Highly stable and Active Cu1 /CeO2 Single-Atom Catalyst for CO Oxidation: A DFT Study

Synthesis of Inexpensive Ternary Metal Oxides by a Co-Precipitation Method for Catalytic Oxidation of Carbon Monoxide

By using a simple co-precipitation method, new Fe2 O3 -based nanocatalysts (samples) were synthesized. The samples were composites of two or three transition metal oxides, MOx (M=Fe, Mn, Co, Ni, and Cu). The average size of CuO crystallites in the composites composed of two oxide components (CuO-Fe2 O3 ) was about 14.3 nm, while in those composed of three (CuO-MnOx -Fe2 O3 ), the composite's phase compositions were almost in the amorphous form when annealing the sample at 300 °C. The latter sample had a specific surface area higher than that of the former, 207.9 and 142.1 g/m2 , respectively, explaining its higher catalytic CO oxidation. The CO conversion over the CuO-MnOx -Fe2 O3 -300 catalyst (1 g of catalyst, 2600 ppm of CO concentration in air, and 1.0 L/min of gas flow rate) begins at about 40 °C; the temperature for 50 % CO conversion (t50 ) is near 82 °C; and CO removal is almost complete at t99 ≈110 °C. The activity of the optimal sample was tested in different catalytic conditions, thereby observing a high durability of 99-100 % CO conversion at 130 °C. The obtained results were derived from XRD, FTIR, BET, SEM, elemental analysis and mapping, as well as catalytic experiments.

Dynamic evolution of the active center driven by hemilabile coordination in Cu/CeO2 single-atom catalyst

Hemilability is an important concept in homogeneous catalysis where both the reactant activation and the product formation can occur simultaneously through a reversible opening and closing of the metal-ligand coordination sphere. However, this effect has rarely been discussed in heterogeneous catalysis. Here, by employing a theoretical study on CO oxidation over substituted Cu₁/CeO₂ single atom catalysts, we show that dynamic evolution of metal-support coordination can significantly change the electronic structure of the active center. The evolution of the active center is shown to either strengthen or weaken the metal-adsorbate bonding as the reaction proceeds from reactants, through intermediates, to products. As a result, the activity of the catalyst can be increased. We explain our observations by extending hemilability effects to single atom heterogenous catalysts and anticipate that introducing this concept can offer a new insight into the important role active site dynamics have in catalysis toward the rational design of more sophisticated single atom catalyst materials.

Structure evolution and durability of Metal-Nitrogen-Carbon (M = Co, Ru, Rh, Pd, Ir) based oxygen evolution reaction electrocatalyst: A theoretical study

Developing low-cost, high activity and stability oxygen evolution reaction (OER) catalysts is significantly important but still challenging for water electrolyzers. In this work, we calculated the OER activity and stability of Metal-Nitrogen-Carbon (MNC, M = Co, Ru, Rh, Pd, Ir) based electrocatalyst with different structures (MN₄C₈, MN₄C₁₀, MN₄C₁₂) using density functional theory (DFT) method. These electrocatalysts were divided into three groups based on the value of ΔG_*OH, that is ΔG_*OH > 1.53 eV (PdN₄C₈, PdN₄C₁₀, PdN₄C₁₂), ΔG_*OH < 1.23 eV (RuN₄C₈, RuN₄C₁₀, RuN₄C₁₂, CoN₄C₈, CoN₄C₁₀) and 1.23 eV < ΔG_*OH < 1.53 eV (RhN₄C₈, RhN₄C₁₀, RhN₄C₁₂, IrN₄C₈, IrN₄C₁₀, IrN₄C₁₂, CoN₄C₁₂), and ΔG_*OH determine whether the structure evolution will appear. The results proved that MNC (M = Rh, Ir) with 1.23 eV < ΔG_*OH < 1.53 eV shows higher OER activity due to moderate binding energy between reaction intermediates and MNC. Furthermore, these catalysts could maintain MNC structure without further oxidation and structural evolution under working conditions (high temperature, dynamic condition, local electric field and strong specific adsorption), therefore show excellent stability. However, MNC electrocatalyst with ΔG_*OH > 1.53 eV or ΔG_*OH < 1.23 eV revealed less stability under working conditions, due to their low intrinsic stability or structural evolution under working conditions, respectively. In conclusion, we proposed a comprehensive evaluation method for MNC electrocatalysts by taking ΔG_*OH as the screening criterion for OER activity and stability, as well as ΔE_b under working condition as descriptor of stability. This is of great significance for the design and screening of ORR, OER and HER electrocatalysts under working conditions.

Hierarchical Porous Carbon Nitride-Crumpled Nanosheet-Embedded Copper Single Atoms: An Efficient Catalyst for Carbon Monoxide Oxidation

Rational design of metal single-site embedded porous graphitic carbon nitride (P-g-C₃N₄) nanostructures exploiting maximum atom utilization is warranted to enhance the thermal CO oxidation (CO_Ox) reaction. Herein, a facile, green, one-pot, and template-free approach is developed to fabricate the hierarchical porous P-g-C₃N₄-crumpled ultrathin nanosheets atomically doped with copper single atoms (Cu-P-g-C₃N₄). Mechanistically, the quick protonation of melamine and pyridine under acidic conditions induces deamination to form melem, which is polycondensed under heating. The interconnected pores, high surface area (240 m²g^-1), and maximized exposed isolated Cu atomic active sites (1.8 wt %) coordinated with nitrogen atom P-g-C₃N₄ are the salient features of Cu- P-g-C₃N₄ that endowed complete conversion to CO₂ at 184 °C. In contrast, P-g-C₃N₄ only converted 3.8% of CO even at 350 °C, implying the electronic effect of Cu single atoms. The abundant Cu-nitrogen moieties can drastically weaken the binding affinity of the CO-oxidation (CO_Ox) intermediates and products, thus accelerating the reaction kinetics at a low temperature. This study may promote the fabrication of P-g-C₃N₄ doped with various single atoms for the oxidation of CO.

Advanced Strategies for Stabilizing Single-Atom Catalysts for Energy Storage and Conversion

Well-defined atomically dispersed metal catalysts (or single-atom catalysts) have been widely studied to fundamentally understand their catalytic mechanisms, improve the catalytic efficiency, increase the abundance of active components, enhance the catalyst utilization, and develop cost-effective catalysts to effectively reduce the usage of noble metals. Such single-atom catalysts have relatively higher selectivity and catalytic activity with maximum atom utilization due to their unique characteristics of high metal dispersion and a low-coordination environment. However, freestanding single atoms are thermodynamically unstable, such that during synthesis and catalytic reactions, they inevitably tend to agglomerate to reduce the system energy associated with their large surface areas. Therefore, developing innovative strategies to stabilize single-atom catalysts, including mass-separated soft landing, one-pot pyrolysis, co-precipitation, impregnation, atomic layer deposition, and organometallic complexation, is critically needed. Many types of supporting materials, including polymers, have been commonly used to stabilize single atoms in these fabrication techniques. Herein, we review the stabilization strategies of single-atom catalyst, including different synthesis methods, specific metals and carriers, specific catalytic reactions, and their advantages and disadvantages. In particular, this review focuses on the application of polymers in the synthesis and stabilization of single-atom catalysts, including their functions as carriers for metal single atoms, synthetic templates, encapsulation agents, and protection agents during the fabrication process. The technical challenges that are currently faced by single-atom catalysts are summarized, and perspectives related to future research directions including catalytic mechanisms, enhancement of the catalyst loading content, and large-scale implementation are proposed to realize their practical applications.

Graphical Abstract

Single-atom catalysts are characterized by high metal dispersibility, weak coordination environments, high catalytic activity and selectivity, and the highest atom utilization. However, due to the free energy of the large surface area, individual atoms are usually unstable and are prone to agglomeration during synthesis and catalytic reactions. Therefore, researchers have developed innovative strategies, such as soft sedimentation, one-pot pyrolysis, coprecipitation, impregnation, step reduction, atomic layer precipitation, and organometallic complexation, to stabilize single-atom catalysts in practical applications. This article summarizes the stabilization strategies for single-atom catalysts from the aspects of their synthesis methods, metal and support types, catalytic reaction types, and its advantages and disadvantages. The focus is on the application of polymers in the preparation and stabilization of single-atom catalysts, including metal single-atom carriers, synthetic templates, encapsulation agents, and the role of polymers as protection agents in the manufacturing process. The main feature of polymers and polymer-derived materials is that they usually contain abundant heteroatoms, such as N, that possess lone-pair electrons. These lone-pair electrons can anchor the single metal atom through strong coordination interactions. The coordination environment of the lone-pair electrons can facilitate the formation of single-atom catalysts because they can enlarge the average distance of a single precursor adsorbed on the polymer matrix. Polymers with nitrogen groups are favorable candidates for dispersing active single atoms by weakening the tendency of metal aggregation and redistributing the charge densities around single atoms to enhance the catalytic performance. This review provides a summary and analysis of the current technical challenges faced by single-atom catalysts and future research directions, such as the catalytic mechanism of single-atom catalysts, sufficiently high loading, and large-scale implementation.

Boosting the Activity of Single-Atom Pt1/CeO2 via Co Doping for Low-Temperature Catalytic Oxidation of CO

The properties of supports have a significant effect on the activity of noble metal single atoms. In this work, Co-doped CeO₂-supported single-atom Pt catalysts (Pt₁/Co-CeO₂) have been acquired by a synchronous pyrolysis/deposition route and demonstrated to promote low-temperature oxidation of CO. Revealed by a model reaction of 1% CO + 1% O₂ + 98% He at a space velocity of 12,000 mL/g_cat/h, CO conversion (100 °C) acquired on a (0.5% Pt)/(10% Co-CeO₂) catalyst (36.6%) was 3.6 and 4.9 times those of 0.5% Pt/CeO₂ (10.0%) and 10% Co-CeO₂ (7.4%) catalysts and 2.1 times that of their conversion sum (17.4%), confirming the positive role of the Co dopant in boosting the low-temperature oxidation of CO. The consistent results are also verified in the comparison of Pt₁/Co-ZnO with Pt₁/ZnO and Pt₁/Co-Al₂O₃ with Pt₁/Al₂O₃. In addition, the activity of single-atom Pt₁/Co-CeO₂ catalysts can be facilely modified by changing the loading of Pt and/or doping amount of Co. These reasonable data will provide a methodology to access more applicable catalysts for CO oxidation at low temperature.

Effects of Subsurface Oxide on Cu1/CeO2 Single-Atom Catalysts for CO Oxidation: A Theoretical Investigation

Supported atomic dispersion metals are of great interest, and the interfacial effect between isolated metal atoms and supports is crucial in heterogeneous catalysis. Herein, the behavior of single-atom Cu catalysts dispersed on CeO₂ (100), (110), and (111) surfaces has been studied by DFT + U calculations. The interactions between ceria crystal planes and isolated Cu atoms together with their corresponding catalytic activities for CO oxidation are investigated. The CeO₂ (100) and (111) surfaces can stabilize active Cu⁺ species, while Cu exists as Cu²⁺ on the (110) surface. Cu⁺ is certified as the most active site for CO adsorption, which can promote the formation of the reaction intermediates and reduce reaction energy barriers. For the CeO₂ (100) surface, the interaction between CO and Cu is weak and the CO adsorbate is more likely to activate the subsurface oxygen. The catalytic performance is closely related to the binding strength of CO to the active Cu single atoms on the different subsurfaces. These results bring a significant insight into the rational design of single metal atoms on ceria and other reducible oxides.

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.

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.

Recent advances in single atom catalysts for the electrochemical carbon dioxide reduction reaction

The electrochemical carbon dioxide reduction reaction (CO2RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels. Single atom catalysts (SACs) are encouraging to catalyze the electrochemical CO2RR due to the tunable electronic structure of the central metal atoms, which can regulate the adsorption energy of reactants and reaction intermediates. Moreover, SACs form a bridge between homogeneous and heterogeneous catalysts, providing an ideal platform to explore the reaction mechanism of electrochemical reactions. In this review, we first discuss the strategies for promoting the CO2RR performance, including suppression of the hydrogen evolution reaction (HER), generation of C1 products and formation of C2+ products. Then, we summarize the recent developments in regulating the structure of SACs toward the CO2RR based on the above aspects. Finally, several issues regarding the development of SACs for the CO2RR are raised and possible solutions are provided.

Reactivity of Single Transition Metal Atoms on a Hydroxylated Amorphous Silica Surface: A Periodic Conceptual DFT Investigation

The drive to develop maximal atom-efficient catalysts coupled to the continuous striving for more sustainable reactions has led to an ever-increasing interest in single-atom catalysis. Based on a periodic conceptual density functional theory (cDFT) approach, fundamental insights into the reactivity and adsorption of single late transition metal atoms supported on a fully hydroxylated amorphous silica surface have been acquired. In particular, this investigation revealed that the influence of van der Waals dispersion forces is especially significant for a silver (98 %) or gold (78 %) atom, whereas the oxophilicity of the Group 8-10 transition metals plays a major role in the interaction strength of these atoms on the irreducible SiO₂ support. The adsorption energies for the less-electronegative row 4 elements (Fe, Co, Ni) ranged from -1.40 to -1.92 eV, whereas for the heavier row 5 and 6 metals, with the exception of Pd, these values are between -2.20 and -2.92 eV. The deviating behavior of Pd can be attributed to a fully filled d-shell and, hence, the absence of the hybridization effects. Through a systematic analysis of cDFT descriptors determined by using three different theoretical schemes, the Fermi weighted density of states approach was identified as the most suitable for describing the reactivity of the studied systems. The main advantage of this scheme is the fact that it is not influenced by fictitious Coulomb interactions between successive, charged reciprocal cells. Moreover, the contribution of the energy levels to the reactivity is simultaneously scaled based on their position relative to the Fermi level. Finally, the obtained Fermi weighted density of states reactivity trends show a good agreement with the chemical characteristics of the investigated metal atoms as well as the experimental data.

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.

Mesoporous Cu-Ce-Ox Solid Solutions from Spray Pyrolysis for Superior Low-Temperature CO Oxidation

Development of Pt group metal-free catalysts for low-temperature CO oxidation remains critical. In this work, active and stable mesoporous Cu-Ce-O_x solid solutions are prepared by using spray pyrolysis. The specific surface areas and pore volumes reach as high as 170 m²  g^-1 and 0.24 cm³  g^-1 , respectively. The results of CO oxidation study suggest that (1) the catalyst obtained by spray pyrolysis possesses much higher activity than those made by co-precipitation, sol-gel, and hydrothermal methods; (2) the optimal Cu_0.2 -Ce_0.8 -O_x solid solution presents a reactivity over 28 times that of both single-component CuO and CeO₂ at 70 °C. Based on the study of pure-phase Cu-Ce-O_x solid solutions by selective leaching of segregated CuO_x species, the active center for CO oxidation is confirmed as the bimetallic Cu-Ce-O site, whereas the individual CuO_x particles not only act as spectators but also block the active Cu-Ce-O sites. A low apparent activation energy of approximately 48 kJ mol^-1 is detected for CO oxidation at the Cu-Ce-O site, making Cu-Ce-O_x solid solutions able to present high activity at low temperature. Furthermore, the Cu-Ce-O_x catalysts exhibit excellent stability and thermal tolerance toward CO oxidation.

Platinum single-atom catalysts: a comparative review towards effective characterization

This review summaries the characterization techniques for Pt single-atom catalysts and focuses on FT-EXAFS spectroscopy to study the coordination environment of Pt–M for atomically dispersed Pt catalysts on diverse supports.