Ultra-small Mo-Pt subnanoparticles enable CO2 hydrogenation at room temperature and atmospheric pressure.

We present a partially-oxidised bimetallic Mo-Pt subnanoparticle (Mo4Pt8Ox) enabling thermally-driven CO2 hydrogenation to CO at room temperature and atmospheric pressure. A mechanistic study explained the full catalytic cycle of the reaction from CO2 activation to catalyst reactivation. DFT calculations revealed that alloying with Mo lowers the activation barrier by weakening the CO adsorption. This finding could be a first step for low-energy CO2 conversion.

Synthesis and magnetic properties of sub-nanosized iron carbides on a carbon support.

Iron carbide clusters with near-sub-nanometer size have been synthesized by employing a tetraphenylmethane-cored phenylazomethine dendrimer generation 4 (TPM-DPAG4) as a molecular template. Magnetic measurements reveal that these iron carbide clusters exhibit a magnetization-field hysteresis loop at 300 K. The data indicate that these iron carbide clusters are ferromagnets at room temperature.

Rhodium-Iodide Complex on a Catalytically Active SiO2 Surface for One-Pot Hydrosilylation-CO2 Cycloaddition.

In this study, a novel Rh-iodide complex was synthesized through a surface reaction between an immobilized Rh cyclooctadiene complex and alkylammonium iodide (N+ I- ) on SiO2 . In the presence of ammonium cations, the SiO2 -supported Rh-iodide complex could be effectively used for the one-pot synthesis of various silylcarbonate derivatives starting from epoxy olefins, hydrosilanes, and CO2 . The maximum turnover numbers (TONs) for the hydrosilylation reaction and the CO2 cycloaddition were 7600 (Rh) and 130 (N+ I- ), respectively. The catalyst exhibited much higher performance for hydrosilylation than solely the Rh complex on SiO2 . The mechanism of the Rh-catalyzed hydrosilylation reaction and the local structure of Rh, which is affected by the co-immobilized N+ I- , were investigated by using Rh and I K-edge XAFS and XPS. Analysis of the XAFS profiles indicated the presence of a Rh-I bond. The Rh unit was in its electron-rich state. Curve-fitting analysis of the Rh K-edge EXAFS profiles suggests dissociation of the cycloocta-1,5-diene (COD) ligand from the Rh center. Results from spectroscopic and kinetic analyses revealed that the high activity of the catalyst (during hydrosilylation) could be attributed to a decrease in steric hindrance and the electron-rich state of the Rh. The decrease in the steric hindrance could be attributed to the absence of COD, and the electron-rich state promoted the oxidative addition of Si-H. To the best of our knowledge, this is the first example of a one-pot silylcarbonate synthesis as well as a determination of a novel surface Rh-iodide complex and its catalysis.

Dehydrogenative Coupling of Alkanes and Benzene Enhanced by Slurry-Phase Interparticle Hydrogen Transfer.

The dehydrogenative coupling reaction of alkanes and benzene has attracted attention as a method of direct conversion of alkanes to raw materials for useful chemical products. Here, we report the first combined catalyst system composed of hydrotalcite-supported palladium and solid acid aluminum-exchanged montmorillonite for the direct alkylation of benzene promoted by slurry-phase interparticle hydrogen transfer at 150 °C. The combination of the two catalytic particles showed excellent activity and achieved the maximum benzene conversion of 21% and target product selectivity of 84% in the reaction of n-heptane and benzene. Our results, thus, provide a feasible strategy to design efficient liquid-phase reaction systems employing simple physical mixing of two catalytic particles.

Low-Temperature H2 Reduction of Copper Oxide Subnanoparticles.

Invited for the cover of this issue is Kimihisa Yamamoto and co-workers at Tokyo Institute of Technology and International Christian University. The image depicts enhanced reactivity of the copper oxide subnanoparticles under low-temperature conditions. Read the full text of the article at 10.1002/chem.202100508.

Low-Temperature H2 Reduction of Copper Oxide Subnanoparticles.

Subnanoparticles (SNPs) with sizes of approximately 1 nm are attractive for enhancing the catalytic performance of transition metals and their oxides. Such SNPs are of particular interest as redox-active catalysts in selective oxidation reactions. However, the electronic states and oxophilicity of copper oxide SNPs are still a subject of debate in terms of their redox properties during oxidation reactions for hydrocarbons. In this work, in situ X-ray absorption fine structure (XAFS) measurements of Cu28 Ox SNPs, which were prepared by using a dendritic phenylazomethine template, during temperature-programmed reduction (TPR) with H2 achieved lowering of the temperature (T50 =138 °C) reported thus far for the CuII →CuI reduction reaction because of Cu-O bond elongation in the ultrasmall copper oxide particles.

Metallic Tungsten Nanoparticles That Exhibit an Electronic State Like Carbides during the Carbothermal Reduction of WCl6 by Hydrogen.

Carbothermal hydrogen reduction (CHR) is a unique dry chemical process used to fabricate metals and carbides on carbon supports. In this study, a stepwise CHR of WCl6 on a graphite support is demonstrated for the first time. Powder X-ray diffraction studies revealed that, at 773 K, metallic tungsten nanoparticles are produced, whereas, at 1073 K, the metastable W2C phase is generated rather than the thermodynamically stable WC phase. X-ray photoelectron spectroscopy and X-ray absorption near edge structure studies showed that the chemical state of the W nanoparticles simultaneously exhibits metallic W(∼0) and carbide W(δ+) character. The obtained results suggest that, although electronic interactions exist between the metallic W atoms and the graphite support, the body-centered cubic structure of the metallic tungsten is maintained, confirmed by the extended X-ray absorption fine structure. In addition, high-resolution scanning transmission electron microscopy observations revealed that the W nanoparticles exhibit a thin flattened shape on the support. These results support the notion that the mechanism for the formation of the W nanoparticles during the CHR is influenced by the electronic interactions between the W nanoparticles and the graphite support. Our work thus suggests that the combination of early-transition-metal atoms and carbon-based supports would afford modulatable electronic systems though the electronic interactions.

Subnano-transformation of molybdenum carbide to oxycarbide.

Ultrasmall particles exhibit structures and/or properties that are different from those of the corresponding bulk materials; in this context especially ultrasmall precious-metal particles have been extensively investigated. In this study, we targeted the transition base-metal Mo and succeeded in systematically producing Mo oxycarbide/carbide particles with diameters of 1.7 ± 0.7, 1.4 ± 0.5, 1.3 ± 0.4, 1.2 ± 0.3, 1.0 ± 0.3, and 0.8 ± 0.2 nm on a carbon support using the carbothermal hydrogen reduction method at 773 K and a diphenylazomethine-type dendrimer as a template. The formation and properties of the particles were confirmed using X-ray photoelectron spectroscopy, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, and X-ray absorption fine structure (XAFS) studies. We found that Mo particles with a diameter of 1.3 nm or greater formed carbides such as ß'-Mo2C, whereas smaller particles formed oxycarbides, indicating a size-dependent transformation in the phase or composition of the particles. Thus, this work demonstrated a new concept, subnano-transformation, which would be a new class of phase transformation based on the concept of the size dependence in such an ultrasmall scale. In addition, the movement of Mo atoms within a cluster and on the fringes of a nanoparticle was also demonstrated during continuous time-course high-resolution HAADF-STEM observation.

Probing the temperature of supported platinum nanoparticles under microwave irradiation by in situ and operando XAFS.

Microwave irradiation can cause high local temperatures at supported metal nanoparticles, which can enhance reaction rates. Here we discuss the temperature of platinum nanoparticles on γ-Al2O3 and SiO2 supports under microwave irradiation using the Debye-Waller factor obtained from in situ extended X-ray absorption fine structure (EXAFS) measurements. Microwave irradiation exhibits considerably smaller Deby-Waller factors than conventional heating, indicating the high local temperature at the nanoparticles. The difference in the average temperatures between the platinum nanoparticles and the bulk under microwaves reaches 26 K and 132 K for Pt/Al2O3 and Pt/SiO2, respectively. As a result, Pt/SiO2 exhibits considerably more reaction acceleration for the catalytic dehydrogenation of 2-propanol under microwave irradiation than Pt/Al2O3. We also find microwaves enhance the reduction of PtOx nanoparticles by using operando X-ray absorption near edge structure (XANES) spectroscopy. The present results indicate that significant local heating of platinum nanoparticles by microwaves is effective for the acceleration of catalytic reactions.

A useful preparation of ultrasmall iron oxide particles by using arc plasma deposition.

Ultrasmall particles, different from the larger size nanoparticles, have recently attracted significant attention in the scientific community in nanotechnology for catalytic, electronic and optical applications; however, their magnetic properties remain unexplored due to the difficult structural analysis. A challenging issue is to develop a preparation method for iron oxide particles (IOPs) with fine size control, and to determine the dependence of magnetic properties on the morphology and crystallinity of the magnetic particles. However, synthetic approaches to obtain IOPs, regarded as one of the new fields of magnetic nanoparticles, have been significantly limited. This article reported a developed synthetic method to prepare IOPs on carbon supports using pulsed arc plasma deposition (APD) in flowing oxygen gas, which clarified the finely-controlled formation of IOPs on graphene nanosheets. Structural characterization of the IOPs revealed the formation of crystalline γ-Fe2O3 ultrasmall particles with oxygen deficiency. The pulsed APD method for IOPs is the first simple and convenient technique to not only prevent significant aggregation and contamination by organic compounds and avoid the need for thermal pretreatment, but also provide uniform crystalline nano-order particles.

Multifunctional Catalytic Surface Design for Concerted Acceleration of One-Pot Hydrosilylation-CO2 Cycloaddition.

Silica-supported Rh-ammonium iodide catalyst showed high performance for hydrosilylation-CO2 cycloaddition reaction sequences. The catalyst was prepared by surface grafting of Rh and the silane-coupling reaction of the ammonium iodide moiety. The acceleration of each catalytic reaction was realized due to the concerted catalysis between Rh species, immobilized organic functions, and surface Si-OH groups. As a result, good to excellent yields of silyl carbonates were obtained from epoxyolefins, hydrosilanes, and CO2 under mild reaction conditions.

Platinum clusters with precise numbers of atoms for preparative-scale catalysis.

Subnanometer noble metal clusters have enormous potential, mainly for catalytic applications. Because a difference of only one atom may cause significant changes in their reactivity, a preparation method with atomic-level precision is essential. Although such a precision with enough scalability has been achieved by gas-phase synthesis, large-scale preparation is still at the frontier, hampering practical applications. We now show the atom-precise and fully scalable synthesis of platinum clusters on a milligram scale from tiara-like platinum complexes with various ring numbers (n = 5-13). Low-temperature calcination of the complexes on a carbon support under hydrogen stream affords monodispersed platinum clusters, whose atomicity is equivalent to that of the precursor complex. One of the clusters (Pt10) exhibits high catalytic activity in the hydrogenation of styrene compared to that of the other clusters. This method opens an avenue for the application of these clusters to preparative-scale catalysis.The catalytic activity of a noble metal nanocluster is tied to its atomicity. Here, the authors report an atom-precise, fully scalable synthesis of platinum clusters from molecular ring precursors, and show that a variation of only one atom can dramatically change a cluster's reactivity.

Finely controlled multimetallic nanocluster catalysts for solvent-free aerobic oxidation of hydrocarbons.

The catalytic activity of alloy nanoparticles depends on the particle size and composition ratio of different metals. Alloy nanoparticles composed of Pd, Pt, and Au are widely used as catalysts for oxidation reactions. The catalytic activities of Pt and Au nanoparticles in oxidation reactions are known to increase as the particle size decreases and to increase on the metal-metal interface of alloy nanoparticles. Therefore, multimetallic nanoclusters (MNCs) around 1 nm in diameter have potential as catalysts for oxidation reactions. However, there have been few reports describing the preparation of uniform alloy nanoclusters. We report the synthesis of finely controlled MNCs (around 1 nm) using a macromolecular template with coordination sites arranged in a gradient of basicity. We reveal that Cu-Pt-Au MNCs supported on graphitized mesoporous carbon show catalytic activity that is 24 times greater than that of a commercially available Pt catalyst for aerobic oxidation of hydrocarbons. In addition, solvent-free aerobic oxidation of hydrocarbons to ketones at room temperature, using small amounts of a radical initiator, was achieved as a heterogeneous catalytic reaction for the first time.

Finding the Most Catalytically Active Platinum Clusters With Low Atomicity.

On a subnanometer scale, an only one-atom difference in a metal cluster may cause significant transitions in the catalytic activity due to the electronic and geometric configurations. We now report the atomicity-specific catalytic activity of platinum clusters with significantly small atomicity, especially below 20. The atomic coordination structure is completely different from that of the larger face-centered cubic (fcc) nanocrystals. Here, an electrochemical study on such small clusters, in which the atomicity ranged between 12 and 20, revealed Pt19 as the most catalytically active species. In combination with a theoretical study, a common structure that leads to a high catalytic performance is proposed.

A solid chelating ligand: periodic mesoporous organosilica containing 2,2'-bipyridine within the pore walls.

Synthesis of a solid chelating ligand for the formation of efficient heterogeneous catalysts is highly desired in the fields of organic transformation and solar energy conversion. Here, we report the surfactant-directed self-assembly of a novel periodic mesoporous organosilica (PMO) containing 2,2'-bipyridine (bpy) ligands within the framework (BPy-PMO) from a newly synthesized organosilane precursor [(i-PrO)3Si-C10H6N2-Si(Oi-Pr)3] without addition of any other silane precursors. BPy-PMO had a unique pore-wall structure in which bipyridine groups were densely and regularly packed and exposed on the surface. The high coordination ability to metals was also preserved. Various bipyridine-based metal complexes were prepared using BPy-PMO as a solid chelating ligand such as Ru(bpy)2(BPy-PMO), Ir(ppy)2(BPy-PMO) (ppy = 2-phenylpyridine), Ir(cod)(OMe)(BPy-PMO) (cod = 1,5-cyclooctadiene), Re(CO)3Cl(BPy-PMO), and Pd(OAc)2(BPy-PMO). BPy-PMO showed excellent ligand properties for heterogeneous Ir-catalyzed direct C-H borylation of arenes, resulting in superior activity, durability, and recyclability to the homogeneous analogous Ir catalyst. An efficient photocatalytic hydrogen evolution system was also constructed by integration of a Ru-complex as a photosensitizer and platinum as a catalyst on the pore surface of BPy-PMO without any electron relay molecules. These results demonstrate the great potential of BPy-PMO as a solid chelating ligand and a useful integration platform for construction of efficient molecular-based heterogeneous catalysis systems.

Preparation and structure of a single Au atom on the TiO2(110) surface: control of the Au-metal oxide surface interaction.

Three-dimensional Au structures on bare and organic-compound-modified TiO2(110) surfaces were interrogated by Au L3-edge polarization dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) spectroscopy. On the bare TiO2(110) surface, icosahedral Au55 nanoclusters were the main product found. When the surfaces were modified with ortho or meso mercaptobenzoic acid (o-MBA or m-MBA), Au was atomically dispersed. Sulfur atoms in the o- and m- MBA formed strong covalent bonds with Au to produce stable Au-MBA (o- and m- forms) surface complexes. On the other hand, only oxygen atoms on the surface did not make a strong enough interaction to stabilize the Au species. We discuss how the Au species formed on the modified TiO2(110) surface and the possibility to control the Au structure by the surface modification method.

Magic number Pt13 and misshapen Pt12 clusters: which one is the better catalyst?

A relationship between the size of metal particles and their catalytic activity has been established over a nanometer scale (2-10 nm). However, application on a subnanometer scale (0.5-2 nm) is difficult, a possible reason being that the activity no longer relies on the size but rather the geometric structure as a cluster (or superatomic) compound. We now report that the catalytic activity for the oxygen reduction reaction (ORR) significantly increased when only one atom was removed from a magic number cluster composed of 13-platinum atoms (Pt13). The synthesis with an atomic-level precision was successfully achieved by using a dendrimer ligand as the macromolecular template strictly defining the number of metal atoms. It was quite surprising that the Pt12 cluster exhibited more than 2-fold catalytic activity compared with that of the Pt13 cluster. ESI-TOF-mass and EXAFS analyses provided information about the structures. These analyses suggested that the Pt12 has a deformed coordination, while the Pt13 has a well-known icosahedral atomic coordination as part of the stable cluster series. Theoretical analyses based on density functional theory (DFT) also supported this idea. The present results suggest potential activity of the metastable clusters although they have been "missing" species in conventional statistical synthesis.

Fine tuning and orientation control of surface Cu complexes on TiO2(110) premodified with mercapto compounds: the effect of different mercapto group positions.

Three-dimensional structures of vacuum-deposited Cu species formed on TiO2(110) surfaces premodified with three mercaptobenzoic acid (MBA) isomers were studied using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). We explored the possibility of fine tuning and orientation control of the surface Cu structures, including their coordination and configuration against the surface, according to the different mercapto group positions of the three MBA isomers (o-, m-, and p-MBA). Almost linear S-Cu-O (lattice O of TiO2) surface compounds were formed on the three MBA-modified TiO2(110) surfaces; however, the orientation of the Cu species on the o- and m-MBA-modified TiO2(110) surfaces (40-45° inclined from the surface normal) was different from that on the p-MBA-modified TiO2(110) surface (60° from the surface normal). This work suggests that the selection of a different MBA isomer for premodification of a single crystal TiO2(110) surface enables fine tuning and orientation control of surface Cu complexes.

Molecular catalysts confined on and within molecular layers formed on a si(111) surface with direct si-C bonds.

Two examples of confined molecular catalysts are presented. PtCl(4) (2-) complexes are attached to a thiol-terminated monolayer by ligand exchange of Cl(-) with a thiolate group and incorporated in a multilayer of viologen moieties by ion exchange. All Cl(-) ligands are replaced by OH(-) or H(2) O before HER takes place. Ex situ and in situ XAFS measurements confirm that the Pt complexes accelerate HER without being converted into Pt particles.

Size-specific catalytic activity of platinum clusters enhances oxygen reduction reactions.

Colloidal platinum nanoparticles with diameters of 2-5 nm on carbon supports are currently regarded as the best catalysts for the oxygen reduction reaction. However, the particle size is limited by the conventional preparation methods that are used to synthesize small platinum particles; the inherent activity of ultrasmall nanoparticles has not yet been revealed. We present a practical synthesis for ultrafine subnanometre platinum clusters using a spherical macromolecular template with no disorder in molecular weight or structure. The template, a phenylazomethine dendrimer, offers control of the number of metal complexes in an assembly through stepwise complexation, allowing the complexes to accumulate in discrete nano-cages. Subsequent reduction of Pt(IV) chloride to Pt(0) results in the formation of platinum clusters composed of a defined number of atoms. As a result of exceptionally small particle size, the clusters exhibit very high catalytic activity for the four-electron reduction of oxygen molecules.