The Structure of Sub-nm Platinum Clusters at Elevated Temperatures

Little is known about metallic clusters consisting merely of a dozen of atoms or even less, despite of their importance in catalysis and crystal nucleation. Scanning transmission electron microscopy (STEM) provides direct atomic structure information but has inherently suffered from limited time resolution. We employ fast dynamic STEM combined with a spatio-temporal image denoising algorithm to explore the structure and stability of Pt clusters on carbon, which represents a highly relevant catalysis system. At room temperature, dynamic amorphous 2D structures are found, while above ≈300 °C, the clusters transform into a crystalline state. Our experimental and theoretical data reveal an unexpected strong trend of the crystalline clusters to exhibit the face-centered cubic, bulk structure of Pt with cuboidal geometries being most prominent.     

Triosmium clusters on a support: determination of structure by X-ray absorption spectroscopy and high-resolution microscopy

The structures of small, robust metal clusters on a solid support were determined by a combination of spectroscopic and microscopic methods: extended X‐ray absorption fine structure (EXAFS) spectroscopy, scanning transmission electron microscopy (STEM), and aberration‐corrected STEM. The samples were synthesized from [Os₃(CO)₁₂] on MgO powder to provide supported clusters intended to be triosmium. The results demonstrate that the supported clusters are robust in the absence of oxidants. Conventional high‐angle annular dark‐field (HAADF) STEM images demonstrate a high degree of uniformity of the clusters, with root‐mean‐square (rms) radii of 2.03±0.06 Å. The EXAFS Os? Os coordination number of 2.1±0.4 confirms the presence of triosmium clusters on average and correspondingly determines an average rms cluster radius of 2.02±0.04 Å. The high‐resolution STEM images show the individual Os atoms in the clusters, confirming the triangular structures of their frames and determining Os? Os distances of 2.80±0.14 Å, matching the EXAFS value of 2.89±0.06 Å. IR and EXAFS spectra demonstrate the presence of CO ligands on the clusters. This set of techniques is recommended as optimal for detailed and reliable structural characterization of supported clusters.     

Heterobimetallic Catalysis: Platinum‐Gold‐Catalyzed Tandem Cyclization/C−X Coupling Reaction of (Hetero)Arylallenes with Nucleophiles

Heterobimetallic catalysis offers new opportunities for reactivity and selectivity but still presents challenges, and only a few metal combinations have been explored so far. Reported here is a Pt‐Au heterobimetallic catalyst system for the synthesis of a family of multi‐heteroaromatic structures through tandem cyclization/C?X coupling reaction. Au‐catalyzed 6‐endo‐cyclization takes place as the first fast step. Pt‐Au clusters are proposed to be responsible for the increased reactivity in the second step, that is, the intermolecular nucleophilic addition which occurs through an outer‐sphere mechanism by hybrid homogeneous‐heterogeneous catalysis.     

Al2O3 Nanosheets Rich in Pentacoordinate Al3+ Ions Stabilize Pt‐Sn Clusters for Propane Dehydrogenation

In heterogeneous catalysis, supports play a crucial role in modulating the geometric and electronic structure of the active metal phase for optimizing the catalytic performance. A γ‐Al₂O₃ nanosheet that contains 27 % pentacoordinate Al³⁺ sites can nicely disperse and stabilize raft‐like Pt‐Sn clusters as a result of strong interactions between metal and support. Consequently, there are strong electronic interactions between the Pt and Sn atoms, resulting in an increase in the electron density of the Pt sites. When used in the propane dehydrogenation reaction, this catalyst displayed an excellent specific activity for propylene formation with >99 % selectivity, and superior anti‐coking and anti‐sintering properties. Its exceptional ability to maintain the high activity and stability at ultrahigh space velocities further showed that the sheet construction of the catalyst facilitated the kinetic transfer process.     

Chemical Dealloying Mechanism of Bimetallic Pt–Co Nanoparticles and Enhancement of Catalytic Activity toward Oxygen Reduction

The chemical dealloying mechanism of bimetallic Pt–Co nanoparticles (NPs) and enhancement of their electrocatalytic activity towards the oxygen reduction reaction (ORR) have been investigated on a fundamental level by the combination of X‐ray absorption spectroscopy (XAS) and aberration‐corrected scanning transmission electron microscopy (STEM). Structural parameters, such as coordination numbers, alloy extent, and the unfilled d states of Pt atoms, are derived from the XAS spectra, together with the compositional variation analyzed by line‐scanning energy‐dispersive X‐ray spectroscopy (EDX) on an atomic scale, to gain new insights into the dealloying process of bimetallic Pt–Co NPs. The XAS results on acid‐treated Pt–Co/C NPs reveal that the Co–Co bonding in the bimetallic NPs dissolves first and the remaining morphology gradually transforms to a Pt‐skin structure. From cyclic voltammetry and mass activity measurements, Pt–Co alloy NPs with a Pt‐skin structure significantly enhance the catalytic performance towards the ORR. Further, it is observed that such an imperfect Pt‐skin surface feature will collapse due to the penetration of electrolyte into layers underneath and cause further dissolution of Co and the loss of Pt. The electrocatalytic activity decreases accordingly, if the dealloying process lasts for 4 h. The findings not only demonstrate the importance of appropriate treatment of bimetallic catalysts, but also can be referred to other Pt bimetallic alloys with transition metals.     

In Situ Single‐Nanoparticle Spectroscopy Study of Bimetallic Nanostructure Formation

Bimetallic nanostructures (NSs), with utility in catalysis, are typically prepared using galvanic exchange (GE), but the final catalyst morphology is dictated by the dynamics of the process. In situ single nanoparticle (NP) optical scattering spectroscopy, coupled with ex situ electron microscopy, is used to capture the dynamic structural evolution of a bimetallic NS formed in a GE reaction between Ag and [PtCl₆]^2?. We identify an early stage involving anisotropic oxidation of Ag to AgCl concomitant with reductive deposition of small Pt clusters on the NS surface. At later stages of GE, unreacted Ag inclusions phase segregate from the overcoated AgCl as a result of lattice strain between Ag and AgCl. The nature of the structural evolution elucidates why multi‐domain Ag/AgCl/Pt NSs result from the GE process. The complex structural dynamics, determined from single‐NP trajectories, would be masked in ensemble studies due to heterogeneity in the response of different NPs.     

Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction

Advances in chemical syntheses have led to the formation of various kinds of nanoparticles (NPs) with more rational control of size, shape, composition, structure and catalysis. This review highlights recent efforts in the development of Pt and non‐Pt based NPs into advanced nanocatalysts for efficient oxygen reduction reaction (ORR) under fuel‐cell reaction conditions. It first outlines the shape controlled synthesis of Pt NPs and their shape‐dependent ORR. Then it summarizes the studies of alloy and core–shell NPs with controlled electronic (alloying) and strain (geometric) effects for tuning ORR catalysis. It further provides a brief overview of ORR catalytic enhancement with Pt‐based NPs supported on graphene and coated with an ionic liquid. The review finally introduces some non‐Pt NPs as a new generation of catalysts for ORR. The reported new syntheses with NP parameter‐tuning capability should pave the way for future development of highly efficient catalysts for applications in fuel cells, metal‐air batteries, and even in other important chemical reactions.     

In‐Situ and Ex‐Situ Microscopic Study of Gas Phase Propylene Polymerization over a High Activity TiCl4‐MgCl2 Heterogeneous Ziegler‐Natta Catalyst

In‐situ gas phase poly(propylene) (PP) formation over a high activity TiCl₄‐MgCl₂‐supported Ziegler‐Natta catalyst has been studied by video microscopy combined with ex‐situ light microscopy, SEM, high‐resolution TEM, and STEM/PEELS/EDX for the first time. In‐situ observation revealed rapid formation of poly(propylene) beads 9–10 μm in size (< 1/30 s) as well as growth of significant amounts of polymer within local regions. Catalyst particles containing 2–5 nm‐sized MgCl₂ crystalline domains are subjected to transformations during catalysis that form PP/catalyst aggregated structures of 30–50 μm in size.     

Direct visualisation, by aberration-corrected electron microscopy, of the crystallisation of bimetallic nanoparticle catalysts

Using a scanning transmission electron microscope, corrected for aberration, ultra-high resolution images of the internal structure of nanoparticle clusters, as well as the number of atoms present per cluster, may be directly determined: it is shown that individual bimetallic clusters of Ru10Pt2 have a "molecular" structure and that when they coalesce into larger entities containing ca. 200 atoms they adopt the regular crystalline, and facetted, state of a bulk metal.     

Mechanical Alloying of Metal–Organic Frameworks

The solvent‐free mechanical milling process for two distinct metal–organic framework (MOF) crystals induced the formation of a solid solution, which is not feasible by conventional solution‐based syntheses. X‐ray and STEM‐EDX studies revealed that performing mechanical milling under an Ar atmosphere promotes the high diffusivity of each metal ion in an amorphous solid matrix; the amorphous state turns into the porous crystalline structure by vapor exposure treatment to form a new phase of a MOF solid solution.     

Water as green oxidant: a highly selective conversion of organosilanes to silanols with water

Selective hydrolytic oxidation of organohydrosilanes was achieved with water in the presence of Pt‐nanoparticle catalyst. The selectivity of the process was established by NMR analysis. In addition, various Pt‐based catalysts were screened to compare the activity and selectivity with Pt‐nanoparticles catalysis. The method was equally applicable to hydrosilanes bearing unsaturated functional groups, which led to corresponding silanols under mild reaction conditions without formation of any side products. Pt‐nanoparticle catalysis was studied in details using UV–vis, TEM and mercury poisoning experiments during the transformation. Copyright © 2009 John Wiley & Sons, Ltd.     

Growth of PtRu Clusters on Ru(0001)‐Supported Monolayer Graphene Films

We report on results of a detailed scanning tunnelling microscopy study on the formation, size and size distribution, and internal structure of small bimetallic PtRu clusters on a graphene monolayer film supported on a Ru(0001) substrate. These clusters, with sizes around ～15 (Ru) or ～40 (Pt) atoms per cluster at the lowest coverage, are interesting model systems for the catalytic behaviour of small metal PtRu particles, for example for application in electrocatalytic oxidation reactions. The clusters were generated by sequential deposition of the two metals at room temperature. The data reveal a distinct influence of the deposition sequence on the cluster formation process, with Ru pre‐deposition followed by Pt deposition leading to predominantly bimetallic clusters, possibly with a core–shell‐type structure, while the reverse sequence results in co‐existent mono‐ and bimetallic clusters, where the latter are likely to intermix at the interface. The observations are related to the nucleation process of the respective metals on the templated surface, and the 2D growth behaviour of the two metals.     

Nafion聚离子修饰纳米Pt的合成及其影响因素

采用化学还原的方法成功地合成了Nafion聚离子修饰的纳米Pt颗粒，平均粒径为～4nm；由于表面缺电子的特性，纳米Pt粒子与Nafion高分子长链上的-SO3^-基团有较强的结合，使粒子随-SO3^的存在状态而分散，在水溶液中呈现与离子团簇相似的分散状态。FTIR显示Pt颗粒存在表面原子暴露在修饰离子之外，因此，这种粒子在具有功能结构的催化领域有着很好的应用前景；合成过程中反应速率随Nafion含量的降低、pH的增加而增大。     

Maximum Noble‐Metal Efficiency in Catalytic Materials: Atomically Dispersed Surface Platinum

Platinum is the most versatile element in catalysis, but it is rare and its high price limits large‐scale applications, for example in fuel‐cell technology. Still, conventional catalysts use only a small fraction of the Pt content, that is, those atoms located at the catalyst’s surface. To maximize the noble‐metal efficiency, the precious metal should be atomically dispersed and exclusively located within the outermost surface layer of the material. Such atomically dispersed Pt surface species can indeed be prepared with exceptionally high stability. Using DFT calculations we identify a specific structural element, a ceria “nanopocket”, which binds Pt²⁺ so strongly that it withstands sintering and bulk diffusion. On model catalysts we experimentally confirm the theoretically predicted stability, and on real Pt‐CeO₂ nanocomposites showing high Pt efficiency in fuel‐cell catalysis we also identify these anchoring sites.     

Distinct Short‐Range Order Is Inherent to Small Amorphous Calcium Carbonate Clusters (<2 nm)

Amorphous intermediate phases are vital precursors in the crystallization of many biogenic minerals. While inherent short‐range orders have been found in amorphous calcium carbonates (ACCs) relating to different crystalline forms, it has never been clarified experimentally whether such orders already exist in very small clusters less than 2 nm in size. Here, we studied the stability and structure of 10,12‐pentacosadiynoic acid (PCDA) protected ACC clusters with a core size of ca. 1.4 nm consisting of only seven CaCO₃ units. Ligand concentration and structure are shown to be key factors in stabilizing the ACC clusters. More importantly, even in such small CaCO₃ entities, a proto‐calcite short‐range order can be identified but with a relatively high degree of disorder that arises from the very small size of the CaCO₃ core. Our findings support the notion of a structural link between prenucleation clusters, amorphous intermediates, and final crystalline polymorphs, which appears central to the understanding of polymorph selection.     

Effects of adsorbates on supported platinum and iridium clusters: Characterization in reactive atmospheres and during alkene hydrogenation catalysis by X-ray absorption spectroscopy

MgO-, SiO2-, and gamma-Al2O3-supported platinum clusters and particles (with average diameters ranging from 11 to 45 A) and zeolite-supported Ir4 clusters (approximately 6 A in diameter) were characterized by extended X-ray absorption fine structure spectroscopy in the presence of H2, O2, ethene, propene, and ethane, as well as under conditions of alkene hydrogenation catalysis. The results indicate that under various atmospheres, the presence of adsorbates affects the smaller platinum clusters (11 A) on gamma-Al2O3 more substantially than the larger platinum particles (i.e., those greater than approximately 21 A in average diameter) on MgO or SiO2. When Pt/gamma-Al2O3 was exposed to H2, the platinum morphology did not change, although the Pt-Pt bond distance increased. In contrast, when the same sample was exposed to O2, complete oxidative fragmentation took place. This processes was reversed following subsequent treatment with H2. Exposure to alkenes changed both the morphology and electron density (as indicated by X-ray absorption near-edge spectra) of the gamma-Al2O3-supported platinum clusters. Under conditions of alkene hydrogenation catalysis at room temperature, the electronic properties and the structure of the platinum clusters were found to depend on the reactant composition and the nature of molecules involved in the reaction process. The effects of the reactant gases on the smaller iridium clusters (Ir4) were substantially less pronounced, apparently as a consequence of the extremely small number of atoms in each iridium cluster.     

Chasing changing nanoparticles with time-resolved pair distribution function methods

When materials are reduced to the nanoscale, their structure and reactivity can deviate greatly from the bulk or extended surface case. Using the archetypal example of supported Pt nanoparticles (ca. 2 nm diameter, 1 wt % Pt on Al(2)O(3)) catalyzing CO oxidation to CO(2) during cyclic redox operation, we show that high energy X-ray total scattering, used with subsecond time resolution, can yield detailed, valuable insights into the dynamic behavior of nanoscale systems. This approach reveals how these nanoparticles respond to their environment and the nature of active sites being formed and consumed within the catalytic process. Specific insight is gained into the structure of the highly active Pt surface oxide that formed on the nanoparticles during catalysis.     

Highly Site Selective Formal [5+2] and [4+2] Annulations of Isoxazoles with Heterosubstituted Alkynes by Platinum Catalysis: Rapid Access to Functionalized 1,3‐Oxazepines and 2,5‐Dihydropyridines

Platinum‐catalyzed formal [5+2] and [4+2] annulations of isoxazoles with heterosubstituted alkynes enabled the atom‐economical synthesis of valuable 1,3‐oxazepines and 2,5‐dihydropyridines, respectively. Importantly, this Pt catalysis not only led to unique reactivity dramatically divergent from that observed under Au catalysis, but also proceeded via unprecedented α‐imino platinum carbene intermediates.     

Thermally Stable and Regenerable Platinum–Tin Clusters for Propane Dehydrogenation Prepared by Atom Trapping on Ceria

Ceria (CeO₂) supports are unique in their ability to trap ionic platinum (Pt), providing exceptional stability for isolated single atoms of Pt. The reactivity and stability of single‐atom Pt species was explored for the industrially important light alkane dehydrogenation reaction. The single‐atom Pt/CeO₂ catalysts are stable during propane dehydrogenation, but are not selective for propylene. DFT calculations show strong adsorption of the olefin produced, leading to further unwanted reactions. In contrast, when tin (Sn) is added to CeO₂, the single‐atom Pt catalyst undergoes an activation phase where it transforms into Pt–Sn clusters under reaction conditions. Formation of small Pt–Sn clusters allows the catalyst to achieve high selectivity towards propylene because of facile desorption of the product. The CeO₂‐supported Pt–Sn clusters are very stable, even during extended reaction at 680 °C. Coke formation is almost completely suppressed by adding water vapor to the feed. Furthermore, upon oxidation the Pt–Sn clusters readily revert to the atomically dispersed species on CeO₂, making Pt–Sn/CeO₂ a fully regenerable catalyst.     

Designed Precursor for the Controlled Synthesis of Highly Active Atomic and Sub‐nanometric Platinum Catalysts on Mesoporous Silica

The development of new methods to synthesize nanometric metal catalysts has always been an important and prerequisite step in advanced catalysis. Herein, we design a stable nitrogen ligated Pt complex for the straightforward synthesis by carbonization of uniformly sized atomic and sub‐nanometric Pt catalysts supported on mesoporous silica. During the carbonization of the Pt precursor into active Pt species, the nitrogen‐containing ligand directed the decomposition in a controlled fashion to maintain uniform sizes of the Pt species. The nitrogen ligand had a key role to stabilize the single Pt atoms on a weak anchoring support like silica. The Pt catalysts exhibited remarkable activities in the hydrogenation of common organic functional groups with turnover frequencies higher than in previous studies. By a simple post‐synthetic treatment, we could selectively remove the Pt nanoparticles to obtain a mixture of single atoms and nanoclusters, extending the applicability of the present method.