Stable Single-atom Catalysts Research Articles

Destabilization of Single-Atom Catalysts: Characterization, Mechanisms, and Regeneration Strategies.

Numerous in situ characterization studies have focused on revealing the catalytic mechanisms of single-atom catalysts (SACs), providing a theoretical basis for their rational design. Although research is relatively limited, the stability of SACs under long-term operating conditions is equally important and a prerequisite for their real-world energy applications, such as fuel cells and water electrolyzers. Recently, there has been a rise in in situ characterization studies on the destabilization and regeneration of SACs; however, timely and comprehensive summaries that provide the catalysis community with valuable insights and research directions are still lacking. This review summarizes recent advances in the destabilization mechanisms and regeneration strategies of SACs, specifically highlighting various state-of-the-art characterization techniques employed in the studies. The factors that induce destabilization in SACs are identified by discussing the failure of active sites, coordination environments, supports, and reaction conditions under long-term operating scenarios. Next, the primary regeneration strategies for SACs are introduced, including redispersion, surface poison desorption, and exposure of subsurface active sites. Additionally, the advantages and limitations of both in situ and ex situ characterization techniques are discussed. Finally, future research directions are proposed, aimed at constructing structure-stability relationships and guiding the design of more stable SACs.

Progress and challenges in structural, in situ and operando characterization of single-atom catalysts by X-ray based synchrotron radiation techniques.

Single-atom catalysts (SACs) represent the ultimate size limit of nanoscale catalysts, combining the advantages of homogeneous and heterogeneous catalysts. SACs have isolated single-atom active sites that exhibit high atomic utilization efficiency, unique catalytic activity, and selectivity. Over the past few decades, synchrotron radiation techniques have played a crucial role in studying single-atom catalysis by identifying catalyst structures and enabling the understanding of reaction mechanisms. The profound comprehension of spectroscopic techniques and characteristics pertaining to SACs is important for exploring their catalytic activity origins and devising high-performance and stable SACs for industrial applications. In this review, we provide a comprehensive overview of the recent advances in X-ray based synchrotron radiation techniques for structural characterization and in situ/operando observation of SACs under reaction conditions. We emphasize the correlation between spectral fine features and structural characteristics of SACs, along with their analytical limitations. The development of IMST with spatial and temporal resolution is also discussed along with their significance in revealing the structural characteristics and reaction mechanisms of SACs. Additionally, this review explores the study of active center states using spectral fine characteristics combined with theoretical simulations, as well as spectroscopic analysis strategies utilizing machine learning methods to address challenges posed by atomic distribution inhomogeneity in SACs while envisaging potential applications integrating artificial intelligence seamlessly with experiments for real-time monitoring of single-atom catalytic processes.

Efficient photocatalytic hydrogen production via single-atom Pt anchored hydrogen-bonded organic frameworks

Constructing single-atom catalysts (SACs) using organic porous framework materials as supports presents a promising approach for developing highly efficient photocatalysts for hydrogen evolution. However, the fabrication of SACs that are both highly stable and active poses a significant challenge, particularly in the precise anchoring of metal single atoms. In this study, we utilized 1,3,6,8-tetra (p-methyl benzoate) pyrene as a ligand to synthesize pyrene-based hydrogen-bonded organic frameworks (denoted as PFC-1) through a self-assembly approach. Subsequently, a liquid-phase photoreduction process was employed to deposit noble metal platinum (Pt) onto PFC-1, resulting in the fabrication of SACs (PFC-1@Pt). Characterization results confirmed that Pt existed in a monatomic state, anchored through PtC and PtO coordination bonds with PFC-1. Serving as electron capture and separation centers, the Pt single atoms effectively suppressed electron-hole recombination, thereby prolonging carrier lifetimes. Consequently, the PFC-1@Pt SAC exhibited efficient hydrogen evolution performance with a rate of 2202.5 μmol g−1 h−1 and maintained photocatalytic activity for over 40 h. Our findings provide a systematic approach for developing efficient and stable SACs based on HOFs, expanding the potential applications of HOF materials in photocatalysis.

Biomimetic Electronic Communication of Iodine Doped Single-Atom Fe Site for Highly Active and Stable Dopamine Oxidation.

Rational design of highly active and stable catalysts for dopamine oxidation is still a great challenge. Herein, inspired by the catalytic pocket of natural enzymes, an iodine (I)-doped single Fe-site catalyst (I/FeSANC) is synthesized to mimic the catalytic center of heme enzymes in both geometrical and electronic structures, aiming to enhance dopamine (DA) oxidation. Experimental studies and theoretical calculations show that electronic communication between I and FeN5 effectively modulates the electronic structure of the active site, greatly optimizing the overlap of Fe 3d and O 2p orbitals, thereby enhancing OH adsorption. In addition, the electronic communication induced by iodine doping attenuates the attack of proton hydrogen on the active center, thereby enhancing the stability of I/FeSANC. This work provides new insights into the design of highly active and stable single-atom catalysts and enhances the understanding of catalytic mechanisms for DA oxidation at the atomic scale.

Cyclo[16]carbon through the lens of density functional theory: Role of impurity decoration in hydrogen evolution reaction

The increasing global demand for sustainable energy sources has intensified the search for cost-effective catalysts in the hydrogen evolution reaction (HER). Single-atom catalysts (SACs) supported on suitable substrates have emerged as promising alternatives. In this study, we investigate the utilization of cyclo[16]carbon (C16 nanocluster) as a support for second-row transition metals (TMs) based SACs. The exploration focuses on analyzing the geometry, electronic properties, stability, and catalytic performance of TM@C16 SACs in the context of HER. Notably, highly negative binding energy values, structural modifications, and altered dipole moments collectively affirm the strong binding between the TMs and the C16 nanocluster. The incorporation of TMs onto the C16 nanocluster substantially enhances the conductivity of the latter. Studied SACs primarily showed H2 evolution via Volmer-Heyrovsky mechanisms. Notably, Pd@C16 and Rh@C16 displayed superior catalytic performance, with ΔGHV values of −0.007 eV and 0.05 eV, respectively. The determination of the exchange current via a volcano plot as a function of Gibbs free energy for hydrogen evolution over these designed SACs further validates their efficacy. Our findings establish Pd@C16 and Rh@C16 as highly stable and efficient SACs for the HER, underscoring their potential in advancing sustainable energy technologies.

Comprehensive review on single-atom catalysts in electrochemical hydrogen-evolution reaction: computational modelling and experimental investigation

ABSTRACT The growing demand for renewable energy sources has driven research efforts towards highly efficient electrochemical processes, especially the Hydrogen Evolution Reaction (HER) which is an essential component in renewable hydrogen production. Single Atom Catalysts (SACs) have been recognised as promising catalysts for exceptional HER activities owing to their unique electronic and geometrical features. This comprehensive review highlights the effective correlation between computational modelling and experimental research in fabricating new SACs and better understanding their catalytic mechanisms in the HER. Furthermore, the review briefly describes the choice of metals and tailoring the electronic structure, as well as exploring the influence of the reaction parameters on the catalytic activity of SACs for HER. Concluding remarks alongside future perspectives on the improvement of SACs for HER are emphasised. It is hoped that this review will pave the way for rational design of highly effective, low-cost and stable SACs for sustainable hydrogen production.

Density functional theory-based screening of Ti4C3O2-loaded single atoms for efficient selective catalytic oxidation of formaldehyde

Indoor formaldehyde (HCHO) pollution poses a major risk to human health. Low-temperature catalytic oxidation is an effective method for HCHO removal. The high activity and selectivity of single atomic catalysts provide a possibility for the development of efficient non-precious metal catalysts. In this study, the most stable single-atom catalyst Ti–Ti4C3O2 was screened by density functional theory among many single atomic catalysts with two-dimensional (2D) monolayer Ti4C3O2 as the support. The computational results show that Ti–Ti4C3O2 is highly selective to HCHO and O2 in complex environments. The HCHO oxidation reaction pathways are proposed based on the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. According to the reaction energy and energy span models, the E-R mechanism has a lower maximum energy barrier and higher catalytic efficiency than the L-H mechanism. In addition, the stability of the Ti–Ti4C3O2 structure and active center was verified by diffusion energy barrier and ab initio molecular dynamics simulations. The above results indicate that Ti–Ti4C3O2 is a promising non-precious metal catalyst. The present study provides detailed theoretical insights into the catalytic oxidation of HCHO by Ti–Ti4C3O2, as well as an idea for the development of efficient non-precious metal catalysts based on 2D materials.

Second-row transition metals decorated cyclo[18]carbon: Single-atom catalysts for excellent hydrogen evolution reaction

To fulfill the huge demand of clean and sustainable energy, scientists are keenly investigating cost-efficient, readily accessible and effective catalysts to replace Pt-based catalysts in hydrogen evolution reaction (HER). Currently, the scientific community has a growing interest in the quest for suitable support for single-atom catalysts (SACs). Here, we have explored the application of cyclo[18]carbon (C18 nanocluster) as a support for the second-row transition metals (TMs) based SACs for HER activity. We have analyzed the geometry, electronic properties, stability and catalytic performance of TMs@C18 SACs for HER activity. The high binding energy values confirmed the stability of SACs. Moreover, structure alterations, changed dipole moments, shifting of electron density towards the adsorption sites and Mulliken charge transfer proved the strong binding of TMs with C18 nanocluster. The decoration of TMs over C18 nanocluster enhances the conductivity of C18 nanocluster. The catalytic performance is best in the Tc@C18, Ru@C18 and Rh@C18 SACs with Gibbs free energy (ΔGH) values of 0.18 eV, −0.18 eV and −0.07 eV, approaching the ideal value. Our findings suggest that Tc@C18, Ru@C18 and Rh@C18 are highly stable and efficient SACs for HER.

Stable anchoring of single rhodium atoms by indium in zeolite alkane dehydrogenation catalysts.

Maintaining the stability of single-atom catalysts in high-temperature reactions remains extremely challenging because of the migration of metal atoms under these conditions. We present a strategy for designing stable single-atom catalysts by harnessing a second metal to anchor the noble metal atom inside zeolite channels. A single-atom rhodium-indium cluster catalyst is formed inside zeolite silicalite-1 through in situ migration of indium during alkane dehydrogenation. This catalyst demonstrates exceptional stability against coke formation for 5500 hours in continuous pure propane dehydrogenation with 99% propylene selectivity and propane conversions close to the thermodynamic equilibrium value at 550°C. Our catalyst also operated stably at 600°C, offering propane conversions of >60% and propylene selectivity of >95%.

Single transition metal anchored on defective boron carbide monolayer for efficient and selective CO2 electrochemical reduction: A theoretical study

Electrochemical reduction of CO2 to produce fuels and industrial chemicals shows significant promise for reducing greenhouse gas emission and mitigation the energy crisis. The exploitation of highly efficient catalysts has long been considered an attractive yet challenging task. Herein, we systematically studied the electrocatalytic performance of a range of transition metal (TM) single atom anchored on defective BC3 monolayer, specifically those with a boron vacancy (VB) and a carbon vacancy (VC), named TM@VB and TM@VC, respectively, as single-atom catalysts (SACs) for CO2 reduction reaction (CO2RR) via density functional theory (DFT). We screened a total of 28 stable SACs by evaluating the formation energy and dissolution potential of metal atoms. Detail reaction mechanisms involved in CO2 reduction process to produce diverse C1 products were considered. Among the 28 candidates, Cu@VB, Mo@VB, Re@VB, Co@VC, and Ir@VC displayed remarkable efficiency in reducing CO2 into HCOOH at nearly zero or extremely low limiting potentials (UL) of -0.01 ∼ 0 V. These SACs exhibit excellent electrocatalytic activity, surpassing most previously reported catalysts. Furthermore, these five SACs effectively suppressed the competing hydrogen evolution reaction, highlighting their high selectivity toward HCOOH. In addition, the volcano relationship between the ∆G*HCOO and UL was established. This work may offer a promising strategy for the research and development of high-efficiency CO2RR electrocatalysts, which has potential implications for addressing environmental and energy challenges.

Atomically dispersed high-loading Pt-Fe/C metal-atom foam catalyst for oxygen reduction in fuel cells

Designing efficient and stable single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) is essential to promote fuel cell commercialization. In this study, the Bayesian Regularization Neural Network (BRNN) has been used to determine the carbothermal shock (CTS) conditions for preparing high-loading Pt-Fe/C SACs. The BRNN model effectively identified a low reaction temperature of 283 °C for achieving high-loading (15 wt %) Pt-Fe/C SACs. Based on this model, we successfully synthesized Pt-Fe/C SACs with a metal mass content of 11.31 wt % using the CTS method. These ultrahigh content metal Pt-Fe/C SACs consisted of dispersed single atoms and foam-like atomic constructions, namely Pt-Fe/C atom foam catalysts (AFCs), rendered them promising candidates for electrocatalysis applications. The Pt-Fe/C AFCs exhibited good catalytic activity and durability toward ORR in alkaline conditions. The Pt-Fe/C AFCs were employed in a direct borohydride fuel cell (DBFC) as cathode catalyst, which resulted in a maximum power density of 214 mW cm−2 at 60 °C and demonstrated stable discharge for 40 h at room temperature. The electrocatalytic performance of the Pt-Fe/C AFCs surpassed that of low-loading Pt-Fe/C SACs and commercial Pt/C catalysts. The rapid ramp-up rate and the rapid cooling rate of the CTS process enabled the incorporation of high-loading Pt and Fe metal atoms into the graphitized carbon layer, resulting in an increased number of active sites of the Pt-Fe/C AFCs. This work provides an effective strategy for manufacturing efficient and stable SACs for practical applications in fuel cells.

First-principles study of the relationship between the formation of single atom catalysts and lattice thermal conductivity

Single atom catalysts (SACs) have been in the forefront of catalysts research because of their high efficiency and low cost and provide new ideas for development of renewable energy conversion and storage technologies. However, the relationship between the intrinsic properties of materials such as lattice thermal conductivity and catalysis remains to be explored. In this work, the lattice thermal conductivity of BN and graphene was calculated by ShengBTE. In addition, the adsorption properties of 3d-TM (TM = V, Cr, Mn, Fe, Co, Ni) on BN and graphene were investigated using first-principles methods, and it was found that Ni atom can form relatively stable SACs compared to other TMs. The molecular dynamics (MD) simulation and migration barrier of Ni loaded on BN and graphene were calculated. Our study found that graphene has higher thermal conductivity and is easier to form SACs than BN, but the SACs formed on BN surface have higher thermodynamic stability.

Single-Atom Cobalt Catalysts Coupled with Peroxidase Biocatalysis for C-H Bond Oxidation.

This paper reports a robust strategy to catalyze in situ C-H oxidation by combining cobalt (Co) single-atom catalysts (SACs) and horseradish peroxidase (HRP). Co SACs were synthesized using the complex of Co phthalocyanine with 3-propanol pyridine at the two axial positions as the Co source to tune the coordination environment of Co by the stepwise removal of axial pyridine moieties under thermal annealing. These structural features of Co sites, as confirmed by infrared and X-ray absorption spectroscopy, were strongly correlated to their reactivity. All Co catalysts synthesized below 300 °C were inactive due to the full coordination of Co sites in octahedral geometry. Increasing the calcination temperature led to an improvement in catalytic activity for reducing O2, although molecular Co species with square planar coordination obtained below 600 °C were less selective to reduce O2 to H2O2 through the two-electron pathway. Co SACs obtained at 800 °C showed superior activity in producing H2O2 with a selectivity of 82-85% in a broad potential range. In situ production of H2O2 was further coupled with HRP to drive the selective C-H bond oxidation in 2-naphthol. Our strategy provides new insights into the design of highly effective, stable SACs for selective C-H bond activation when coupled with natural enzymes.

Theoretical Insights on ORR Activity of Sn-N-C Single-Atom Catalysts.

The advancement of efficient and stable single-atom catalysts (SACs) has become a pivotal pursuit in the field of proton exchange membrane fuel cells (PEMFCs) and metal-air batteries (MABs), aiming to enhance the utilization of clean and sustainable energy sources. The development of such SACs has been greatly significant in facilitating the oxygen reduction reaction (ORR) process, thereby contributing to the progress of these energy conversion technologies. However, while transition metal-based SACs have been extensively studied, there has been comparatively less exploration of SACs based on p-block main-group metals. In this study, we conducted an investigation into the potential of p-block main-group Sn-based SACs as a cost-effective and efficient alternative to platinum-based catalysts for the ORR. Our approach involved employing density functional theory (DFT) calculations to systematically examine the catalyst properties of Sn-based N-doped graphene SACs, the ORR mechanism, and their electrocatalytic performance. Notably, we employed an H atom-decorated N-based graphene matrix as a support to anchor single Sn atoms, creating a contrast catalyst to elucidate the differences in activity and properties compared to pristine Sn-based N-doped graphene SACs. Through our theoretical analysis, we gained a comprehensive understanding of the active structure of Sn-based N-doped graphene electrocatalysts, which provided a rational explanation for the observed high four-electron reactivity in the ORR process. Additionally, we analyzed the relationship between the estimated overpotential and the electronic structure properties, revealing that the single Sn atom was in a +2 oxidation state based on electronic analysis. Overall, this work represented a significant step towards the development of efficient and cost-effective SACs for ORR which could alleviate environmental crises, advance clean and sustainable energy sources, and contribute to a more sustainable future.

Molecular self-assembled synthesis of highly dispersed Co single-atom coordinated 2-methylimidazole modified carbon nitride for peroxymonosulfate activation

Although single-atom catalysts are pioneers in peroxymonosulfate (PMS) activation, selection of substrates and enhanced binding remains ambiguous. Here, a novel highly dispersed cobalt single-atom loaded 2-methylimidazole (MeIm) ligand-modified carbon nitride catalyst (Co-MCAMeIm) was prepared by a molecular self-assembly strategy. The densely distributed SA-Co with Co-N sites were corroborated by HAADF-STEM and XAFS analysis. The strong chelation of pre-dispersed ligands and triazine ring of g-C3N4 confers higher stability to the catalyst, reducing Co leaching. Meanwhile, the g-C3N4 substrate offers additional adsorption sites to shorten the reaction distance. The embedded Co-N functions as an electron donor capable of bridging N-rich carbon networks, hence accelerating the electron transport. Experimental findings demonstrated that sulfamethoxazole (SMX) could be effectively cleared through Co-MCAMeIm/PMS process (99.1%, 0.13 min−1). Several key influencing factors like Co to MeIm molar ratio, PMS concentration, Co-MCAMeIm dosage, pH value, anions, and humic acid were examined. The catalytic pathway is dominated by the activation of PMS with SA-Co-N as active sites to form 1O2. Meanwhile, a multi-ROS process with occurrence of SO4•-, O2•- and •OH was confirmed. Additionally, the SMX degradation routes and toxicity variations were derived and analyzed. Herein, the research may provide new insights for stable single-atom catalyst towards water treatment.

A highly active and stable single-atom catalyst for oxygen reduction with axial Fe-O coordination prepared through a fast medium-temperature pyrolysis process

In this study, we developed a highly active and stable Fe single atomic catalyst with axial Fe–O coordination through a fast medium-temperature pyrolysis process. The catalyst exhibited a high ORR performance, with a high half-wave potential of 0.93 V (vs. RHE), as well as excellent stability. When incorporated in a zinc–air battery's air electrode, a high power density of 214.5 mW cm−2 was obtained. After 333 h cyclic charging-discharging test, no obvious performance decay was observed. Density functional theory calculations revealed that the axial Fe–O coordination optimized the interactions between Fe and intermediates, inhibited the demetallization of Fe single atomic active sites and eventually enhanced the performance and stability of the obtained catalyst. We believe that the present strategy is easily translatable to the rational design of highly active and stable ORR catalysts for eco-friendly and future-oriented energy applications.

Transition Metal Single-Atom Catalysts for the Electrocatalytic Nitrate Reduction: Mechanism, Synthesis, Characterization, Application, and Prospects.

Excessive accumulation of nitrate in the environment will affect human health. To combat nitrate pollution, chemical, biological, and physical technologies have been developed recently. The researcher favors electrocatalytic reduction nitrate reaction (NO3 RR) because of the low post-treatment cost and simple treatment conditions. Single-atom catalysts (SACs) offer great activity, exceptional selectivity, and enhanced stability in the field of NO3 RR because of their high atomic usage and distinctive structural characteristics. Recently, efficient transition metal-based SACs (TM-SACs) have emerged as promising candidates for NO3 RR. However, the real active sites of TM-SACs applied to NO3 RR and the key factors controlling catalytic performance in the reaction process remain ambiguous. Further understanding of the catalytic mechanism of TM-SACs applied to NO3 RR is of practical significance for exploring the design of stable and efficient SACs. In this review, from experimental and theoretical studies, the reaction mechanism, rate-determining steps, and essential variables affecting activity and selectivity are examined. The performance of SACs in terms of NO3 RR, characterization, and synthesis is then discussed. In order to promote and comprehend NO3 RR on TM-SACs, the design of TM-SACs is finally highlighted, together with the current problems, their remedies, and the way forward.

Identifying the Activity Origin of a Single-Atom Au1/Nb2O5 Catalyst for Hydrodeoxygenation of Methylcatechol: A Stable Substitutional Au+ Site

Developing effective and stable single-atom catalysts (SACs) is a significant but challenging task, which requires the identification of single-atom metal sites and their configurations. Herein, we report a single-atom Au1/Nb2O5 catalyst for the hydrodeoxygenation of methylcatechol, a lignin model compound, which maintains stability in five consecutive conversions. Combined with the density functional theory (DFT) calculations, the Au+ located at the Nb lattice site with two concomitant oxygen vacancies (Au@Nbv–2Ov) is identified as the thermodynamically favorable Au single-atom species and dominates the activity for molecular H2 dissociation via a heterolytic mechanism. Moreover, the presence of oxygen vacancies in Au@Nbv–2Ov facilitates the adsorption of methylcatechol, resulting in efficient cleavage of C–O bonds. Consequently, we design a catalyst with more Au+ sites, which exhibits enhanced catalytic activity. This work systematically elucidates the structure–activity relationship and provides a promising strategy to optimize SACs for the hydrodeoxygenation reaction.

Turning the Inert Element Zinc into an Active Single-Atom Catalyst for Efficient Fenton-Like Chemistry.

Amongst various Fenton-like single-atom catalysts (SACs), the zinc (Zn)-related SACs have been barely reported due to the fully occupied 3d10 configuration of Zn2+ being inactive for the Fenton-like reaction. Herein, the inert element Zn is turned into an active single-atom catalyst (SA-Zn-NC) for Fenton-like chemistry by forming an atomic Zn-N4 coordination structure. The SA-Zn-NC shows admirable Fenton-like activity in organic pollutant remediation, including self-oxidation and catalytic degradation by superoxide radical (O2 ⋅- ) and singlet oxygen (1 O2 ). Experimental and theoretical results unveiled that the single-atomic Zn-N4 site with electron acquisition can transfer electrons donated by electron-rich pollutants and low-concentration PMS toward dissolved oxygen (DO) to actuate DO reduction into O2 ⋅- and successive conversion into 1 O2 . This work inspires an exploration of efficient and stable Fenton-like SACs for sustainable and resource-saving environmental applications.

Operando Stability of Single-Atom Electrocatalysts.

Single-atom catalysts (SACs) are appealing next-generation catalysts for various electrochemical technologies. Along with significant breakthroughs in their initial activity, SACs now face the next challenge for their viable applications, insufficient operational stability. In this Minireview, we summarize the current knowledge of SAC degradation mechanisms mainly based on Fe-N-C SACs, some of the most investigated SACs. Recent studies on isolated metal, ligand, and support degradations are introduced, and the underlying fundamentals of each degradation path are categorized into active site density (SD) and turnover frequency (TOF) losses. Finally, we discuss the challenges and prospects for the future outlook of stable SACs.