The single metal atom (Ni, Pd, Pt) anchored on defective hexagonal boron nitride for oxidative desulfurization

Single-atom catalysts (SACs) have attracted great attention for various chemical reactions because of their strong activity, high metal utilization ratio, and low cost. Here, by using the density functional theory (DFT) method, the stability of a single VIII-group metal atom (M = Ni, Pd, Pt) anchored on the defective hexagonal boron nitride (h-BN) sheet and its possible application in oxidative desulfurization (ODS) are investigated. Calculations show that the stability of the single M atom embedded in the h-BN surface with B and N vacancies is strikingly enhanced compared to that on the perfect h-BN surface. The catalytic activities of the defective h-BN-supported single metal atom are further studied by the activation of molecular oxygen and subsequent oxidation of dibenzothiophene (DBT). O2 is activated to the super-oxo state with large interaction energies on three M/VN surfaces. However, among the three M/VB surfaces, only Pt/VB performs efficient activation of O2. The oxidation of DBT proceeds in two steps; the rate-determining step is the initial step, in which activated O2 oxidizes DBT to produce sulfoxide. By comparing the energy barrier in the first reaction step, both Ni/VN and Pt/VB are revealed as promising candidates for the ODS reaction.

Ag-N-C single atom catalyst with resistance for Ag loss in acetylene hydrochlorination

Ag-N-C catalyst was synthesized by the calcination process with AgNO3as precursors, active carbon as support, and melamine as an N source. Series of characterizations showed that Ag was transferred into AgCl during the active phase by HCl, and pyridinic structure in the support was bonded with Ag components. Then, Ag-N-C single atom catalyst (SAC) was obtained by washing Ag-N-C with acid, aberration-correction high-angle-annular-dark-field scanning transmission electron microscopy showed that Ag presented in single atoms form, and Ag coordinated with the nitrogen atom in the support. Ag loss rate for Ag-N-C SAC was only 0.09% after running 10 h in acetylene hydrochlorination process, which was much smaller than Ag-N-C (57%), indicating that the presence of the Ag-N bond could be inhibiting Ag species loss.

Single-atom site catalysts for environmental remediation: Recent advances

Single-atom site catalysts (SACs) can maximize the utilization of active metal species and provide an attractive way to regulate the activity and selectivity of catalytic reactions. The adjustable coordination configuration and atomic structure of SACs enable them to be an ideal candidate for revealing reaction mechanisms in various catalytic processes. The minimum use of metals and relatively tight anchoring of the metal atoms significantly reduce leaching and environmental risks. Additionally, the unique physicochemical properties of single atom sites endow SACs with superior activity in various catalytic processes for environmental remediation (ER). Generally, SACs are burgeoning and promising materials in the application of ER. However, a systematic and critical review on the mechanism and broad application of SACs-based ER is lacking. Herein, we review emerging studies applying SACs for different ERs, such as eliminating organic pollutants in water, removing volatile organic compounds, purifying automobile exhaust, and others (hydrodefluorination and disinfection). We have summarized the synthesis, characterization, reaction mechanism and structural-function relationship of SACs in ER. In addition, the perspectives and challenges of SACs for ER are also analyzed. We expect that this review can provide constructive inspiration for discoveries and applications of SACs in environmental catalysis in the future.

Recent Progress in Fabrication and Application of BN Nanostructures and BN-Based Nanohybrids

Due to its unique physical, chemical, and mechanical properties, such as a low specific density, large specific surface area, excellent thermal stability, oxidation resistance, low friction, good dispersion stability, enhanced adsorbing capacity, large interlayer shear force, and wide bandgap, hexagonal boron nitride (h-BN) nanostructures are of great interest in many fields. These include, but are not limited to, (i) heterogeneous catalysts, (ii) promising nanocarriers for targeted drug delivery to tumor cells and nanoparticles containing therapeutic agents to fight bacterial and fungal infections, (iii) reinforcing phases in metal, ceramics, and polymer matrix composites, (iv) additives to liquid lubricants, (v) substrates for surface enhanced Raman spectroscopy, (vi) agents for boron neutron capture therapy, (vii) water purifiers, (viii) gas and biological sensors, and (ix) quantum dots, single photon emitters, and heterostructures for electronic, plasmonic, optical, optoelectronic, semiconductor, and magnetic devices. All of these areas are developing rapidly. Thus, the goal of this review is to analyze the critical mass of knowledge and the current state-of-the-art in the field of BN-based nanomaterial fabrication and application based on their amazing properties.

Ag Atom Anchored on Defective Hexagonal Boron Nitride Nanosheets As Single Atom Adsorbents for Enhanced Adsorptive Desulfurization via S-Ag Bonds

Single atom adsorbents (SAAs) are a novel class of materials that have great potential in various fields, especially in the field of adsorptive desulfurization. However, it is still challenging to gain a fundamental understanding of the complicated behaviors on SAAs for adsorbing thiophenic compounds, such as 1-Benzothiophene (BT), Dibenzothiophene (DBT), and 4,6-Dimethyldibenzothiophene (4,6-DMDBT). Herein, we investigated the mechanisms of adsorptive desulfurization over a single Ag atom supported on defective hexagonal boron nitride nanosheets via density functional theory calculations. The Ag atom can be anchored onto three typical sites on the pristine h-BN, including the monoatomic defect vacancy (B-vacancy and N-vacancy) and the boron-nitrogen diatomic defect vacancy (B-N-divacancy). These three Ag-doped hexagonal boron nitride nanosheets all exhibit enhanced adsorption capacity for thiophenic compounds primarily by the S-Ag bond with π-π interaction maintaining. Furthermore, from the perspective of interaction energy, all three SAAs show a high selectivity to 4,6-DMDBT with the strong interaction energy (-33.9 kcal mol^-1, -29.1 kcal mol^-1, and -39.2 kcal mol^-1, respectively). Notably, a little charge transfer demonstrated that the dominant driving force of such S-Ag bond is electrostatic interaction rather than coordination effect. These findings may shed light on the principles for modeling and designing high-performance and selective SAAs for adsorptive desulfurization.

Modulating the Local Coordination Environment of Single-Atom Catalysts for Enhanced Catalytic Performance in Hydrogen/Oxygen Evolution Reaction

Single-atom catalysts (SACs) hold the promise of utilizing 100% of the participating atoms in a reaction as active catalytic sites, achieving a remarkable boost in catalytic efficiency. Thus, they present great potential for noble metal-based electrochemical application systems, such as water electrolyzers and fuel cells. However, their practical applications are severely hindered by intrinsic complications, namely atom agglomeration and relocation, originating from the uncontrollably high surface energy of isolated single-atoms (SAs) during postsynthetic treatment processes or catalytic reactions. Extensive efforts have been made to develop new methodologies for strengthening the interactions between SAs and supports, which could ensure the desired stability of the active catalytic sites and their full utilization by SACs. This review covers the recent progress in SACs development while emphasizing the association between the regulation of coordination environments (e.g., coordination atoms, numbers, sites, structures) and the electrocatalytic performance of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The crucial role of coordination chemistry in modifying the intrinsic properties of SACs and manipulating their metal-loading, stability, and catalytic properties is elucidated. Finally, the future challenges of SACS development and the industrial outlook of this field are discussed.

Ca functionalized N-doped porphyrin-like porous C60 as an efficient material for storage of molecular hydrogen

It is widely known that decorating metal atoms on defective carbon nanomaterials is a useful approach to enhance the hydrogen storage capacity of these systems. Herein, density functional theory calculations are used to determine the H₂ storage capacity of Ca functionalized nitrogen incorporated defective C₆₀ fullerenes (Ca₆C₂₄N₂₄). The strong binding, uniform distribution, and significant positive charges of the Ca atoms make this system effective material for storage of H₂. Ca₆C₂₄N₂₄ may adsorb a maximum of 6 hydrogen molecules per Ca atom, yielding a total gravimetric density of 7.7 wt %.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Significant Role of Oxygen Dopants in Photocatalytic PFCA Degradation over h-BN

The activation of the C(sp³)-F bond is extremely difficult due to its unreactive nature. The importance of this bond activation is recently highlighted because extensive distribution of perfluorocarboxylic acids (PFCAs) (C_nF_2n+1COOH) has emerged as a challenging environmental issue. Photocatalytic degradation of PFCAs over a few semiconducting light absorbers is known to remove these water and soil resilient contaminants but with limited efficiency. This work reports density functional theory calculations, through which we present a detailed mechanistic study of photocatalytic degradation of CF₃COOH (the shortest member of the PFCA family) over hexagonal boron nitride (h-BN). Our results clearly demonstrate that the existence of point defects is necessary to activate the h-BN plane for photocatalytic dissociation of the C-F bond. Specifically, we show that vacancies create strong Lewis acid or base sites (B or N vacancy, respectively) that facilitate the activation of the C(sp³)-F bond considerably. Furthermore, this study presents vivid theoretical evidence for the significant role of oxygen dopants, which mitigate the strength of the active sites and promote PFCA degradation over h-BN. Our calculations suggest that while the very stable intermediates generated during the reaction, in the case of h-BN with B or N vacancies, practically poison the catalyst, oxygen dopants make the degradation much more plausible and controllable. This work thus provides both an explanation for recently observed photocatalytic activity of h-BN to decompose PFCAs and valuable insights for exploring defected two-dimensional materials for activating and removing the fluorine-containing contaminants from water and soil.

Catalytic CO oxidation reaction over N-substituted graphene nanoribbon with edge defects

Density functional theory calculations, including dispersion effects, are used to demonstrate how substitutional nitrogen atoms can improve the catalytic reactivity of graphene nanoribbons (GNR) with edge defects in the CO oxidation process. It is demonstrated that the addition of nitrogen impurities significantly enhances O₂ adsorption on GNR. Carbon atoms near the edges of defects are the most active sites for capturing O₂ molecules. The lower adsorption energy of CO relative to O₂ implies that the N-modified GNR is resistant to CO poisoning. The Eley-Rideal (E-R) mechanism has activation energies as low as 0.38 eV, making it the most energetically relevant pathway for the CO + O₂ reaction. The findings of this study might help in the design of catalysts for metal-free catalysis of CO oxidation.

CO oxidation over defective and nonmetal doped MoS2monolayers

Defective (missing S atoms) and nonmetal (C- and N-) doped MoS₂monolayers in the 2H and 1T' phases have been evaluated for catalyzing CO oxidation based on first-principles calculations. For the reaction 2CO + O₂→ 2CO₂, the oxidization of the first CO molecule is fairly easy and sometimes is even spontaneous, as the O₂ molecule is highly activated or dissociates upon adsorption. However, for the defective (2H-), C-doped (1T'-), and N-doped (2H- and 1T'-) MoS₂monolayers, the remaining O^*adatom often refuses to react with other CO molecules and is hard to be removed (barrier > 1.20 eV). Only when over the C-doped 2H- and defective 1T'-MoS₂monolayers, the removal of the second O^*adatom requires to overcome moderate barriers (0.74 and 0.88 eV, respectively) by reacting with another CO molecule via the Eley-Rideal mechanism and the catalysts are recovered. The barriers can be further reduced by applying either tensile or compressive strain to the MoS₂nanosheet. In contrast, the Langmuir-Hinshelwood mechanism is followed over the metal-containing MoS₂nanosheets, as the bigger size of metal dopants allow the co-adsorption of CO and O₂. Therefore, the C-doped 2H- and defective 1T'-MoS₂monolayers are promising nonmetal-doped catalysts for CO oxidation.

Highly efficient catalytic properties of Sc and Fe single atoms stabilized on a honeycomb borophene/Al(111) heterostructure via a dual charge transfer effect

Theoretical design and experimental fabrication of highly efficient single-atom catalysts (SACs) containing isolated metal atoms monodispersed on appropriate substrates have surged to the forefront of heterogeneous catalysis in recent years. Nevertheless, the instability of SACs, i.e., preferential clustering in chemical reaction processes, dramatically hinders their practical applications. In this paper, using first-principles calculations, we predict that a honeycomb borophene/Al(111) heterostructure can be an ideal candidate to stabilize and enhance the catalysis of many transition metal (TM) SACs via a dual charge transfer mechanism. The Al(111) substrate donates electrons to the pre-covered two-dimensional honeycomb borophene (h-B) to stabilize the latter, and the deposited TM atoms further provide electrons to the h-B, enhancing the covalent binding between the h-B and the Al(111) substrate. Intriguingly, during CO oxidation, the h-B/Al(111) heterostructure can in turn serve as an efficient electron reservoir to accept electrons from or donate electrons to the deposited TM-SACs and the reactants. Such a flexible dual charge transfer mechanism not only facilitates stabilizing the TM-SACs rather than clustering, but also effectively reduces the reaction barriers. Particularly, in contrast to expensive noble metal atoms such as Pd and Pt, low-cost Sc- and Fe-SACs are found to be the most promising SAC candidates that can be stabilized on h-B/Al(111) for O₂ activation and CO oxidation, with fairly low reaction barriers (around 0.50-0.65 eV). The present findings may provide important theoretical guidance for the experimental fabrication of highly stable, efficient, and economic SACs stabilized on various heterostructure substrates.

Phosphorene Supported Single-Atom Catalysts for CO Oxidation: A Computational Study

Single-atom catalysts (SACs) have attracted extensive attention owing to their high catalytic activity. The development of efficient SACs is crucial for applications in heterogeneous catalysis. In this article, the geometric configuration, electronic structure, stabilitiy and catalytic performance of phosphorene (Pn) supported single metal atoms (M=Ru, Rh, Pd, Ir, Pt, and Au) have been systematically investigated using density functional theory calculations and ab initio molecular dynamics simulations. The single atoms are found to occupy the hollow site of phosphorene. Among the catalysts studied, Ru-decorated phosphorene is determined to be a potential catalyst by evaluating adsorption energies of gaseous molecules. Various mechanisms including the Eley-Rideal (ER), Langmuir-Hinshelwood (LH) and trimolecular Eley-Rideal (TER) mechanisms are considered to validate the most favourable reaction pathway. Our results reveal that Ru-Pn exhibits outstanding catalytic activity toward CO oxidation reaction via TER mechanism with the corresponding rate-determining energy barrier of 0.44 eV, making it a very promising SAC for CO oxidation under mild conditions. Overall, this work may provide a new avenue for the design and fabrication of two-dimensional materials supported SACs for low-temperature CO oxidation.

Single transition metal anchored C9N4 sheets as an efficient catalyst for CO oxidation: a first-principles study

Single Ni/Co anchored C₉N₄ sheet works as an efficient catalyst for CO oxidation.

Catalytic role of graphitic nitrogen atoms in the CO oxidation reaction over N-containing graphene: a first-principles mechanistic evaluation

The catalytic role of graphitic nitrogen atoms of a series of nitrogen-doped graphene surfaces is explored for low-temperature oxidation of CO using periodic DFT calculations.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Understanding the Activity of Single-Atom Catalysis from Frontier Orbitals

The d-band center and charge states are often used to analyze the catalytic activity of noble or transition metal surfaces and clusters, but their applicability for single-atom catalysts (SACs) is unsure. This work suggests that the spatial structure and orientation of frontier orbitals which are closest to the Fermi level of SACs play a vital role. Taking adsorption of several molecules and CO oxidization on C_{3}N-supported single-atom Au as examples, we demonstrate that adsorption and catalytic activities are well correlated with the characteristics of frontier orbitals. This work provides an effective guidance for understanding the performance of single-atom catalysts.

Improving the Catalytic Activity of Carbon-Supported Single Atom Catalysts by Polynary Metal or Heteroatom Doping

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.

An effective approach for tuning catalytic activity of C3N nanosheets: Chemical-doping with the Si atom

It is well-known that the catalytic oxidation of CO molecule into CO₂ is one of the most important strategies for the removing of this toxic gas from the atmosphere. In the present study, we investigate the reaction pathways and energy barriers for the oxidation of CO by O₂ molecule over the Si-doped C₃N nanosheet. According to our results, doping of C₃N nanosheet with a Si atom could greatly modify its surface reactivity and electronic structure. Due to the large positive charge on the Si, this atom acts as the most active site to adsorb CO and O₂ molecules. Three possible reaction mechanisms are studied for the CO oxidation, namely the Eley-Rideal (ER), Langmuir-Hinshelwood (LH) and new Eley-Rideal (NER). Comparing the activation energies indicates that the CO oxidation reaction proceeds via the LH mechanism over the title surface. The energy barrier needed to remove the activated oxygen atom (O*) from the Si atom is only 0.22 eV, which is most likely to overcome at room temperature. The results of this study may be useful to fabricate noble-metal free catalysts to remove toxic CO molecules from the atmosphere.

First-Principles Investigations of Single Metal Atoms (Sc, Ti, V, Cr, Mn, and Ni) Embedded in Hexagonal Boron Nitride Nanosheets for the Catalysis of CO Oxidation

We evaluated isolated transition metal atoms (Sc, Ti, V, Cr, Mn, and Ni) embedded in hexagonal-BN as novel single atom catalysts for CO oxidation. We predicted that embedded Ni atoms should have superior performance for this task. Ti, V, and Mn bind CO2 too strongly and so the reaction will not proceed smoothly. We studied the detailed reaction processes for Sc, Cr, and Ni. The Langmuir–Hinshelwood (LH), Eley–Rideal (ER), and the new termolecular Eley–Rideal (TER) processes for CO oxidation were investigated. Sc was not effective. Cr primarily used the ER process, although the barrier was relatively large at 1.30 eV. Ni was the best of the group, with a 0.44 eV barrier for LH, and a 0.47 eV barrier for TER. Therefore, we predicted that the LH and TER processes could operate at relatively low temperatures between 300 and 500 K.

In situ spectroscopy-guided engineering of rhodium single-atom catalysts for CO oxidation

Single-atom catalysts have recently been applied in many applications such as CO oxidation. Experimental in situ investigations into this reaction, however, are limited. Hereby, we present a suite of operando/in situ spectroscopic experiments for structurally well-defined atomically dispersed Rh on phosphotungstic acid during CO oxidation. The identification of several key intermediates and the steady-state catalyst structure indicate that the reactions follow an unconventional Mars-van Krevelen mechanism and that the activation of O₂ is rate-limiting. In situ XPS confirms the contribution of the heteropoly acid support while in situ DRIFT spectroscopy consolidates the oxidation state and CO adsorption of Rh. As such, direct observation of three key components, i.e., metal center, support and substrate, is achieved, providing a clearer picture on CO oxidation on atomically dispersed Rh sites. The obtained information are used to engineer structurally similar catalysts that exhibit T₂₀ values up to 130 °C below the previously reported Rh₁/NPTA.

Single-atom catalysts have been studied for CO oxidation, but experimental in situ investigations are limited. Here, the authors use a suite of in situ/operando spectroscopy to identify key intermediates and define design principles to enhance the CO oxidation activity of atomically dispersed Rh on heteropoly acids.

A DFT study on NO reduction to N2O using Al- and P-doped hexagonal boron nitride nanosheets

Using the dispersion-corrected DFT calculations, the catalytic reduction of NO molecules to N₂O is investigated over Al- and P-doped hexagonal boron nitride nanosheets (h-BNNS). It is found that NO dissociation over both these surfaces needs a very large energy barrier, which indicates it cannot proceed at normal temperature. In contrast, the results show that NO molecules can be easily reduced into N₂O via a dimer mechanism. The obtained activation energies reveal that the catalytic activity of Al-doped h-BNNS is better than that of P-doped one, mainly due to the moderate coadsorption energies of NO molecules over this surface.

Single-atom Pt on non-metal modified graphene sheets as efficient catalysts for CO oxidation

By the density functional theory (DFT) calculations, the formation geometries, electronic structures and catalytic properties of metal Pt and nonmetal (NM) atom-co-modified graphene (Pt–3NM–graphene, NM = N, Si, P) as reactive substrates were investigated.

Theoretical analysis of oxygen reduction reaction activity on single metal (Ni, Pd, Pt, Cu, Ag, Au) atom supported on defective two-dimensional boron nitride materials

Single metal atom supported by a defective two-dimensional boron nitride material is a promising ORR catalyst.

Structural evolution of Ag/BN hybrids via a polyol-assisted fabrication process and their catalytic activity in CO oxidation

Enhanced catalytic activity of Ag/BN nanohybrids is ascribed to the formation of a thin intermediate Ag–O–B layer.

Single Pt atom supported on penta-graphene as an efficient catalyst for CO oxidation

We performed a systematic study of CO oxidation on a single Pt atom supported on penta-graphene (Pt/PG) by utilizing spin-polarized first-principles calculations. The results manifested that Pt/PG, as a single-atom catalyst, exhibited excellent catalytic activity toward CO oxidation and provided a novel strategy for the design of single-atom catalysts based on penta-graphene.