Single-Atom Catalysts for CO2 Electroreduction with Significant Activity and Selectivity Improvements

Discovering electrocatalysts that can efficiently convert carbon dioxide (CO2) to valuable fuels and feedstocks using excess renewable electricity is an emergent carbon-neutral technology. A single metal atom embedded in doped graphene, i.e., single-atom catalyst (SAC), possesses high activity and selectivity for electrochemical CO2 reduction (CO2R) to CO, yet further reduction to hydrocarbons is challenging. Here, using density functional theory calculations, we investigate stability and reactivity of a broad SAC chemical space with various metal centers (3d transition metals) and dopants (2p dopants of B, N, O; 3p dopants of P, S) as electrocatalysts for CO2R to methane and methanol. We observe that the rigidities of these SACs depend on the type of dopants, with 3p-coordinating SACs exhibiting more severe out-of-plane distortion than 2p-coordinating SACs. Using CO adsorption energy as a descriptor for CO2R reactivity, we narrow down the candidates and identify seven SACs with near-optimal CO binding strength. We then elucidate full reaction mechanisms towards methane and methanol generation on these identified candidates and observe highly dopant-dependent activity and rate-limiting steps, divergent from conventional mechanistic understanding on metallic surfaces, calling into question whether previous design principles established on metals are directly transferrable to SACs. Consequently, we find that zinc embedded in boron-doped graphene (Zn-B-C) is a highly active catalyst for electrochemical CO2R to C1 hydrocarbons. Our work reveals the opportunities of tuning SAC reactivity via engineering dopants and metals and highlights the importance of re-elucidating CO2R reaction mechanisms on SACs towards unearthing new design principles for SAC chemistry.

Propane Dehydrogenation on Single-Site [PtZn4] Intermetallic Catalysts

Based on first-principles calculations, the potential of Ti2 CO2 monolayer (MXene) as a single-atom catalyst (SAC) support for 3d transition metal (TM) atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) is studied for CO oxidation. We first screen the support effect according to the stability of a single metal atom and find that Sc and Ti supported on Ti2 CO2 have stronger adsorption energy than the cohesive energy of their bulk counterparts and therefore, we selected Sc and Ti supported on Ti2 CO2 for further catalytic reactions. The stability and the potential catalytic reactivity are verified by electronic structure and charge transfer analysis. Both Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms are considered in this study, and lower energy barriers of 0.002 and 0.37 eV were found in the ER mechanism compared to the LH mechanism, which are 0.25 and 0.34 eV for Sc and Ti catalysts, respectively. Moreover, kinetic ER and LH mechanisms are favorable for both Sc- and Ti/Ti2 CO2 because of the comparable energy barrier to other metals and SAC supported on 2D materials. However, Ti/Ti2 CO2 catalyst is thermodynamically unfavorable. Based on these calculations, we propose that Sc supported on Ti2 CO2 is the best catalyst for CO-oxidation. The current study not only broadens the scope of the single-atom Sc catalyst but also extends the consideration of MXene support for catalyst optimization.

Influence of uncoordinated N content on Ni[sbnd]N[sbnd]C electrocatalyst for CO2 reduction: Combining first principle with machine learning

Revealing the underlying relationship between the loading of metal atoms and catalytic behavior is crucial for guiding the design of optimal single-atom catalysts (SACs) in heterogeneous catalysis. However, the apparent catalytic behavior controlled by interactions between individual active sites remains elusive. Here, we successfully synthesized a series of atomically dispersed Ir SACs with the Ir-PN3 configuration through a straightforward P atom anchoring strategy, which effectively prevents metal aggregation due to its strong metal-coordination capabilities. These SACs are loaded at varying percentages, ranging from 5% to 21 wt %, and exhibited an unforeseen “volcano-type” relationship between the Ir loadings and their acidic oxygen evolution reaction activity. Notably, the highest activity is observed at a moderate loading level of 14%. This phenomenon can be attributed to the competition between the number and electronic structure of Ir active sites. Electronic structural characterizations and theoretical calculations reveal that the key for establishing the “volcano-type” relationship is the strong electronic interactions among Ir active sites at high densities, leading to charge rearrangement. This study highlights the importance of the relationship between the metal loading and activity of SACs, which may not exhibit a positive linear regression.

The electrochemical reduction of CO2 has the potential to store energy from intermittent renewable sources and to produce carbon-neutral fuels and chemicals1. In recent years, theoretical studies of CO2 reduction have usually applied the computational hydrogen electrode model, which allows for the determination of the energies of reaction intermediates without explicitly treating the potential and the ions in solution2. This thermochemical approach has been shown to correlate well with experimental onset potentials 3,4 and applied to computational screening of new catalysts 5,6. However, an understanding of charge transfer barriers, kinetics, selectivity, and pH effects all require explicit consideration of solvent and charge. In this talk, I will discuss new developments in the explicit treatment of the electrochemical interface, which is based on a simple capacitor model that uses the interfacial charge to obtain barriers at constant potential 7,8. I will then discuss the application of such a scheme to CO2 reduction: the determination of reaction pathways on transition metals, field and solvation effects9,10, the resultant kinetics, and catalyst design criteria. 1 Whipple, D. T. & Kenis, P. J. A. Prospects of CO2 Utilization via Direct Heterogeneous Electrochemical Reduction. The Journal of Physical Chemistry Letters 1, 3451-3458, doi:10.1021/jz1012627 (2010). 2 Nørskov, J. K. et al. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 108, 17886--17892 (2004). 3 Shi, C., Hansen, H. A., Lausche, A. C. & Nørskov, J. K. Trends in electrochemical CO2 reduction activity for open and close-packed metal surfaces. Physical Chemistry Chemical Physics 16, 4720-4727, doi:10.1039/C3CP54822H (2014). 4 Kuhl, K. P. et al. Electrocatalytic Conversion of Carbon Dioxide to Methane and Methanol on Transition Metal Surfaces. Journal of the American Chemical Society, doi:10.1021/ja505791r (2014). 5 Chan, K., Tsai, C., Hansen, H. A. & Nørskov, J. K. Molybdenum Sulfides and Selenides as Possible Electrocatalysts for CO2 Reduction. ChemCatChem 6, 1899-1905, doi:10.1002/cctc.201402128 (2014). 6 Asadi, M. et al. Robust carbon dioxide reduction on molybdenum disulphide edges. Nature communications 5 (2014). 7 Chan, K. & Nørskov, J. K. Electrochemical Barriers Made Simple. J. Phys. Chem. Lett., 2663--2668 (2015). 8 Chan, K. & Nørskov, J. K. Potential Dependence of Electrochemical Barriers from ab Initio Calculations. The Journal of Physical Chemistry Letters, 1686-1690, doi:10.1021/acs.jpclett.6b00382 (2016). 9 Montoya, J. H., Shi, C., Chan, K. & Norskov, J. K. Theoretical Insights into a CO Dimerization Mechanism in CO2 Electroreduction. The journal of physical chemistry letters 6, 2032--2037 (2015). 10 Chen, L. D., Urushihara, M., Chan, K. & Nørskov, J. Electric Field Effects in Electrochemical CO2 Reduction. ACS Catalysis (2016). 11 Liu, X. et al. Understanding trends in CO2 reductin on transition metals. (2016).

Single atom catalysts (SACs) can achieve a maximum atom utilization efficiency of 100%, which provides significantly increased active sites compared with traditional catalysts during catalytic reactions. Synchrotron radiation technology is an important characterization method for identifying single-atom catalysts. Several types of internal information, such as the coordination number, bond length and electronic structure of metals, can all be analyzed. This review will focus on the introduction of synchrotron radiation techniques and their applications in SACs. First, the fundamentals of synchrotron radiation and the corresponding techniques applied in characterization of SACs will be briefly introduced, such as X-ray absorption near edge spectroscopy and extended X-ray absorption fine structure spectroscopy and in situ techniques. The detailed information obtained from synchrotron radiation X-ray characterization is described through four routes: 1) the local environment of a specific atom; 2) the oxidation state of SACs; 3) electronic structures at different orbitals; and 4) the in situ structure modification during catalytic reaction. In addition, a systematic summary of synchrotron radiation X-ray characterization on different types of SACs (noble metals and transition metals) will be discussed.

Single atom catalysts (SACs) have shown their noticeable potential and gradually become a new favorite in catalytic field due to the particular selectivity, high catalytic performance, and strong durability. The most important factor in the synthesis of SACs is the selection of appropriate support and formation of metal-support interaction. Among a large number of nanomaterials, MXenes can be utilized as benign supports for fixing SACs because of their expansive specific surface area, regulable bandgap, superior electronic conductivity, and strong mechanical stability. The structure and property of MXenes can be manipulated by changing transition metal elements and surface termination. Here, the uniqueness and superiority of MXenes as superexcellent supports for confining SACs are analyzed from structure and property. The synthetic strategy of MXene-supported SACs is also summarized, especially emphasizing the immobilization of isolated atom against aggregation by utilizing the formidable metal-support covalent coordination interaction. In addition, the applications of MXene-supported SACs in electrocatalytic field are highlighted, including hydrogen evolution reaction, oxygen evolution reaction, overall water splitting, oxygen reduction reaction, and nitrogen reduction reaction. Finally, the challenges and prospects are pointed out for the further understanding and practical application of MXene-supported SACs in electrocatalysis.

Low-cost single-atom transition metals on two-dimensional SnO nanosheets for efficient hydrogen evolution catalysis in all pH-range

Single-atom catalysts are proven to be an effective strategy for suppressing shuttle effect at the source by accelerating the redox kinetics of intermediate polysulfides in lithium-sulfur (Li-S) batteries. However, only a few 3d transition metal single-atom catalysts (Ti, Fe, Co, Ni) are currently applied for sulfur reduction/oxidation reactions (SRR/SOR), which remains challenging for screening new efficient catalysts and understanding the relationship between structure-activity of catalysts. Herein, N-doped defective graphene (NG) supported 3d, 4d, and 5d transition metals are used as single-atom catalyst models to explore electrocatalytic SRR/SOR in Li-S batteries by using density functional theory calculations. The results show that M1 /NG (M1 = Ru, Rh, Ir, Os) exhibits lower free energy change of rate-determining step and Li2 S decomposition energy barrier, which significantly enhance the SRR and SOR activity compared to other single-atom catalysts. Furthermore, the study accurately predicts the by machine learning based on various descriptors and reveals the origin of the catalyst activity by analyzing the importance of the descriptors. This work provides great significance for understanding the relationships between the structure-activity of catalysts, and manifests that the employed machine learning approach is instructive for theoretical studies of single-atom catalytic reactions.

Single atom catalysts (SACs) have emerged as a novel class of heterogeneous catalysts which exhibit superior catalytic performance in various reaction, CO oxidation, CO2 conversion, oxygen reduction reaction (ORR), etc. It is challenging to explore electronic structure and catalytic mechanics of SACs when the single metal atom is bonding with different dopants via experimental methods, which influence the catalytic efficiency directly. In this paper, Density Functional Theory (DFT) calculations, an ab initio simulation methods, have been carried out to investigate the properties of Fe-N-C, Fe-O-C, Fe-P-C, and Fe-S-C with oxo or hydroxyl group, four common SACs models. Geometry structure of Fe-N-C, Fe-O-C, Fe-P-C, Fe-S-C, and after each has bonded with oxo or hydroxyl group are studied. Formation energy of the Single Atom Catalyst (SAC), oxygen formation energy, and hydrogen atom transfer energy of the four SACs models are also calculated. In order to investigate the electronic structure of the four SACs, we examined the HOMO LUMO gap values for each system. In addition, we plotted the corresponding HOMO and LUMO orbitals of each SACs models and with oxo or hydroxyl group. Through analyzing the data, I anticipated that Fe-N-C model is the easiest to synthesis and the most stable among the four models. Our work discloses the microscopic property of SACs and provides valuable suggestions to experimental scientists.

Precise electronic structure modulation on MXene-based single atom catalysts for high-performance electrocatalytic CO2 reduction reaction: A first-principle study

Developing high-efficiency single-atom catalysts (SACs) is essential for the carbon dioxide reduction reaction (CO2RR) to produce fuels and chemicals, yet achieving optimal performance remains challenging. In this work, we applied a grain boundary (GB) strategy to rationally design a series of MoS2 supported SACs for electrocatalytic CO2RR. Concretely, different 3d transition metal (TM) atoms were respectively anchored at three types of MoS2 GBs (e.g., 5|7, 8|8, and 4|8 GBs) to form various TM@GB SACs. Our density-functional theory (DFT) calculations revealed that, in comparison with the MoS2 perfect monolayer, strong interactions between TM atoms and MoS2 GBs shift down the d-band center of these atoms, hence improving the CO2RR activity of TM@GB SACs. Among TM@GBs, V@5|7, Cr@8|8, and V@4|8 GBs were demonstrated to possess excellent catalytic activity, enabling spontaneous CO2 reduction to CO at applied electrode potentials of -0.30, -0.10, and -0.26 V (vs SHE), respectively. Further, a pivotal descriptor (ψ) based on the inherent structural properties of GB and TM was proposed to correlate the structure with the CO2RR activity. Our findings highlight the potential of GB engineering as a strategic tool for modulating the properties of SACs, broadening the applications in catalyst design and optimization.

Emerging single-atom catalysts for efficient electrocatalytic CO2 reduction and water splitting: Recent advances

Strategy to weaken the oxygen adsorption on single-atom catalysts towards oxygen-involved reactions