Single-atom catalysts for CO2 electroreduction with significant activity and selectivity improvements.

Single-atom catalyst (SAC) has an electronic structure that is very different from its bulk counterparts, and has shown unexpectedly high specific activity with a significant reduction of noble metal usages although physical origins of such performance enhancements are still poorly understood. Herein, by means of density functional theory calculations, we for the first time investigate the great potential of single atom catalyst for CO2 electroreduction applications. In particular, we study a single transition metal atom anchored on the defective graphene with single or double vacancies, denoted as M@sv-Gr or M@dv-Gr, where M are various transition metals, as a CO2 reduction catalyst. Many SACs are indeed shown to be highly selective for CO2 reduction reaction over a competitive H2 evolution reaction due to a favorable adsorption of carboxyl or formate over hydrogen on the catalysts. On the basis of free energies, we found that the Pt@dv-Gr catalyst shows a remarkable reduction in the limiting potential for CH3OH production compared to any existing catalysts, synthesized or predicted (Figure 1). We focus on the Pt@dv-Gr to investigate the origin of improvement on the SACs compared to the transition metal. As shown in Figure 1, with the Pt@dv-Gr catalyst, all the intermediates are destabilized compared to those on Pt (211), but most importantly, the destabilization of *CO (0.98 eV) is much more noticeable than that of *CHO (0.42 eV), leading to a 0.49 V reduction in the limiting potential. We also observe that with SACs, the conventional scaling relation between the *CO binding and *CHO binding established for the bulk transition metal significantly deviates from linearity (Figure 2). We discuss features of SACs which contribute to the breakdown of scaling relation between *CO and *CHO, namely, the lack of atomic ensemble for adsorbates binding and the metal-support interactions that lead to the electronic structures conducive to the catalysis. Atomic Ensemble: The optimized geometries of the bare catalysts are shown for Pt@dv-Gr and Pt (211) in Figure 3. One can see that, for Pt (211), two surface Pt atoms are involved in the *CO bonding while only one Pt atom is bonding with *CHO, leading to a large destabilization in the relative free energies when going from *CO to *CHO. On the other hand, for Pt@dv-Gr, only one Pt atom is utilized for both *CO and *CHO binding, resulting in a much more moderate destabilization in relative free energies compared to Pt (211). Thus, the lack of Pt ensemble in the Pt@dv-Gr is responsible for a significantly weaker binding of *CO on the Pt@dv-Gr compared to the Pt (211) surface. Electronic Structure: The strong metal-support interaction affects the electronic structure of a metal atom in the SACs greatly. In Figure 4A, the Pt 5d density of states (DOS) in Pt@dv-Gr shows a significant orbital overlap with the C 2p orbitals of the graphene. The electron density isosurfaces (Figure 4B) illustrate that electron clouds of four carbon atoms surrounding the Pt atom are significantly hybridized with the Pt atom. The differential charge density map (Figure 4C) between the defective graphene and Pt@dv-Gr also suggests that the Pt atom is positively charged by electron transfer from the Pt atom to the defective graphene support. In this paper, we investigated the single atom catalysts (SACs) as a promising CO2 electroreduction catalyst using DFT calculations. The main findings of this work are as follows. (i) By comparing the free energies of the initial protonation steps for the CRR and HER, we found that all SACs can selectively reduce CO2 rather than producing H2. In particular, the predicted limiting potential for Pt@dv-Gr (-0.27 V) for CH3OH production is considerably less negative than the conventional transition metal catalysts. (ii) To understand the origin of the improvements in SACs, we investigated two aspects of the Pt@dv-Gr that affect the relative stability of *CO vs. *CHO. A one-fold bonding of *CO on Pt@dv-Gr due to a lack of atomic ensemble, as compared to the two-fold *CO bonding on the Pt (211) is responsible for the significant weakening of *CO binding on the Pt@dv-Gr. (iii) We investigated the electronic structure of Pt atom in the SAC to find the origin of the deviation of SACs from the conventional scaling relation of transition metals, which arises from the d-band center theory. We suggest that the strong electronic interaction between the d-orbital of metal atom and the p-orbital of graphene is responsible for the different behavior from the transition metal surfaces, as evidenced by the electron transfer and the overlap in the DOS. Figure 1

ZnO monolayer supported single atom catalysts for efficient nitrogen electroreduction to ammonia

Ammonia synthesis through electrochemical reduction of nitrogen molecules is a promising strategy to significantly reduce the energy consumption in traditional industrial process. Detailed mechanism study of multistep complex nitrogen reduction reaction is prerequisite for the design of highly efficient catalyst. Stable atomically dispersed catalyst with unique geometric and electronic structure is suitable for the mechanism clarification of such a complex reaction. In this study, d‐block transition‐metal (TM) anchored C2N single layer catalyst is investigated by the density functional theory (DFT) calculation. Both single TM‐anchored single atom catalyst (SAC) and double TM‐anchored double atom catalyst (DAC) exhibit good thermodynamic stability in atomically dispersed catalyst. In the case of SACs, IVB metals (Ti, Zr, Hf) exhibit the highest reactivity and lowest overpotential. While in the case of DACs, Cr─Cr system leads to the NH3 formation, but V─V system leads to the N2H4 formation. The SACs show much lower overpotential and stronger activation of N2 molecule than the DACs due to the different activation mechanisms: traditional σ‐donation/π‐backdonation N2 activation mechanism is found in SACs, while a new π‐donation/π‐backdonation N2 activation mechanism is found in the DACs. The present work demonstrates that the different catalytic effect for NRR between SAC and DAC and their corresponding electronic structure origin, which gives more insight into the single atom catalyst.image

Recently, single atom catalysts (SACs) with isolated metal atom as the active site have received extensive attention for their excellent catalytic performance. However, limited by the strong aggregation tendency of monometallic atoms, the construction of SACs remains a formidable challenge. Herein, we developed a facile ternary copolymerization‐pyrolysis approach to synthesize a single cobalt atom catalyst (Co 1 @NC) by employing amino‐functionalized cobalt phthalocyanine (CoPc(NH 2 ) 4 ) as the metal precursor. Specially, CoPc(NH 2 ) 4 was copolymerized with melamine and 1,4‐phthalaldehyde to yield a terpolymer, thereby allowing CoPc to be more uniformly and stably distributed in the polymer network. Subsequently, the obtained terpolymer was pyrolyzed to afford Co 1 @NC. Aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (AC HAADF‐STEM) and X‐ray absorption spectroscopy (XAS) directly confirmed that the catalyst Co 1 @NC was a single cobalt atom catalyst. Furthermore, the large specific surface area ( S BET = 418.8 m 2 /g) and high cobalt content (2.71 wt%) of Co 1 @NC provided more cobalt active sites and therefore displayed excellent catalytic activity in the hydrogenation of nitrobenzene. In addition, the catalyst showed remarkable cyclic stability in five cycles and remained as the single atom catalyst.

Iron-nitrogen-carbon single atom catalyst (SAC) is regarded as one of the promising electrocatalysts for NO3 - reduction reaction (NO3 RR) to NH3 due to its high activity and selectivity. However, synergistic effects of topological defects and FeN4 active moiety in Fe-N-C SAC have rarely been investigated. By performing density functional theory (DFT) calculations, 13 defective graphene FeN4 with 585, 484, and 5775 topological line defects are constructed, yielding 585-68-FeN4 with optimal NO3 RR catalytic activity, high selectivity, as well as robust anti-dissolution stability. The high NO3 RR activity on 585-68-FeN4 is well explained by the high valence state of Fe center as well as asymmetric charge distribution on FeN4 moiety influenced by 5- and 8-member rings. This DFT work provides theoretical guidance for engineering NO3 RR performance of iron-nitrogen-carbon catalysts by modulating periodic topological defects.

Design and development of a single atomic catalyst with high activity is desirable but proved to be very challenging in the renewable energy conversion and storage technologies. As a classic carbon material, graphene has many excellent properties and thus may be a good support to stabilize the isolated metal atoms. However, the oxygen evolution activity of a single cobalt atom supported on graphene is still very low. To improve its performance, support modification has been carried out based on a density functional theory framework for the design predication. In our theoretical study, two nitrogen formats are incorporated to the graphene substrates, including graphitic nitrogen and pyridine-like nitrogen, which are usually observed in experiment. The oxygen evolution process has been envisaged on these single cobalt atom catalysts via gas phase adsorption calculation. The electronic structure on the single Co active site can be effectively regulated by the support modification, which will contribute to its enhanced performance. Henceforth, free energy change diagrams, partial density of states, Raman spectra, and charge density difference are discussed. It is suggested that incorporating pyridine-like nitrogen on graphene is an ideal approach for the supported Co atom to achieve high OER activity, opening up new opportunity for the preparation and application of highly active and stable single atomic catalysts.

Single-atom catalysts (SACs) are recently emerging as a new frontier in heterogenous catalysis for their effectiveness in promoting catalytic reactions. We report herein the study of carbon-supported single-atom catalysts synthesized by a novelty method exhibit distinctive selectivity for some special electrochemical reactions during the electrolysis process. The aberration-corrected HAADF-STEM images combined with X-ray absorption spectroscopy identified the presence of single atoms. Theoretical density functional theory simulations of the best performing catalytic activity and selectivity could be attributed to the existence of single atoms in the catalysts. Introduction CO2 is a cheap resource of carbon that can be used as raw materials for fuel synthesis using renewable energy In 2005, 36.2 billion metric tons of CO2 was emitted to the atmosphere from anthropogenic sources, [1] which is corresponding to an increase in global mean temperature of around 0.1oC. New carbon capture, sequestration and conversion technologies are widely pursued as potential solutions to mitigate the concerns. The electrochemical CO2 reduction reaction (CO2RR) can combine carbon capture storage with renewable energy to convert CO2 into fuels and chemical feedstocks. Which is extraordinary significance for industry and is highly competitive with water electrolysis. Therefore, efficient electrocatalysts are required to reduce the reaction overpotential and improve the faradaic efficiency and selectivity for high value chemical products further. [2] The earth-rare, high price and low selectivity of precious metals prevent further large-scale commercial applications. Furthermore, these precious metal catalysts exhibit desirable products at highly cathodic potential window with high overpotential, which causes lot of energy consuming problem in application. Recently, single-atom catalysts (SACs) are emerging as a new frontier in heterogenous catalysis for their effectiveness in promoting catalytic reactions. [3] The unique electronic structure and unsaturated coordination environments in single atom catalysts (SACs) have been proven to enhance catalytic activity and excellent selectivity in a variety of reactions. In this work, the catalyst was prepared by a proprietary method currently under patent examination. Results and Discussion We report herein the study of carbon-supported SACs synthesized by a novel method that exhibit distinctive selectivity in electrochemical synthesis. The aberration-corrected HAADF-STEM images combined with X-ray absorption spectroscopy identified the presence of single atoms. Theoretical density functional theory simulations of the best performing catalytic activity and selectivity could be attributed to the existence of single atoms in the catalysts. The prepared metal single atoms dispersed on substrate catalysts showed high faradaic efficiency and selectivity of CO2 to C2 products conversion with low onset potential. Fig. 1a shows linear sweep voltammetry (LSVs) over M/C-0.1 before and after chronoamperometry measurements at different potentials. CV curves (Figure 1b) also displayed obvious activity toward CO2RR in the potential range from 0.5 V to -1.5 V vs RHE, and the current density remained stable with no apparent decay before and after electrolysis in CO2 saturated KHCO3 solution. The FE reached to as high as ~91.8% for ethanol at a low potential of -0.6 V when the metal loading is 0.1 wt% on carbon support. Figure 1. Electrochemical CO2RR over M/C-0.1. (a) LSV in a potential range of 0 to -1.5 V vs RHE in Ar and CO2 saturated 0.1 M KHCO3. (Scan rate: 50 mV s-1). (b) CV in a potential range of 0.5 to -1.5 V vs RHE in Ar and CO2 saturated 0.1 M KHCO3. (Scan rate: 50 mV s-1, (c) FEs of key byproduct as the function of potential over the catalysts of M/C-0.1 Significance Understanding the role of metal single atoms on the CO2 electrochemical reduction reaction is fundamental for design of an appropriated catalyst for the large-scale application of CO2 RR that aims the production of high energy liquid chemicals.

Single atom catalysts (SACs) are widely researched in various chemical transformations due to the high atomic utilization and catalytic activity. Carbon-supported SACs are the largest class because of the many excellent properties of carbon derivatives. The single metal atoms are usually immobilized by doped N atoms and in some cases by C geometrical defects on carbon materials. To explore the catalytic mechanisms and improve the catalytic performance, many efforts have been devoted to modulating the electronic structure of metal single atomic sites. Doping with polynary metals and heteroatoms has been recently proposed to be a simple and effective strategy, derived from the modulating mechanisms of metal alloy structure for metal catalysts and from the donating/withdrawing heteroatom doping for carbon supports, respectively. Polynary metals SACs involve two types of metal with atomical dispersion. The bimetal atom pairs act as dual catalytic sites leading to higher catalytic activity and selectivity. Polynary heteroatoms generally have two types of heteroatoms in which N always couples with another heteroatom, including B, S, P, etc. In this Review, the recent progress of polynary metals and heteroatoms SACs is summarized. Finally, the barriers to tune the activity/selectivity of SACs are discussed and further perspectives presented.

Coordination Number Regulation of Molybdenum Single-Atom Nanozyme Peroxidase-like Specificity

Designing C3N-supported single atom catalysts for efficient nitrogen reduction based on descriptor of catalytic activity

Highly efficient photocatalytic reduction of nitrogen into ammonia by single Ru atom catalyst supported by BeO monolayer

Precious Pt metals deposited on reducible CeO2 oxide have been widely used in exhaust gas treatment. However, the high cost of Pt metals is the main barrier that prevents from applying the Pt metals. Therefore, a lot of efforts have been dedicated to reducing the Pt size as small as possible, aiming at maximizing Pt metal utilization efficiency. The smallest ultimate size that Pt metal can be deposited on CeO2 as single atom structures. Furthermore, the supported single Pt atom catalysts enhance catalytic performance and selectivity compared to Pt cluster or Pt particle counterparts. However, the electronic structures of single Pt atom catalysts at the atomic level are a challenge for the experimentalist, leading to a lack of important information in using these catalysts. Here, we aim to figure out the nature of the high thermal stability of single Pt atom catalysts on CeO2 support using density theory functional and compared to experimental data from previous studies. This study will provide useful information on single Pt atom catalysts on CeO2 support aiming at boosting synthesis and applications applying CeO2‐supported Pt.

Single atom-based catalysts for electrochemical CO2 reduction

Single atom catalysts (SACs) have attracted great attention as promising catalysts that integrate the benefits of both heterogeneous and homogeneous catalysts. SACs exhibit unique properties that are otherwise difficult to achieve, such as high atom utilization efficiency, unprecedentedly high catalytic activity and selectivity. However, it still remains a great challenge to prepare stable SACs without particle aggregation and sintering. Among the various fabrication methods for SACs, metal-organic framework (MOF)-derived synthesis routes have shown great potential by taking advantage of MOFs’ high structural/chemical tunability, large surface area and high porosity. In this review, the synthesis strategies for MOF-derived SACs are comprehensively summarized and classified into five classes, metal node modification, ligand modification, guest encapsulation, migration and trapping, and others. The current challenges and future opportunities of MOF-derived SACs are further discussed. This review will be useful for the rational design of MOF-derived SACs for various catalytic reactions.

Electrochemical carbon dioxide (CO2) reduction is emerging as a promising technique to decrease atmospheric CO2 concentration and relieve energy pressure. Besides the single-carbon (C1) species, multi-carbon (C2+) products are more preferred because of their elevated energy density and/or larger economic value. Single atom catalysts (SACs) have been widely used in the field of catalysis due to their tunable active center and unique electronic structure. So far, extensive research progresses have been achieved in utilizing SACs to promote the CO2 reduction toward C1 products, but little attention is paid to the formation of high-value C2+ products. In this review, we present the recent advances of electrochemical reduction of CO2 to C2+ products with SACs. Firstly, the reaction mechanism of converting CO2 to C2+ products is briefly introduced. Then the general design principles of SACs toward C2+ products are systematically discussed. After that, we highlight the representative studies on the C2+ generation and the corresponding mechanism with SACs, including the copper and non-copper based SACs. Finally, we summarize the latest progresses and provide personal perspectives for the future design and target preparation of advanced SACs for the high-performance CO2 electrolysis to specific C2+ products.