Active Site Configuration Research Articles

Ambient Electrosynthesis toward Single-Atom Sites for Electrocatalytic Green Hydrogen Cycling.

With the ultimate atomic utilization, well-defined configuration of active sites and unique electronic properties, catalysts with single-atom sites (SASs) exhibit appealing performance for electrocatalytic green hydrogen generation from water splitting and further utilization via hydrogen-oxygen fuel cells, such that a vast majority of synthetic strategies toward SAS-based catalysts (SASCs) are exploited. In particular, room-temperature electrosynthesis under atmospheric pressure offers a novel, safe, and effective route to access SASs. Herein, the recent progress in ambient electrosynthesis toward SASs for electrocatalytic sustainable hydrogen generation and utilization, and future opportunities are discussed. A systematic summary is started on three kinds of ambient electrochemically synthetic routes for SASs, including electrochemical etching (ECE), direct electrodeposition (DED), and electrochemical leaching-redeposition (ELR), associated with advanced characterization techniques. Next, their electrocatalytic applications for hydrogen energy conversion including hydrogen evolution reaction, oxygen evolution reaction, overall water splitting, and oxygen reduction reaction are reviewed. Finally, a brief conclusion and remarkson future challenges regarding further development of ambient electrosynthesis of high-performance and cost-effective SASCs for many other electrocatalytic applications are presented.

Emergence of a proton exchange-based isomerization and lactonization mechanism in the plant coumarin synthase COSY

Plants contain rapidly evolving specialized enzymes that support the biosynthesis of functionally diverse natural products. In coumarin biosynthesis, a BAHD acyltransferase-family enzyme COSY was recently discovered to accelerate coumarin formation as the only known BAHD enzyme to catalyze an intramolecular acyl transfer reaction. Here we investigate the structural and mechanistic basis for COSY’s coumarin synthase activity. Our structural analyses reveal an unconventional active-site configuration adapted to COSY’s specialized activity. Through mutagenesis studies and deuterium exchange experiments, we identify a unique proton exchange mechanism at the α-carbon of the o-hydroxylated trans-hydroxycinnamoyl-CoA substrates during the catalytic cycle of COSY. Quantum mechanical cluster modeling and molecular dynamics further support this key mechanism for lowering the activation energy of the rate-limiting trans-to-cis isomerization step in coumarin production. This study unveils an unconventional catalytic mechanism mediated by a BAHD-family enzyme, and sheds light on COSY’s evolutionary origin and its recruitment to coumarin biosynthesis in eudicots.

N, S heteroatom co-doped carbon-based materials carrying Mn and Co atoms as bifunctional catalysts for stable rechargeable zinc-air batteries

Rational designing of efficient bifunctional oxygen electrocatalysts is significantly important but rather challenging for rechargeable zinc-air batteries. Herein, a novel high-performance bifunctional oxygen electrocatalyst for rechargeable zinc–air batteries are constructed based on bimetal Mn@Co-N-C encapsulated in in situ grown N,S-doped graphitic carbon framework with within a porous three-dimensional (3D) structure. The final catalyst shows a high half-wave potential of 0.89 V for oxygen reduction reaction, and a low operating overpotential of 0.38 V to achieve a 10 mA cm−2 current density for oxygen evolution reaction. The physical characterizations reveal that a strong synergetic coupling between bimetal Mn@Co-N-C and S dopant can effectively increase the active sites, meanwhile achieve a rational manipulation of the active site configuration toward a favorable electronic structure. Impressively, the assembled zinc–air batteries using liquid electrolytes and the all-solid-state batteries with this catalyst exhibit excellent charging–discharging performance, long lifetime, and high flexibility.

Porous Covalent Organic Polymer Coordinated Single Co Site Nanofibers for Efficient Oxygen-Reduction Cathodes in Polymer Electrolyte Fuel Cells.

Nitrogen-coordinated single-cobalt-atom electrocatalysts, particularly ones derived from high-temperature pyrolysis of cobalt-based zeolitic imidazolate frameworks (ZIFs), have emerged as a new frontier in the design of oxygen reduction cathodes in polymer electrolyte fuel cells (PEFCs) due to their enhanced durability and smaller Fenton effects related to the degradation of membranes and ionomers compared with emphasized iron-based electrocatalysts. However, pyrolysis techniques lead to obscure active-site configurations, undesirably defined porosity and morphology, and fewer exposed active sites. Herein, a highly stable cross-linked nanofiber electrode is directly prepared by electrospinning using a liquid processability cobalt-based covalent organic polymer (Co-COP) obtained via pyrolysis-free strategy. The resultant fibers can be facilely organized into a free-standing large-area film with a uniform hierarchical porous texture and a full dispersion of atomic Co active sites on the catalyst surface. Focused ion beam-field emission scanning electron microscopy and computational fluid dynamics experiments confirm that the relative diffusion coefficient is enhanced by 3.5 times, which can provide an efficient route both for reactants to enter the active sites, and drain away the produced water efficiently. Resultingly, the peak power density of the integrated Co-COP nanofiber electrode is remarkably enhanced by 1.72 times along with significantly higher durability compared with conventional spraying methods. Notably, this nanofabrication technique also maintains excellent scalability and uniformity.

Emerging dual-atomic-site catalysts for electrocatalytic CO2 reduction

The electrochemical CO2 reduction reaction (CO2RR) to yield high-value added fuels and chemicals provides a promising approach towards global carbon neutrality. Constant endeavors have been devoted to the exploration of high-efficiency catalyst with rapid reaction kinetics, low energy input, and high selectivity. In addition to the maximum metal atomic utilization and unique catalytic performance of single-atom catalyst (SAC), dual-atomic-site catalysts (DASCs) offer more sophisticated and tunable atomic structure through the modulations of another adjacent metal atom, which can bring new opportunities for CO2RR as a deeper extension of SACs and have recently aroused surging interest. In this review, we highlight the recent advances on DASCs for enhancing CO2RR. First, the classification, synthesis, and identification of DASCs are provided according to the geometric structure and electronic configuration of dual-atomic active sites. Then, the catalytic applications of DASCs in CO2RR are categorized based on marriage-type, hetero-nuclear, and homo-nuclear dual-atomic sites. Particularly, the structure-activity relationship of DASCs in CO2RR is elaborately summarized through systematically analyzing the reaction pathways and the atom structures. Finally, the opportunities and challenges are proposed for inspiring the design of future DASCs with high structural accuracy and high CO2RR activity and selectivity.

Enhancing oxygen reduction reaction activity of pyrolyzed Fe–N–C catalyst by the inclusion of BN dopant at the graphitic edges

We study the detailed mechanism of oxygen reduction reaction (ORR) at the graphitic edges of BN-doped pyrolyzed Fe–N–C catalyst (FeN4-BN-edge) by utilizing the combination of DFT and microkinetic calculations. We find that the ORR mechanism on FeN4-BN-edge active sites is more selective toward the associative reduction pathway for all relevant potential ranges instead of the dissociative reduction pathway. This situation arises because the O2 dissociation energies on the FeN4-BN-edge active sites are too high for the dissociative reduction pathway to take place with sufficient ORR rates. The inclusion of BN-dopant at the graphitic edge of pyrolyzed Fe–N–C is found to improve the ORR activity of the catalyst. One factor contributed to this improvement originates from the easy formation of active site configurations at the graphitic edges, which is induced by the interaction between neighboring FeN4 and BN configurations. Another factor is the formation of the FeN4-BN configuration at the zigzag edge of graphene. These active sites could improve the onset potential for the associative reduction pathway up to 0.12 V larger than that at the standard FeN4 active site in the basal plane of graphene.

Synergic effect of Fe-doping and Ni3S2/MnS heterointerface to boost efficient oxygen evolution reaction

The high overpotential of oxygen evolution reaction (OER) is the main limitation of electrolytic water splitting efficiency for hydrogen production. Herein, Fe doped Ni3S2/MnS heterostructured nanosheets were grown on Ni foam (Fe-Ni3S2/MnS@NF) via a simple one-step solvothermal reaction. As-synthesized Fe-Ni3S2/MnS@NF exhibits excellent OER electrocatalytic activity in 1 M KOH, with an extremely low overpotential of 259 mV at 50 mA cm−2 as well as a small Tafel slope (39 mV dec−1), much better than Ni3S2-, Fe-doped Ni3S2-, and Ni3S2/MnS- nanosheets grown on NF. Results clearly reveal that the better electrocatalytic performance of Fe-Ni3S2/MnS@NF than its rivals is mainly due to the synergistic effect of Fe-doping and Ni3S2/MnS heterojunctions. In details, heterojunction combination with Fe-doping not only improves the conductivity of catalysts but also modulates the electronic configuration of active sites and optimizes the adsorption capacity of oxygen-containing intermediates, thus achieving rapid reaction kinetics and high electrocatalytic performance. This work provides insights into the synergistic regulation of electrocatalytic activity of non-noble catalysts toward high efficiency energy conversion.

Modulating the Electronic Coordination Configuration and d-Band Center in Homo-Diatomic Fe2N6 Catalysts for Enhanced Peroxymonosulfate Activation.

The electronic coordination configuration of metal active sites and the reaction mechanism were investigated by constructing homo-diatomic Fe sites for visible-light-assisted heterogeneous peroxymonosulfate (PMS) activation. A novel Fe2N6 catalyst was synthesized by selecting uniform pyridinic-N of graphitic carbon nitride (g-C3N4) as anchoring sites. The results demonstrated that homo-diatomic Fe sites modulated the d-band center and electron delocalization and thus enhanced the PMS activation kinetics (3.58 times vs single-atom Fe catalyst) with kobs of 0.111 min-1 owing to the synergistic effect between adjacent Fe atoms. New Fe-Fe coordination significantly decreased the contribution of the antibonding state in the Fe-O bond due to the coupling of the Fe-3d orbitals, which facilitated the O-O bond cleavage of the Fe2-HOO-SO3 complex with a reduced thermodynamic energy barrier of only -0.29 eV. This work provided comprehensive mechanistic insights into developing homo-diatomic catalysts governed by the coordination configuration and radical pathway for efficient heterogeneous PMS catalysis.

Efficient polysulfide conversion by Fe-N/C active sites anchored in N, P- doped carbon for high-performance lithium-sulfur batteries

Lithium-Sulfur batteries have become one of the most promising energy storage systems due to their ultra-high energy density, environmental friendliness, and low cost. However, the sluggish redox kinetics of lithium polysulfide (LPS) and its shuttling effects have impeded the practical applications of Li-S batteries. Single-atom catalysts (SACs) with atomically dispersed metal-based sites have been applied as a promising candidate for electrocatalysts for Li-S batteries. Herein, we design and demonstrate a single atom Fe-N/C catalyst impregnated with N, P–doped carbon (FeN6–NPC), N, P–doped carbon material (NPC), as well as Fe2P nanoparticles anchored with N, P–doped carbon (Fe2P–NPC). In-depth XANES and FT-EXAFS analyses were used to characterize the precise architecture of SACs containing Fe-N/C active sites. The electrochemical results of SACs with Fe-N/C active site configuration reveal the highest catalytic lithium polysulfide conversion compared to the NPC and nanoparticle anchored samples. In addition, the SACs enable FeN6–NPC/S electrode to deliver a high discharge capacity of 1115 mAh g−1 at 0.1 C and maintain 570 mAh g−1 after 200 cycles with excellent Columbic efficiency (>99%). This work successfully develops organic-based strategies for SACs that suppress lithium polysulfide formation, improve cyclic stability, and increase Coulombic efficiency for lithium-sulfur batteries.

Synergistically tuning the graphitic degree, porosity, and the configuration of active sites for highly active bifunctional catalysts and Zn-air batteries

Synergistically tuning the graphitic degree, porosity, and the configuration of active sites for highly active bifunctional catalysts and Zn-air batteries

Ultrafine PdZn bimetallic nanoparticles anchored on sulfur-doped mesoporous carbon for the partial hydrogenation of alkynols

The catalytic partial hydrogenation of alkynols is a key approach for the synthesis of enols, which are crucial chemical intermediates for pharmaceuticals and essences. Low selectivity is the main challenge of the partial hydrogenation of alkynols due to the easy over-hydrogenation of alkynyl groups. Herein, ultrafine PdZn bimetallic nanoparticles (NPs) were anchored on a S-doped mesoporous carbon catalyst (PdZn/Meso_S-C) and used for the partial hydrogenation of alkynols. Zn metal with a high electron density was used to modify the active metal Pd to change the electronic and geometric configuration of the Pd active sites, which improve the selectivity of enols during the partial hydrogenation of alkynols. Furthermore, the use of a S-doped mesoporous carbon support dispersed and stabilized the ultrafine PdZn NPs due to strong interactions between the metal atoms and S sites. This also prevented PdZn ultrafine NPs from aggregating and increased the partial hydrogenation selectivity. The partial hydrogenation of propynol ethoxylate (PME) catalyzed by PdZn/Meso_S-C reached 96.0% enol selectivity, while the alkynol conversion reached 97.5%. The general applicability of PdZn/Meso_S-C for the partial hydrogenation of various alkynols has also been demonstrated. Thus, this work has great application prospects for the high-efficiency catalytic partial hydrogenation of alkynols.

Atomically Dispersed Dual-Metal Site Catalysts for Enhanced CO2 Reduction: Mechanistic Insight into Active Site Structures.

Carbon-supported nitrogen-coordinated single-metal site catalysts (i.e., M-N-C, M: Fe, Co, or Ni) are active for the electrochemical CO2 reduction reaction (CO2 RR) to CO. Further improving their intrinsic activity and selectivity by tuning their N-M bond structures and coordination is limited. Herein, we expand the coordination environments of M-N-C catalysts by designing dual-metal active sites. The Ni-Fe catalyst exhibited the most efficient CO2RR activity and promising stability compared to other combinations. Advanced structural characterization and theoretical prediction suggest that the most active N-coordinated dual-metal site configurations are 2N-bridged (Fe-Ni)N6 , in which FeN4 and NiN4 moieties are shared with two N atoms. Two metals (i.e., Fe and Ni) in the dual-metal site likely generate a synergy to enable more optimal *COOH adsorption and *CO desorption than single-metal sites (FeN4 or NiN4 ) with improved intrinsic catalytic activity and selectivity.

Atomically-dispersed Mn-(N-C2)2(O-C2)2 sites on carbon for efficient oxygen reduction reaction

Owing to their ultimate mass-catalytic activity, simple active site configuration and readily tunable electronic structures, transition-metal single atoms (SAs) on carbon support have emerged as a new category of electrocatalysts for oxygen reduction reaction (ORR). Here, we exemplify the use of atomically dispersed Mn with N, O heteroatoms-coordinated Mn sites to enhance alkaline ORR performance. A combined sol-gel/carbonization approach is developed to controllably anchor Mn-SAs onto graphitic carbon nanosheets via a hybridized N, O coordination configuration of Mn-(N-C2)2(O-C2)2 (denoted as Mn-SA@CNSs). The obtained Mn-SA@CNSs exhibits superior alkaline ORR electrocatalytic activity with a half-wave potential of 0.88 V vs. RHE in 0.1 M KOH. The rechargeable Zn–air battery assembled using Mn-SA@CNSs air cathode can readily attain a high power density of 177 mW cm−2 with a narrow voltage gap of 0.76 V at 5 mA cm−2. Density functional theory calculations unveil that altering the coordination environment of Mn-SAs from N-coordinated Mn-(N-C2)4 to N-/O-coordinated Mn-(N-C2)2(O-C2)2 alters the d-band electronic structures and regulates the binding strength of ORR intermediates on Mn-SA sites to dramatically reduce the energy barrier and enhance ORR activity. The exemplified hybridization coordination approach in this work would be applicable to alter the electronic structures of other transition-metal SAs for ORR and other reactions.

The Interactive Dynamics of Nanocatalyst Structure and Microenvironment during Electrochemical CO2 Conversion.

In the pursuit of a decarbonized society, electrocatalytic CO2 conversion has drawn tremendous research interest in recent years as a promising route to recycling CO2 into more valuable chemicals. To achieve high catalytic activity and selectivity, nanocatalysts of diverse structures and compositions have been designed. However, the dynamic structural transformation of the nanocatalysts taking place under operating conditions makes it difficult to study active site configurations present during the CO2 reduction reaction (CO2RR). In addition, although recognized as consequential to the catalytic performance, the reaction microenvironment generated near the nanocatalyst surface during CO2RR and its impact are still an understudied research area. In this Perspective, we discuss current understandings and difficulties associated with investigating such dynamic aspects of both the surface reaction site and its surrounding reaction environment as a whole. We further highlight the interactive influence of the structural transformation and the microenvironment on the catalytic performance of nanocatalysts. We also present future research directions to control the structural evolution of nanocatalysts and tailor their reaction microenvironment to achieve an ideal catalyst for improved electrochemical CO2RR.

Reversing the Nucleophilicity of Active Sites in CoP2 Enables Exceptional Hydrogen Evolution Catalysis.

Precisely constructing the local configurations of active sites to achieve on-demand catalytic functions is highly critical yet challenging. Herein, an anion-deficient strategy to precisely capture Ru single atoms on the anion vacancies of CoP2 (Ru-SA/Pv-CoP2 ) is developed. Refined structural characterizations reveal that the Ru single atoms preferably bind to the anion vacancy sites and consequently build a superior catalytic surface with neighboring CoP and CoRu coordination states for the hydrogen evolution reaction (HER) catalysis. The prepared Ru-SA/Pv-CoP2 nanowires exhibit an unprecedented overpotential of 17mV at 10mA cm-2geo , and the corresponding mass activity is 52.2 times higher than the benchmark Pt/C catalyst at the overpotential of 50mV. Theoretical analysis illustrates that the introduced Ru-SAs can reverse electrons state distribution (from nucleophilic P sites to electrophilic Ru sites) and boost the activation of water molecules and hydrogen production. More importantly, such a construction strategy is also applicable for Pt single atom coupling, suggesting its generality in building catalytic sites. The capability to precisely construct active sites offers a powerful platform to manipulate the catalytic performance of HER catalysts and beyond.

Activating MoS2 via electronic structure modulation and phase engineering for hydrogen evolution reaction

The local electronic configuration of active sites in molybdenum disulfide (MoS2) could significantly alter the intrinsic activity towards hydrogen evolution reaction (HER). Herein, we report that the HER performance of MoS2 electrocatalysts was boosted via bismuth and palladium co-doping. The introduction of heteroatom Bi successfully altered the local electronic configuration of S atoms in the MoS2 inert basal plane to form an electron-enriched environment, conducive to boost active site density and proton adsorption, as experimentally verified by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Furthermore, after activating the MoS2 via Pd doping, a phase transition to stable 1 T phase occurred which was verified by Raman. In acidic media, the as-prepared Bi/Pd-MoS2 catalyst exhibited excellent electrochemical HER performance in terms of a low overpotential of -114 mV at a current density of 10 mA cm−2, a small Tafel slope of 65 mV dec−1, and outstanding long-term stability.

Identification of molecular basis that underlie enzymatic specificity of AzoRo from Rhodococcus opacus 1CP: A potential NADH:quinone oxidoreductase

Azo dyes are important to various industries such as textile industries. However, these dyes are known to comprise toxic, mutagenic, and carcinogenic representatives. Several approaches have already been employed to mitigate the problem such as the use of enzymes. Azoreductases have been well-studied in its capability to reduce azo dyes. AzoRo from Rhodococcus opacus 1CP has been found to be accepting only methyl red as a substrate, surmising that the enzyme may have a narrow active site. To determine the active site configuration of AzoRo at atomic level and identify the key residues involved in substrate binding and enzyme specificity, we have determined the crystal structure of holo-AzoRo and employed a rational design approach to generate AzoRo variants. The results reported here show that AzoRo has a different configuration of the active site when compared with other bacterial NAD(P)H azoreductases, having other key residues playing a role in the substrate binding and restricting the enzyme activity towards different azo dyes. Moreover, it was observed that AzoRo has only about 50% coupling yield to methyl red and p-benzoquinone – giving rise to the possibility that NADH oxidation still occurs even during catalysis. Results also showed that AzoRo is more active and more efficient towards quinones (about four times higher than methyl red).

Rapid internal conversion harvested in Co/Mo dichalcogenides hollow nanocages of polysulfides for stable Lithium-Sulfur batteries

The sluggish conversion kinetics of the lithium polysulfide (LiPS) intermediates are hindering the practical application of lithium sulfur batteries (LSB) by their low efficiency and low cycle stability. To tackle these challenges, a rapid internal conversion (RIC) mechanism is proposed for the first time to accelerate the liquid–solid conversion in the sulfur reduction reaction (SRR) kinetics. It is reaveled that the reaction energy barrier in the SRR lies in the liquid–solid conversion, from Li2S4 to LiS2* to Li2S2. To accelerate the liquid–solid conversion, the CoS2@MoS2 hollow nanocage, a novel sulfur cathode, with the rational synergistic active site configuration is specifically designed toward a well-managed reaction path and sulfurphilic structure to regulate and modulate Li(LiPS)-S(sulfide) and S(LiPS)-M(sulfide) bonds prominently, affecting on activating reactants and stablizing transition states. Based on the RIC mechanism in the SRR, the stronger S-Co bonds between CoS2 and LiPS traps Li2S4 and catalytically facilitates S-S bond scission while the MoS2 is conducive to the migration of Li2S4 by weaker Li-S bonds. The unique synergetic conversion and catalytic functions is confirmed by density functional theory (DFT) calculations. The RIC mechanism could provide a new idea for exploring the conversion-catalysis material of Li-S batteries in the specific material design.

Isotopic transient kinetic analysis of CO2 hydrogenation to methanol on Cu/SiO2 promoted by Ga and Zn

Cu-based catalysts promoted with elements such as Zn or Ga are the most studied for CO2 hydrogenation to methanol due to their low cost and high activity. However, the fundamental causes of promotion are still not fully understood. In this work we assessed the effect that these promoters have on the number and intrinsic activity of surface sites using steady state isotopic transient kinetic analysis (SSITKA). We found an order of magnitude increase in the concentration of surface intermediates leading to CO when Zn and Ga are added. At the same time, it reduced the intrinsic rate of CO formation by a similar magnitude, leading to a similar macroscopic CO formation rate as compared with non-promoted Cu. The quantification of surface intermediates showed that only a small fraction of the surface is occupied by intermediates leading to CO and methanol, experimentally confirming theoretical studies who described the unique configuration of active sites for this reaction.

The effect of CO treatment on the surface structure of bimetallic Pd-Au/HOPG and Pd-In/HOPG nanoparticles: A comparative study

Bimetallic nanoparticles have been attracting more and more attention as catalysts in recent years since they often provide improved catalytic performance when compared to their monometallic analogues. Trying to design the optimum configuration of active sites in such catalytic systems, researchers resort to different strategies, one of which consists of a specific pretreatment of pre-synthesized nanoparticles in a particular gaseous environment prior to the catalytic reaction, resulting in adsorption-induced segregation. However, such a strategy does not always lead to any significant changes in the surface structure, so the identification of all factors responsible for the segregation process is of high fundamental and applied interest for the targeted design of bimetallic catalytically active nanoparticles. Here we show that for two types of bimetallic nanosystems, a stoichiometric intermetallic compound of Pd and In, and a substitutional solid solution of Pd and Au, the exposure to a CO atmosphere leads to completely different results, i.e., a noticeable surface segregation of Pd atoms being observed exclusively for the latter system. Driven by adsorption of CO molecules, the process of segregation is enhanced in the studied Pd-Au/HOPG catalyst (initial Pd/Au ratio of 1.48) when the temperature of the CO treatment is increased to 150 °C. In contrast, the Pd-In/HOPG nanoparticles (initial Pd/In ratio of 1.35) show no considerable change in the apparent surface atomic composition under a CO atmosphere, irrespective of temperature. Our results, alongside previous findings, demonstrate that CO adsorption-induced segregation as a tool for target-oriented modification of active sites in bimetallic nanoparticles for catalytic applications is appropriate only for substitutional solid solutions having a reasonable number of contacts between atoms of one type prone to segregation.