Construction of Active Site in a Sintered Copper–Ceria Nanorod Catalyst

Rational Design of Rare Earth-Based Nanomaterials for Electrocatalytic Reactions

Rare earth-based nanomaterials hold great promise for applications in the electrocatalysis field owing to their unique 4f electronic structure, adjustable coordination modes, and high oxophilicity. As a cocatalyst, the location of rare earth elements can alter the intrinsic properties of support, including coordination environments, electronic structure, and structure evolution under applied potentials in a variable manner, to potentially impact catalytic performance with respect to their activity, stability, and selectivity. Therefore, a comprehensive understanding of the effects of rare earth elements' location on local structure and reaction mechanisms is a prerequisite for designing advanced rare earth-based nanomaterials. In this review, the rare earth-based nanomaterials have been categorized into three main groups based upon the location of rare earth elements in the support, namely lattice, surface, and interface structure. We initially discuss recent advances and representing breakthroughs to realize controllable synthesis of rare earth-based nanomaterials. Next, we discuss the state-of-the-art rare earth-based nanomaterials and the structure modulation strategy employed to enhance their catalytic performance. Combined with advanced characterizations, the role of rare earth elements in reaction mechanisms and structure evolution process is also discussed. Finally, we further highlight the future research directions and remaining challenges for the development of rare earth-based nanomaterials in practical applications.

Engineering Unsaturated Cu1-O3 Coordination to Boost Oxygen Species Activation for Low-Temperature Catalysis in CO Oxidation

The activation of lattice oxygen at low temperatures is essential for heterogeneous catalytic oxidation, but exactly how this is achieved by adjusting the coordination structure of atomic sites is still elusive. Herein, the Cu1O3-CeO2 catalyst with highly dispersed unsaturated Cu1-O3 coordination was creatively engineered, which remarkably enhanced the low-temperature oxidation of CO (a typical model reaction) from 12% to 90% at 66 °C compared to conventional CuCeO x catalyst. The preservation of atomic coordination-deficient Cu sites enables the transfer of electron cloud density from Cu atoms to O atoms, hence, facilitating the activation of lattice oxygen. Further electron transfer from O atom to Cu species results in charge back-donation to form sufficient Cu+ and metal per-oxy species, contributing to weaken O-O bonds. We determined that the increasing number of electron donors induced by unsaturated atomic Cu1-O3 coordination is an efficient strategy to develop highly active and stable catalysts for lattice oxygen activation. The catalyst synthesis strategies and oxygen activation mechanism demonstrated in this work provide a generalizable platform for the future design of well-defined functional catalysts for low-temperature oxidation reactions.

Hemilabile Coordination in Single-Atom Catalyst: A Strategy To Overcome the Limitation of the Scaling Relationship

Traditional catalyst optimization, based on the Sabatier principle, encounters performance limits due to the scaling relationship between binding energies for a series of adsorbates. This restriction prevents independent optimization of the reactant activation and product desorption. Single-atom catalysts (SACs) offer a unique advantage, with their ability to dynamically adjust the metal-support coordination environment. This flexibility allows us to apply hemilability, a concept from homogeneous catalysis, to modulate catalytic activity. Hemilability, which involves the reversible opening and closing of the coordination site, enables SACs to dynamically alter their electronic structure, effectively decoupling the competing requirements of activation and desorption. In this Perspective, we highlight how SACs, with hemilabile metal-support coordination, represent a promising strategy to bypass the limitations imposed by the scaling relationship. We also discuss the experimental challenges and future opportunities for directly observing and controlling these dynamic processes in SACs, thus presenting a powerful way for developing more efficient catalytic systems.

Oxide Support Inert in Its Interaction with Metal but Active in Its Interaction with Oxide and Vice Versa

Supported metal or oxide nanostructures catalyze many industrial reactions, where the interaction of metal or oxide overlayer with its support can have a substantial influence on catalytic performance. In this work, we show that small Pt species can be well stabilized on CeO2 under both H2-containing and O2-containing atmospheres but sintering happens on SiO2, indicating that CeO2 is active whereas SiO2 is inert in Pt-support interaction. On the other hand, Co oxide (CoOx) supported on SiO2 can maintain a low-valence Co2+ state both in air and during CO2 hydrogenation to CO, indicating a strong interaction of CoOx with SiO2. However, the CoOx overlayer has a weak interaction with CeO2 and is easily reduced to metallic Co during the CO2 hydrogenation reaction producing CH4. Thus, SiO2 is active, but CeO2 is inert for CoOx-support interaction, which is counter to the common sense from the Pt/oxide systems. Systematic studies in stability behaviors of Pt and CoOx nanocatalysts supported on various oxides show that the reducibility of the oxide supports can be used to describe the catalyst-support interaction. Oxide supports with high reducibility or low metal-oxygen bond strength interact strongly with Pt and other metals, showing high metalphilicity. Conversely, oxide supports with low reducibility or high metal-oxygen bond strength have strong interaction with CoOx and other oxides, having high oxidephilicity.

Optimizing low-temperature CO oxidation under realistic combustion conditions: The impact of CeO2 morphology on Au/CeO2 catalysts

The development of carbon monoxide oxidation catalysts for complex gas environments faces significant challenges in fire scenarios. Only a few representative gases are used as interfering components in simulated real smoke under laboratory conditions, which cannot accurately reflect the performance of catalysts in a real fire. Herein, Au/CeO2 catalysts with high activity were prepared by adjusting the morphology (rod, cube, polyhedron and irregular particles) and exposed crystal surface ratio of CeO2. Rod-like Au/CeO2 (Au/CeO2-NR) achieved 99 % CO conversion at 25 °C and demonstrated excellent water resistance. This excellent activity originates from the high oxygen vacancy concentration of the CeO2-NR and the interaction between Au species and the carrier. A testbed was established by connecting a steady-state tube furnace with a catalytic fixed-bed reactor to evaluate the CO elimination performance of the catalyst under realistic combustion conditions. Despite competitive adsorption of small molecules (H2O, acetone, etc.) on the active sites, Au/CeO2-NR eliminates carbon monoxide in real combustion atmospheres at only 60 °C. This study provides a method for evaluating the catalytic activity of CO in realistic environments, which is promising for practical use in application scenarios dealing with toxic fumes.

Structure-activity relationships in the development of single atom catalysts for sustainable organic transformations

Single atom catalysts (SACs), which can provide the combined benefits of homogeneous and heterogeneous catalysts, are a revolutionary concept in the field of material research. The highly exposed catalytic surfaces, unsaturated sites, as well as unique structural and electronic properties of SACs have the potential to catalyze numerous reactions with unmatched efficiency and durability when stabilized on a suitable support. In this review, we have provided an intuitive insight into the strategies adopted in the last 5 years for morphology control of SACs to know about its impact on metal-support interaction and various organic transformations with special reference to metal oxides, alloys, metal-organic frameworks (MOFs) and carbon-based supports. This review also includes a brief description of unparalleled potentials of SACs and the recent advances in the catalysis of industrially important organic transformations, with special emphasis on the C-C cross-coupling reaction, biomass conversion, hydrogenation, oxidation and click chemistry. This unprecedented and unique perspective will highlight the interactions occurring within SACs that are responsible for their high catalytic efficiency, which will potentially benefit various organic transformations. We have also suggested plausible synergy of various other concepts such as defect engineering and piezocatalysis with SACs, which can provide a new direction to sustainable chemistry. A good understanding of the different types of metal-support interactions will help researchers develop morphology-controlled SACs with tunable properties and establish mechanisms for their exceptional catalytic behaviour in industrially important organic transformations.

Promoting effect of Cu as electron transfer medium on NH3-SCO reaction in asymmetric Ag-Ov-Ti-Sm-Cu ring active site

Selective catalytic oxidation of ammonia (NH3-SCO) has become an effective method to reduce ammonia (NH3) emissions, and is a key part to solve the problem of NH3 pollution. Nevertheless, the optimization of this technology's performance relies heavily on innovation and the development of catalyst design. In this study, a SmCuAgTiOx catalyst with an asymmetric Ag-Ov-Ti-Sm-Cu ring active site was prepared and applied to the NH3-SCO reaction. The low conversion of Cu-based catalysts in NH3 at low temperature and the inherent low N₂ selectivity of Ag-based catalysts were solved. The successful creation of the asymmetric ring active site improved the catalyst's reduction performance. Additionally, Cu, acting as an electron transfer medium, plays a crucial role in enhancing electron transfer within the asymmetric ring active site, thus increasing the redox cycle of the catalyst during the reaction. In addition, some lattice oxygen is lost in the catalyst, resulting in the formation of a large number of oxygen vacancies. This process stimulates the adsorption and activation of surface-adsorbed oxygen, facilitating the conversion of NH3 to an amide (NH2) intermediate during the reaction and reducing non-selective oxidation. The N2 selectivity was improved without significantly affecting the performance of Ag-based catalyst. In-situ diffuse reflectance fourier transform infrared spectroscopy (In-situ DRIFTS) analysis reveals that the SmCuAgTiOx catalyst primarily follows an "internal" selective catalytic reduction (iSCR) mechanism in the NH3-SCO reaction, complemented by the imide mechanism. The asymmetric Ag-Ov-Ti-Sm-Cu ring active site developed in this study provides a new perspective for efficiently solving NH3 pollution in the future.

CeO2-δ as Electron Donor in Co0.07Ce0.93O2-δ Solid Solution Boosts Alkaline Water Splitting

Optimizing the electronic structure with increasing intrinsic stability is a usual method to enhance the catalysts' performance. Herein, a series of cerium dioxide (CeO2-δ) based solid solution materials is synthesized via substituting Ce atoms with transition metal (Co, Cu, Ni, etc.), in which Co0.07Ce0.93O2-δ shows optimized band structure because of electron transition in the reaction, namely Co3+ (3d64s0) + Ce3+ (4f15d 06s0) → Co2+ (3d74s0) + Ce4+ (4f05d06s0), with more stable electronic configuration. The in situ Raman spectra show a stable F2g peak at ≈452 cm-1 of Co0.07Ce0.93O2-δ, while the F2g peak in CeO2-δ almost disappeared during HER progress, demonstrating the charge distribution of *H adsorbed on Co0.07Ce0.93O2-δ is more stable than *H adsorbed on CeO2-δ. Density functional theory calculations reveal that Co0.07Ce0.93O2-δ solid solution increases protonation capacity and favors for formation of *H in alkaline media. General guidelines are formulated for optimizing adsorption capacity and the volcano plot demonstrates the excellent catalytic performance of Co0.07Ce0.93O2-δ solid solution. The alkaline anion exchange membrane water electrolysis based on Co0.07Ce0.93O2-δ/NiFe LDH realizes a current density of 1000 mA cm-2 at ≈1.86 V in alkaline seawater at 80 °C and exhibits long-term stability for 450 h.

Ceria-based supported metal catalysts for the low-temperature water-gas shift reaction

Water-gas shift (WGS) reaction is a crucial step for the industrial production of hydrogen or upgrading the hydrogen generated from fossil or biomass sources by removing the residual CO. However, current industrial catalysts for this process, comprising Cu/ZnO and Fe2O3-Cr2O3, suffer from safety or environmental issues. In the past decades, ceria-based materials have attracted wide attention as WGS catalysts due to their abundant oxygen vacancies and tunable metal-support interaction. Strategies through engineering the shape or crystal facet, size of both metal and ceria, interfacial-structure, etc., to alter the performances of ceria-based catalysts have been extensively studied. Additionally, the developments in the in situ techniques and DFT calculations are favorable for deepening the understanding of the reaction mechanism and structure-function relationship at the molecular level, comprising active sites, reaction path/intermediates, and inducements for deactivation. This article critically reviews the literature on ceria-based catalysts toward the WGS reaction, covering the fundamental insight of the reaction path and development in precisely designing catalysts.

Direct cleavage of C=O double bond in CO2 by the subnano MoOx surface on Mo2N

Compared to H2-assisted activation mode, the direct dissociation of CO2 into carbonyl (*CO) with a simplified reaction route is advantageous for CO2-related synthetic processes and catalyst upgrading, while the stable C = O double bond makes it very challenging. Herein, we construct a subnano MoO3 layer on the surface of Mo2N, which provides a dynamically changing surface of MoO3↔MoOx (x < 3) for catalyzing CO2 hydrogenation. Rich oxygen vacancies on the subnano MoOx surface with a high electron donating capacity served as a scissor to directly shear the C = O double bond of CO2 to form CO at a high rate. The O atoms leached in CO2 dissociation are removed timely by H2 to regenerate active oxygen vacancies. Owing to the greatly enhanced dissociative activation of CO2, this MoOx/Mo2N catalyst without any supported active metals shows excellent performance for catalyzing CO2 hydrogenation to CO. The construction of highly disordered defective surface on heterostructures paves a new pathway for molecule activation.

Subsurface Single-Atom Catalyst Enabled by Mechanochemical Synthesis for Oxidation Chemistry

Single-atom catalysts have garnered significant attention due to their exceptional atom utilization and unique properties. However, the practical application of these catalysts is often impeded by challenges such as sintering-induced instability and poisoning of isolated atoms due to strong gas adsorption. In this study, we employed the mechanochemical method to insert single Cu atoms into the subsurface of Fe2O3 support. By manipulating the location of single atoms at the surface or subsurface, catalysts with distinct adsorption properties and reaction mechanisms can be achieved. It was observed that the subsurface Cu single atoms in Fe2O3 remained isolated under both oxidation and reduction environments, whereas surface Cu single atoms on Fe2O3 experienced sintering under reduction conditions. The unique properties of these subsurface single-atom catalysts call for innovations and new understandings in catalyst design.

Facet-Dependent Diversity of Pt-O Coordination for Pt1/CeO2 Catalysts Achieved by Oriented Atomic Deposition

The surface chemistry of CeO2 is dictated by the well-defined facets, which exert great influence on the supported metal species and the catalytic performance. Here we report Pt1/CeO2 catalysts exhibiting specific structures of Pt-O coordination on different facets by using adequate preparation methods. The simple impregnation method results in Pt-O3 coordination on the predominantly exposed {111} facets, while the photo-deposition method achieves oriented atomic deposition for Pt-O4 coordination into the "nano-pocket" structure of {100} facets at the top. Compared to the impregnated Pt1/CeO2 catalyst showing normal redox properties and low-temperature activity for CO oxidation, the photo-deposited Pt1/CeO2 exhibits uncustomary strong metal-support interaction and extraordinary high-temperature stability. The preparation methods dictate the facet-dependent diversity of Pt-O coordination, resulting in the further activity-selectivity trade-off. By applying specific preparation routes, our work provides an example of disentangling the effects of support facets and coordination environments for nano-catalysts.

Catalytic properties of trivalent rare-earth oxides with intrinsic surface oxygen vacancy

Oxygen vacancy (Ov) is an anionic defect widely existed in metal oxide lattice, as exemplified by CeO2, TiO2, and ZnO. As Ov can modify the band structure of solid, it improves the physicochemical properties such as the semiconducting performance and catalytic behaviours. We report here a new type of Ov as an intrinsic part of a perfect crystalline surface. Such non-defect Ov stems from the irregular hexagonal sawtooth-shaped structure in the (111) plane of trivalent rare earth oxides (RE2O3). The materials with such intrinsic Ov structure exhibit excellent performance in ammonia decomposition reaction with surface Ru active sites. Extremely high H2 formation rate has been achieved at ~1 wt% of Ru loading over Sm2O3, Y2O3 and Gd2O3 surface, which is 1.5-20 times higher than reported values in the literature. The discovery of intrinsic Ov suggests great potentials of applying RE oxides in heterogeneous catalysis and surface chemistry.

Unraveling distinct effects between CuOx and PtCu alloy sites in Pt-Cu bimetallic catalysts for CO oxidation at different temperatures

In situ exploration of the dynamic structure evolution of catalysts plays a key role in revealing reaction mechanisms and designing efficient catalysts. In this work, PtCu/MgO catalysts, synthesized via the co-impregnation method, outperforms monometallic Pt/MgO and Cu/MgO. Utilizing quasi/in-situ characterization techniques, it is discovered that there is an obvious structural evolution over PtCu/MgO from PtxCuyOz oxide cluster to PtCu alloy with surface CuOx species under different redox and CO oxidation reaction conditions. The synergistic effect between PtCu alloy and CuOx species enables good CO oxidation activity through the regulation of CO adsorption and O2 dissociation. At low temperatures, CO oxidation is predominantly catalyzed by surface CuOx species via the Mars-van Krevelen mechanism, in which CuOx can provide abundant active oxygen species. As the reaction temperature increases, both surface CuOx species and PtCu alloy collaborate to activate gaseous oxygen, facilitating CO oxidation mainly through the Langmuir-Hinshelwood mechanism.

Oxygen Coordination Promotes Single-Atom Cu(II)-Catalyzed Azide-Alkyne Click Chemistry without Reducing Agents

Unique active sites make single-atom (SA) catalysts promising to overcome obstacles in homogeneous catalysis but challenging due to their fixed coordination environment. Click chemistry is restricted by the low activity of more available Cu(II) catalysts without reducing agents. Herein, we develop efficient, O-coordinated SA Cu(II) directly catalyzed click chemistry. As revealed by theoretical calculations of the superior coordination structure to promote the click reaction, an organic molecule-assisted strategy is applied to prepare the corresponding SA Cu catalysts with respective O and N coordination. Although they both belong to Cu(II) centers, the O-coordinated one exhibits a 5-fold higher activity than the other and even much better activity than traditional homogeneous and heterogeneous Cu(II) catalysts. Control experiments further proved that the O-coordinated SA Cu(II) catalyst tends to be reduced by alkyne into Cu acetylide rather than the N-coordinated catalyst and thus facilitates click chemistry.

Structure-Function Relationship of Highly Reactive CuOx Clusters on Co3 O4 for Selective Formaldehyde Sensing at Low Temperatures

Designing reactive surface clusters at the nanoscale on metal-oxide supports enables selective molecular interactions in low-temperature catalysis and chemical sensing. Yet, finding effective material combinations and identifying the reactive site remains challenging and an obstacle for rational catalyst/sensor design. Here, the low-temperature oxidation of formaldehyde with CuOx clusters on Co3 O4 nanoparticles is demonstrated yielding an excellent sensor for this critical air pollutant. When fabricated by flame-aerosol technology, such CuOx clusters are finely dispersed, while some Cu ions are incorporated into the Co3 O4 lattice enhancing thermal stability. Importantly, infrared spectroscopy of adsorbed CO, near edge X-ray absorption fine structure spectroscopy and temperature-programmed reduction in H2 identified Cu+ and Cu2+ species in these clusters as active sites. Remarkably, the Cu+ surface concentration correlated with the apparent activation energy of formaldehyde oxidation (Spearman's coefficient ρ = 0.89) and sensor response (0.96), rendering it a performance descriptor. At optimal composition, such sensors detected even the lowest formaldehyde levels of 3 parts-per-billion (ppb) at 75°C, superior to state-of-the-art sensors. Also, selectivity to other aldehydes, ketones, alcohols, and inorganic compounds, robustness to humidity and stable performance over 4 weeks are achieved, rendering such sensors promising as gas detectors in health monitoring, air and food quality control.

Tuning oxidant and antioxidant activities of ceria by anchoring copper single-site for antibacterial application

The reaction system of hydrogen peroxide (H2O2) catalyzed by nanozyme has a broad prospect in antibacterial treatment. However, the complex catalytic activities of nanozymes lead to multiple pathways reacting in parallel, causing uncertain antibacterial results. New approach to effectively regulate the multiple catalytic activities of nanozyme is in urgent need. Herein, Cu single site is modified on nanoceria with various catalytic activities, such as peroxidase-like activity (POD) and hydroxyl radical antioxidant capacity (HORAC). Benefiting from the interaction between coordinated Cu and CeO2 substrate, POD is enhanced while HORAC is inhibited, which is further confirmed by density functional theory (DFT) calculations. Cu-CeO2 + H2O2 system shows good antibacterial properties both in vitro and in vivo. In this work, the strategy based on the interaction between coordinated metal and carrier provides a general clue for optimizing the complex activities of nanozymes.

Boosting the catalytic performance of Al2O3-supported Pd catalysts by introducing CeO2 promoters

Maintaining the stability of noble metals is the key to the long-term stability of supported catalysts. In response to the instability of noble metal species at high temperatures, we developed a synergistic strategy of dual oxide supports. By designing and constructing ceria components with small sizes, we have achieved unity in the ability of catalytic materials to supply oxygen and stabilize metal species. In this study, we prepared Al2O3-CeO2-Pd (AlCePd) catalysts containing trace amounts of Ce through the hydrolysis of cerium acetate, which achieved 100% CO conversion at 160 °C. More importantly, the activity remained at its initial 100% in the long-term durability testing, demonstrating the high stability of AlCePd. In contrast, the CO conversion of the CeO2-Pd (CePd) catalyst decreased from 100% to 54% within 3 h. Through comprehensive studies, we found that this excellent catalytic performance stems from the stabilizing effect of an alumina support and the possible reverse oxygen spillover effect of small-sized ceria components, where small-sized ceria components provide active oxygen for independent Pd species, making it possible for the CO adsorbed on Pd to react with this oxygen species.

Ceo2 /Cus Nanoplates Electroreduce Co2 to Ethanol with Stabilized Cu+ Species

Copper-based electrocatalysts effectively produce multicarbon (C2+ ) compounds during the electrochemical CO2 reduction (CO2 RR). However, big challenges still remain because of the chemically unstable active sites. Here, cerium is used as a self-sacrificing agent to stabilize the Cu+ of CuS, due to the facile Ce3+ /Ce4+ redox. CeO2 -modified CuS nanoplates achieve high ethanol selectivity, with FE up to 54% and FEC2+ ≈ 75% in a flow cell. Moreover, in situ Raman spectroscopy and in situ Fourier-transform infrared spectroscopy indicate that the stable Cu+ species promote CC coupling step under CO2 RR. Density functional theory calculations further reveal that the stronger * CO adsorption and lower CC coupling energy, which is conducive to the selective generation of ethanol products. This work provides a facile strategy to convert CO2 into ethanol by retaining Cu+ species.

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.

Probing and Leveraging the Structural Heterogeneity of Nanomaterials for Enhanced Catalysis

The marriage between nanoscience and heterogeneous catalysis has introduced transformative opportunities for accessing better nanocatalysts. However, the structural heterogeneity of nanoscale solids stemming from distinct atomic configurations makes it challenging to realize atomic-level engineering of nanocatalysts in the way that is attained for homogeneous catalysis. Here, we discuss recent efforts in unveiling and exploiting the structural heterogeneity of nanomaterials for enhanced catalysis. Size and facet control of nanoscale domains produce well-defined nanostructures that facilitate mechanistic studies. Differentiation of surface and bulk characteristics for ceria-based nanocatalysts guides new thoughts toward lattice oxygen activation. Manipulating the compositional and species heterogeneity between local and average structures allows regulation of catalytically active sites via the ensemble effect. Studies on catalyst restructurings further highlight the necessity to assess the reactivity and stability of nanocatalysts under reaction conditions. These advances promote the development of novel nanocatalysts with expanded functionalities and bring atomistic insights into heterogeneous catalysis.

DRIFTS-MS Investigation of Low-Temperature CO Oxidation on Cu-Doped Manganese Oxide Prepared Using Nitrate Aerosol Decomposition

Cu-doped manganese oxide (Cu-Mn₂O₄) prepared using aerosol decomposition was used as a CO oxidation catalyst. Cu was successfully doped into Mn₂O₄ due to their nitrate precursors having closed thermal decomposition properties, which ensured the atomic ratio of Cu/(Cu + Mn) in Cu-Mn₂O₄ close to that in their nitrate precursors. The 0.5Cu-Mn₂O₄ catalyst of 0.48 Cu/(Cu + Mn) atomic ratio had the best CO oxidation performance, with T₅₀ and T₉₀ as low as 48 and 69 °C, respectively. The 0.5Cu-Mn₂O₄ catalyst also had (1) a hollow sphere morphology, where the sphere wall was composed of a large number of nanospheres (about 10 nm), (2) the largest specific surface area and defects on the interfacing of the nanospheres, and (3) the highest Mn³⁺, Cu⁺, and Oads ratios, which facilitated oxygen vacancy formation, CO adsorption, and CO oxidation, respectively, yielding a synergetic effect on CO oxidation. DRIFTS-MS analysis results showed that terminal-type oxygen (M=O) and bridge-type oxygen (M-O-M) on 0.5Cu-Mn₂O₄ were reactive at a low temperature, resulting in-good low-temperature CO oxidation performance. Water could adsorb on 0.5Cu-Mn₂O₄ and inhibited M=O and M-O-M reaction with CO. Water could not inhibit O₂ decomposition to M=O and M-O-M. The 0.5Cu-Mn₂O₄ catalyst had excellent water resistance at 150 °C, at which the influence of water (up to 5%) on CO oxidation could be completely eliminated.

Designing Reactive Bridging O2- at the Atomic Cu-O-Fe Site for Selective NH3 Oxidation

Surface oxidation chemistry involves the formation and breaking of metal-oxygen (M-O) bonds. Ideally, the M-O bonding strength determines the rate of oxygen absorption and dissociation. Here, we design reactive bridging O^2- species within the atomic Cu-O-Fe site to accelerate such oxidation chemistry. Using in situ X-ray absorption spectroscopy at the O K-edge and density functional theory calculations, it is found that such bridging O^2- has a lower antibonding orbital energy and thus weaker Cu-O/Fe-O strength. In selective NH₃ oxidation, the weak Cu-O/Fe-O bond enables fast Cu redox for NH₃ conversion and direct NO adsorption via Cu-O-NO to promote N-N coupling toward N₂. As a result, 99% N₂ selectivity at 100% conversion is achieved at 573 K, exceeding most of the reported results. This result suggests the importance to design, determine, and utilize the unique features of bridging O^2- in catalysis.

Single-Atom Catalysts: Preparation and Applications in Environmental Catalysis

Due to the expensive price and the low reserve of noble metals in nature, much attention has been paid to single-atom catalysts (SACs)—especially single-atom noble metal catalysts—owing to their maximum atomic utilization and dispersion. The emergence of SACs greatly decreases the amount of precious metals, improves the catalytic activity, and makes the catalytic process progressively economic and sustainable. However, the most remarkable challenge is the active sites and their stability against migration and aggregation under practical conditions. This review article summarizes the preparation strategies of SACs and their catalytic applications for the oxidation of methane, carbon monoxide, and volatile organic compounds (VOCs) and the reduction of nitrogen oxides. Furthermore, the perspectives and challenges of SACs in future research and practical applications are proposed. It is envisioned that the results summarized in this review will stimulate the interest of more researchers in developing SACs that are effective in catalyzing the reactions related to the environmental pollution control.

Isolating Single and Few Atoms for Enhanced Catalysis

Atomically dispersed metal catalysts have triggered great interest in the field of catalysis owing to their unique features. Isolated single or few metal atoms can be anchored on substrates via chemical bonding or space confinement to maximize atom utilization efficiency. The key challenge lies in precisely regulating the geometric and electronic structure of the active metal centers, thus significantly influencing the catalytic properties. Although several reviews have been published on the preparation, characterization, and application of single-atom catalysts (SACs), the comprehensive understanding of SACs, dual-atom catalysts (DACs), and atomic clusters has never been systematically summarized. Here, recent advances in the engineering of local environments of state-of-the-art SACs, DACs, and atomic clusters for enhanced catalytic performance are highlighted. Firstly, various synthesis approaches for SACs, DACs, and atomic clusters are presented. Then, special attention is focused on the elucidation of local environments in terms of electronic state and coordination structure. Furthermore, a comprehensive summary of isolated single and few atoms for the applications of thermocatalysis, electrocatalysis, and photocatalysis is provided. Finally, the potential challenges and future opportunities in this emerging field are presented. This review will pave the way to regulate the microenvironment of the active site for boosting catalytic processes.

Defect Engineering of Ceria Nanocrystals for Enhanced Catalysis via a High-Entropy Oxide Strategy

Introducing transition-metal components to ceria (CeO₂) is important to tailor the surface redox properties for a broad scope of applications. The emergence of high-entropy oxides (HEOs) has brought transformative opportunities for oxygen defect engineering in ceria yet has been hindered by the difficulty in controllably introducing transition metals to the bulk lattice of ceria. Here, we report the fabrication of ceria-based nanocrystals with surface-confined atomic HEO layers for enhanced catalysis. The increased covalency of the transition-metal-oxygen bonds at the HEO-CeO₂ interface promotes the formation of surface oxygen vacancies, enabling efficient oxygen activation and replenishment for enhanced CO oxidation capabilities. Understanding the structural heterogeneity involving bulk and surface oxygen defects in nanostructured HEOs provides useful insights into rational design of atomically precise metal oxides, whose increased compositional and structural complexities give rise to expanded functionalities.

Support stabilized PtCu single-atom alloys for propane dehydrogenation

PtCu single-atom alloys (SAAs) open an extensive prospect for heterogeneous catalysis. However, as the host of SAAs, Cu suffers from severe sintering at elevated temperature, resulting in poor stability of catalysts. This paper describes the suppression of the agglomeration of Cu nanoparticles under high temperature conditions using copper phyllosilicate (CuSiO3) as the support of PtCu SAAs. Based on quasi in situ XPS, in situ CO-DRIFTS, in situ Raman spectroscopy and in situ XRD, we demonstrated that the interfacial Cu+-O-Si formed upon reduction at 680 °C serves as the adhesive between Cu nanoparticles and the silicon dioxide matrix, strengthening the metal-support interaction. Consequently, the resistance to sintering of PtCu SAAs was improved, leading to high catalytic stability during propane dehydrogenation without sacrificing conversion and selectivity. The optimized PtCu SAA catalyst achieved more than 42% propane conversion and 93% propylene selectivity at 580 °C for at least 30 hours. It paves a way for the design and development of highly active supported single-atom alloy catalysts with excellent thermal stability.

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.

Hydrothermal synthesis and characterization of transition metal (Mn/Fe/Cu) co-doped cerium oxide-based nano-additives for potential use in the reduction of exhaust emission from spark ignition engines

The goal of this work was to synthesize new cerium oxide-based nano-additives to minimise emissions from spark ignition (SI) engines fueled with gasoline blends, such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NO_x). To investigate the effect of transition metal dopants on their respective catalytic oxidation activity, nano-sized CeO₂ catalysts co-doped with Mn, Fe, Cu and Ag ions were successfully produced by a simple hydrothermal technique. The synthesis of nano-catalysts with cubic fluorite geometry was confirmed by XRD data. The addition of transition metal ions to the CeO₂ lattice increased the concentration of structural defects like oxygen vacancies and Ce³⁺ ions, which are advantageous for the catalytic oxidation reaction, as also supported by XAFS and RAMAN analysis. Further, nano-gasoline fuel emission parameters are measured and compared to straight gasoline fuel. The results demonstrated that harmful exhaust pollutants such as CO, HC and NO_x were significantly reduced. The high surface area, better redox characteristics and presence of additional oxygen vacancy sites or Ce³⁺ ions have been linked to the improved catalytic performance of the synthesized catalyst.

Illustrating the synthesis of doped and undoped CeO₂ nanomaterial and its potential application as a promising catalyst for additives to minimize emissions from spark ignition (SI) engines fueled with gasoline blends.

Recent advances in hydrothermal synthesis of facet-controlled CeO2-based nanomaterials

CeO₂-based nanomaterials have received tremendous attention due to their variety of applications. This paper is focused on the recent advances in facet-controlled CeO₂-based nanomaterials by the hydrothermal method. CeO₂-based nanomaterials with controllable size and exposed facets can be prepared by adjusting the reaction parameters. Moreover, doping and loading metals can improve the oxygen storage capacity (OSC) of CeO₂ and its catalytic activity. Various research studies on catalytic applications such as CO oxidation, water-gas shift reaction (WGSR), decomposition of hydrocarbons, and photocatalytic reaction have been carried out to exhibit the high potential of facet-controlled CeO₂ nanomaterials. This review will provide readers with various ideas for facet-controlled CeO₂-based nanomaterials.

Manipulating Copper Dispersion on Ceria for Enhanced Catalysis: A Nanocrystal-Based Atom-Trapping Strategy

Due to tunable redox properties and cost-effectiveness, copper-ceria (Cu-CeO2 ) materials have been investigated for a wide scope of catalytic reactions. However, accurately identifying and rationally tuning the local structures in Cu-CeO2 have remained challenging, especially for nanomaterials with inherent structural complexities involving surfaces, interfaces, and defects. Here, a nanocrystal-based atom-trapping strategy to access atomically precise Cu-CeO2 nanostructures for enhanced catalysis is reported. Driven by the interfacial interactions between the presynthesized Cu and CeO2 nanocrystals, Cu atoms migrate and redisperse onto the CeO2 surface via a solid-solid route. This interfacial restructuring behavior facilitates tuning of the copper dispersion and the associated creation of surface oxygen defects on CeO2 , which gives rise to enhanced activities and stabilities catalyzing water-gas shift reaction. Combining soft and solid-state chemistry of colloidal nanocrystals provide a well-defined platform to understand, elucidate, and harness metal-support interactions. The dynamic behavior of the supported metal species can be further exploited to realize exquisite control and rational design of multicomponent nanocatalysts.

Heterostructured Ceria-Titania-Supported Platinum Catalysts for the Water Gas Shift Reaction

The water gas shift (WGS) reaction is a key process in the industrial hydrogen production and the development and application of the proton exchange membrane fuel cell. Metal oxide-supported highly dispersed Pt has been proved as an efficient catalyst for the WGS reaction. In this work, a series of supported 0.5Pt/xCe-10Ti (x = 1, 3, or 5) catalysts with different Ce/Ti molar ratios were prepared by a simple deposition-precipitation method. Compared with single TiO₂- or CeO₂-supported Pt catalysts, it was found that the 0.5Pt/3Ce-10Ti catalyst showed an obvious advantage in activity for the WGS reaction. In this catalyst, dispersed CeO₂ nanoparticles were supported on the TiO₂ sheets, and Pt single atoms and nanoparticles were located on CeO₂ and at the boundary of TiO₂ and CeO₂, respectively. It found that the reduction ability of the supported Pt catalyst was remarkably improved; meanwhile, the adsorption strength of CO on the surface of 0.5Pt/3Ce-10Ti was moderate. The heterostructured CeO₂-TiO₂ support gave an effective regulation on the Pt status and further influenced the CO adsorption ability, inducing excellent WGS reaction activity. This work provides a reference for the development and application of heterostructured materials in heterogeneous catalysis.

Partially sintered copper‒ceria as excellent catalyst for the high-temperature reverse water gas shift reaction

For high-temperature catalytic reaction, it is of significant importance and challenge to construct stable active sites in catalysts. Herein, we report the construction of sufficient and stable copper clusters in the copper‒ceria catalyst with high Cu loading (15 wt.%) for the high-temperature reverse water gas shift (RWGS) reaction. Under very harsh working conditions, the ceria nanorods suffered a partial sintering, on which the 2D and 3D copper clusters were formed. This partially sintered catalyst exhibits unmatched activity and excellent durability at high temperature. The interaction between the copper and ceria ensures the copper clusters stably anchored on the surface of ceria. Abundant in situ generated and consumed surface oxygen vacancies form synergistic effect with adjacent copper clusters to promote the reaction process. This work investigates the structure-function relation of the catalyst with sintered and inhomogeneous structure and explores the potential application of the sintered catalyst in C1 chemistry.

Oxygen vacancy distributions and electron localization in a CeO2(100) nanocube

Oxygen vacancy distributions in a 5 nm CeO2 nanocube were determined using the Reverse Monte Carlo method. The oxygen vacancies tend to be located on the surface of the CeO2 nanocube, with far fewer in subsurface and internal regions.

Thermally Stable Single-Atom Heterogeneous Catalysts

Single-atom catalysts (SACs) have attracted extensive attention in fields related to energy, environment, and material sciences because of the high atom efficiency and the unique properties of these materials. Many approaches have hitherto been successfully established to prepare SACs, including impregnation, pyrolysis-involved processes, atom trapping, and coprecipitation. However, under typical reaction conditions, single atoms on catalysts tend to migrate or agglomerate, forming nanoclusters or nanoparticles, which lowers their surface free energy. Efforts are required to develop strategies for improving the thermal stability of SACs while achieving excellent catalytic performance. In this Progress Report, recent advances in the development of thermally durable single-atom heterogeneous catalysts are discussed. Several important preparation approaches for thermally stable SACs are described in this article. Fundamental understanding of the coordination structures of thermally stable single atom prepared by these methods is discussed. Furthermore, the catalytic performances of these thermally stable SACs are reviewed, including their activity and stability. Finally, a perspective of this important and rapidly evolving research field is provided.

Elucidating the Role of Single-Atom Pd for Electrocatalytic Hydrodechlorination

In this study, we loaded Pd catalysts onto a reduced graphene oxide (rGO) support in an atomically dispersed fashion [i.e., Pd single-atom catalysts (SACs) on rGO or Pd₁/rGO] via a facile and scalable synthesis based on anchor-site and photoreduction techniques. The as-synthesized Pd₁/rGO significantly outperformed the Pd nanoparticle (Pd_nano) counterparts in the electrocatalytic hydrodechlorination of chlorinated phenols. Downsizing Pd_nano to Pd₁ leads to a substantially higher Pd atomic efficiency (14 times that of Pd_nano), remarkably reducing the cost for practical applications. The unique single-atom architecture of Pd₁ additionally affects the desorption energy of the intermediate, suppressing the catalyst poisoning by Cl^-, which is a prevalent challenge with Pd_nano. Characterization and experimental results demonstrate that the superior performance of Pd₁/rGO originates from (1) enhanced interfacial electron transfer through Pd-O bonds due to the electronic metal-support interaction and (2) increased atomic H (H*) utilization efficiency by inhibiting H₂ evolution on Pd₁. This work presents an important example of how the unique geometric and electronic structure of SACs can tune their catalytic performance toward beneficial use in environmental remediation applications.

In Situ Generation of the Surface Oxygen Vacancies in a Copper-Ceria Catalyst for the Water-Gas Shift Reaction

The dissociation of H₂O is a crucial aspect for the water-gas shift reaction, which often occurs on the vacancies of a reducible oxide support. However, the vacancies sometimes run off, thus inhibiting H₂O dissociation. After high-temperature treatment, the ceria supports were lacking vacancies because of sintering. Unexpectedly, the in situ generation of surface oxygen vacancies was observed, ensuring the efficient dissociation of H₂O. Due to the surface reconstruction of ceria nanorods, the copper species sustained were highly dispersed on the sintered support, on which CO was adsorbed efficiently to react with hydroxyls from H₂O dissociation. In contrast, no surface reconstruction occurred in ceria nanoparticles, leading to the sintering of copper species. The sintered copper species were averse to adsorb CO, so the copper-ceria nanoparticle catalyst had poor reactivity even when surface oxygen vacancies could be generated in situ.

Elucidating the structure, redox properties and active entities of high-temperature thermally aged CuOx-CeO2 catalysts for CO-PROX

CuOx-CeO2 catalysts with different copper contents are synthesized via a coprecipitation method and thermally treated at 700 °C. Various characterization techniques including X-ray diffraction (XRD) Rietveld refinement, N2 adsorption-desorption isotherms, X-ray photoelectron spectra (XPS), UV-Raman, high-resolution transmission electron microscopy (HRTEM), temperature-programmed reduction (TPR) and in situ diffuse reflectance infrared Fourier transform spectra (DRIFTs) were adopted to investigate the structure/texture properties, oxygen vacancies, Cu-Ce interaction and redox properties of the catalysts. After the thermal treatment, the catalysts exhibited outstanding catalytic properties for the preferential oxidation (PROX) of CO (with the T50% of 62 °C and the widest operation temperature window of 85-140 °C), which provided a new strategy for the design of Cu-Ce based catalysts with high catalytic performance. The characterization results indicated that moderately elevating the copper content (below 5%) increases the amount of highly dispersed Cu species in the catalysts, including highly dispersed surface CuOx species and strongly bonded Cu-[Ox]-Ce species, strengthening the Cu-Ce interaction, increasing oxygen vacancies and promoting redox properties, but a further increase in copper content causes the agglomeration of crystalline CuO and decreases the highly dispersed Cu species. This work also provides evidence from the perspective that the catalytic performance of CuOx-CeO2 catalysts for CO-PROX at low and high reaction temperatures is dependent on the redox properties of highly dispersed CuOx species and strongly bonded Cu-[Ox]-Ce species, respectively.