Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles.

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

Single-Atom Ce-Modified α-Fe2O3 for Selective Catalytic Reduction of NO with NH3.

A single-atom Ce-modified α-Fe2O3 catalyst (Fe0.93Ce0.07Ox catalyst with 7% atomic percentage of Ce) was synthesized by a citric acid-assisted sol-gel method, which exhibited excellent performance for selective catalytic reduction of NOx with NH3 (NH3-SCR) over a wide operating temperature window. Remarkably, it maintained ∼93% NO conversion efficiency for 168 h in the presence of 200 ppm SO2 and 5 vol % H2O at 250 °C. The structural characterizations suggested that the introduction of Ce leads to the generation of local Fe-O-Ce sites in the FeOx matrix. Furthermore, it is critical to maintain the atomic dispersion of the Ce species to maximize the amounts of Fe-O-Ce sites in the Ce-doped FeOx catalyst. The formation of CeO2 nanoparticles due to a high doping amount of Ce species leads to a decline in catalytic performance, indicating a size-dependent catalytic behavior. Density functional theory (DFT) calculation results indicate that the formation of oxygen vacancies in the Fe-O-Ce sites is more favorable than that in the Fe-O-Fe sites in the Ce-free α-Fe2O3 catalyst. The Fe-O-Ce sites can promote the oxidation of NO to NO2 on the Fe0.93Ce0.07Ox catalyst and further facilitate the reduction of NOx by NH3. In addition, the decomposition of NH4HSO4 can occur at lower temperatures on the Fe0.93Ce0.07Ox catalyst containing atomically dispersed Ce species than on the α-Fe2O3 reference catalyst, resulting in the good SO2/H2O resistance ability in the NH3-SCR reaction.

Type of mandibular asymmetry affects changes and outcomes of bimaxillary surgery for class III asymmetry.

OBJECTIVES: Various methods have been used to classify class III asymmetry. There is little information on the use of an asymmetry index to examine soft tissue changes and outcomes for patients with class III asymmetry. This study aimed to (1) evaluate soft tissue changes and outcomes for three types of mandibular asymmetry and (2) determine if measures are associated with type of asymmetry. MATERIALS AND METHODS: Adults who consecutively underwent bimaxillary surgery using surgery-first approach for correction of class III asymmetry were divided into three groups based on type of mandibular asymmetry. This previously reported classification system is simple and mutually independent, categorizing mandibular asymmetry according to the amount and direction of ramus asymmetry relative to menton deviation: patients with a larger transverse ramus distance on the menton deviation side were divided into group 1 and group 2; group 1 (n = 45) exhibited a menton deviation larger than ramus discrepancy; group 2 (n = 11) exhibited a menton deviation less than ramus discrepancy; group 3 (n = 22) had larger transverse ramus distance contralateral to the side of the menton deviation. Soft tissue facial asymmetry indices, calculated from cone beam computed tomography images, assessed midline and contour asymmetry presurgery, changes postsurgery, and outcomes. RESULTS: Compared with groups 1 and 2, the presurgery index for contour and midline asymmetry was smallest for group 3. All the three groups had significant improvement in midline asymmetry postsurgery, and outcome measures were good; there were no differences between groups. However, contour asymmetry only improved significantly for groups 1 and 2. The outcome for contour asymmetry was fair for groups 1 and 3 and poor for group 2. CONCLUSIONS: Bimaxillary surgery significantly improved facial midline asymmetry. The type of mandibular asymmetry was associated with postsurgical changes and outcomes for contour asymmetry. CLINICAL RELEVANCE: Understanding the types of mandibular asymmetry could help clinicians to develop treatment plans and predict treatment changes and outcomes.

Regioselective generation and reactivity control of subnanometric platinum clusters in zeolites for high-temperature catalysis.

Subnanometric metal species (single atoms and clusters) have been demonstrated to be unique compared with their nanoparticulate counterparts. However, the poor stabilization of subnanometric metal species towards sintering at high temperature (>500 °C) under oxidative or reductive reaction conditions limits their catalytic application. Zeolites can serve as an ideal support to stabilize subnanometric metal catalysts, but it is challenging to localize subnanometric metal species on specific sites and modulate their reactivity. We have achieved a very high preference for localization of highly stable subnanometric Pt and PtSn clusters in the sinusoidal channels of purely siliceous MFI zeolite, as revealed by atomically resolved electron microscopy combining high-angle annular dark-field and integrated differential phase contrast imaging techniques. These catalysts show very high stability, selectivity and activity for the industrially important dehydrogenation of propane to form propylene. This stabilization strategy could be extended to other crystalline porous materials.

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal-support interaction, and metal-reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results obtained from different systems and try to give a picture on how different types of metal species work in different reactions and give perspectives on the future directions toward better understanding of the catalytic behavior of different metal entities (single atoms, nanoclusters, and nanoparticles) in a unifying manner.

Regioselective Generation of Single-Site Iridium Atoms and Their Evolution into Stabilized Subnanometric Iridium Clusters in MWW Zeolite.

Preparation of supported metal catalysts with uniform particle size and coordination environment is a challenging and important topic in materials chemistry and catalysis. In this work, we report the regioselective generation of single-site Ir atoms and their evolution into stabilized subnanometric Ir clusters in MWW zeolite, which are located at the 10MR window connecting the two neighboring 12MR supercages. The size of the subnanometric Ir clusters can be controlled by the post-synthesis treatments and maintain below 1 nm even after being reduced at 650 °C, which cannot be readily achieved with samples prepared by conventional impregnation methods. The high structure sensitivity, size-dependence, of catalytic performance in the alkane hydrogenolysis reaction of Ir clusters in the subnanometric regime is evidenced.

Hydrothermal Synthesis of Ruthenium Nanoparticles with a Metallic Core and a Ruthenium Carbide Shell for Low-Temperature Activation of CO2 to Methane.

Ruthenium nanoparticles with a core-shell structure formed by a core of metallic ruthenium and a shell of ruthenium carbide have been synthesized by a mild and easy hydrothermal treatment. The dual structure and composition of the nanoparticles have been determined by synchrotron X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) analysis, and transmission electron microscopy (TEM) imaging. According to depth profile synchrotron XPS and X-ray diffraction (XRD) analysis, metallic ruthenium species predominate in the inner layers of the material, ruthenium carbide species being located on the upper surface layers. The ruthenium carbon catalysts presented herein are able to activate both CO2 and H2, exhibiting exceptional high activity for CO2 hydrogenation at low temperatures (160-200 °C) with 100% selectivity to methane, surpassing by far the most active Ru catalysts reported up to now. On the basis of catalytic studies and isotopic 13CO/12CO2/H2 experiments, the active sites responsible for this unprecedented activity can be associated with surface ruthenium carbide (RuC) species, which enable CO2 activation and transformation to methane via a direct CO2 hydrogenation mechanism. Both the high activity and the absence of CO in the gas effluent confer relevance to these catalysts for the Sabatier reaction, a chemical process with renewed interest for storing surplus renewable energy in the form of methane.

Base-Controlled Heck, Suzuki, and Sonogashira Reactions Catalyzed by Ligand-Free Platinum or Palladium Single Atom and Sub-Nanometer Clusters.

The assumption that oxidative addition is the key step during the cross-coupling reaction of aryl halides has led to the development of a plethora of increasingly complex metal catalysts, thereby obviating in many cases the exact influence of the base, which is a simple, inexpensive, and necessary reagent for this paramount transformation. Here, a combined experimental and computational study shows that the oxidative addition is not the single kinetically relevant step in different cross-coupling reactions catalyzed by sub-nanometer Pt or Pd species, since the reactivity control is shifted toward subtle changes in the base. The exposed metal atoms in the cluster cooperate to enable an extremely easy oxidative addition of the aryl halide, even chlorides, and allow the base to bifurcate the coupling. With sub-nanometer Pd species, amines drive to the Heck reaction, carbonate drives to the Sonogahira reaction, and phosphate drives to the Suzuki reaction, while for Pt clusters and single atoms, good conversion is only achieved using acetate as a base. This base-controlled orthogonal reactivity with ligand-free catalysts opens new avenues in the design of cross-coupling reactions in organic synthesis.

Corrigendum: Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D.

This corrects the article DOI: 10.1038/nmat4757.

Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D.

Single metal atoms and metal clusters have attracted much attention thanks to their advantageous capabilities as heterogeneous catalysts. However, the generation of stable single atoms and clusters on a solid support is still challenging. Herein, we report a new strategy for the generation of single Pt atoms and Pt clusters with exceptionally high thermal stability, formed within purely siliceous MCM-22 during the growth of a two-dimensional zeolite into three dimensions. These subnanometric Pt species are stabilized by MCM-22, even after treatment in air up to 540 °C. Furthermore, these stable Pt species confined within internal framework cavities show size-selective catalysis for the hydrogenation of alkenes. High-temperature oxidation-reduction treatments result in the growth of encapsulated Pt species to small nanoparticles in the approximate size range of 1 to 2 nm. The stability and catalytic activity of encapsulated Pt species is also reflected in the dehydrogenation of propane to propylene.

Stabilized naked sub-nanometric Cu clusters within a polymeric film catalyze C-N, C-C, C-O, C-S, and C-P bond-forming reactions.

Sub-nanometric Cu clusters formed by endogenous reduction of Cu salts and Cu nanoparticles are active and selective catalysts for C-N, C-C, C-O, C-S, and C-P bond-forming reactions. Sub-nanometric Cu clusters have also been generated within a polymeric film and stored with full stability for months. In this way, they are ready to be used on demand and maintain high activity (TONs up to 10(4)) and selectivity for the above reactions. A potential mechanism for the formation of the sub-nanometric clusters and their electronic nature is presented.

Crystal-plane-dependent metal-support interaction in Au/TiO2.

Metal-support interactions between Au and TiO2 are studied based on Au/TiO2 catalysts with different TiO2 crystal planes exposed. With ex situ XPS, TEM and in situ DRIFTS, we have investigated the crystal-plane-dependent metal-support interaction effects on the physiochemical properties of Au/TiO2 catalysts. Based on the structural characterization and spectroscopic results, we can observe chemical oscillations (including the electronic structures of Au nanoparticles and the interaction between Au/TiO2 catalysts and CO molecules) during alternate H2 and O2 pre-treatments. Their variation tendencies of oscillations are greatly dependent on the crystal planes of TiO2 and the pre-treatment temperature. Furthermore, their surface and electronic changes after H2 and O2 pre-treatments can be well correlated with their catalytic activities in CO oxidation. Electron-transfer processes across the Au-TiO2 interface are proved to be the origin accounting for their changes after H2 and O2 pre-treatments. The different electronic structures of different TiO2 crystal planes should have relationships with the crystal-plane-dependent metal-support interaction effects in Au/TiO2.

Complete photocatalytic reduction of CO2 to methane by H2 under solar light irradiation.

Nickel supported on silica-alumina is an efficient and reusable photocatalyst for the reduction of CO2 to methane by H2, reaching selectivity above 95% at CO2 conversion over 90%. Although NiO behaves similarly, it undergoes a gradual deactivation upon reuse. About 26% of the photocatalytic activity of Ni/silica-alumina under solar light derives from the visible light photoresponse.

Reactant-Induced Structural Evolution of Pt Catalysts Confined in Zeolite.

Reactant-induced structural evolutions of heterogeneous metal catalysts are frequently observed in numerous catalytic systems, which can be associated with the formation or deactivation of active sites. In this work, we will show the structural transformation of subnanometer Pt clusters in pure-silica MFI zeolite structure in the presence of CO, O2, and/or H2O and the catalytic consequences of the Pt-zeolite materials derived from various treatment conditions. By applying the appropriate pretreatment under a reactant atmosphere, we can precisely modulate the size distribution of Pt species spanning from single Pt atoms to small Pt nanoparticles (1-5 nm) in the zeolite matrix, resulting in the desirably active and stable Pt species for CO oxidation. We also show the incorporation of Fe into the zeolite framework greatly promotes the stability of Pt species against undesired sintering under harsh conditions (up to 650 °C in the presence of CO, O2, and moisture).

Integrated immediate postmortem and acute bereavement care: Competency-based entrustable professional activities for nursing.

BACKGROUND: Integrated immediate postmortem and acute bereavement care alleviates emotional distress due to losing a loved one; however, the provision of effective nursing care remains insufficient. Therefore, preparing nursing students with such skills is essential in end-of-life care education, and entrustable professional activities (EPAs) offer potential to address this gap. OBJECTIVES: To establish EPAs concerning immediate postmortem and acute bereavement care with a seven-category description for EPAs, milestones, and assessment tools. DESIGN: We used a modified Delphi method and four-step consensus-building approach to i) identifying the list of possible EPA items related to immediate postmortem and acute bereavement care based on a literature review and clinical experiences, ii) select an expert panel, iii) pool, review, and revise the EPAs, and iv) validate EPA quality using the Queen's EPA Quality rubric. Data analysis was performed via modes and quartile deviations. RESULTS: The following four major EPA components were identified: i) cultural and religious ritual assessment; ii) death preparation; iii) postmortem care; and iv) acute bereavement care. Three essential competencies were identified as highly correlated: general clinical skills, communication and teamwork capabilities, and caring. Consensus was achieved after three survey rounds. A 100 % questionnaire response rate was obtained. In the third round, all items received 4 or 5 points from >95 % of the panel members and were found to meet the quartile deviation cutoff score of <0.6, indicating that a high consensus level was established. The average Queen's EPA Quality rubric score was 62.5, with an average item score of 4.46, which was higher than the cut-off score of 4.07. Three major parts of EPAs were developed: task descriptions, milestones, and the assessment tool. CONCLUSION: The development of EPAs assessments concerning immediate postmortem and acute bereavement care may guide nursing curricula planning to bridge the gap between competencies and clinical practice.

Generation of Subnanometer Metal Clusters in Silicoaluminate Zeolites as Bifunctional Catalysts.

Zeolite-encapsulated subnanometer metal catalysts are an emerging class of solid catalysts with superior performances in comparison to metal catalysts supported on open-structure solid carriers. Currently, there is no general synthesis methodology for the encapsulation of subnanometer metal catalysts in different zeolite structures. In this work, we will show a general synthesis method for the encapsulation of subnanometer metal clusters (Pt, Pd, and Rh) within various silicoaluminate zeolites with different topologies (MFI, CHA, TON, MOR). The successful generation of subnanometer metal species in silicoaluminate zeolites relies on the introduction of Sn, which can suppress the migration of subnanometer metal species during high-temperature oxidation-reduction treatments according to advanced electron microscopy and spectroscopy characterizations. The advantage of encapsulated subnanometer Pt catalysts in silicoaluminate zeolites is reflected in the direct coupling of ethane and benzene for production of ethylbenzene, in which the Pt and the acid sites work in a synergistic way.

Dominance of Influencing Factors on Cooling Effect of Urban Parks in Different Climatic Regions.

The enhancement of the park cooling effect (PCE) is one method used to alleviate the urban heat island (UHI). The cooling effect is affected by park factors; however, the importance of these factors in the case of the PCE is still unclear. Optimizing or planning urban parks according to the importance of the influencing factors can effectively enhance the PCE. Herein, we selected 502 urban parks in 29 cities in China with three different climatic regions and quantified the PCE based on the park cooling intensity (PCI) and park cooling area (PCA). Subsequently, the relative importance of the influencing factors for the PCE was compared to identify the main factors. Consequently, certain park planning suggestions were proposed to enhance the cooling effect. The results show that: (1) the PCE increased in the order of arid/semi-arid, semi-humid, and humid regions. (2) The main factors of the PCI differed significantly in different climatic regions; however, the waterbody within a park significantly affected the PCI in all three climates. However, for the PCA, park patch characteristics were the dominant factor, contributing approximately 80% in the three climates regions. (3) In arid/semi-arid and semi-humid regions, the optimal area proportion of waterbody and vegetation within the park were approximately 1:2 and 1:1, respectively, and the threshold value of the park area was 16 ha. In contrast, in the humid region, the addition of a waterbody area within the park, to the best extent possible, enhanced the PCI, and the threshold value of the park area was 19 ha. The unique results of this study are expected to function as a guide to future urban park planning on a regional scale to maximize ecological benefits while mitigating the UHI.

Direct assessment of confinement effect in zeolite-encapsulated subnanometric metal species.

Subnanometric metal species confined inside the microporous channels/cavities of zeolites have been demonstrated as stable and efficient catalysts. The confinement interaction between the metal species and zeolite framework has been proposed to play the key role for stabilization, though the confinement interaction is elusive to be identified and measured. By combining theoretical calculations, imaging simulation and experimental measurements based on the scanning transmission electron microscopy-integrated differential phase contrast imaging technique, we have studied the location and coordination environment of isolated iridium atoms and clusters confined in zeolite. The image analysis results indicate that the local strain is intimately related to the strength of metal-zeolite interaction and a good correlation is found between the zeolite deformation energy, the charge state of the iridium species and the local absolute strain. The direct observation of confinement with subnanometric metal species encapsulated in zeolites provides insights to understand their structural features and catalytic consequences.

Structural transformations of solid electrocatalysts and photocatalysts.

Heterogeneous catalysts often undergo structural transformations when they operate under thermal reaction conditions. These transformations are reflected in their evolving catalytic activity, and a fundamental understanding of the changing nature of active sites is vital for the rational design of solid materials for applications. Beyond thermal catalysis, both photocatalysis and electrocatalysis are topical because they can harness renewable energy to drive uphill reactions that afford commodity chemicals and fuels. Although structural transformations of photocatalysts and electrocatalysts have been observed in operando, the resulting implications for catalytic behaviour are not fully understood. In this Review, we summarize and compare the structural evolution of solid thermal catalysts, electrocatalysts and photocatalysts. We suggest that well-established knowledge of thermal catalysis offers a good basis to understand emerging photocatalysis and electrocatalysis research.