CO Oxidation on Metal Oxide Supported Single Pt atoms: The Role of the Support

Synthetic Pt-Fe(OH) x catalysts by one-pot method for CO catalytic oxidation

Catalytic oxidation is used to control carbon monoxide (CO) emissions from industrial exhaust. In this work, The prepared Pta-Fe(OH) x catalysts (x represents the mass fraction of Pt loading (%), a = 0.5, 1 and 2) by the one-pot reduction method exhibited excellent CO catalytic activity, with the Pt2-Fe(OH) x catalyst, 70% and ∼100% CO conversion was achieved at 30°C and 60°C, respectively. In addition, the Pt2-Fe(OH) x catalyst also showed excellent H2O resistance and hydrothermal stability in comparison to the Pt2/Fe(OH) x catalyst prepared by impregnation method. Characterization results showed that the excellent catalytic performance of the catalysts was mainly attributed to the abundant surface oxygen species and Pt0 the presence of H2O, which promoted the catalytic reaction of CO, and Density functional theory (DFT) calculation showed that this was mainly attributed to the catalytic activity of the hydroxyl (-OH) species on Pt2-Fe(OH) x surface, which could easily oxidize CO to -COOH, which could be further decomposed into CO2 and H atoms. This study provides valuable insights into the design of high-efficiency non-precious metal catalysts for CO catalytic oxidation catalysts with high efficiency.

Sandwich-type electrochemical aptasensor for sensitive detection of myoglobin based on Pt@CuCo-oxide nanoparticles as a signal marker

When an acute myocardial infarction (AMI) occurs, myoglobin (Mb) is the biomarker whose concentration firstly increases, and the high sensitive detection of Mb is critical for early diagnosis of AMI. Herein, a sandwich-type electrochemical aptasensor for the sensitive detection of Mb was constructed by using Pt@Cu1.33OCo0.83O as the signal marker. On one hand, nano-flower-like Cu1.33OCo0.83O was synthesized by hydrothermal method and Pt nanoparticles (Pt NPs) were loaded on its surface. Pt@Cu1.33OCo0.83O could immobilize aptamer 2 (Apt2) successfully by the Pt-S bond. And because of the synergistic effect between Pt and bimetallic oxide, Pt@Cu1.33OCo0.83O had an excellent catalytic effect on the signal source of hydrogen peroxide (H2O2) to amplify the current signal, which enhance the sensitivity of the aptasensor. On the other hand, the screen-printed gold electrode (SPGE) was used as the sensing base, which had good conductivity and ensured the immobilization of aptamer 1 (Apt1). The quantitative detection of Mb was achieved by specific recognition between Mb and Apt1, Apt2. As a result, the constructed electrochemical aptasensor had a good linear range (1-1500 ng/mL) with a low detection limit (LOD) of 0.128 ng/mL (S/N = 3), and a high sensitivity of 29.47 μA dec-1. The aptasensor also realized the detection of Mb in human serum samples with good accuracy, and the results were consistent with the hospital's biochemical indicators, which demonstrated the potential application of the prepared sensor in the clinical detection of Mb.

Hydrogen Spillover Induced PtCo/CoOx Interfaces with Enhanced Catalytic Activity for CO Oxidation at Low Temperatures in Humid Conditions

The development of catalysts with abundant active interfaces for superior low-temperature catalytic CO oxidation is critical to meet increasingly rigorous emission requirements, yet still challenging. Herein, this work reports a PtCo/CoOx/Al2O3 catalyst with PtCo clusters and enriched Pt─O─Co interfaces induced by hydrogen spillover from the Pt sites and self-oxidation process in air, exhibiting excellent performance for CO oxidation at low temperatures and humid conditions. The combination of structural characterizations and in situ Fourier transform infrared spectroscopy reveals that the PtCo cluster effectively prevents CO saturation/poisoning on the Pt surface. Additionally, the presence of Pt─O─Co interfaces in the PtCo/CoOx/Al2O3 catalyst provides a significant number of active sites for oxygen activation and ─OH formation. This facilitates efficient generation of CO2 at ambient temperature by coupling with nearby adsorbed CO molecules, resulting in superior low-temperature activity and long-term stability for CO oxidation under humid conditions. This work provides a facile route toward rationalizing the design of catalysts with more active interfaces for superior low-temperature CO oxidation under humid conditions for practical applications.

Remotely Bonded Bridging Dioxygen Ligands Enhance Hydrogen Transfer in a Silica-Supported Tetrairidium Cluster Catalyst

A longstanding challenge in catalysis by noble metals has been to understand the origin of enhancements of rates of hydrogen transfer that result from the bonding of oxygen near metal sites. We investigated structurally well-defined catalysts consisting of supported tetrairidium carbonyl clusters with single-atom (apical iridium) catalytic sites for ethylene hydrogenation. Reaction of the clusters with ethylene and H2 followed by O2 led to the onset of catalytic activity as a terminal CO ligand at each apical Ir atom was removed and bridging dioxygen ligands replaced CO ligands at neighboring (basal-plane) sites. The presence of the dioxygen ligands caused a 6-fold increase in the catalytic reaction rate, which is explained by the electron-withdrawing capability induced by the bridging dioxygen ligands, consistent with the inference that reductive elimination is rate-determining. Electronic-structure calculations demonstrate an additional role of the dioxygen ligands, changing the mechanism of hydrogen transfer from one involving equatorial hydride ligands to that involving bridging hydride ligands. This mechanism is made evident by an inverse kinetic isotope effect observed in ethylene hydrogenation reactions with H2 and, alternatively, with D2 on the cluster incorporating the dioxygen ligands and is a consequence of quasi-equilibrated hydrogen transfer in this catalyst. The same mechanism accounts for rate enhancements induced by the bridging dioxygen ligands for the catalytic reaction of H2 with D2 to give HD. We posit that the mechanism involving bridging hydride ligands facilitated by oxygen ligands remote from the catalytic site may have some generality in catalysis by oxide-supported noble metals.

Enhancing oxidation reaction over Pt-MnO2 catalyst by activation of surface oxygen

Formaldehyde (HCHO) and carbon monoxide (CO) are both common air pollutants and hazardous to human body. It is imperative to develop the catalyst that is able to efficiently remove these pollutants. In this work, we activated Pt-MnO2 under different conditions for highly active oxidation of HCHO and CO, and the catalyst activated under CO displayed superior performance. A suite of complementary characterizations revealed that the catalyst activated with CO created the highly dispersed Pt nanoparticles to maintain a more positively charged state of Pt, which appropriately weakens the Mn-O bonding strength in the adjacent region of Pt for efficient supply of active oxygen during the reaction. Compared with other catalysts activated under different conditions, the CO-activated Pt-MnO2 displays much higher activity for oxidation of HCHO and CO. This research contributes to elucidating the mechanism for regulating the oxidation activity of Pt-based catalyst.

A Review of Mn-Based Catalysts for Abating NOx and CO in Low-Temperature Flue Gas: Performance and Mechanisms

Mn-based catalysts have attracted significant attention in the field of catalytic research, particularly in NOx catalytic reductions and CO catalytic oxidation, owing to their good catalytic activity at low temperatures. In this review, we summarize the recent progress of Mn-based catalysts for the removal of NOx and CO. The effects of crystallinity, valence states, morphology, and active component dispersion on the catalytic performance of Mn-based catalysts are thoroughly reviewed. This review delves into the reaction mechanisms of Mn-based catalysts for NOx reduction, CO oxidation, and the simultaneous removal of NOx and CO. Finally, according to the catalytic performance of Mn-based catalysts and the challenges faced, a possible perspective and direction for Mn-based catalysts for abating NOx and CO is proposed. And we expect that this review can serve as a reference for the catalytic treatment of NOx and CO in future studies and applications.

Rational design of trimetallic AgPt-Fe3O4 nanozyme for catalyst poisoning-mediated CO colorimetric detection

Carbon monoxide (CO) is not only a highly poisonous gas that brings great health risk, but also a significant signaling molecule in body. However, it is still challengeable for development of alternative colorimetric probes to traditional organic chromophores for simple, sensitive and convenient CO sensing. Here, for the first time, we rationally design a novel hydrophilic AgPt-Fe₃O₄ nanozyme with a unique heterodimeric nanostructure for colorimetric sensing of CO based on the excellent peroxidase-like catalytic activity as well as highly poisonous effect of CO on the nanozyme's catalytic activity. Both experimental evidence and theoretical calculations reveal the trimetallic AgPt-Fe₃O₄ nanozyme is susceptible to poisoning with the strongest affinity towards CO compared to individual Fe₃O₄ or Ag-Fe₃O₄, which is attributed to the adequate exposure of the active metallic sites and efficient interfacial synergy of unique heterodimeric nanostructure. Accordingly, a novel nanozyme-based colorimetric strategy is developed for CO detection with a low detection limit of 5.6 ppb in solution. Furthermore, the probe can be prepared as very convenient test strips and integrated with the portable smartphone platforms for detecting CO gas samples with a low detection limit of 8.9 ppm. Overall, our work proposes guidelines for the rational design of metallic heterogeneous nanostructure to expand the analytical application of nanozyme.

Design of Single-Atom Catalysts and Tracking Their Fate Using Operando and Advanced X-ray Spectroscopic Tools

The potential of operando X-ray techniques for following the structure, fate, and active site of single-atom catalysts (SACs) is highlighted with emphasis on a synergetic approach of both topics. X-ray absorption spectroscopy (XAS) and related X-ray techniques have become fascinating tools to characterize solids and they can be applied to almost all the transition metals deriving information about the symmetry, oxidation state, local coordination, and many more structural and electronic properties. SACs, a newly coined concept, recently gained much attention in the field of heterogeneous catalysis. In this way, one can achieve a minimum use of the metal, theoretically highest efficiency, and the design of only one active site-so-called single site catalysts. While single sites are not easy to characterize especially under operating conditions, XAS as local probe together with complementary methods (infrared spectroscopy, electron microscopy) is ideal in this research area to prove the structure of these sites and the dynamic changes during reaction. In this review, starting from their fundamentals, various techniques related to conventional XAS and X-ray photon in/out techniques applied to single sites are discussed with detailed mechanistic and in situ/operando studies. We systematically summarize the design strategies of SACs and outline their exploration with XAS supported by density functional theory (DFT) calculations and recent machine learning tools.

Theoretical insights into the oxidation of elemental mercury by O2 on graphene-based Pt single-atom catalysts

Pt single-atom catalysts (SACs) exhibit good performance for oxygen activation, which plays a significant role in the oxidation of Hg⁰ by O₂ in flue gas. Density functional theory calculations are carried out to reveal the interfacial behavior of Hg⁰, O₂ and HgO on Pt SACs (single vacancy and 3 N doped defected graphene, Pt/SV-GN and Pt/3N-GN) and the mechanism of Hg⁰ oxidation by O₂. The results show that the flue gas components are chemically adsorbed and bond with the Pt of the Pt SACs with adsorption energies ranging from -0.555 to -5.154 eV. Electronic structure analysis indicates that Hg⁰ is an electron donor and transfers 0.114-0.128 e^- to the Pt SACs. Both O₂ and HgO are electron acceptors and obtain 0.184-0.303 e^- from the slabs. Pt/3N-GN has a higher activity than that of Pt/SV-GN for these three flue gas compositions. The significant charge transfer and orbital hybridization between the gas molecules and atomic catalysts lead to a strong interaction. Furthermore, the Pt-3C and Pt-3N states can increase the band gap compared with pristine graphene, corresponding to 0.195 and 0.129 eV, respectively. Narrow band gaps indicate easier electron excitation properties, which enhance the activity of the reaction. Through a transition states (TSs) search, the lower O₂ dissociation barrier is found to correspond to the lower Hg⁰ oxidation barrier. Pt/3N-GN has higher catalytic oxidation performance for Hg⁰ in the presence of O₂, with a rate determining reaction barrier of 2.016 eV. Compared to traditional selective catalytic reduction and Fe-based SACs, the Pt/3N-GN catalyst has a good oxidation reaction capability with a lower activation energy, indicating that it is a promising catalyst for the oxidation of Hg⁰ by O₂.

A minireview on the synthesis of single atom catalysts

Single atom catalysis is a prosperous and rapidly growing research field, owing to the remarkable advantages of single atom catalysts (SACs), such as maximized atom utilization efficiency, tailorable catalytic activities as well as supremely high catalytic selectivity. Synthesis approaches play crucial roles in determining the properties and performance of SACs. Over the past few years, versatile methods have been adopted to synthesize SACs. Herein, we give a thorough and up-to-date review on the progress of approaches for the synthesis of SACs, outline the general principles and list the advantages and disadvantages of each synthesis approach, with the aim to give the readers a clear picture and inspire more studies to exploit novel approaches to synthesize SACs effectively.

Single Atom Catalysts: An Overview of the Coordination and Interactions with Metallic Supports

Catalyst utilization is a key economic factor in heterogeneous catalysis, particularly, when noble metals are used as the active phase. A huge saving on catalyst cost can be achieved with developing a single atomic layer of the active catalyst on a given cheap support. Besides the economic benefit, single atom catalysts (SACs) have also shown superior activity and selectivity relative to catalytic particles or nanoparticles; yet they are prone to aggregation and deactivation. The development of effective, stable, and commercially viable SACs is still a huge challenge. One of the remaining key obstacles is the ability to easily and effectively tune SACs-support interactions and coordination in a way that enables the production of robust, stable, and versatile SACs. Accordingly, the coordination and interactions between metallic supports and SACs and their impacts on SACs stability and activity are reviewed in this article.

Exploring the materials space in the smallest particle size range: From heterogeneous catalysis to electrocatalysis and photocatalysis

Ultrasmall clusters of subnanometer size can possess unique and even unexpected physical and chemical propensities which make them interesting in various fields of basic science and for potential applications, such...

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.

Porous Materials Confining Single Atoms for Catalysis

In recent years, single-atom catalysts (SACs) have received extensive attention due to their unique structure and excellent performance. Currently, a variety of porous materials are used as confined single-atom catalysts, such as zeolites, metal-organic frameworks (MOFs), or carbon nitride (CN). The support plays a key role in determining the coordination structure of the catalytic metal center and its catalytic performance. For example, the strong interaction between the metal and the carrier induces the charge transfer between the metal and the carrier, and ultimately affects the catalytic behavior of the single-atom catalyst. Porous materials have unique chemical and physical properties including high specific surface area, adjustable acidity and shape selectivity (such as zeolites), and are rational support materials for confined single atoms, which arouse research interest in this field. This review surveys the latest research progress of confined single-atom catalysts for porous materials, which mainly include zeolites, CN and MOFs. The preparation methods, characterizations, application fields, and the interaction between metal atoms and porous support materials of porous material confined single-atom catalysts are discussed. And we prospect for the application prospects and challenges of porous material confined single-atom catalysts.

Surface Density Dependent Catalytic Activity of Single Palladium Atoms Supported on Ceria*

The analogy between single-atom catalysts (SACs) and molecular catalysts predicts that the specific catalytic activity of these systems is constant. We provide evidence that this prediction is not necessarily true. As a case in point, we show that the specific activity over ceria-supported single Pd atoms linearly increases with metal atom density, originating from the cumulative enhancement of CeO₂ reducibility. The long-range electrostatic footprints (≈1.5 nm) around each Pd site overlap with each other as surface Pd density increases, resulting in an observed deviation from constant specific activity. These cooperative effects exhaust previously active O atoms above a certain Pd density, leading to their permanent removal and a consequent drop in reaction rate. The findings of our combined experimental and computational study show that the specific catalytic activity of reducible oxide-supported single-atom catalysts can be tuned by varying the surface density of single metal atoms.

Thermal CO Oxidation and Photocatalytic CO2 Reduction over Bare and M-Al2O3 (M = Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) Cotton-Like Nanosheets

Aluminum oxide (Al₂O₃) has abundantly been used as a catalyst, and its catalytic activity has been tailored by loading transition metals. Herein, γ-Al₂O₃ nanosheets were prepared by the solvothermal method, and transition metals (M = Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) were loaded onto the nanosheets. Big data sets of thermal CO oxidation and photocatalytic CO₂ reduction activities were fully examined for the transition metal-loaded Al₂O₃ nanosheets. Their physicochemical properties were examined by scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction crystallography, and X-ray photoelectron spectroscopy. It was found that Rh, Pd, Ir, and Pt-loading showed a great enhancement in CO oxidation activity while other metals negated the activity of bare Al₂O₃ nanosheets. Rh-Al₂O₃ showed the lowest CO oxidation onset temperature of 172 °C, 201 °C lower than that of bare γ-Al₂O₃. CO₂ reduction experiments were also performed to show that CO, CH₃OH, and CH₄ were common products. Ag-Al₂O₃ nanosheets showed the highest performances with yields of 237.3 ppm for CO, 36.3 ppm for CH₃OH, and 30.9 ppm for CH₄, 2.2×, 1.2×, and 1.6× enhancements, respectively, compared with those for bare Al₂O₃. Hydrogen production was found to be maximized to 20.7 ppm during CO₂ reduction for Rh-loaded Al₂O₃. The present unique pre-screening test results provided very useful information for the selection of transition metals on Al₂O₃-based energy and environmental catalysts.

Catalytic oxidation of CO on noble metal-based catalysts

Carbon monoxide (CO) catalytic oxidation has gained increasing interest in recent years due to its application prospects. The noble metal catalysts commonly exhibit outstanding CO catalytic oxidation activity. Therefore, this article reviewed the recent research on the application of noble metal catalysts in the catalytic oxidation of CO. The effects of catalyst support, dopant, and physicochemical properties on the catalytic activity for CO oxidation are summarized. The influence of the presence of water vapor and sulfur dioxide in the reaction atmosphere on the catalytic activity in CO oxidation is emphatically discussed. Moreover, this paper discussed several reaction mechanisms of CO catalytic oxidation on noble metal catalysts. Finally, the challenges of removing CO by catalytic oxidation in practical industrial flue gas are proposed.

Theoretical Study on a Potential Oxygen Reduction Reaction Electrocatalyst: Single Fe Atoms Supported on Graphite Carbonitride

In recent years, one of the research directions of proton-exchange membrane fuel cells (PEMFCs) was to exploit efficient electrocatalysts for oxygen reduction reaction (ORR) instead of precious metals. In this study, on the basis of the density-functional theory (DFT) calculations, we designed a new type of single-atom ORR electrocatalyst by doping single iron atoms into the N-coordination cavity of the substrate graphite carbonitride (Fe/g-C₃N₄). The adsorption site and the adsorption energy of all the intermediates, the reaction energy barriers, potential energy surface, and Mulliken charges have been analyzed. The feasible ORR reaction paths and the most favorable ORR reaction mechanism were performed. Our calculation results prove that Fe/g-C₃N₄ is a potential electrocatalyst toward ORR. This work proposes a novel notion for the development of cathode materials in PEMFCs.

A perspective on oxide-supported single-atom catalysts

Single-atom catalysts (SACs) can not only maximize the metal atom utilization efficiency, but also show drastically improved catalytic performance for various important catalytic processes. Insights into the working principles of SACs provide rational guidance to design and prepare advanced catalysts. Many factors have been claimed to affect the performance of SACs, which makes it very challenging to clarify the correlation between the catalytic performance and physicochemical characteristics of SACs. Oxide-supported SACs are one of the most extensively explored systems. In this minireview, some latest developments on the determining factors of the stability, activity and selectivity of SACs on oxide supports are overviewed. Discussed also are the reaction mechanisms for different systems and methods that are employed to correlate the properties with the catalyst structures at the atomic level. In particular, a recently proposed surface free energy approach is introduced to fabricate well-defined modelled SACs that may help address some key issues in the development of SACs in the future.

Atomic Layer Deposition for Preparing Isolated Co Sites on SiO2 for Ethane Dehydrogenation Catalysis

Unlike Co clusters, isolated Co atoms have been shown to be selective for catalytic dehydrogenation of ethane to ethylene; however, preparation of isolated Co sites requires special preparation procedures. Here, we demonstrate that Atomic Layer Deposition (ALD) of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)cobalt(III) (Co(TMHD)₃) on silica and other supports is effective in producing these isolated species. Silica-supported catalysts prepared with one ALD cycle showed ethylene selectivities greater than 96% at 923 K and were stable when CO₂ was co-fed with the ethane. Co catalysts prepared by impregnation formed clusters that were significantly less active, selective, and stable. Rates and selectivities also decreased for catalysts with multiple ALD cycles. Isolated Co catalysts prepared on Al₂O₃ and MgAl₂O₄ showed reasonable selectivity for ethane dehydrogenation but were not as effective as their silica counterpart.

Surface chemistry dictates stability and oxidation state of supported single metal catalyst atoms

Single atom catalysts receive considerable attention due to reducing noble metal utilization and potentially eliminating certain side reactions. Yet, the rational design of highly reactive and stable single atom catalysts is hampered by the current lack of fundamental insights at the single atom limit. Here, density functional theory calculations are performed for a prototype reaction, namely CO oxidation, over different single metal atoms supported on alumina. The governing reaction mechanisms and scaling relations are identified using microkinetic modeling and principal component analysis, respectively. A large change in the oxophilicity of the supported single metal atom leads to changes in the rate-determining step and the catalyst resting state. Multi-response surfaces are introduced and built cheaply using a descriptor-based, closed form kinetic model to describe simultaneously the activity, stability, and oxidation state of single metal atom catalysts. A double peaked volcano in activity is observed due to competing rate-determining steps and catalytic cycles. Reaction orders of reactants provide excellent kinetic signatures of the catalyst state. Importantly, the surface chemistry determines the stability, oxidation, and resting state of the catalyst.

Linear Activation Energy-Reaction Energy Relations for LaBO3 (B = Mn, Fe, Co, Ni) Supported Single-Atom Platinum Group Metal Catalysts for CO Oxidation

Single-atom catalysts are at the center of attention of the heterogeneous catalysis community because they exhibit unique electronic structures distinct from nanoparticulate forms, resulting in very different catalytic performance combined with increased usage of often costly transition metals. Proper selection of a support that can stably keep the metal in a high dispersion is crucial. Here, we employ spin-polarized density functional theory and microkinetics simulations to identify optimum LaBO₃ (B = Mn, Fe, Co, Ni) supported catalysts dispersing platinum group metals as atoms on their surface. We identify a strong correlation between the CO adsorption energy and the d-band center of the doped metal atom. These CO adsorption strength differences are explained in terms of the electronic structure. In general, Pd-doped surfaces exhibit substantially lower activation barriers for CO₂ formation than the Rh- and Pt-doped surfaces. Strong Brønsted-Evans-Polanyi correlations are found for CO oxidation on these single-atom catalysts, providing a tool to predict promising compositions. Microkinetics simulations show that Pd-doped LaCoO₃ is the most active catalyst for low-temperature CO oxidation. Moderate CO adsorption strength and low reaction barriers explain the high activity of this composition. Our approach provides guidelines for the design of highly active and cost-effective perovskite supported single-atom catalysts.

Modulating the surface defects of titanium oxides and consequent reactivity of Pt catalysts

In heterogeneous catalysis, it is widely believed that the surface states of catalyst supports can strongly influence the catalytic performance, because active components are generally anchored on supports. This paper describes a detailed understanding of the influence of surface defects of TiO2 supports on the catalytic properties of Pt catalysts. Pt was deposited on reduced (r-), hydroxylated (h-), and oxidized (o-) TiO2 surfaces, respectively, and the different surface states of TiO2 not only lead to differences in metal dispersion, but also distinct electronic interactions between the metal and the support. The highest reactivity for catalytic CO oxidation can be achieved over the Pt catalyst supported on reduced TiO2 with surface oxygen vacancies. The turnover frequency (TOF) of this catalyst is determined to be ∼11 times higher than that of Pt supported on oxidized TiO2. More importantly, the reactivity is seen to increase in the sequence of Pt/o-TiO2 < Pt/h-TiO2 < Pt/r-TiO2, which is well consistent with the trend of the calculated Bader charge of Pt.

Determination of the Evolution of Heterogeneous Single Metal Atoms and Nanoclusters under Reaction Conditions: Which Are the Working Catalytic Sites?

Identification of active sites in heterogeneous metal catalysts is critical for understanding the reaction mechanism at the molecular level and for designing more efficient catalysts. Because of their structural flexibility, subnanometric metal catalysts, including single atoms and clusters with a few atoms, can exhibit dynamic structural evolution when interacting with substrate molecules, making it difficult to determine the catalytically active sites. In this work, Pt catalysts containing selected types of Pt entities (from single atoms to clusters and nanoparticles) have been prepared, and their evolution has been followed, while they were reacting in a variety of heterogeneous catalytic reactions, including selective hydrogenation reactions, CO oxidation, dehydrogenation of propane, and photocatalytic H₂ evolution reaction. By in situ X-ray absorption spectroscopy, in situ IR spectroscopy, and high-resolution electron microscopy techniques, we will show that some characterization techniques carried out in an inadequate way can introduce confusion on the interpretation of coordination environment of highly dispersed Pt species. Finally, the combination of catalytic reactivity and in situ characterization techniques shows that, depending on the catalyst–reactant interaction and metal–support interaction, singly dispersed metal atoms can rapidly evolve into metal clusters or nanoparticles, being the working active sites for those abovementioned heterogeneous reactions.

High-Density Isolated Fe1O3 Sites on a Single-Crystal Cu2O(100) Surface

Single-atom catalysts have recently been subject to considerable attention within applied catalysis. However, complications in the preparation of well-defined single-atom model systems have hampered efforts to determine the reaction mechanisms underpinning the reported activity. By means of an atomic layer deposition method utilizing the steric hindrance of the ligands, isolated Fe₁O₃ motifs were grown on a single-crystal Cu₂O(100) surface at densities up to 0.21 sites per surface unit cell. Ambient pressure X-ray photoelectron spectroscopy shows a strong metal-support interaction with Fe in a chemical state close to 3+. Results from scanning tunneling microscopy and density functional calculations demonstrate that isolated Fe₁O₃ is exclusively formed and occupies a single site per surface unit cell, coordinating to two oxygen atoms from the Cu₂O lattice and another through abstraction from O₂. The isolated Fe₁O₃ motif is active for CO oxidation at 473 K. The growth method holds promise for extension to other catalytic systems.

Supported Noble-Metal Single Atoms for Heterogeneous Catalysis

Single-atom catalysts (SACs), with atomically distributed active metal sites on supports, serve as a newly advanced material in catalysis, and open broad prospects for a wide variety of catalytic processes owing to their unique catalytic behaviors. To construct SACs with precise structures and high density of accessible single-atom sites, while preventing aggregation to large nanoparticles, various strategies for their chemical synthesis have been recently developed by improving the distribution and chemical bonding of active sites on supports, which results in excellent activity and selectivity in a variety of catalytic reactions. Noble-metal-based SACs are discussed, and their structural properties, chemical synthesis, and catalytic applications are highlighted. The structure-activity relationships and the underlying catalytic mechanisms are addressed, including the influences of surface species and reducibility of supports on the activity and stability, impact of the unique structural and electronic properties of single-atom centers modulated by metal/support interactions on catalytic activity and selectivity, and how the modified catalytic mechanism obtained by inhibiting the multiatoms involves catalytic pathways. Finally, the prospects and challenges for development in this field are highlighted.

Remarkable active-site dependent H2O promoting effect in CO oxidation

The interfacial sites of supported metal catalysts are often critical in determining their performance. Single-atom catalysts (SACs), with every atom contacted to the support, can maximize the number of interfacial sites. However, it is still an open question whether the single-atom sites possess similar catalytic properties to those of the interfacial sites of nanocatalysts. Herein, we report an active-site dependent catalytic performance on supported gold single atoms and nanoparticles (NPs), where CO oxidation on the single-atom sites is dramatically promoted by the presence of H₂O whereas on NPs’ interfacial sites the promoting effect is much weaker. The remarkable H₂O promoting effect makes the Au SAC two orders of magnitude more active than the commercial three-way catalyst. Theoretical studies reveal that the dramatic promoting effect of water on SACs originates from their unique local atomic structure and electronic properties that facilitate an efficient reaction channel of CO + OH.

The issue that whether single-atom sites possess similar catalytic properties to the interfacial sites of nanocatalysts remains unresolved. Here, the authors demonstrate a large H₂O promotional effect on CO oxidation over Au single-atom sites due to their unique local atomic structure and electronic properties.

Cu2O-Supported Atomically Dispersed Pd Catalysts for Semihydrogenation of Terminal Alkynes: Critical Role of Oxide Supports

Atomically dispersed catalysts have demonstrated superior catalytic performance in many chemical transformations. However, limited success has been achieved in applying oxide-supported atomically dispersed catalysts to semihydrogenation of alkynes under mild conditions. By utilizing various metal oxides (e.g., Cu2O, Al2O3, ZnO, and TiO2) as supports for atomically dispersed Pd catalysts, we demonstrate herein the critical role of the oxidation state and coordinate environment of Pd centers in their catalytic performance, thus leading to the discovery of an “oxide-support effect” on atomically dispersed metal catalysts. Pd atomically dispersed on Cu2O exhibits far better catalytic activity in the hydrogenation of alkynes, with an extremely high selectivity toward alkenes, compared to catalysts on other oxides. Pd species galvanically displace surface Cu(I) sites on Cu2O to create two-coordinated Pd(I), which is a critical step for the activation and heterolytic splitting of H2 into Pd-H− and O-H+ species for the selective hydrogenation of alkynes. Moreover, the adsorption of alkenes on H2-preadsorbed Pd(I) is relatively weak, preventing deeper hydrogenation and increased selectivity during semihydrogenation. We demonstrate that the local coordinate environment of active metal centers plays a crucial role in determining the catalytic performance of an oxide-supported atomically dispersed catalyst.

Atmosphere-dependent stability and mobility of catalytic Pt single atoms and clusters on γ-Al2O3

Atomically dispersed metals promise the ultimate catalytic efficiency, but their stabilization onto suitable supports remains challenging owing to their aggregation tendency. Focusing on the industrially-relevant Pt/γ-Al2O3 catalyst, in situ X-ray absorption spectroscopy and environmental scanning transmission electron microscopy allow us to monitor the stabilization of Pt single atoms under O2 atmosphere, as well as their aggregation into mobile reduced subnanometric clusters under H2. Density functional theory calculations reveal that oxygen from the gas phase directly contributes to metal-support adhesion, maximal for single Pt atoms, whereas hydrogen only adsorbs on Pt, and thereby leads to Pt clustering. Finally, Pt cluster mobility is shown to be activated at low temperature and high H2 pressure. Our results highlight the crucial importance of the reactive atmosphere on the stability of single-atom versus cluster catalysts.

Highlights of Major Progress on Single-Atom Catalysis in 2017

Single-atom catalysis has rapidly progressed during the last few years. In 2017, single-atom catalysts (SACs) were fabricated with higher metal loadings and designed into more delicate structures. SACs also found wide applications in C1 chemical conversion, such as selective oxidation of methane and conversion of carbon dioxide. Both experimental characterizations and computational modeling revealed the presence of tunable interactions between single atom species and their surrounding chemical environment, and thus SACs may be more effective and more stable than their nanoparticle counterparts. In this mini-review, we summarize the major achievements of SACs into three main aspects: a) the advanced synthetic methodologies, b) catalytic performance in C1 chemistry, and c) strong metal-support interaction induced unexpected durability. These accomplishments will shed new light on the recognition of single-atom catalysis and encourage more efforts to explore potential applications of SACs.