Strong interactions through the highly polar "Early-Late" metal-metal bonds enable single-atom catalysts good durability and superior bifunctional ORR/OER activity.

Simultaneously enhancing the durability and catalytic performance of metal-nitrogen-carbon (M-Nx-C) single-atom catalysts is critical to boost oxygen electrocatalysis for energy conversion and storage, yet it remains a grand challenge. Herein, through the combination of early and late metals, we proposed to enhance the stability and tune the catalytic activity of M-Nx-C SACs in oxygen electrocatalysis by their strong interaction with the M2'C-type MXene substrate. Our density functional theory (DFT) computations revealed that the strong interaction between "early-late" metal-metal bonds significantly improves thermal and electrochemical stability. Due to considerable charge transfer and shift of the d-band center, the electronic properties of these SACs can be extensively modified, thereby optimizing their adsorption strength with oxygenated intermediates and achieving eight promising bifunctional catalysts for ORR/OER with low overpotentials. More importantly, the constant-potential analysis demonstrated the excellent bifunctional activity of SACs supported on MXene substrate across a broad pH range, especially in strongly alkaline media with record-low overpotentials. Further machine learning analysis shows that the d-band center, the charge of the active site, and the work function of the formed heterojunctions are critical to revealing the ORR/OER activity origin. Our results underscore the vast potential of strong interactions between different metal species in enhancing the durability and catalytic performance of SACs.

Si3C Monolayer as an Efficient Metal-Free Catalyst for Nitrate Electrochemical Reduction: A Computational Study.

Nitrate electroreduction reaction to ammonia (NO3ER) holds great promise for both nitrogen pollution removal and valuable ammonia synthesis, which are still dependent on transition-metal-based catalysts at present. However, metal-free catalysts with multiple advantages for such processes have been rarely reported. Herein, by means of density functional theory (DFT) computations, in which the Perdew-Burke-Ernzerhof (PBE) functional is obtained by considering the possible van der Waals (vdW) interaction using the DFT+D3 method, we explored the potential of several two-dimensional (2D) silicon carbide monolayers as metal-free NO3ER catalysts. Our results revealed that the excellent synergistic effect between the three Si active sites within the Si3C monolayer enables the sufficient activation of NO3- and promotes its further hydrogenation into NO2*, NO*, and NH3, making the Si3C monolayer exhibit high NO3ER activity with a low limiting potential of -0.43 V. In particular, such an electrochemical process is highly dependent on the pH value of the electrolytes, in which acidic conditions are more favorable for NO3ER. Moreover, ab initio molecular dynamics (AIMD) simulations demonstrated the high stability of the Si3C monolayer. In addition, the Si3C monolayer shows a low formation energy, excellent electronic properties, a superior suppression effect on competing reactions, and high stability, offering significant advantages for its experimental synthesis and practical applications in electrocatalysis. Thus, a Si3C monolayer can perform as a promising NO3ER catalyst, which would open a new avenue to further develop novel metal-free catalysts for NO3ER.

Interface engineering of transition metal-nitrogen-carbon by graphdiyne for boosting the oxygen reduction/evolution reactions: A computational study.

Exploring high-efficiency electrocatalysts to boost the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is pivotal to the large-scale applications for clean and renewable energy technologies, such as fuel cells, water splitting, and metal-air batteries. Herein, by means of density functional theory (DFT) computations, we proposed a strategy to modulate the catalytic activity of transition metal-nitrogen-carbon catalysts through their interface engineering with graphdiyne (TMNC/GDY). Our results revealed that these hybrid structures exhibit good stability and excellent electrical conductivity. Especially, CoNC/GDY was identified as a promising bifunctional catalyst for ORR/OER with rather low overpotentials in acidic conditions according to the constant-potential energy analysis. Moreover, the volcano plots were established to describe the activity trend of the ORR/OER on TMNC/GDY using the adsorption strength of the oxygenated intermediates. Remarkably, the d-band center and charge transfer of the TM active sites can be utilized to correlate the ORR/OER catalytic activity and their electronic properties. Our findings not only suggested an ideal bifunctional oxygen electrocatalyst, but also provided a useful strategy to obtain highly efficient catalysts by interface engineering of two-dimensional heterostructures.

Machine-learning-accelerated screening of single metal atoms anchored on MnPS3 monolayers as promising bifunctional oxygen electrocatalysts.

Searching for bifunctional oxygen electrocatalysts with good catalytic performance to promote the oxygen evolution/reduction reactions (OER/ORR) is of great significance to the development of sustainable and renewable clean energy. Herein, we performed density functional theory (DFT) and machine-learning (DFT-ML) hybrid computations to investigate the potential of a series of single transition metal atoms anchored on the experimentally available MnPS3 monolayer (TM/MnPS3) as the bifunctional electrocatalysts for the ORR/OER. The results revealed that the interactions of these metal atoms with MnPS3 are rather strong, thus guaranteeing their high stability for practical applications. Remarkably, the highly efficient ORR/OER can be achieved on Rh/MnPS3 and Ni/MnPS3 with lower overpotentials than those of metal benchmarks, which can be further rationalized by establishing the volcano and contour plots. Furthermore, the ML results showed that the bond length of TM atoms with the adsorbed O species (dTM-O), the number of d electrons (Ne), the d-center (εd), the radius (rTM) and the first ionization energy (Im) of the TM atoms are the primary descriptors featuring the adsorption behavior. Our findings not only suggest novel highly efficient bifunctional oxygen electrocatalysts, but also provide cost-effective opportunities for the design of single-atom catalysts using the DFT-ML hybrid method.

N-Doped CrS2 Monolayer as a Highly-Efficient Catalyst for Oxygen Reduction Reaction: A Computational Study.

Searching for low-cost and highly-efficient oxygen reduction reaction (ORR) catalysts is crucial to the large-scale application of fuel cells. Herein, by means of density functional theory (DFT) computations, we proposed a new class of ORR catalysts by doping the CrS2 monolayer with non-metal atoms (X@CrS2, X = B, C, N, O, Si, P, Cl, As, Se, and Br). Our results revealed that most of the X@CrS2 candidates exhibit negative formation energy and large binding energy, thus ensuring their high stability and offering great promise for experimental synthesis. Moreover, based on the computed free energy profiles, we predicted that N@CrS2 exhibits the best ORR catalytic activity among all considered candidates due to its lowest overpotential (0.41 V), which is even lower than that of the state-of-the-art Pt catalyst (0.45 V). Remarkably, the excellent catalytic performance of N@CrS2 for ORR can be ascribed to its optimal binding strength with the oxygenated intermediates, according to the computed linear scaling relationships and volcano plot, which can be well verified by the analysis of the p-band center as well as the charge transfer between oxygenated species and catalysts. Therefore, by carefully modulating the incorporated non-metal dopants, the CrS2 monolayer can be utilized as a promising ORR catalyst, which may offer a new strategy to further develop eligible electrocatalysts in fuel cells.

Engineering d-band center of iron single atom site through boron incorporation to trigger the efficient bifunctional oxygen electrocatalysis.

Modulating the microenvironment of single-metal active sites holds excellent promises for developing highly efficiently oxygen electrocatalysts. Herein, by combining theoretical predictions and experiments, we reported a general strategy to engineer the electronic properties of iron-nitrogen-carbon (FeN4/C) catalysts via the incorporation of the boron (B) atom for achieving improved catalytic activity in oxygen electrocatalysis. Our theoretical results revealed that B modulation effectively tunes the d-band center of the iron (Fe) active site to optimize its adsorption strength with oxygenated species, greatly enhancing oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) activity. Our experimental measurements then confirmed the above theoretical predictions: the as-synthesized B-doped FeN4/C (Fe-N4-B) material can perform as an eligible bifunctional catalyst for ORR and OER in alkaline media, and its catalytic activity even outperforms the commercial noble metal benchmarks. The present findings provide a promising strategy to further design the advanced catalysts for a wide range of electrochemical applications.

Single Ir atom anchored in pyrrolic-N4 doped graphene as a promising bifunctional electrocatalyst for the ORR/OER: a computational study.

The development of highly-efficient electrocatalysts with bifunctional catalytic activity for oxygen reduction reaction (ORR) and oxygen evolution reaction. (OER) still remains a great challenge for the large-scale application of renewable energy conversion and storage technologies. Herein, by means of comprehensive density functional theory (DFT) computations, we systematically explored the potential of pyrrolic-N doped graphene (pyrrolic-N4-G) supported various transition metal atoms (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ru, Pd, W, Os, Ir, and Pt) as electrocatalysts for the ORR and OER. Our results revealed that these TM/pyrrolic-N4-G candidates exhibit high electrochemical stability due to their positive dissolution potentials. Especially, the Ir/pyrrolic-N4-G can perform as a promising bifunctional electrocatalyst for both ORR and OER with the low overpotentials (ηORR = 0.34 V and ηOER = 0.32 V). Interestingly, multiple-level descriptors, including energy descriptor (ΔGOH* - ΔGO*), (ΔGOH*), structure descriptor (φ), and d-band center (ε) can well rationalize the origin of the high catalytic activity of Ir/pyrrolic-N4-G for the ORR/OER. Our findings not only further enrich the SACs, but also open a new avenue to develop novel 2D materials-based SACs for highly efficient oxygen electrocatalysts.

Two-dimensional IrN2 monolayer: An efficient bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions.

The development of bifunctional electrocatalysts with good stability and high efficiency for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for renewable energy conversion and storage. Herein, by means of swarm-intelligence structure search and density functional theory (DFT) computations, we proposed a novel kind of two-dimensional (2D) monolayer with hypercoordinate structure as electrocatalysts for ORR/OER, namely, transition dinitride (TMN2, TM = V, Co, Rh, Pd, W, Re, and Ir) monolayer. Our result revealed that these TMN2 monolayers have excellent thermal, dynamic and chemical stability, as well as inherent metallic nature for their practical applications in electrocatalysis. More interestingly, among all 2D TMN2 materials, the IrN2 monolayer was suggested to perform as an ideal bifunctional electrocatalyst for ORR/OER with a low overpotential of 0.47 and 0.27 V, respectively, which is comparable to Pt and Ir- or Ru-based oxides. Furthermore, by examining the d-band centers of the active sites in different TMN2 monolayers, we well rationalized the superior catalytic activity of IrN2 monolayer for ORR/OER. Our findings not only further enrich 2D nanomaterials with hypercoordinate structure, but also open a new door to develop bifunctional oxygen electrocatalysts with high efficiency.

Enhanced catalytic activity of MXene for nitrogen electoreduction reaction by carbon doping.

The electrocatalytic performance of nitrogen reduction reaction (NRR) is seriously hindered by the lack of cost-effective electrocatalysts with high-efficiency and high-selectivity. In this work, the NRR catalytic activity of single carbon (C) atom embedded into two-dimensional (2D) transition metal carbides (M2CO2, M = Ti, Zr, Hf, Nb, Ta, Mo, and W) with oxygen vacancy was systematically evaluated by means of comprehensive density functional theory (DFT) computations. Our results revealed that the embedded single C atom possesses good durability due to its strong interaction with metal atoms around vacancy in these MXenes. Interestingly, through high-throughput screening, the single C atoms anchored on Nb2CO2, Mo2CO2, and W2CO2 nanosheets are identified as promise NRR catalysts with high-activity due to their low limiting potentials (-0.14 to  -0.38 V) via a distal mechanism and outstanding selectivity again the competing hydrogen evolution reaction. Remarkably, the intrinsic activity of the C single atom supported by these MXenes mainly originates from the activation degree of the adsorbed N2 molecule, which is greatly dependent on the electron filling degree of pz orbital in C atom. Thus, by carefully choosing suitable substrates, the single C catalyst can be utilized as ideal NRR catalysts for NH3 synthesis.

Single transition metal atoms anchored on a C2N monolayer as efficient catalysts for hydrazine electrooxidation.

Searching for highly-efficient and low-cost electrocatalysts for the hydrazine oxidation reaction (HzOR) is a key issue in the development of direct hydrazine fuel cells for hydrogen production, which is a promising energy-efficient conversion technology to replace the sluggish oxygen evolution reaction in water splitting. Herein, the potential of a series of single transition metal atoms anchored on nitrogenated holey graphene (TM@C2N, TM = Ti, Mn, Fe, Co, Ni, Cu, Mo, Rh, Ru, Pd, Pt, Au, Ag, and W) as catalysts for the HzOR was systematically explored by means of comprehensive density functional theory (DFT) computations. Our results revealed that these TM atoms anchored on a C2N monolayer exhibit high stability due to their strong interactions with the N atoms on the C2N monolayer. Furthermore, on the basis of the computed free energy profiles, Ru@C2N, Mo@C2N, Ti@C2N, Co@C2N, and Fe@C2N were shown to display high HzOR catalytic activity due to their lower (or comparable) limiting potential compared to the well-established Fe-doped CoS2 nanosheet. In particular, Ru@C2N is identified as the best catalyst with the lowest limiting potential of -0.24 V due to its optimum difference between the adsorption strength of N2H3* and N2H2* species. More interestingly, we found that single Mo and Ti atoms also exhibit excellent catalytic performance for the hydrogen evolution reaction, suggesting their bifunctional activity towards hydrazine splitting for H2 production. Our findings provide a new avenue to develop an efficient single-atom electrocatalyst for experimental validation to convert hydrazine into hydrogen.

Comparison of PAH content, potential risk in vegetation, and bare soil near Daqing oil well and evaluating the effects of soil properties on PAHs.

As the largest oil field in China, Daqing oil field has been developed in the past six decades. The objectives of this study were to measure the levels of polycyclic aromatic hydrocarbons (PAHs) and assess their ecological risk of PAHs in vegetation soil and bare soil near oil well in Daqing and surrounding soil. Ten sites were selected from two types of soil in grassland: vegetation soil (VS, n = 5) and bare soil (BS, n = 5). The mean concentration of 16 PAHs (∑16 PAHs) was 2240.2 µg/kg. The mean concentrations of eight carcinogenic PAHs (∑8c PAHs) was 1312.3 µg/kg which accounts for 59% of ∑16 PAHs. The sampling sites had higher proportions of high weight molecular ringed PAHs with higher proportions of benzo (a) pyren (BaP) and benzo (k) fluoranthene (BkF). The main source of PAHs was petroleum, coal/biomass combustion, and vehicular emission in these sampling sites. According to Canadian soil quality guidelines, 60% sites had a significant risk to human health. Moreover, 50% sites had high ecological risk and 30% sites were close to this critical value. Notably, PAH levels were significantly higher in VS than BS; moreover, VS had higher organic matter (OM) content, soil dehydrogenase (sDHA) activity, and lower pH and salt content. A structural equation model was established to explore the effects of soil properties on PAH concentration in VS. The result revealed that OM and sDHA were meaningful to enhance the adsorption and biological fixation of PAHs. This study will provide basic information on PAH level and potential application for phytoremediation.

Photodegradation of naphthalene over Fe3O4 under visible light irradiation.

Using FeCl3 and FeSO4 as precursors, Fe3O4 were prepared by co-precipitation method via FeCl3 and FeSO4 aqueous solutions successively added to NaOH solution. The sample was proved by X-ray powder diffraction, transmission electron microscope, ultraviolet-visible spectrophotometry and magnetic measurement. The results showed that the prepared Fe3O4 material was composed of an average diameter of about 15 nm particles and nano rods with well-crystallized magnetite and stronger superparamagnetic, getting a saturation magnetization of 49.5 emu g-1. This Fe3O4 material was found to be an effective catalyst for photodegradation of naphthalene with or without H2O2 under visible light irradiation, getting 81.1% and 74.3% degradation rate in these two cases, respectively. The degradation pathway in the absence and presence of H2O2 was analysed via measurement of the distribution of degradation products by GC-MS and adsorption of reactants on the surface of the catalyst by in situ DRIFTS spectra.

Single transition metal atom embedded into a MoS2 nanosheet as a promising catalyst for electrochemical ammonia synthesis.

The electrochemical reduction of N2 to NH3 (NRR) under ambient conditions is significant for sustainable agriculture. Here, by means of density functional theory (DFT) computations, the potential of a series of single transition metal (TM) atoms embedded into a MoS2 monolayer with an S-vacancy (TM/MoS2) as electrocatalysts for NRR was systematically investigated. Our DFT results revealed that among all these considered candidate catalysts, the single Mo atom embedded into the MoS2 nanosheet was found to be the most active catalyst for NRR with an onset potential of -0.53 V, in which the hydrogenation of the adsorbed N2* to N2H* is the potential-determining step. The high stabilization of the N2H* species is responsible for the superior performance of the embedded Mo atom for the NRR, which is well consistent with its d-band center. Our findings may facilitate the further design of single-atom electrocatalysts with high efficiency for NH3 synthesis at room temperature.

CO2 electroreduction performance of a single transition metal atom supported on porphyrin-like graphene: a computational study.

Searching for low-cost, efficient, and stable electrocatalysts for CO2 electroreduction (CO2ER) reactions is highly desirable for the reduction of CO2 emission and its conversion into useful products, but remains a great challenge. In this work, single transition metal atoms supported on porphyrin-like graphene catalysts, i.e., TMN4/graphene, acting as electrocatalysts for CO2 reduction were explored by means of comprehensive density functional theory (DFT) computations. Our results revealed that these anchored TM atoms possess high stability due to their strong hybridization with the unsaturated N atoms of the substrate and function as the active sites. On the basis of the calculated adsorption strength of CO2ER intermediates, we have identified that single Co, Rh, and Ir atoms exhibit superior catalytic activity towards CO2 reduction. In particular, CH3OH is the preferred product of CO2ER on the CoN4/graphene catalyst with an overpotential of 0.59 V, while the RhN4/graphene and IrN4/graphene catalysts prefer to reduce CO2 to CH2O with an overpotential of 0.35 and 0.29 V, respectively. Our work may open a new avenue for the development of catalytic materials with high efficiency for CO2 electroreduction.

Pyrrolic-nitrogen doped graphene: a metal-free electrocatalyst with high efficiency and selectivity for the reduction of carbon dioxide to formic acid: a computational study.

Searching for metal-free catalysts for the carbon dioxide reduction reaction (CO2RR) has been a key challenge in the electrosynthesis of fuels for CO2 utilization. In this work, we investigated the potential of N-doped graphene as the electrocatalyst of CO2RR by means of comprehensive density functional theory (DFT) computations. The computations revealed that N-doping can modify the electronic properties of graphene for enhancing the electrochemical reduction of CO2 into CO and HCOOH, resulting in a low free energy barrier for the potential-limiting step to form the key intermediate COOH as well as the strong adsorption energy of adsorbed COOH and the weak adsorption energy of CO or HCOOH. The highest catalytic activity toward CO2RR is shown by pyrrolic N-doped graphene due to its lowest overpotential of 0.24 V among all N-doped graphenes, and leads exclusively to HCOOH as the product. Therefore, our results demonstrated that N-doped graphene holds great promise as an electrocatalyst for the CO2RR with high efficiency and selectivity by suitably tuning its N species.

Pyridine derivative/graphene nanoribbon composites as molecularly tunable heterogeneous electrocatalysts for the oxygen reduction reaction.

In this study, a strategy to design a new class of metal-free electrocatalysts for the oxygen reduction reaction (ORR) was proposed by means of density functional theory (DFT) computations. The electrocatalysts consist of various pyridine derivatives that are anchored on the edge sites of armchair graphene nanoribbons (AGNRs). Our results revealed that these anchored pyridine derivatives have considerably high stability, and the C atoms around the "external" N-dopant possess the largest positive charge, thus facilitating the ORR though a more efficient 4e pathway, in which the first electron is transferred into O2 molecules over a long range in the outer Helmholtz plane (i.e., the ET-OHP mechanism). Among these designed catalysts, the pyrimidine/AGNR exhibits the highest catalytic activity, which can be comparable to that of Pt-based catalysts. Therefore, our computations suggested that the combination of pyridine derivatives with graphene nanoribbons can constitute a novel and well-defined heterogeneous electrocatalyst with good stability and tunable active sites for the ORR, which provides a useful guidance to develop the next-generation of low-cost and metal-free electrocatalysts with accurate N species and content for the ORR in fuel cells.

Stability and properties of the two-dimensional hexagonal boron nitride monolayer functionalized by hydroxyl (OH) radicals: a theoretical study.

Motivated by the great advance in graphene hydroxide--a versatile material with various applications--we performed density functional theory (DFT) calculations to study the functionalization of the two-dimensional hexagonal boron nitride (h-BN) sheet with hydroxyl (OH) radicals, which has been achieved experimentally recently. Particular attention was paid to searching for the most favorable site(s) for the adsorbed OH radicals on a h-BN sheet and addressing the roles of OH radical coverage on the stability and properties of functionalized h-BN sheet. The results indicate that, for an individual OH radica, the most stable configuration is that it is adsorbed on the B site of the h-BN surface with an adsorption energy of -0.88 eV and a magnetic moment of 1.00 µ(B). Upon adsorption of more than one OH radical on a h-BN sheet, however, these adsorbates prefer to adsorb in pairs on the B and its nearest N atoms from both sides of h-BN sheet without magnetic moment. An energy diagram of the average adsorption energy of OH radicals on h-BN sheet as a function of its coverage indicates that when the OH radical coverage reaches to 60 %, the functionalized h-BN sheet is the most stable among all studied configurations. More importantly, this configuration exhibits good thermal and dynamical stability at room temperature. Owing to the introduction of certain impurity levels, the band gap of h-BN sheet gradually decreases with increasing OH coverage, thereby enhancing its electrical conductivity.

Silicon-doped graphene: an effective and metal-free catalyst for NO reduction to N2O?

Density functional theory (DFT) calculations were performed on the NO reduction on the silicon (Si)-doped graphene. The results showed that monomeric NO dissociation is subject to a high barrier and large endothermicity and thus is unlikely to occur. In contrast, it was found that NO can easily be converted into N2O through a dimer mechanism. In this process, a two-step mechanism was identified: (i) the coupling of two NO molecules into a (NO)2 dimer, followed by (ii) the dissociation of (NO)2 dimer into N2O + O(ad). In the energetically most favorable pathway, the trans-(NO)2 dimer was shown to be a necessary intermediate with a total energy barrier of 0.464 eV. The catalytic reactivity of Si-doped graphene to NO reduction was interpreted on the basis of the projected density of states and charge transfer.

Can Si-doped graphene activate or dissociate O2 molecule?

Recently, the adsorption and dissociation of oxygen molecule on a metal-free catalyst has attracted considerable attention due to the fundamental and industrial importance. In the present work, we have investigated the adsorption and dissociation of O(2) molecule on pristine and silicon-doped graphene, using density functional theory calculations. We found that O(2) is firstly adsorbed on Si-doped graphene by [2+1] or [2+2] cycloaddition, with adsorption energies of -1.439 and -0.856eV, respectively. Following this, the molecularly adsorbed O(2) can be dissociated in different pathways. In the most favorable reaction path, the dissociation barrier of adsorbed O(2) is significantly reduced from 3.180 to 0.206eV due to the doping of silicon into graphene. Our results may be useful to further develop effective metal-free catalysts for the oxygen reduction reactions (ORRs), thus greatly widening the potential applications of graphene.

Can trans-polyacetylene be formed on single-walled carbon-doped boron nitride nanotubes?

Recently, the grafting of polymer chains onto nanotubes has attracted increasing attention as it can potentially be used to enhance the solubility of nanotubes and in the development of novel nanotube-based devices. In this article, based on density functional theory (DFT) calculations, we report the formation of trans-polyacetylene on single-walled carbon-doped boron nitride nanotubes (BNNTs) through their adsorption of a series of C(2)H(2) molecules. The results show that, rather than through [2 + 2] cycloaddition, an individualmolecule would preferentially attach to a carbon-doped BNNT via "carbon attack" (i.e., a carbon in the C(2)H(2) attacks a site on the BNNT). The adsorption energy gradually decreases with increasing tube diameter. The free radical of the carbon-doped BNNT is almost completely transferred to the carbon atom at the end of the adsorbed C(2)H(2) molecule. When another C(2)H(2) molecule approaches the carbon-doped BNNT, it is most energetically favorable for this C(2)H(2) molecule to be adsorbed at the end of the previously adsorbed C(2)H(2) molecule, and so on with extra C(2)H(2) molecules, leading to the formation of polyacetylene on the nanotube. The spin of the whole system is always localized at the tip of the polyacetylene formed, which initiates the adsorption of the incoming species. The present results imply that carbon-doped BNNT is an effective "metal-free" initiator for the formation of polyacetylene.