd(II) migration from vacant sites to edge sites, while a high concentration of S2- largely converted Cd(II) adsorbed on the surface of δ-MnO2 to CdS. At low pH, the cysteine addition led to the release of Cd(II) initially adsorbed at the δ-MnO2 vacant sites into the solution and caused the migration of a small portion of Cd(II) to the δ-MnO2 edge sites. At high pH, a high concentration of cysteine led to the detachment of Cd(II) from δ-MnO2, Cd(II) readsorption by Mn(III)-bearing minerals, and Cd-cysteine formation. Changes of Cd(II) speciation were caused by δ-MnO2 dissolution induced by reduced sulfur, the competition of generated Mn(II/III) for the adsorption sites, and the precipitation of Cd(II) with reduced sulfur. This study indicates that reduced sulfur is a critical factor controlling the fate of Cd(II) immobilized on Mn oxides in the aquatic environment.

Direct Hybridization of Noble Metal Nanostructures on 2D Metal-Organic Framework Nanosheets To Catalyze Hydrogen Evolution.

Rational hybridization of two-dimensional (2D) nanomaterials with extrinsic species has shown great promise for a wide range of applications. To date, rational design and engineering of heterostructures based on 2D metal-organic frameworks (MOFs) has been rather limited. Herein, we report an efficient strategy to construct noble metal/2D MOF heterostructures, featuring the utilization of surface oxygen sites from uncoordinated MOF ligands. The incorporation of highly dispersed noble metal nanoparticles (e.g., Pt and Pd) with modulated electronic structure is enabled on a surfactant-free MOF surface. As a proof-of-concept demonstration, the 2D Ni-MOF@Pt hybrid with well-defined interfaces is applied to boost the electrochemical hydrogen evolution reaction (HER) and delivers decent electrocatalytic activity under both acidic and alkaline conditions. The present results are expected to provide new insights into furnishing MOFs with extended functionalities and applications.

Multifunctional Active-Center-Transferable Platinum/Lithium Cobalt Oxide Heterostructured Electrocatalysts towards Superior Water Splitting.

Designing cost-effective and efficient electrocatalysts plays a pivotal role in advancing the development of electrochemical water splitting for hydrogen generation. Herein, multifunctional active-center-transferable heterostructured electrocatalysts, platinum/lithium cobalt oxide (Pt/LiCoO2 ) composites with Pt nanoparticles (Pt NPs) anchored on LiCoO2 nanosheets, are designed towards highly efficient water splitting. In this electrocatalyst system, the active center can be alternatively switched between Pt species and LiCoO2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Specifically, Pt species are the active centers and LiCoO2 acts as the co-catalyst for HER, whereas the active center transfers to LiCoO2 and Pt turns into the co-catalyst for OER. The unique architecture of Pt/LiCoO2 heterostructure provides abundant interfaces with favorable electronic structure and coordination environment towards optimal adsorption behavior of reaction intermediates. The 30 % Pt/LiCoO2 heterostructured electrocatalyst delivers low overpotentials of 61 and 285 mV to achieve 10 mA cm-2 for HER and OER in alkaline medium, respectively.