Photoinduced loading of electron-rich Cu single atoms by moderate coordination for hydrogen evolution

Single-atom catalysts offer maximal atom utilization efficiencies and high-electronegativity heteroatoms play a crucial role in coordinating reactive single metal atoms to prevent agglomeration. However, these strong coordination bonds withdraw electron density for coordinated metal atoms and consequently affect their catalytic activity. Herein we reveal the high loading (11.3 wt%) and stabilization of moderately coordinated Cu-P₃ structure on black phosphorus support by a photochemical strategy with auxiliary hydrogen. Single-atom Cu sites with an exceptional electron-rich feature show the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\triangle {G}_{{{{{{\rm{H}}}}}}*}$$\end{document}△GH* close to zero to favor catalysis. Neighboring Cu atoms work in synergy to lower the energy of key water adsorption and dissociation intermediates. The reported catalyst shows a low overpotential of only 41 mV at 10 mA cm⁻² and Tafel slope of 53.4 mV dec⁻¹ for the alkaline hydrogen evolution reaction, surpassing both isolated Cu single atoms and Cu nanoclusters. The promising materials design strategy sheds light on the design and fabrication of high-loading single metal atoms and the role of neighboring single atoms for enhanced reaction kinetics.

While atomically dispersed metals can maximize reaction catalytic sites, it is challenging to achieve high atomic densities without agglomeration. Here, authors prepared Cu single-atoms on black phosphorous using a photochemical strategy and auxiliary H₂ as proton reduction electrocatalysts.

In Situ DRIFTS Study of Single-Atom, 2D, and 3D Pt on γ-Al2O3 Nanoflakes and Nanowires for C2H4 Oxidation

Up to now, a great number of catalysts have been reported that are active in the oxidation of volatile organic compounds (VOCs). However, supported noble-metal catalysts (especially Pt-based catalysts) are still the most excellent ones for this reaction. In this study, Pt species supported on γ-Al2O3 and ranging from single-atom sites to clusters (less than 1 nm) and 1–2 nm nanoparticles were prepared and investigated for oxidizing C2H4. The Pt-loaded γ-Al2O3 nanoflakes (PtAl-NF) and Pt-loaded γ-Al2O3 nanowires (PtAl-NW) were successfully prepared. The samples were characterized using XRD, TEM, XPS, HAADF-STEM, and in situ DRIFTS. Based on in situ DRIFTS, a simple surface reaction mechanism was developed. The stable intermediates CO on single-atom Pt, subnanometer Pt particles, and fully exposed Pt clusters could be explained by the strong binding of CO molecule poisoning Pt sites. Moreover, the oxidation of C2H4 was best achieved by Pt particles that were 1–2 nm in size and the catalytic activity of PtAl-NF was better when it had less Pt. Lastly, the most exposed (110) facets of γ-Al2O3 nanoflakes were more resistant to water than the majorly exposed (100) facets of γ-Al2O3 nanowires.

Butterfly Effect of Electron Donor from Monoatomic Cobalt in Few-Atom Platinum Clusters: Boosting Electrocatalysis

Few-atom metal clusters feature an extremely large surface area and abundant active sites, which are particularly important for electrocatalysis. Herein, we report a monoatomic cobalt tailoring strategy to boost the performance of platinum clusters (ca. <1 nm) via hetero-charge-trapping chemistry by ultraviolet light reducing Pt-based anions anchored on target Co cations. The created Co₁Pt_x clusters exhibit a mass activity of 2.27 A mg_Pt^-1, which is about 1621% higher than that obtained by state-of-the-art Pt/C (2 nm) for the oxygen reduction reaction (ORR). This can be attributed to the butterfly effect of electron donor from monoatomic cobalt in the platinum clusters. Moreover, the improved stability results from the Co located at the bottom position of the Pt host, possessing high resistance to Co leaching. Therefore, this offers a general strategy to optimize the high performance of platinum group metal (PGM) clusters for electrocatalysis.

High-Entropy Intermetallics Serve Ultrastable Single-Atom Pt for Propane Dehydrogenation

Propane dehydrogenation has been a promising propylene production process that can compensate for the increasing global demand for propylene. However, Pt-based catalysts with high stability at ≥600 °C have barely been reported because the catalysts typically result in short catalyst life owing to side reactions and coke formation. Herein, we report a new class of heterogeneous catalysts using high-entropy intermetallics (HEIs). Pt-Pt ensembles, which cause side reactions, are entirely diluted by the component inert metals in PtGe-type HEIs. The resultant HEI (PtCoCu) (GeGeSn)/Ca-SiO₂ exhibited an outstandingly high catalytic stability, even at 600 °C (k_d^-1 = τ = 4146 h = 173 d), and almost no deactivation of the catalyst was observed for 2 months for the first time. Detailed experimental studies and theoretical calculations demonstrated that the combination of the site-isolation and entropy effects upon multi-metallization of PtGe drastically enhanced the desorption of propylene and the thermal stability, eventually suppressing the side reactions even at high reaction temperatures.

Effect of Hydroxyl Groups on Metal Anchoring and Formaldehyde Oxidation Performance of Pt/Al2O3

Pt/Al₂O₃ catalysts showing excellent activity and stability have been used in various reactions, including HCHO oxidation. Herein, we prepared Pt-Na/Al₂O₃ catalysts with a Pt content of 0.05 wt % to reveal the key factors determining the anchoring of Pt as well as the catalytic activity and mechanism of HCHO oxidation. Pt-Na/nano-Al₂O₃ (denoted as Pt-Na/nAl₂O₃) catalysts with 0.05 wt % Pt content could completely oxidize HCHO to CO₂ at room temperature, which is the lowest Pt content used in HCHO catalytic oxidation to our knowledge. After Na addition, terminal hydroxyl groups (denoted as HO-μ_ter) on nano-Al₂O₃ were transformed to doubly bridging hydroxyl groups between Na and Al (denoted as HO-μ_bri(Na-Al)), which atomically dispersed Pt species. Pt anchoring further promoted the regeneration of HO-μ_bri(Na-Al) by activating O₂ and H₂O, oxidizing HCHO to CO₂ directly by the fast reaction step ([HCOO^-] + [OH]_a → CO₂ + H₂O). Our study revealed that the HO-μ_bri(Na-Al) synergistically generated by HO-μ_ter and Na species provided anchoring sites for Pt species.

Atomic-Level Pt Electrocatalyst Synthesized via Iced Photochemical Method for Hydrogen Evolution Reaction with High Efficiency

In heterogeneous catalysis, metal particle morphology and size can influence markedly the activity. It is of great significance to rationally design and control the synthesis of Pt at the atomic level to demonstrate the structure-activity relationship toward electrocatalysis. Herein, a powerful strategy is reported to synthesize graphene-supported platinum-based electrocatalyst, that is, nanocatalysts with controllable size can be prepared by iced photochemical method, including single atoms (Pt-SA@HG), nanoclusters (Pt-Clu@HG), and nanocrystalline (Pt-Nc@HG). The Pt-SA@HG exhibits unexpected electrocatalytic hydrogen evolution reaction (HER) performances with 13 mV overpotential at 10 mA cm^-2 current densities which surpass Pt-Clu@HG and Pt-Nc@HG. The in situ X-ray absorption fine structure spectroscopy (XAFS) and density functional theory (DFT) calculations determine the Pt-C₃ active site is linchpin to the excellent HER performance of Pt-SA@HG. Compared with the traditional Pt-N_x coordination structure, the pure carbon coordinated Pt-C₃ site is more favorable for HER. This work opens up a new way to adjust the metal particle size and catalytic performance of graphene at a multiscale level.

Distinct Role of Surface Hydroxyls in Single-Atom Pt1/CeO2 Catalyst for Room-Temperature Formaldehyde Oxidation: Acid-Base Versus Redox

The development of highly efficient catalysts for room-temperature formaldehyde (HCHO) oxidation is of great interest for indoor air purification. In this work, it was found that the single-atom Pt₁/CeO₂ catalyst exhibits a remarkable activity with complete removal of HCHO even at 288 K. Combining density functional theory calculations and in situ DRIFTS experiments, it was revealed that the active O_latticeH site generated on CeO₂ in the vicinity of Pt²⁺ via steam treatment plays a key role in the oxidation of HCHO to formate and its further oxidation to CO₂. Such involvement of hydroxyls is fundamentally different from that of cofeeding water which dissociates on metal oxide and catalyzes the acid-base-related chemistry. This study provides an important implication for the design and synthesis of supported Pt catalysts with atom efficiency for a very important practical application-room-temperature HCHO oxidation.

Interconversion of Atomically Dispersed Platinum Cations and Platinum Clusters in Zeolite ZSM-5 and Formation of Platinum gem-Dicarbonyls

Catalysts composed of platinum dispersed on zeolite supports are widely applied in industry, and coking and sintering of platinum during operation under reactive conditions require their oxidative regeneration, with the platinum cycling between clusters and cations. The intermediate platinum species have remained only incompletely understood. Here, we report an experimental and theoretical investigation of the structure, bonding, and local environment of cationic platinum species in zeolite ZSM-5, which are key intermediates in this cycling. Upon exposure of platinum clusters to O₂ at 700 °C, oxidative fragmentation occurs, and Pt²⁺ ions are stabilized at six-membered rings in the zeolite that contain paired aluminum sites. When exposed to CO under mild conditions, these Pt²⁺ ions form highly uniform platinum gem-dicarbonyls, which can be converted in H₂ to Pt^δ+ monocarbonyls. This conversion, which weakens the platinum-zeolite bonding, is a first step toward platinum migration and aggregation into clusters. X-ray absorption and infrared spectra provide evidence of the reductive and oxidative transformations in various gas environments. The chemistry is general, as shown by the observation of platinum gem-dicarbonyls in several commercially used zeolites (ZSM-5, Beta, mordenite, and Y).

Rational Design of Synergistic Structure Between Single-Atoms and Nanoparticles for CO2 Hydrogenation to Formate Under Ambient Conditions

Single-atom catalysts (SACs) as the new frontier in heterogeneous catalysis have attracted increasing attention. However, the rational design of SACs with high catalytic activities for specified reactions still remains challenging. Herein, we report the rational design of a Pd1-PdNPs synergistic structure on 2,6-pyridinedicarbonitrile-derived covalent triazine framework (CTF) as an efficient active site for CO2 hydrogenation to formate under ambient conditions. Compared with the catalysts mainly comprising Pd1 and PdNPs, this hybrid catalyst presented significantly improved catalytic activity. By regulating the ratio of Pd1 to PdNPs, we obtained the optimal catalytic activity with a formate formation rate of 3.66 molHCOOM·molPd−1·h−1 under ambient conditions (30°C, 0.1 MPa). Moreover, as a heterogeneous catalyst, this hybrid catalyst is easily recovered and exhibits about a 20% decrease in the catalytic activity after five cycles. These findings are significant in elucidating new rational design principles for CO2 hydrogenation catalysts with superior activity and may open up the possibilities of converting CO2 under ambient conditions.

Observation and Characterization of Dicarbonyls on a RhCu Single-Atom Alloy

Dicarbonyl species are ubiquitous on Rh/oxide catalysts and are known to form on Rh⁺ centers. However, dicarbonyl species have never been directly observed on single-atom alloys (SAAs) where the active site is metallic. Herein, using surface science and theoretical modeling, we provide evidence of dicarbonyl species at isolated Rh sites on a RhCu(100) SAA. This approach not only enables us to directly visualize dicarbonyl species at Rh sites but also demonstrates that the transition between the mono- and dicarbonyl configuration can be achieved by changing surface temperature and CO pressure. Density functional theory calculations further support the mono- and dicarbonyl assignments and provide evidence that these species should be stable on other SAA combinations. Together, these results provide a picture of the structure and energetics of both the mono- and dicarbonyl configurations on the RhCu(100) SAA surface and should aid with IR assignments on SAA nanoparticle catalysts.

Direct Visualization of the Evolution of a Single-Atomic Cobalt Catalyst from Melting Nanoparticles with Carbon Dissolution

Transition metal single-atom catalysts (SACs) are of immense interest, but how exactly they are evolved upon pyrolysis of the corresponding precursors remains unclear as transition metal ions in the complex precursor undergo a series of morphological changes accompanied with changes in oxidation state as a result of the interactions with the carbon support. Herein, the authors record the complete evolution process of Co SAC during the pyrolysis a Co/Zn-containing zeolitic imidazolate framework. Aberration-corrected environmental TEM coupled with in-situ EELS is used for direct visualization of the evolution process at 200-1000 °C. Dissolution of carbon into the nanoparticles of Co is found to be key to modulating the wetting behavior of nanoparticles on the carbon support; melting of Co nanoparticles and their motion within the zeolitic architecture leads to the etching of the framework structure, yielding porous C/N support onto which Co-single atoms reside. This uniquely structured Co SAC is found to be effective for the oxidation of a series of aromatic alkanes to produce selective ketones among other possible products. The carbon dissolution and melting/sublimation-driven structural dynamics of transition metal revealed here will expand the methodology in synthesizing SACs and other high-temperature processes.

Identification of the Stable Pt Single Sites in the Environment of Ions: From Mechanism to Design Principle

For single-atom (SA)-based catalysis, it is urgent to understand the nature and dynamic evolution of SA active sites during the reactions. In this work, an example of Pt SA-Zn_0.5 Cd_0.5 S (Pt SA-ZCS) is found to display interesting phenomena when facing the Brownian collision of ions in aqueous photocatalysis. Via synchrotron radiation, surface techniques, microscopy, and theory calculations, the results show that two kinds of Pt sites exist: Pt_Zn-sub -S₃ (Pt substituting the Zn site) and Pt_ads -S₂ (Pt adsorbing on the surface). In Na₂ S, the S^2- can coordinate with Pt atoms and peel them from the Pt_ads -S₂ sites, but leaves more stable Pt_Zn-sub -S₃ sites, bringing a low but stable catalytic activity (19.40 mmol g^-1 h^-1 ). Meanwhile, in ascorbic acid, the ascorbic acid ions show less complex ability with Pt atoms, but can decrease the migration barrier of Pt_ads -S₂ sites (67.18 down to 35.96 mmol g^-1 h^-1 , 52.03% drop after 6 h). Therefore, the Pt_ads -S₂ sites gradually aggregate into nanoclusters, bringing a high but decayed catalytic activity. Moreover, a Pt SA-ZCS-Sulfur composite is designed mainly covered by Pt_Zn-sub -S₃ sites accordingly (max: 79.09 mmol g^-1 h^-1 , 5% drop after 6 h and QE: 14.0% at 420 nm), showing a beneficial strategy "from mechanism to design principle."

Atomically Dispersed Manganese on Biochar Derived from a Hyperaccumulator for Photocatalysis in Organic Pollution Remediation

Phytoremediation is a potentially cost-effective and environmentally friendly remediation method for environmental pollution. However, the safe treatment and resource utilization of harvested biomass has become a limitation in practical applications. To address this, a novel manganese-carbon-based single-atom catalyst (SAC) method has been developed based on the pyrolysis of a manganese hyperaccumulator, Phytolacca americana. In this method, manganese atoms are dispersed atomically in the carbon matrix and coordinate with N atoms to form a Mn-N₄ structure. The SAC developed exhibited a high photooxidation efficiency and excellent stability during the degradation of a common organic pollutant, rhodamine B. The Mn-N₄ site was the active center in the transformation of photoelectrons via the transfer of photoelectrons between adsorbed O₂ and Mn to produce reactive oxygen species, identified by in situ X-ray absorption fine structure spectroscopy and density functional theory calculations. This work demonstrates an approach that increases potential utilization of biomass during phytoremediation and provides a promising design strategy to synthesize cost-effective SACs for environmental applications.

Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO 2 Catalysts

Aiming at knowledge‐driven design of novel metal–ceria catalysts for automotive exhaust abatement, current efforts mostly pertain to the synthesis and understanding of well‐defined systems. In contrast, technical catalysts are often heterogeneous in their metal speciation. Here, we unveiled rich structural dynamics of a conventional impregnated Pd/CeO₂ catalyst during CO oxidation. In situ X‐ray photoelectron spectroscopy and operando X‐ray absorption spectroscopy revealed the presence of metallic and oxidic Pd states during the reaction. Using transient operando infrared spectroscopy, we probed the nature and reactivity of the surface intermediates involved in CO oxidation. We found that while low‐temperature activity is associated with sub‐oxidized and interfacial Pd sites, the reaction at elevated temperatures involves metallic Pd. These results highlight the utility of the multi‐technique operando approach for establishing structure–activity relationships of technical catalysts.

Engineering gold-platinum core-shell nanoparticles by self-limitation in solution

Core-shell particles with thin noble metal shells represent an attractive material class with potential for various applications ranging from catalysis to biomedical and pharmaceutical applications to optical crystals. The synthesis of well-defined core-shell architectures remains, however, highly challenging. Here, we demonstrate that atomically-thin and homogeneous platinum shells can be grown via a colloidal synthesis method on a variety of gold nanostructures ranging from spherical nanoparticles to nanorods and nanocubes. The synthesis is based on the exchange of low binding citrate ligands on gold, the reduction of platinum and the subsequent kinetically hindered growth by carbon monoxide as strong binding ligand. The prerequisites for homogeneous growth are low core-binding ligands with moderate fast ligand exchange in solution, a mild reducing agent to mitigate homonucleation and a strong affinity of a second ligand system that can bind to the shell's surface. The simplicity of the described synthetic route can potentially be adapted to various other material libraries to obtain atomically smooth core-shell systems.

Coadsorption Interfered CO Oxidation over Atomically Dispersed Au on h-BN

Similar to the metal centers in biocatalysis and homogeneous catalysis, the metal species in single atom catalysts (SACs) are charged, atomically dispersed and stabilized by support and substrate. The reaction condition dependent catalytic performance of SACs has long been realized, but seldom investigated before. We investigated CO oxidation pathways over SACs in reaction conditions using atomically dispersed Au on h-BN (AuBN) as a model with extensive first-principles-based calculations. We demonstrated that the adsorption of reactants, namely CO, O2 and CO2, and their coadsorption with reaction species on AuBN would be condition dependent, leading to various reaction species with different reactivity and impact the CO conversion. Specifically, the revised Langmuir-Hinshelwood pathway with the CO-mediated activation of O2 and dissociation of cyclic peroxide intermediate followed by the Eley-Rideal type reduction is dominant at high temperatures, while the coadsorbed CO-mediated dissociation of peroxide intermediate becomes plausible at low temperatures and high CO partial pressures. Carbonate species would also form in existence of CO2, react with coadsorbed CO and benefit the conversion. The findings highlight the origin of the condition-dependent CO oxidation performance of SACs in detailed conditions and may help to rationalize the current understanding of the superior catalytic performance of SACs.

Towards the rational design of Pt-based alloy catalysts for the low-temperature water-gas shift reaction: from extended surfaces to single atom alloys

The rational design of Pt-based catalysts for the low-temperature water-gas-shift (LT-WGS) reaction is an active research field because of its important role played in the fuel cell-based hydrogen economy, especially in mobile applications. Previous theoretical analyses have suggested that Pt alloys, leading to a weaker CO binding affinity than the Pt metal, could help alleviate CO poisoning and thus should be promising catalysts of the LT-WGS reaction. However, experimental research along this line was rather ineffective in the past decade. In the present work, we employed the state-of-the-art kinetic Monte Carlo (KMC) simulations to examine the influences of the electronic effect by introducing sub-surface alloys and/or core–shell structures, and the synergetic effect by introducing single atom alloys on the catalytic performance of Pt-alloy catalysts. Our KMC simulations have highlighted the importance of the OH binding affinity on the catalyst surfaces to reduce the barrier of water dissociation as the rate determining step, instead of the CO binding affinity as has been emphasized before in conventional mean-field kinetic models. Along this new direction of catalyst design, we found that Pt–Ru synergetic effects can significantly increase the activity of the Pt metal, leading to Ru_1–3@Pt alloys with a tetrahedron site of one surface-three subsurface Ru atoms on the Pt host, showing a turnover frequency of about five orders of magnitude higher than the Pt metal.

KMC simulations show that decreasing the barrier of H₂O decomposition is more beneficial than decreasing the CO binding affinity in LT-WGS, while the latter was overemphasized by MF-MKM. Here Ru_1–3@Pt alloy is proposed as a promising catalyst.

Nanoengineering of Catalysts for Enhanced Hydrogen Production

Hydrogen (H2) has emerged as a sustainable energy carrier capable of replacing/complementing the global carbon-based energy matrix. Although studies in this area have often focused on the fundamental understanding of catalytic processes and the demonstration of their activities towards different strategies, much effort is still needed to develop high-performance technologies and advanced materials to accomplish widespread utilization. The main goal of this review is to discuss the recent contributions in the H2 production field by employing nanomaterials with well-defined and controllable physicochemical features. Nanoengineering approaches at the sub-nano or atomic scale are especially interesting, as they allow us to unravel how activity varies as a function of these parameters (shape, size, composition, structure, electronic, and support interaction) and obtain insights into structure–performance relationships in the field of H2 production, allowing not only the optimization of performances but also enabling the rational design of nanocatalysts with desired activities and selectivity for H2 production. Herein, we start with a brief description of preparing such materials, emphasizing the importance of accomplishing the physicochemical control of nanostructures. The review finally culminates in the leading technologies for H2 production, identifying the promising applications of controlled nanomaterials.

Considering single-atom catalysts as photocatalysts from synthesis to application

With the ever-increased greenhouse effect and energy crisis, developing novel photocatalysts to realize high-efficient solar-driven chemicals/fuel production is of great scientific and practical significance. Recently, single-atom photocatalysts (SAPs) are promising catalysts with maximized metal dispersion and tuneable coordination environments. SAPs exhibit boosted photocatalytic performance by enhancing optical response, facilitating charge carrier transfer behaviors or directly manipulating surface reaction processes. In this regard, this article systematically reviews the state-of-the-art progress in the development and application of SAPs, especially the mechanism and performance of SAPs on various reaction processes. Some future challenges and potential research directions over SAPs are outlined at the final stage.

Optimizing the synergy between alloy and alloy-oxide interface for CO oxidation in bimetallic catalysts

Creating synergetic metal-oxide interfaces is a promising strategy to promote the catalytic performance of heterogeneous catalysts. However, this strategy has been mainly applied to monometallic catalysts, while scarcely applied to alloy catalysts. In this work, we present a comprehensive study on the synergetic alloy-oxide interfaces in the bimetallic Pt-Co/Al₂O₃ catalysts for CO oxidation. A series of Pt₁Co_x/Al₂O₃ catalysts with various Co/Pt molar ratios with x ranging from 0.5 to 3.8 was synthesized via a facile wet-chemistry strategy. Among them, the Pt₁Co_0.5/Al₂O₃ catalyst exhibits the best catalytic performance for CO oxidation, with the lowest CO complete conversion temperature of -10 °C and the highest mass specific rate of 2.61 (mol CO) h^-1 (g Pt)^-1. From in situ X-ray absorption fine structure and diffuse reflectance infrared Fourier-transform spectroscopy studies, the superior catalytic performance of Pt₁Co_0.5/Al₂O₃ originates from the optimal length of the three-dimensional alloy-oxide perimeter sites. We further extended this strategy to other bimetallic systems of Pt-Fe and Pt-Ni, which also show similar structural properties and remarkable promotional effects on the catalytic activity.

Electronic metal-support interaction constructed for preparing sinter-resistant nano-platinum catalyst with redox property

Generally, support materials with particular structural properties could effectively anchor metal nanoparticles and provide lower activation barriers in heterogeneous catalysis. To tailor the structure of stable iron oxide, NiFe₂O₄ of inverse spinel structure was obtained by combining nickel with iron element under an alkaline environment and high-temperature calcination. The p-type conductivity of NiFe₂O₄ provides the possibility of constructing electronic interfacial interaction with Pt nanoparticles by electron transfer. The constructed metal-support interaction could effectively stabilize Pt nanoparticles and be further enhanced during long-term harsh calcination (700 °C for 48 h) even under an O₂ atmosphere. Meanwhile, the abundant structural defects of NiFe₂O₄ are beneficial for constructing low-temperature redox centers with the aid of Pt nanoparticles. Pt/NiFe₂O₄ exhibited not only excellent activity in room-temperature oxidation (CO and HCHO) and reduction reactions (chemo-selective hydrogenation of nitroarenes), but also high stability even after storage for more than 6 months. A self-adjusting mechanism triggered by structural defects is disclosed by in situ characterization and systematic reaction results. This work demonstrates an alternative concept to construct sinter-resistant and highly-effective nano-platinum catalysts robust for oxidation and reduction reactions.

Synergetic Charge Transfer and Spin Selection in CO Oxidation at Neighboring Magnetic Single-Atom Catalyst Sites

Deciphering the precise physical mechanism of interaction between an adsorbed species and a reactive site in heterogeneous catalysis is crucial for predictive design of highly efficient catalysts. Here, using first-principles calculations we identify that the two-dimensional ferromagnetic metal organic framework of Mn₂C₁₈H₁₂ can serve as a highly efficient single-atom catalyst for spin-triplet O₂ activation and CO oxidation. The underlying mechanism is via "concerted charge-spin catalysis", involving a delicate synergetic process of charge transfer, provided by the hosting Mn atom, and spin selection, preserved through active participation of its nearest neighboring Mn atoms for the crucial step of O₂ activation. The synergetic mechanism is further found to be broadly applicable in O₂ adsorption on magnetic X₂C₁₈H₁₂ (X = Mn, Fe, Co, and Ni) with a well-defined linear scaling dependence between the chemical activity and spin excitation energy. The present findings provide new insights into chemical reactions wherein spin selection plays a vital role.

Promotional Effect of Pt-Doping on the Catalytic Performance of Pt−CeO2 Catalyst for CO Oxidation

Growing interest in the development of a hydrogen economy means that CO oxidation is increasingly important for upgrading H2-rich fuel gas streams for fuel cells. CeO2-supported catalysts are the most promising candidates for the catalytic oxidation of CO because of their high activity. In the present work, DFT+U calculations were performed to investigate the stability and CO oxidation reactivity of Ptn (n = 1−4) clusters supported on CeO2(111) (Pt/CeO2) and Pt-doped CeO2(111) (Pt/(Pt−Ce)O2) surfaces. The Pt clusters showed similar nucleation behavior on both CeO2 and (Pt−Ce)O2 surfaces. Further, the formation of oxygen vacancies (Ov) was facilitated because of surface charge depletion caused by the dopant Pt. Our DFT results suggest that the interfacial OV plays an important role in the CO oxidation reaction cycle, and the calculated energy barrier for the CO oxidation reaction on the Pt/(Pt−Ce)O2 surface is approximately 0.43 eV lower than that on the surface of the undoped catalyst, suggesting enhanced CO oxidation reactivity. Therefore, the chemical modification of the CeO2 support via doping is an effective strategy for improving the catalytic performance of Pt/CeO2.

Atomically Dispersed Platinum in Surface and Subsurface Sites on MgO Have Contrasting Catalytic Properties for CO Oxidation

Atomically dispersed metals on metal oxide supports are a rapidly growing class of catalysts. Developing an understanding of where and how the metals are bonded to the supports is challenging because support surfaces are heterogeneous, and most reports lack a detailed consideration of these points. Herein, we report two atomically dispersed CO oxidation catalysts having markedly different metal-support interactions: platinum in the first layer of crystalline MgO powder and platinum in the second layer of this support. Structural models have been determined on the basis of data and computations, including those determined by extended X-ray absorption fine structure and X-ray absorption near edge structure spectroscopies, infrared spectroscopy of adsorbed CO, and scanning transmission electron microscopy. The data demonstrate the transformation of surface to subsurface platinum as the temperature of sample calcination increased. Catalyst performance data demonstrate the lower activity but greater stability of the subsurface platinum than of the surface platinum.

Pt Atom on the Wall of Atomic Layer Deposition (ALD)-Made MoS2 Nanotubes for Efficient Hydrogen Evolution

Single-atom catalysts (SACs) can achieve excellent catalytic efficiency at ultralow catalyst consumptions. Herein, platinum (Pt) atoms are fixed on the wall of atomic layer deposition (ALD)-made molybdenum disulfide nanotube arrays (MoS₂ -NTA) for efficient hydrogen evolution reaction (HER). More concretely, MoS₂ -NTA with different nanotube diameters and wall thicknesses are fabricated by a sacrificial strategy of anodic aluminum oxide (AAO) template via ALD; then Pt atoms are fixed on the wall of Ti₃ C₂ -supported MoS₂ -NTA as a catalytic system. The MoS₂ -NTA/Ti₃ C₂ decorated with 0.13 wt.% of Pt results in a low overpotential of 32 mV to deliver a current density of 10 mA cm^-2 , which is superior to 20 wt.% commercial Pt/C (41 mV). Ordered MoS₂ -NTA instead of 2D MoS₂ prevents Pt atoms from aggregating and then exerts catalytic activities. The density functional theory calculations suggest that the Pt atoms are more likely to occupy the sites on the tubular MoS₂ than the planar MoS₂ , and the Pt atoms accumulated at the Mo site of MoS₂ -NT have a moderate Gibbs free energy (close to zero).

MXene-Supported, Atomic-Layered Iridium Catalysts Created by Nanoparticle Re-Dispersion for Efficient Alkaline Hydrogen Evolution

Tailoring the structure of metal components and interaction with their anchored substrates is essential for improving the catalytic performance of supported metal catalysts; the ideal catalytic configuration, especially down to the range of atomic layers, clusters, and even single atoms, remains a subject under intensive study. Here, an Ir-on-MXene (Mo₂ TiC₂ T_x ) catalyst with controlled morphology changing from nanoparticles down to flattened atomic layers, and finally ultrathin layers and single atoms dispersed on MXene nanosheets at elevated temperature, is presented. The intermediate structure, consisting of mostly Ir atomic layers, shows the highest activity toward the hydrogen evolution reaction (HER) under industry-compatible alkaline conditions. In addition, the better HER activity of Ir atomic layers than that of single atoms suggests that the former serves as the main active sites. Detailed mechanism analysis reveals that the nanoparticle re-dispersion process and Ir atomic layers with a moderate interaction to the substrate associate with unconventional electron transfer from MXene to Ir, leading to suitable H* adsorption. The results indicate that the structural design is important for the development of highly efficient catalysts.

CO oxidation by Pt2/Fe3O4: Metastable dimer and support configurations facilitate lattice oxygen extraction

Heterogeneous catalysts based on subnanometer metal clusters often exhibit strongly size-dependent properties, and the addition or removal of a single atom can make all the difference. Identifying the most active species and deciphering the reaction mechanism is extremely difficult, however, because it is often not clear how the catalyst evolves in operando. Here, we use a combination of atomically resolved scanning probe microscopies, spectroscopic techniques, and density functional theory (DFT)-based calculations to study CO oxidation by a model Pt/Fe3O4(001) "single-atom" catalyst. We demonstrate that (PtCO)2 dimers, formed dynamically through the agglomeration of mobile Pt-carbonyl species, catalyze a reaction involving the oxide support to form CO2. Pt2 dimers produce one CO2 molecule before falling apart into two adatoms, releasing the second CO. Olattice extraction only becomes facile when both the Pt-dimer and the Fe3O4 support can access metastable configurations, suggesting that substantial, concerted rearrangements of both cluster and support must be considered for reactions occurring at elevated temperature.

Enhanced catalytic performance of atomically dispersed Pd on Pr-doped CeO2 nanorod in CO oxidation

Single-atom noble metal catalysts have been widely studied for catalytic oxidation of CO. Regulating the coordination environment of single metal atom site is an effective strategy to improve the intrinsic catalytic activity of single atom catalyst. In this work, single atom Pd catalyst supported on Pr-doped CeO₂ nanorods was prepared, and the performance and nature of Pr-coordinated atomic Pd site in CO catalytic oxidation are systematically investigated. The structure characterization using AC-HAADF-STEM, EXAFS, XRD and Raman spectroscopy demonstrate the formation of single atom Pd site and abundant surface oxygen vacancies on the surface of Pr-doped CeO₂ nanorod. With the combination of the XPS characterization and DFT calculations, the oxidation state of Pd on Pr-doped CeO₂ nanorod is determined lower than that on CeO₂ nanorod. The turnover frequency of CO oxidation is markedly increased from 8.4 × 10^-3 to 31.9 × 10^-3 s with Pr-doping at 130 ºC and GHSV of 70,000 h^-1. Combined with kinetic studies, DRIFT and DFT calculations, the doped-Pr atoms reduced the formation energy of oxygen vacancies and generate more oxygen vacancies around the atomically dispersed Pd sites on the surface of cerium oxide, which reduces the dissociation energy of oxygen, thereby accelerating the reaction rate of CO oxidation.

Probing active sites for carbon oxides hydrogenation on Cu/TiO2 using infrared spectroscopy

The valorization of carbon oxides on metal/metal oxide catalysts has been extensively investigated because of its ecological and economical relevance. However, the ambiguity surrounding the active sites in such catalysts hampers their rational development. Here, in situ infrared spectroscopy in combination with isotope labeling revealed that CO molecules adsorbed on Ti3+ and Cu+ interfacial sites in Cu/TiO2 gave two disparate carbonyl peaks. Monitoring each of these peaks under various conditions enabled tracking the adsorption of CO, CO2, H2, and H2O molecules on the surface. At room temperature, CO was initially adsorbed on the oxygen vacancies to produce a high frequency CO peak, Ti3+-CO. Competitive adsorption of water molecules on the oxygen vacancies eventually promoted CO migration to copper sites to produce a low-frequency CO peak. In comparison, the presence of gaseous CO2 inhibits such migration by competitive adsorption on the copper sites. At temperatures necessary to drive CO2 and CO hydrogenation reactions, oxygen vacancies can still bind CO molecules, and H2 spilled-over from copper also competed for adsorption on such sites. Our spectroscopic observations demonstrate the existence of bifunctional active sites in which the metal sites catalyze CO2 dissociation whereas oxygen vacancies bind and activate CO molecules.

Interspersing CeOx Clusters to the Pt-TiO2 Interfaces for Catalytic Promotion of TiO2-Supported Pt Nanoparticles

We propose an interface-engineered oxide-supported Pt nanoparticle-based catalyst with improved low-temperature activity toward CO oxidation. By wet-impregnating 1 wt % Ce on TiO₂, we synthesized hybrid oxide support of CeO_x-TiO₂, in which dense CeO_x clusters formed on the surface of TiO₂. Then, the Pt/CeO_x-TiO₂ catalyst was synthesized by impregnating 2 wt % Pt on the CeO_x-TiO₂ supporting oxide. Pt-CeO_x-TiO₂ triphase interfaces were eventually formed upon impregnation of Pt on CeO_x-TiO₂. The Pt-CeO_x-TiO₂ interfaces open up the interface-mediated Mars-van Krevelen CO oxidation pathway, thus providing additional interfacial reaction sites for CO oxidation. Consequently, the specific reaction rate of Pt/CeO_x-TiO₂ for CO oxidation was increased by 3.2 times compared with that of Pt/TiO₂ at 140 °C. Our results demonstrate a widely applicable and straightforward method of catalytic activation of the interfaces between metal nanoparticles and supporting oxides, which enabled fine-tuning of the catalytic performance of oxide-supported metal nanoparticle classes of heterogeneous catalysts.