Ni-N4 sites in a single-atom Ni catalyst on N-doped carbon for hydrogen production from formic acid

Bimetal loaded graphitic carbon nitride with synergistic enhanced peroxidase-like activity for colorimetric detection of p-phenylenediamine

In recent years, great progress has been made on the study of nanozymes with enzyme-like properties. Here, bimetallic Fe and Ni nanoclusters were anchored on the nanosheets of nitrogen-rich layered graphitic carbon nitride by one-step pyrolysis at high temperature (Fe/Ni-CN). The loading content of Fe and Ni on Fe/Ni-CN is as high as 8.0%, and Fe/Ni-CN has a high specific surface area of 121.86 m2 g-1. The Fe/Ni-CN can effectively oxidize 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2, and exhibits efficient peroxidase-like activity, leading to a 17.2-fold increase compared to pure graphitic carbon nitride (CN). Similar to the natural horseradish peroxidase (HRP), the Fe/Ni-CN nanozyme follows catalytic kinetics. The Michaelis-Menten constant (Km) value of the Fe/Ni-CN nanozyme for TMB is about 8.3-fold lower than that for HRP, which means that the Fe/Ni-CN nanozyme has better affinity for TMB. In addition, the catalytic mechanism was investigated by combination of free radical quenching experiments and density-functional theory (DFT) calculations. The results show that the high peroxidase-like activity is due to the easy adsorption of H2O2 after bimetal loading, which is conducive to the production of hydroxyl radicals. Based on the extraordinary peroxidase-like activity, the colorimetric detection of p-phenylenediamine (PPD) was constructed with a wide linear range of 0.2-30 μM and a low detection limit of 0.02 μM. The sensor system has been successfully applied to the detection of residual PPD in real dyed hair samples. The results show that the colorimetric method is sensitive, highly selective and accurate. This study provides a new idea for the efficient enhancement of nanozyme activity and effective detection of PPD by a bimetallic synergistic strategy.

Single Atom Catalyst in Persulfate Oxidation Reaction: From Atom Species to Substance

With maximum utilization of active metal sites, more and more researchers have reported using single atom catalysts (SACs) to activate persulfate (PS) for organic pollutants removal. In SACs, single metal atoms (Fe, Co, Cu, Mn, etc.) and different substrates (porous carbon, biochar, graphene oxide, carbon nitride, MOF, MoS2, and others) are the basic structural. Metal single atoms, substances, and connected chemical bonds all have a great influence on the electronic structures that directly affect the activation process of PS and degradation efficiency to organic pollutants. However, there are few relevant reviews about the interaction between metal single atoms and substances during PS activation process. In this review, the SACs with different metal species and substrates are summarized to investigate the metal-support interaction and evaluate their effects on PS oxidation reaction process. Furthermore, how metal atoms and substrates affect the reactive species and degradation pathways are also discussed. Finally, the challenges and prospects of SACs in PS-AOPs are proposed.

Carbon-based single-atom catalysts derived from biomass: Fabrication and application

Single-atom catalysts (SACs) with active metals dispersed atomically have shown great potential in heterogeneous catalysis due to the high atomic utilization and superior selectivity/stability. Synthesis of SACs using carbon-neutral biomass and its components as the feedstocks provides a promising strategy to realize the sustainable and cost-effective SACs preparation as well as the valorization of underused biomass resources. Herein, we begin by describing the general background and status quo of carbon-based SACs derived from biomass. A detailed enumeration of the common biomass feedstocks (e.g., lignin, cellulose, chitosan, etc.) for the SACs preparation is then offered. The interactions between metal atoms and biomass-derived carbon carriers are summarized to give general rules on how to stabilize the atomic metal centers and rationalize porous carbon structures. Furthermore, the widespread adoption of catalysts in diverse domains (e.g., chemocatalysis, electrocatalysis and photocatalysis, etc.) is comprehensively introduced. The structure-property relationships and the underlying catalytic mechanisms are also addressed, including the influences of metal sites on the activity and stability, and the impact of the unique structure of single-atom centers modulated by metal/biomass feedstocks interactions on catalytic activity and selectivity. Finally, we end this review with a look into the remaining challenges and future perspectives of biomass-based SACs. We expect to shed some light on the forthcoming research of carbon-based SACs derived from biomass, manifestly stimulating the development in this emerging research area.

Enhanced electron-transfer for peroxymonosulfate activation by Ni single sites adjacent to Ni nanoparticles

Oriented generation of specific reactive oxygen species (ROS) has been challenging in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs). In this work, we constructed a multifunctional catalyst composed of Ni NPs embedded in N-doped carbon nanotubes (NCNTs) with exposed Ni single-atom sites (Ni-NCNTs). The Ni-N4 single sites adjacent to the Ni NPs are more efficient for PMS adsorption and activation, resulting in enhanced production of singlet oxygen (1O2). More interesting, we demonstrated that the superoxide anion radical (O2•-) was generated from 1O2 reduction via the electron transfer from the graphitic-N sites of Ni-NCNTs rather than from O2 reduction or PMS decomposition as reported in previous studies. Thus, Ni-NCNTs can act as both electron acceptor and donor to trigger the cascade production of 1O2 and O2•-, respectively, leading to fast and selective degradation of aqueous organic pollutants. The graphitic-N adjacent to the aromatic π-conjugation of NCNTs facilitated chemisorption of 1O2 onto NCNTs via the strong π*-π interactions, and more importantly, donated the lone pair electrons to trigger the reduction of 1O2 to O2•-. This study unravels the mechanisms for enhanced production of ROS in the nanoconfined Fenton-like systems and shed new light on the application of multifunctional nanocatalyst for rapid wastewater decontamination.

Defect-driven nanostructuring of low-nuclearity Pt-Mo ensembles for continuous gas-phase formic acid dehydrogenation

Supported metal clusters comprising of well-tailored low-nuclearity heteroatoms have great potentials in catalysis owing to the maximized exposure of active sites and metal synergy. However, atomically precise design of these architectures is still challenging for the lack of practical approaches. Here, we report a defect-driven nanostructuring strategy through combining defect engineering of nitrogen-doped carbons and sequential metal depositions to prepare a series of Pt and Mo ensembles ranging from single atoms to sub-nanoclusters. When applied in continuous gas-phase decomposition of formic acid, the low-nuclearity ensembles with unique Pt3Mo1N3 configuration deliver high-purity hydrogen at full conversion with unexpected high activity of 0.62 molHCOOH molPt-1 s-1 and remarkable stability, significantly outperforming the previously reported catalysts. The remarkable performance is rationalized by a joint operando dual-beam Fourier transformed infrared spectroscopy and density functional theory modeling study, pointing to the Pt-Mo synergy in creating a new reaction path for consecutive HCOOH dissociations.

Zeolite encapsulated organometallic complexes as model catalysts

Heterogeneities in the structure of active centers in metal-containing porous materials are unavoidable and complicate the description of chemical events occurring along reaction coordinates at the atomic level. Metal containing zeolites include sites of varied local coordination and secondary confining environments, requiring careful titration protocols to quantify the predominant active sites. Hybrid organometallic-zeolite catalysts are useful well-defined platform materials for spectroscopic, kinetic, and computational studies of heterogeneous catalysis that avoid the complications of conventional metal-containing porous materials. Such materials have been synthesized and studied previously, but catalytic applications were mostly limited to liquid-phase oxidation and electrochemical reactions. The hydrothermal stability, time-on-stream stability, and utility of these materials in gas-phase oxidation reactions are under-studied. The potential applications for single-site heterogeneous catalysts in fundamental research are abundant and motivate future synthetic, spectroscopic, kinetic, and computational studies.

Temperature-Induced Low-Coordinate Ni Single-Atom Catalyst for Boosted CO2 Electroreduction Activity

Single-atom catalysts (SACs) exhibit remarkable potential for electrochemical reduction of CO2 to value-added products. However, the commonly pursued methods for preparing SACs are hard to scale up, and sometimes, lack general applicability because of expensive raw materials and complex synthetic procedures. In addition, the fine tuning of coordination environment of SACs remains challenging due to their structural vulnerability. Herein, a simple and universal strategy is developed to fabricate Ni SACs with different nitrogen coordination numbers through one-step pyrolysis of melamine, Ni(NO3 )∙6H2 O, and polyvinylpyrrolidone at different temperatures. Experimental measurements and theoretical calculations reveal that the low-coordinate Ni SACs exhibit outstanding CO2 reduction performance and stability, achieving a Faradic efficiency (FECO ) of 98.5% at -0.76 V with CO current density of 24.6 mA cm-2 , and maintaining FECO of over 91.0% at all applied potential windows from -0.56 to -1.16 V, benefiting from its coordinatively unsaturated structure to afford high catalytic activity and low barrier for the formation of *COOH intermediate. No significant performance degradation is observed over 50 h of continuous operation. Additionally, several other metallic single-atom catalysts are successfully prepared by this synthetic method, demonstrating the universality of this strategy.

Edge Coordination of Ni Single Atoms on Hard Carbon Promotes the Potassium Storage and Reversibility

Hard carbon is the most promising anode for potassium-ion batteries (PIBs) due to its low cost and abundance, but its limited storage capacity remains a major challenge. Herein, edge coordination of metal single atoms is proved to be an effective strategy for promoting potassium storage in hard carbon for the first time, taking B, N co-doped hard carbon nanotubes anchored by edge Ni-N₄ -B atomic sites (Ni@BNHC) as an example. It is revealed that edge Ni-N₄ -B can provide active sites for interlayer adsorption of K⁺ and that Ni atoms can facilitate the reversibility of K⁺ storage on N and B atoms. Furthermore, an unprecedentedly reversible K⁺ storage capacity of 694 mAh g^-1 at 0.05 A g^-1 is realized by introducing commercial carbon nanotubes. This work provides a new perspective for the application of single-atom engineering and the design of high-performance carbon anodes for PIBs.

Soft-template derived Ni/Mo2C hetero-sheet arrays for large current density hydrogen evolution reaction

Practical structural design and electronic regulation are significant for synthesising efficient electrocatalysts. Therefore, a facile soft-template approach has been applied to successfully grow Ni/Mo₂C heterojunction nanosheet arrays on nickel foam (NF) skeleton (NS-Ni/Mo₂C@NF) using polyvinylpyrrolidone (PVP) as a soft template. The density functional theory (DFT) calculations reveal that abundant Ni/Mo₂C heterojunction in NS-Ni/Mo₂C@NF can provide many active sites with a moderate hydrogen adsorption free energy (ΔG_H*, 0.037 eV). Benefiting from this nanosheet array structure and abundant Ni/Mo₂C heterojunctions, the NS-Ni/Mo₂C@NF catalyst can efficiently catalyze HER, especially at large current densities. As a result, only 151 and 271 mV overpotentials are needed to deliver 100 and 1000 mA/cm² HER, respectively. More importantly, the hydrogen production testing with NS-Ni/Mo₂C@NF as the working electrode can run stably for 500 h without activity decay under the current density of 500 mA/cm² commonly used in industrial water electrolyzers, indicating that NS-Ni/Mo₂C@NF has broad application prospects.

Emerging materials for electrochemical CO2 reduction: progress and optimization strategies of carbon-based single-atom catalysts

The electrochemical CO₂ reduction reaction can effectively convert CO₂ into promising fuels and chemicals, which is helpful in establishing a low-carbon emission economy. Compared with other types of electrocatalysts, single-atom catalysts (SACs) immobilized on carbon substrates are considered to be promising candidate catalysts. Atomically dispersed SACs exhibit excellent catalytic performance in CO₂RR due to their maximum atomic utilization, unique electronic structure, and coordination environment. In this paper, we first briefly introduce the synthetic strategies and characterization techniques of SACs. Then, we focus on the optimization strategies of the atomic structure of carbon-based SACs, including adjusting the coordination atoms and coordination numbers, constructing the axial chemical environment, and regulating the carbon substrate, focusing on exploring the structure-performance relationship of SACs in the CO₂RR process. In addition, this paper also briefly introduces the diatomic catalysts (DACs) as an extension of SACs. At the end of the paper, we summarize the article with an exciting outlook discussing the current challenges and prospects for research on the application of SACs in CO₂RR.

Highly Dispersed Ni on Nitrogen-Doped Carbon for Stable and Selective Hydrogen Generation from Gaseous Formic Acid

Ni supported on N-doped carbon is rarely studied in traditional catalytic reactions. To fill this gap, we compared the structure of 1 and 6 wt% Ni species on porous N-free and N-doped carbon and their efficiency in hydrogen generation from gaseous formic acid. On the N-free carbon support, Ni formed nanoparticles with a mean size of 3.2 nm. N-doped carbon support contained Ni single-atoms stabilized by four pyridinic N atoms (N₄-site) and sub-nanosized Ni clusters. Density functional theory calculations confirmed the clustering of Ni when the N₄-sites were fully occupied. Kinetic studies revealed the same specific Ni mass-based reaction rate for single-atoms and clusters. The N-doped catalyst with 6 wt% of Ni showed higher selectivity in hydrogen production and did not lose activity as compared to the N-free 6 wt% Ni catalyst. The presented results can be used to develop stable Ni catalysts supported on N-doped carbon for various reactions.

Development of Mo-Modified Pseudoboehmite Supported Ni Catalysts for Efficient Hydrogen Production from Formic Acid

Formic acid (FA), as a safe and renewable liquid hydrogen storage material, has attracted extensive attention. In this paper, a series of Mo-modified pseudoboehmite supported Ni catalysts were developed and evaluated for efficient hydrogen production from formic acid. Pseudoboehmite (PB) as a catalyst carrier was used for the first time. Ni/PB and NiMo/PB possessed a mesostructure, and the pore size distribution was mainly concentrated between 2 and 20 nm. The oxygen vacancies caused by Mo enhanced Ni anchoring, thus inhibiting Ni sintering. Compared with Ni10/PB (7.62 nm), Ni10Mo1/PB had smaller Ni particles (5.08 nm). The Ni-O-Al solid solutions formed through the interaction of Ni with the PB improved the catalytic performance. Ni10Mo1/PB gave the highest conversion of 92.8% with a H₂ selectivity of 98% at 300 °C, and the catalyst activity hardly decreased during the 50 h stability test. In short, Ni10Mo1/PB was a promising catalyst for hydrogen production from formic acid because of the oxygen vacancy anchoring effect as well as the formation of Ni-O-Al solid solutions which could effectively suppress the Ni sintering.

Electrochemical reduction of NO catalyzed by boron-doped C60 fullerene: a first-principles study

The electrochemical reduction of nitrogen monoxide (NO) is one of the most promising approaches for converting this harmful gas into useful chemicals. Using density functional theory calculations, the work examines the potential of a single B atom doped C60 fullerene (C59B) for catalytic reduction of NO molecules. The results demonstrate that the NO may be strongly activated over the B atom of C59B, and that the subsequent reduction process can result in the formation of NH3 and N2O molecules at low and high coverages, respectively. Based on the Gibbs free energy diagram, it is inferred that the C59B has excellent catalytic activity for NO reduction at ambient conditions with no potential-limiting. At normal temperature, the efficient interaction between the *NOH and NO species might lead to the spontaneous formation of the N2O molecule. Thus, the findings of this study provide new insights into NO electrochemical reduction on heteroatom doped fullerenes, as well as a unique strategy for fabricating low-cost NO reduction electrocatalysts with high efficiency.

Efficient and Controlled H2 Release from Sodium Formate

Sodium formate (SF) has been used for a long time as a technological additive for H2 release from the dehydrogenation of formic acid . Formic acid is often synthesized from...

Unique catalytic properties of Ni–Ir alloy for the hydrogenation of N-heteroaromatics

SiO2-supported Ni–Ir alloy catalysts showed much higher catalytic activity for the hydrogenation of N-heteroaromatics including pyridines and quinolines than monometallic Ir/SiO2 and Ni/SiO2.

Single-Atom Catalysts: A Review of Synthesis Strategies and Their Potential for Biofuel Production

Biofuels have been derived from various feedstocks by using thermochemical or biochemical procedures. In order to synthesise liquid and gas biofuel efficiently, single-atom catalysts (SACs) and single-atom alloys (SAAs) have been used in the reaction to promote it. SACs are made up of single metal atoms that are anchored or confined to a suitable support to keep them stable, while SAAs are materials generated by bi- and multi-metallic complexes, where one of these metals is atomically distributed in such a material. The structure of SACs and SAAs influences their catalytic performance. The challenge to practically using SACs in biofuel production is to design SACs and SAAs that are stable and able to operate efficiently during reaction. Hence, the present study reviews the system and configuration of SACs and SAAs, stabilisation strategies such as mutual metal support interaction and geometric coordination, and the synthesis strategies. This paper aims to provide useful and informative knowledge about the current synthesis strategies of SACs and SAAs for future development in the field of biofuel production.