Graphene-supported Single-atom Research Articles

Catalytic mechanism of sulfur-nitrogen doping metal single-atom graphene-based catalysts for the conversion of CO2 to CH4: A computational study

The conversion of CO2 into renewable fuel (CH4) has significant environmental and economic benefits. Graphene-supported single-atom catalysts (SACs) show promise as electrocatalysts for the carbon dioxide reduction reaction (CO2RR). In this study, using spin-polarized density functional theory (DFT) calculations, we investigate the potential for converting CO2 to CH4 on the representative SACs, specifically graphene-supported TM-N-S (transition metal-nitrogen-sulfur). The results show that TM-N-S can break the scaling relationship between the adsorption energy of *CHO and *OCH2. The free energy profiles indicate that among the candidate TM-N-S SACs, Fe, Co, and Ni-N-S stand out with low overpotentials of 0.23, 0.40, and 0.39 V, respectively. Compared with the competing hydrogen evolution reaction (HER), they exhibit higher CO2 selectivity and catalytic activity. Analysis of electronic structure and charge variations reveals the origin of CO2RR activity and the binding strength between catalyst and intermediate. This work uncovers the potential mechanism of CO2RR, providing valuable insights into the conversion of CO2 to CH4 via TM-N-S catalysis and offering novel perspectives for the design and screening of efficient SACs for CO2RR.

Activation of peroxymonopulfate by graphene-supported single-atom FeN5 with an ultra-low dose at low temperature: Nonradical mechanism and antibacterial activity

The catalytic activity of single-atom catalysts (SACs) is closely related to the coordination environment of metal center. Here, a new peroxymonosulfate (PMS) activation strategy based on graphitic carbon nitride-supported Fe SACs (FeCN) containing a five-coordinated structure (FeN5) exhibited remarkable catalytic performance towards doxorubicin hydrochloride (DOX) with an ultra-low-dose catalyst (10 mg·L−1) at a low temperature (4 °C). Singlet oxygen (1O2) and high-valent iron-oxo species (HV-Fe)-dominated nonradical processes are responsible for the degradation and rapid inactivation of Escherichia coli (E. coil) and S. aureus. Density functional theory (DFT) calculations demonstrated that the terminal O of PMS adsorbed onto Fe-N5 sites facilitated the subsequent homolysis of the OO bond to genarate 1O2 and HV-Fe. This work not only develops an efficient catalyst by fine-tuning the local coordination environment but also provides an insight into the mechanism of the nonradical-species-mediated PMS process as well as potential application at the device level.

Fine-Tuning Dual Single-Atom Metal Sites on Graphene toward Enhanced Oxygen Reduction Reaction Activity.

The oxygen reduction reaction (ORR) remains at the forefront of research in diverse energy and sustainability domains. While graphene-supported single-atom catalysts (SACs) have garnered attention for optimizing ORR efficiency, tailoring the interactions between adjacent single-atom sites presents intricate challenges. In this study, we leveraged density functional theory (DFT) calculations and cutting-edge machine learning (ML) techniques to explore 144 graphene-supported SACs, featuring interacting M1-N4 and M2-N4 moieties (M1, M2 = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag Ir, Pt, Au), denoted as M1-M2. By tailoring these interactions, we discovered 13 exceptional SACs outperforming the benchmark catalyst Fe(OH)-N4, including the best-performing Fe-Pd and several non-noble-metal SACs like Fe-Ag, Ag-Cu, and Ag-Ag. Venturing further, our ML models effectively capture the correlation between single-atom metal properties and overpotential, offering tools for rational electrocatalyst design. Our study illuminates the path to efficient SAC-catalyzed ORR, fostering a sustainable, energy-efficient future.

Graphene-Supported Single-Atom Ni as High-Performance Catalyst for Direct Methane-to-Methanol Conversion

Graphene-Supported Single-Atom Ni as High-Performance Catalyst for Direct Methane-to-Methanol Conversion

Proximity effects in graphene-supported single-atom catalysts for hydrogen evolution reaction.

The interaction between adjacent active sites is crucial to balance the efficiency and utilization of functional atoms in single-atom catalysts. Herein, the catalytic activity of hydrogen evolution reaction at different site (nitrogen coordinated transition metal centers embedded in graphene) distances was comprehensively investigated by density functional theory calculations. The results show that a proximity effect of reactivity and site spacing can be identified in the Co-series single-atom catalysts. Although the proximity effect is more linearly responded with the site spacing along x direction, an optimal distance of ∼0.8 and ∼2.8nm are found for Co and Rh, Ir atoms, respectively. An in-depth analysis of the electronic property reveals that the proximity effect is caused by the distinct net charge of the active site, which is affected by the dz2 position relative to EF. Subsequently, an excess electron nodal channel in x direction was found to serve as a communication pathway between the active sites. Through the finding in this work, an optimal Fe-N2C2 structure was deliberately designed and has shown prominent proximity effect as Co-series do. The results reported in this work provide a simple and effective tuning method for the reactivity of a single-atom catalyst.

Understanding unique orbital matching in graphene-supported single-atom sites for enhanced gas adsorption sensitivity

The understanding and prediction of the adsorption sensitivity of molecules in single-atom systems is an instructive and also challenging research topic. We here performed theoretical studies on interaction between isolated molecules and single transition-metal (TM) atoms in typical TM-N-C systems. This work demonstrates “unique orbital matching”, e.g. CO/NO-2π* and TM-dxz,yz, or lone pair orbital of H2S/H2Se and TM-dz2, in determining adsorption mode. The charge transfer between adsorbents and substrate could be well described by orbital energy difference and shows a linear relationship with after adsorption local work function. Our studies could provide instructive guidance for efficient design of sensitive materials.

Role of Peripheral Coordination Boron in Electrocatalytic Nitrogen Reduction over N-Doped Graphene-Supported Single-Atom Catalysts.

In this work, we investigate the effect of peripheral B doping on the electrocatalytic nitrogen reduction reaction (NRR) performance of N-doped graphene-supported single-metal atoms using density functional theory (DFT) calculations. Our results showed that the peripheral coordination of B atoms could improve the stability of the single-atom catalysts (SACs) and weaken the binding of nitrogen to the central atom. Interestingly, it was found that there was a linear correlation between the change in the magnetic moment (μ) of single-metal atoms and the change in the limiting potential (UL) of the optimum NRR pathway before and after B doping. It was also found that the introduction of the B atom suppressed the hydrogen evolution reaction, thereby enhancing the NRR selectivity of the SACs. This work provides useful insights into the design of efficient SACs for electrocatalytic NRR.

A Surrogate Machine Learning Model for the Design of Single-Atom Catalyst on Carbon and Porphyrin Supports towards Electrochemistry

We apply the machine learning (ML) tool to calculate the Gibbs free energy (ΔG) of reaction intermediates rapidly and accurately as a guide for designing porphyrin- and graphene-supported single-atom catalysts (SACs) toward electrochemical reactions. Based on the 2105 DFT calculation data from the literature, we trained a support vector machine (SVR) algorithm. The hyperparameters were optimized using Bayesian optimization along with 10-fold cross-validation to avoid overfitting. Based on the Shapley Additive exPlanation (SHAP) and permutation methods, the feature importance analysis suggests that the most important parameters are the number of pyridinic nitrogen (Npy), the number of d electrons (θd), and the number of valence electrons of reaction intermediates. Inspired by this feature importance analysis and the Pearson correlation coefficient, we found a linear dependent, simple, and general descriptor (φ) to describe ΔG of reaction intermediates (e.g., ΔGOH* = 0.020φ – 2.190). Using the trained SVR algorithm, ΔGOH*, ΔGO*, ΔGOOH*, ΔGOO*, ΔGH*, ΔGCOOH*, ΔGCO*, and ΔGN2* intermediates are predicted for the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), the hydrogen evolution reaction (HER), and the CO2 reduction reaction (CO2RR). The SVR model predicts an ORR overpotential of 0.51 V and an HER overpotential of 0.22 V for FeN4-SAC. Moreover, we used the SVR algorithm for high-throughput screening of SACs, suggesting new SACs with low ORR overpotentials. This strategy provides a data-driven catalyst design method that significantly reduces the costs of DFT calculations while providing the means for designing SACs for electrocatalysis and beyond.

Advances in Graphene-Supported Single-Atom Catalysts for Clean Energy Conversion

Advances in Graphene-Supported Single-Atom Catalysts for Clean Energy Conversion

A nitrogen-doped graphene-supported nickel-single-atom catalyst in the flow cell meets the industrial criteria of carbon dioxide reduction reaction to carbon monoxide

Carbon dioxide reduction reaction (CO2RR) is a promising approach to accomplishing net zero CO2 emissions. Among CO2RR catalysts, nitrogen-doped graphene-supported single-atom catalysts show a remarkable conversion rate from CO2 to CO; however, the low production amount has been limited using the conversion H cell, hindering its industrial development. In this work, we synthesize a nitrogen-doped graphene-supported nickel-single-atom catalyst and conduct CO2RR in a flow cell, exhibiting a CO2-to-CO Faradaic efficiency of 96% and a partial current density of 144 mA cm−2. It can also achieve the highest partial current density of 204 mA cm−2 with a turnover frequency of 7,852 h−1. According to the techno-economic analysis, these preeminent activities meet the industrial criteria (Faradaic efficiency >60% and partial current density >100 mA cm−2). This activity enhancement using a flow system can significantly accelerate net-zero CO2 emission realization.

Engineering the Morphology and Microenvironment of a Graphene-Supported Co-N-C Single-Atom Electrocatalyst for Enhanced Hydrogen Evolution.

Graphene-supported single-atom catalysts (SACs) are promising alternatives to precious metals for catalyzing the technologically important hydrogen evolution reaction (HER), but their performances are limited by the low intrinsic activity and insufficient mass transport. Herein, a highly HER-active graphene-supported Co-N-C SAC is reported with unique design features in the morphology of the substrate and the microenvironment of the single metal sites: i) the crumpled and scrolled morphology of the graphene substrate circumvents the issues encountered by stacked nanoplatelets, resulting in improved exposure of the electrode/electrolyte interfaces (≈10 times enhancement); ii) the in-plane holes in graphene preferentially orientate the Co atoms at the edge sites with low-coordinated Co-N3 configuration that exhibits enhanced intrinsic activity (≈2.6 times enhancement compared to the conventional Co-N4 moiety), as evidenced by detailed experiments and density functional theory calculations. As a result, this catalyst exhibits significantly improved HER activity with an overpotential (η) of merely 82mV at 10mA cm-2 , a small Tafel slope of 59.0mV dec-1 and a turnover frequency of 0.81 s-1 at η= 100mV, ranking it among the best Co-N-C SACs.

Formulating electronic descriptors to rationally design graphene-supported single-atom catalysts for oxygen electrocatalysis

Descriptors indicating activity and stability of G-SACs and a guideline for material design are suggested through pre hoc analysis.

Rational Design of Graphene-Supported Single-Atom Catalysts for Electroreduction of Nitrogen.

Critically, the central metal atoms along with their coordination environment play a significant role in the catalytic performance of single-atom catalysts (SACs). Herein, 12 single Fe, Mo, and Ru atoms supported on defective graphene are theoretically deigned for investigation of their structural and electronic properties and catalytic nitrogen reduction reaction (NRR) performance using first-principles calculations. Our results reveal that graphene with vacancies can be an ideal anchoring site for stabilizing isolated metal atoms owing to the strong metal-support interaction, forming stable TMCx or TMNx active centers (x = 3 or 4). Six SACs are screened as promising NRR catalyst candidates with excellent activity and selectivity during NRR, and RuN3 is identified as the optimal one with an overpotential of ≥0.10 V via the distal mechanism.

Constructing FeN4/graphitic nitrogen atomic interface for high-efficiency electrochemical CO2 reduction over a broad potential window

Summary Atomically dispersed Fe–N–C catalyst has shown great performance in the CO2-to-CO conversion, yet the high CO selectivity is achieved only within a rather narrow potential range, which cannot well meet the requirements for the following CO dimerization or hydrogenation. Here, we developed a hydrogen-pyrolysis etching strategy to precisely manipulate the uncoordinated N dopants in the Fe–N–C catalyst. This strategy could preferentially eliminate pyridinic and pyrrolic N atoms while leaving graphitic N. The resulting catalyst gave a CO faradaic efficiency (FECO) above 90% over a broad window from −0.3 to −0.8 V (versus RHE) (in particular, FECO > 97% at −0.6 V). In situ ATR-SEIRAS and first-principle calculations further revealed that this strategy can not only suppress the parasitic H2 generation but also promote CO2 activation and protonation with the assistance of co-adsorbed H2O on the “atomic interface” of “FeN4/graphitic N.”

Universal Approach to Fabricating Graphene-Supported Single-Atom Catalysts from Doped ZnO Solid Solutions.

Single-atom catalysts (SACs) have attracted widespread interest for many catalytic applications because of their distinguishing properties. However, general and scalable synthesis of efficient SACs remains significantly challenging, which limits their applications. Here we report an efficient and universal approach to fabricating a series of high-content metal atoms anchored into hollow nitrogen-doped graphene frameworks (M-N-Grs; M represents Fe, Co, Ni, Cu, etc.) at gram-scale. The highly compatible doped ZnO templates, acting as the dispersants of targeted metal heteroatoms, can react with the incoming gaseous organic ligands to form doped metal–organic framework thin shells, whose composition determines the heteroatom species and contents in M-N-Grs. We achieved over 1.2 atom % (5.85 wt %) metal loading content, superior oxygen reduction activity over commercial Pt/C catalyst, and a very high diffusion-limiting current (6.82 mA cm–2). Both experimental analyses and theoretical calculations reveal the oxygen reduction activity sequence of M-N-Grs. Additionally, the superior performance in Fe-N-Gr is mainly attributed to its unique electron structure, rich exposed active sites, and robust hollow framework. This synthesis strategy will stimulate the rapid development of SACs for diverse energy-related fields.

Synthesis, Characterization and Analysis of Graphene-Supported Single-Atom Catalysts

Synthesis, Characterization and Analysis of Graphene-Supported Single-Atom Catalysts

Directly predicting limiting potentials from easily obtainable physical properties of graphene-supported single-atom electrocatalysts by machine learning

The oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are three critical reactions for energy-related applications, such as water electrolyzers and metal–air batteries.

Theoretical investigation on graphene-supported single-atom catalysts for electrochemical CO2 reduction

TM atoms supported on the graphene sheet (TM@Grs) as promising CO2 catalysts were investigated by first-principles calculations. Cr-, Co- and Rh@Grs show remarkable performance with the low limiting potentials for CO2RR.

Grand Canonical Potential Kinetics of CO2 Reduction Reaction over Graphene-Supported Single-Atom Catalysts

The Electrochemical reduction of CO2 to CO offers a promising strategy for managing the global carbon balance, but lack of selectivity towards traditional elctrocatlysts forces us to find an effective catalysts with high selectivity and stability. The single atom catalysts has emerged as potential candidate in the field of heterogeneous catalysis by offering single active site for different chemical reaction.1-3 Here, we studied the CO2 reduction mechanism over graphene supported Ni-single atom catalysts with the help of grand canonocal potential kinetics (GCP-K) formulation based on thermodynamics from quantum calculations.4 The GCP-K formulation obtained from the minimization of free enrgy using a legendare transformation relating the net charge of the system and applied potential. Using the transformation we are able to make a relation between GCP-K formulation with traditional Butler-Volmer kinetics, which allows us to describe the CO2 to CO reduction process elaborately at neutral medium and different applied voltages. We found that the geometry of transition states are continuously changes towards reaction coordinates with the applied potential as in Figure 1. The figure shows the transition states (TS) at lower potential looks similar to product geometry with high energy barrier while when we applied potential the TS moves towards reactant with lowering the reaction barrier. We also observed fraction of charges transferred from electrode to the reaction speices during the reaction at specific potential. Moreover, according to our GCP-K formulation, we describe the charge transfer from electrode to adsorbed species are continuous throughout the reaction path over traditional Butler-Volmer kinetics where a complete electron transfer from electrode to product shows discontinuty within reaction plane. The calcualtion results showed that the trans intermediate is more stable at higher potential than cis form of *COOH and the reaction barrier is lower along this pathways. We have shown that HER reaction is kinetically hindered on single Ni active sites, which provides almost 100% faradic CO efficiency. Finally, we found that the potential reaquied to produce 10 mA/cm2 current density is about -0.85 V, showing excellect aggrement with experimental literatures.5, 6 L. Yang, D. Cheng, H. Xu, X. Zeng, X. Wan, J. Shui, Z. Xiang and D. Cao, Proceedings of the National Academy of Sciences, 2018, 115, 6626-6631.M. D. Hossain, Z. Liu, M. Zhuang, X. Yan, G.-L. Xu, C. A. Gadre, A. Tyagi, I. H. Abidi, C.-J. Sun, H. Wong, A. Guda, Y. Hao, X. Pan, K. Amine and Z. Luo, Advanced Energy Materials, 2019, 9, 1803689.Y. Chen, S. Ji, C. Chen, Q. Peng, D. Wang and Y. Li, Joule, 2018, 2, 1242-1264.Y. Huang, R. J. Nielsen and W. A. Goddard, Journal of the American Chemical Society, 2018, 140, 16773-16782.H. B. Yang, S.-F. Hung, S. Liu, K. Yuan, S. Miao, L. Zhang, X. Huang, H.-Y. Wang, W. Cai, R. Chen, J. Gao, X. Yang, W. Chen, Y. Huang, H. M. Chen, C. M. Li, T. Zhang and B. Liu, Nature Energy, 2018, 3, 140-147.C. Zhao, X. Dai, T. Yao, W. Chen, X. Wang, J. Wang, J. Yang, S. Wei, Y. Wu and Y. Li, Journal of the American Chemical Society, 2017, 139, 8078-8081. Figure 1. Transition state (TS) changes with applied potential for trans-COOH to CO formation step. (a) TS moving towards reactant with decreasing reaction barrier as a function of potential, (b), (c) Reaction coordinates changes linearly with charges within the TS when potential is applied to begin the conversion process. Figure 1

Single Atoms for Energy Applications

Single Atoms for Energy Applications