Single non-noble-metal cobalt atom stabilized by pyridinic vacancy graphene: An efficient catalyst for CO oxidation

Hydroxyls on CeO2 Support Promoting CuO/CeO2 Catalyst for Efficient CO Oxidation and NO Reduction by CO

Transition metal catalysts, such as copper oxide, are more attractive alternatives to noble metal catalysts for emission control due to their higher abundance, lower cost, and excellent catalytic activity. In this study, we report the preparation and application of a novel CuO/CeO2 catalyst using a hydroxyl-rich Ce(OH)x support for CO oxidation and NO reduction by CO. Compared to the catalyst prepared from a regular CeO2 support, the new CuO/CeO2 catalyst prepared from the OH-rich Ce(OH)x (CuO/CeO2-OH) showed significantly higher catalytic activity under different testing conditions. The effect of OH species in the CeO2 support on the catalytic performance and physicochemical properties of the CuO/CeO2 catalyst was characterized in detail. It is demonstrated that the abundant OH species enhanced the CuOx dispersion on CeO2, increased the CuOx-CeO2 interfaces and surface defects, promoted the oxygen activation and mobility, and boosted the NO adsorption and dissociation on CuO/CeO2-OH, thus contributing to its superior catalytic activity for both CO oxidation and NO reduction by CO. These results suggest that the OH-rich Ce(OH)x is a superior support for the preparation of highly efficient metal catalysts for different applications.

Activity Quantification of Fuel Cell Catalysts via Sequential Poisoning by Multiple Reaction Inhibitors

The development of non-Pt or carbon-based catalysts for anion exchange membrane fuel cells (AEMFCs) requires identification of the active sites of the catalyst. Since not only metals but also carbon materials exhibit oxygen reduction reaction (ORR) activity in alkaline conditions, the contribution of carbon-based materials to ORR performance should also be thoroughly analyzed. However, the conventional CN^- poisoning experiments, which are mainly used to explain the main active site of M-N-C catalysts, are limited to only qualitative discussions, having the potential to make fundamental errors. Here, we report a modified electrochemical analysis to quantitatively investigate the contribution of the metal and carbon active sites to ORR currents at a fixed potential by sequentially performing chronoamperometry with two reaction inhibitors, CN^- and benzyl trimethylammonium (BTMA⁺). As a result, we discover how to quantify the individual contributions of two active sites (Pt nanoparticles and carbon support) of carbon-supported Pt (Pt/C) nanoparticles as a model catalyst. This study is expected to provide important clues for the active site analysis of carbon-supported non-Pt catalysts, such as M-N-C catalysts composed of heterogeneous elements.

Electronic Properties and Electrocatalytic Water Splitting Activity for Precious-Metal-Adsorbed Silicene with Nonmetal Doping

Since nonmetal (NM)-doped two-dimensional (2D) materials can effectively modulate their physical properties and chemical activities, they have received a lot of attention from researchers. Therefore, the stability, electronic properties, and electrocatalytic water splitting activity of precious-metal (PM)-adsorbed silicene doped with two NM atoms are investigated based on density functional theory (DFT) in this paper. The results show that NM doping can effectively improve the stability of PM-adsorbed silicene and exhibit rich electronic properties. Meanwhile, by comparing the free energies of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) intermediates of 15 more stable NM-doped systems, it can be concluded that the electrocatalytic water splitting activity of the NM-doped systems is more influenced by the temperature. Moreover, the Si-S2-Ir-doped system exhibits good HER performance when the temperature is 300 K, while the Si-N2-Pt-doped system shows excellent OER activity. Our theoretical study shows that NM doping can effectively promote the stability and electrocatalytic water splitting of PM-adsorbed silicene, which can help in the application of silicene in electrocatalytic water splitting.

Fe@χ3-borophene as a promising catalyst for CO oxidation reaction: A first-principles study

A novel single-atom catalyst of Fe adsorbed on χ₃-borophene has been proposed as a potential catalyst for CO oxidation reaction (COOR). Quantitative pictures have been provided of both the stability of Fe@χ₃-borophene and various kinetic reaction pathways using first-principles calculations. Strong adsorption energy of -3.19 eV and large diffusion potential of 3.51 eV indicates that Fe@χ₃-borophene is highly stable. By exploring reaction mechanisms for COOR, both Eley-Ridel (E-R) and trimolecule E-R (TER) were identified as possible reaction paths. Low reaction barriers with 0.49 eV of E-R and 0.57 eV of TER suggest that Fe@χ₃-borophene is a very promising catalyst for COOR. Charge transfer between the χ₃-borophene and CO, O₂ and CO₂ gas molecules plays a key role in lowering the energy barrier during the reactions. Our results propose that Fe@χ₃-borophene can be a good candidate of single-atom catalyst for COOR with both high stability and catalytic activity.

Oxidation of methane and ethylene over Al incorporated N-doped graphene: A comparative mechanistic DFT study

It is generally recognized that developing effective methods for selective oxidation of hydrocarbons to generate more useful chemicals is a major challenge for the chemical industry. In the present study, density functional theory calculations are conducted to examine the catalytic partial oxidation of methane (CH₄) and ethylene (C₂H₄) by nitrous oxide (N₂O) over Al-incorporated porphyrin-like N-doped graphene (AlN₄-Gr). Adsorption energies for the most stable configurations of CH₄, C₂H₄, and N₂O molecules over the AlN₄-Gr catalyst are determined to be -0.25, -0.64, and -0.40 eV, respectively. According to our findings, N₂O can be efficiently split into N₂ and O_ads species with a negligible activation energy on the AlN₄-Gr surface. Meanwhile, CH₄ and C₂H₄ molecules compete for reaction with the activated oxygen atom (O_ads) that stays on the surface. The energy barriers for partial methane oxidation through the CH₄ + O_ads → CH₃° + HO_ads and CH₃° + HO_ads → CH₃OH reaction steps are 0.16 eV and 0.27 eV, respectively. Furthermore, the produced CH₃OH may be overoxidized by O_ads to give formaldehyde and water molecules by overcoming a relatively low activation barrier. The activation barriers for C₂H₄ epoxidation are small and comparable to those for CH₄ oxidation, implying that AlN₄-Gr is highly active for both reactions. The high energy barrier for the 1,2-hydrogen shift in the OCH₂CH₂ intermediate, on the other hand, makes the production of acetaldehyde impossible under normal conditions. According to the population analysis, the AlN₄-Gr serves as a strong electron donor to aid in the charge transfer between the Al atom and the O_ads moiety, which is necessary for the activation of CH₄ and C₂H₄. The findings of the present study may pave the way for a better understanding of the catalytic oxidation the CH₄ and C₂H₄, as well as for the development of highly efficient noble-metal free catalysts for these reactions.

The Adsorption Behavior of Gas Molecules on Co/N Co-Doped Graphene

Herein, we have used density functional theory (DFT) to investigate the adsorption behavior of gas molecules on Co/N3 co-doped graphene (Co/N3-gra). We have investigated the geometric stability, electric properties, and magnetic properties comprehensively upon the interaction between Co/N3-gra and gas molecules. The binding energy of Co is -5.13 eV, which is big enough for application in gas adsorption. For the adsorption of C2H4, CO, NO2, and SO2 on Co/N-gra, the molecules may act as donors or acceptors of electrons, which can lead to charge transfer (range from 0.38 to 0.7 e) and eventually change the conductivity of Co/N-gra. The CO adsorbed Co/N3-gra complex exhibits a semiconductor property and the NO2/SO2 adsorption can regulate the magnetic properties of Co/N3-gra. Moreover, the Co/N3-gra system can be applied as a gas sensor of CO and SO2 with high stability. Thus, we assume that our results can pave the way for the further study of gas sensor and spintronic devices.

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.

Reversible CO2 storage and efficient separation using Ca decorated porphyrin-like porous C24N24 fullerene: a DFT study

The search for novel materials for effective storage and separation of CO₂ molecules is a critical issue for eliminating or lowering this harmful greenhouse gas. In this paper, we investigate the potential application of a porphyrin-like porous fullerene (C₂₄N₂₄) as a promising material for CO₂ storage and separation using thorough density functional theory calculations. The results show that CO₂ is physisorbed on bare C₂₄N₂₄, implying that this material cannot be used for efficient CO₂ storage. Coating C₂₄N₂₄ with Ca atoms, on the other hand, can greatly improve the adsorption strength of CO₂ molecules due to polarization and charge-transfer effects. Furthermore, the average adsorption energy for each of the maximum 24 absorbed CO₂ molecules on the fully decorated Ca₆C₂₄N₂₄ fullerene is −0.40 eV, which fulfills the requirement needed for efficient CO₂ storage (−0.40 to −0.80 eV). The Ca coated C₂₄N₂₄ fullerene also have a strong potential for CO₂ separation from CO₂/H₂, CO₂/CH₄, and CO₂/N₂ mixtures.

Using dispersion-corrected DFT calculations, the potential application of a porphyrin-like porous fullerene (C₂₄N₂₄) as an efficient material for CO₂ storage and separation was investigated.

Theoretical insights into the thermal reduction of N2 to NH3 over a single metal atom incorporated nitrogen-doped graphene

Density functional theory calculations have been performed to study the reaction mechanism of N₂ thermal reduction (N₂TR) over a single metal atom incorporated nitrogen-doped graphene. Our results reveal that the type of metal atoms and their coordination environments have a significant effect on the catalytic activity of N₂TR. Regarding CoN₄- and FeN₄-embedded graphene sheets that the metal atom is fourfold coordinated, they are inactive for N₂TR owing to the poor stability of the adsorbed H₂ and N₂ molecules. In contrast, if the monodisperse metal atom is surrounded by three N atoms, namely, CoN₃/G and FeN₃/G show activity toward N₂TR, and catalytic conversion of N₂ into ammonia is achieved through the associative mechanism rather than the dissociative mechanism. Further investigations show that the synthesis of NH₃ over the two surfaces is mainly through the formation of an NHNH^* intermediate; however, the detailed reaction mechanisms are sensitive to the type of metal atom introduced into N-doped graphene. Based on the calculated kinetic barriers, FeN₃/G exhibits a better catalytic activity for N₂TR. The superior performance of FeN₃/G can be attributed to the fact that this surface prefers a high spin-polarized state during the whole process of N₂TR, while the non-spin polarized state is predicted as the ground state for most of the elementary steps of N₂-fixation over CoN₃/G. The present study provides theoretical insights into developing graphene-based single atom catalysts with a high activity toward ammonia synthesis through N₂TR.

Ultrasonic Plasma Engineering Toward Facile Synthesis of Single-Atom M-N4/N-Doped Carbon (M = Fe, Co) as Superior Oxygen Electrocatalyst in Rechargeable Zinc-Air Batteries

As bifunctional oxygen evolution/reduction electrocatalysts, transition-metal-based single-atom-doped nitrogen-carbon (NC) matrices are promising successors of the corresponding noble-metal-based catalysts, offering the advantages of ultrahigh atom utilization efficiency and surface active energy. However, the fabrication of such matrices (e.g., well-dispersed single-atom-doped M-N₄/NCs) often requires numerous steps and tedious processes. Herein, ultrasonic plasma engineering allows direct carbonization in a precursor solution containing metal phthalocyanine and aniline. When combining with the dispersion effect of ultrasonic waves, we successfully fabricated uniform single-atom M-N₄ (M = Fe, Co) carbon catalysts with a production rate as high as 10 mg min^-1. The Co-N₄/NC presented a bifunctional potential drop of ΔE = 0.79 V, outperforming the benchmark Pt/C-Ru/C catalyst (ΔE = 0.88 V) at the same catalyst loading. Theoretical calculations revealed that Co-N₄ was the major active site with superior O₂ adsorption-desorption mechanisms. In a practical Zn-air battery test, the air electrode coated with Co-N₄/NC exhibited a specific capacity (762.8 mAh g^-1) and power density (101.62 mW cm^-2), exceeding those of Pt/C-Ru/C (700.8 mAh g^-1 and 89.16 mW cm^-2, respectively) at the same catalyst loading. Moreover, for Co-N₄/NC, the potential difference increased from 1.16 to 1.47 V after 100 charge-discharge cycles. The proposed innovative and scalable strategy was concluded to be well suited for the fabrication of single-atom-doped carbons as promising bifunctional oxygen evolution/reduction electrocatalysts for metal-air batteries.

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.

Theoretical Understandings of Graphene-based Metal Single-Atom Catalysts: Stability and Catalytic Performance

Research on heterogeneous single-atom catalysts (SACs) has become an emerging frontier in catalysis science because of their advantages in high utilization of noble metals, precisely identified active sites, high selectivity, and tunable activity. Graphene, as a one-atom-thick two-dimensional carbon material with unique structural and electronic properties, has been reported to be a superb support for SACs. Herein, we provide an overview of recent progress in investigations of graphene-based SACs. Among the large number of publications, we will selectively focus on the stability of metal single-atoms (SAs) anchored on different sites of graphene support and the catalytic performances of graphene-based SACs for different chemical reactions, including thermocatalysis and electrocatalysis. We will summarize the fundamental understandings on the electronic structures and their intrinsic connection with catalytic properties of graphene-based SACs, and also provide a brief perspective on the future design of efficient SACs with graphene and graphene-like materials.

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.

Surface vacancy on PtTe2 for promoting CO oxidation through efficiently activating O2

Platinum group metal dichalcogenides (PtTe₂) with controllable thickness have been synthesized and confirmed to be promising electric and spintronic materials. Here, using the first-principles calculations, we demonstrate the potential application of PtTe₂ as catalyst substrate. Taking CO oxidation as model reaction, the importance of surface vacancy is clarified. It is found that surface vacancy on PtTe₂ could improve the stability and catalytic activity of the supported Pt atom. The details of CO oxidation processes indicate that surface vacancy could weaken the adsorption of reactants and speed up the formation and decomposition of OOCO intermediate on Pt catalysts. The underlying mechanisms for the improved activity are unveiled through comprehensively analyzing the charge transfer, density of states, and charge density difference. We hope that the current findings were beneficial for the research and development of efficient catalysts by collocating various single atom/cluster catalysts with different platinum group metal dichalcogenides.

Single-atom metal-modified graphenylene as a high-activity catalyst for CO and NO oxidation

Herein, the adsorption behaviors and interactions of different gas species on single-metal atom-anchored graphenylene (M–graphenylene, M = Mn, Co, Ni, and Cu) sheets were investigated by first-principles calculations.

AlN4-Graphene as an efficient catalyst for CO oxidation: a DFT study

DFT investigations suggest that AlN₄-Gr shows high stability and superior catalytic performance towards CO oxidation without CO poisoning.

Efficient DBT removal from diesel oil by CVD synthesized N-doped graphene as a nanoadsorbent: Equilibrium, kinetic and DFT study

Adsorptive Dibenzothiophene (DBT) removal from diesel oil stream on nitrogen doped graphene (N-doped graphene) was considered. The N-doped graphene was synthesized by chemical vapor deposition (CVD) method at 1000 °C using camphor and urea. The adsorbent was characterized by Field Emission Scanning Electron Microscopy (FE-SEM), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR) and Nitrogen adsorption/desorption technique. Adsorption parameters such as temperature, time, concentration and mass loaded were optimized by experimental design method. Experimental kinetic data was fitted to Pseudo second order model successfully. Frendulich model was recommended for experimental isotherm data. However, Tempkin model was presented because of the importance of interaction between pyridinic nitrogen and DBT aromatic structure. The results indicate that not only the pore volume and surface area but also types of surface functionalities have an important role for DBT adsorption process, especially for the adsorbates with aromatic structures. The adsorption capacity was calculated up to 73.4 mg/g which is 1.25 times higher than the adsorption capacity of pristine. Thermal regeneration stability, fast adsorption kinetics and high adsorption capacity make N-G4 a potential promising adsorbent for DBT removal. Besides, density functional theory calculations revealed that an increase in the number of doped N atoms as well as the presence of a mono or divacancy defect in N-doped graphene can enhance the adsorption energy of DBT.

Strong metal-support interactions impart activity in the oxygen reduction reaction: Au monolayer on Mo2C (MXene)

The rational design of low-cost, high-efficiency, corrosion-resistant and persistent-activity oxygen reduction reaction (ORR) electrocatalysts is a common goal for the large-scale application of fuel cells. Inspired by the excellent characteristics of MXenes when used as substrate materials and recent experiments of depositing metal nanoparticles on MXenes, we systematically investigated monolayer metal thin films decorated by Mo₂C (MXene) (M_ML/Mo₂C, M  =  Cu, Pd, Pt, Ag and Au) as ORR catalysts using density functional theory. According to the stability and adsorption properties, we speculate that Au_ML/Mo₂C possesses outstanding ORR performance and enhanced durability in comparison with Pt/C catalysts. The ORR on Au_ML/Mo₂C proceeds through a four-electron reduction pathway with comparable or even better activity than Pt(1 0 0), Pt(1 1 1) and commercial Pt/C catalysts both kinetically and thermodynamically. Strong metal-support interactions give rise to larger electronic perturbations in the supported Au monolayer in contact with Mo₂C, which strengthen the adsorption of oxygen-containing species and enhance the catalytic activity. Our current results indicate that Au_ML/Mo₂C is a promising ORR catalyst candidate to replace precious Pt/C catalysts due to its good stability, enhanced durability, low cost and high activity. We hope our results will inspire more experimental and theoretical research to further design, explore and apply advanced metal monolayer-supported MXene composites.

A comparative DFT study on single-atom catalysis of CO oxidation over Al- and P-embedded hexagonal boron-nitride nanosheets

Density functional theory calculations are performed to compare catalytic oxidation of CO molecule over Al- and P-embedded hexagonal boron nitride nanosheet (h-BN). It is found that the Al and P adatom can be stably anchored on the boron-vacancy site of h-BN, as evidenced by a relatively large adsorption energy and charge-transfer value. According to our findings, the oxidation of CO over these surfaces proceeds via the Langmuir-Hinshelwood mechanism, followed by the elimination of the remaining atomic O by another CO molecule. Meanwhile, the stronger adsorption of O₂ than CO avoids poisoning of the active site of both surfaces. The results of the present study indicate that Al-doped h-BN exhibits higher catalytic activity for CO oxidation than P-doped one, which may provide a valuable guidance on design metal-free catalysts to remove toxic CO molecules.

Cu3-Cluster-Doped Monolayer Mo2CO2 (MXene) as an Electron Reservoir for Catalyzing a CO Oxidation Reaction

The catalytic oxidation of CO on Cu₃-cluster-decorated pristine and defective Mo₂CO₂ (MXene) monolayers (Cu₃/p-Mo₂CO₂ and Cu₃/d-Mo₂CO₂) was investigated by first-principles calculations. The stability of the designed catalysts was comprehensively demonstrated via analysis of the energies, geometry distortion, and molecular dynamics simulations at finite temperatures. The difference in the individual adsorption energies, as well as the oxidation and poisoning of Cu₃/p(d)-Mo₂CO₂ under CO and O₂ gas atmospheres, was tested to estimate the catalytic ability. We found that Cu₃/d-Mo₂CO₂ might be a superior catalyst with good stability and reactivity for CO oxidation. The active sites of the Cu₃ cluster acting as an electron reservoir governed its electron-donating and -accepting ability. Different adsorption configurations of O₂ on Cu₃/d-Mo₂CO₂ also gave rise to different reaction activities. The facile rate-limiting energy barrier was attributed to the charge buffer capacity of the Cu₃ cluster that mediates the reaction. Our results may provide clues to fabricate MXene-based materials by depositing small clusters on MXenes and exploring the advanced applications of these materials.

Single Pd atomic catalyst on Mo2CO2 monolayer (MXene): unusual activity for CO oxidation by trimolecular Eley–Rideal mechanism

A Pd atom Mo₂CO₂ exhibits excellent stability and high activity to CO oxidation.

Catalytic activity of corrole complexes with post-transition elements for the oxidation of carbon monoxide: a first-principles study

Our theoretical investigation shows that the aluminum and gallium complexes of corrole can be considered as potential high-performance catalysts for the oxidation of CO.

Support effects on adsorption and catalytic activation of O2 in single atom iron catalysts with graphene-based substrates

The adsorption and catalytic activation of O₂ on single atom iron catalysts with graphene-based substrates were investigated systematically by density functional theory calculation.

Structural, electronic and catalytic performances of single-atom Fe stabilized by divacancy-nitrogen-doped graphene

The geometric, electronic and catalytic properties of a single-atom Fe embedded GN4 sheet (Fe–GN4) were systematically studied using first-principles calculations.

Single Ni atom incorporated with pyridinic nitrogen graphene as an efficient catalyst for CO oxidation: first-principles investigation

We have investigated the potential catalytic activity of a single Ni atom incorporated with pyridinic nitrogen graphene (Ni-3N-G) in CO oxidation with first-principles calculations.