Sub-nanometer Clusters Research Articles

Application of a parallel genetic algorithm to the global optimization of medium-sized Au\u2013Pd sub-nanometre clusters

To contribute to the discussion of the high activity and reactivity of Au–Pd system, we have adopted the BPGA-DFT approach to study the structural and energetic properties of medium-sized Au–Pd sub-nanometre clusters with 11–18 atoms. We have examined the structural behaviour and stability as a function of cluster size and composition. The study suggests 2D–3D crossover points for pure Au clusters at 14 and 16 atoms, whereas pure Pd clusters are all found to be 3D. For Au–Pd nanoalloys, the role of cluster size and the influence of doping were found to be extensive and non-monotonic in altering cluster structures. Various stability criteria (e.g. binding energies, second differences in energy, and mixing energies) are used to evaluate the energetics, structures, and tendency of segregation in sub-nanometre Au–Pd clusters. HOMO–LUMO gaps were calculated to give additional information on cluster stability and a systematic homotop search was used to evaluate the energies of the generated global minima of mono-substituted clusters and the preferred doping sites, as well as confirming the validity of the BPGA-DFT approach.

Supercritical diethylamine facilitated loading of ultrafine Ru particles on few-layer graphene for solvent-free hydrogenation of levulinic acid to γ-valerolactone

We demonstrate a facile and versatile method to grow Ru particles uniformly on pristine few-layer graphene (FLG) in supercritical diethylamine. In particular, a large number of Ru subnanometer clusters less than 1.0 nm were observed. The particle size can be tuned by manipulating the loading content of Ru and controlling the reaction temperature. The resulting Ru/FLG showed remarkably high activity, selectivity, and reusability towards the hydrogenation of levulinic acid to γ-valerolactone. This method is flexible, and can be extended to the synthesis of a variety of other ultrafine metal particles supported on FLG.

Sub-nanometer Co3O4 clusters anchored on TiO2(B) nano-sheets: Pt replaceable Co-catalysts for H2 evolution.

Cobalt-based heterogeneous cocatalysts are important substitutions of noble metal cocatalysts in many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the atoms located at surface active-sites participate in the chemical reaction. Thus, cocatalysts with small cluster dispersions are highly desirable to maximize the amount of active-sites and enhance the per atom efficiency. Here, we report the synthesis of sub-nanometer Co3O4 clusters which are anchored to 2D ultrathin TiO2(B) nanosheets, as a cocatalyst for H2 evolution reaction (HER). It was found that the conduction type of Co3O4 clusters turns from P-type to N-type, and the heterojunction band structure between TiO2(B) and Co3O4 clusters changes from type II to type I, when the cluster size is reduced from nanometer scale to the sub-nanometer scale. With a suitable energy band matching between TiO2(B) and sub-nanometer Co3O4 clusters, the electrons generated in TiO2(B) during the photocatalytic process reduce the Co ions into metallic Co atoms, which produce excellent photocatalytic stability and extremely high HER efficiency comparable to that of the noble Pt cocatalyst.

Hydrogen evolution reaction (HER) on Au@Ag ultrananoclusters as electro-catalysts.

We propose and investigate computationally Ag-Au subnanometer clusters as catalysts for the hydrogen evolution reaction (HER). Focusing on Ag12Au, we conduct a complete first-principles study of the HER process on this Au@Ag ultrananocatalyst. After determining the hydrogen-saturated resting state under standard conditions as Ag12AuH11, HER reaction energies and barriers are predicted also including solvent effects using both implicit and explicit models. We find that Ag12Au is a good candidate as a HER catalyst, with good stability and an overall reaction energy barrier of 0.89 eV as an upper bound. We also draw indications for the design of HER subnanometer catalysts.

Bimetallic Ag-Pt Sub-nanometer Supported Clusters as Highly Efficient and Robust Oxidation Catalysts.

A combined experimental and theoretical investigation of Ag-Pt sub-nanometer clusters as heterogeneous catalysts in the CO→CO2 reaction (COox) is presented. Ag9 Pt2 and Ag9 Pt3 clusters are size-selected in the gas phase, deposited on an ultrathin amorphous alumina support, and tested as catalysts experimentally under realistic conditions and by first-principles simulations at realistic coverage. In situ GISAXS/TPRx demonstrates that the clusters do not sinter or deactivate even after prolonged exposure to reactants at high temperature, and present comparable, extremely high COox catalytic efficiency. Such high activity and stability are ascribed to a synergic role of Ag and Pt in ultranano-aggregates, in which Pt anchors the clusters to the support and binds and activates two CO molecules, while Ag binds and activates O2 , and Ag/Pt surface proximity disfavors poisoning by CO or oxidized species.

Surface modification of TiO2 with copper clusters for band gap narrowing

Surface modification of photocatalytic materials to give better activity and potentially extending the response into the visible spectrum, is an area of active research. In this work, DFT modelling suggests that surface modification of rutile and anatase TiO2 with partially oxidised copper clusters can induce a red shift in the photo-action spectrum. Copper clusters were synthesised and characterised separately before TiO2 nanoparticle surface modification. Characterisation of copper clusters and photocatalysts modified with copper clusters showed that ex-situ synthesis can control the size of surface clusters. Sub-nanometre clusters of copper maintained their size and morphology upon attachment to the photocatalyst surface. The copper clusters were determined to be a mixture of Cu(0) and Cu(I), and no significant change in the oxidation state was observed following surface modification or following photoelectrochemical measurements. Experimental measurements including UV–Vis spectroscopy and valence band XPS showed a small red shift the band gap correlating to the DFT predictions. Photoelectrochemical characterisation showed an enhancement in the UV photocurrent response and a small red shift in the effective band gap for the surface modified TiO2.

Support effects and reaction mechanism of acetylene trimerization over silica-supported Cu4 clusters: A DFT study

Oxide-supported Cu nanoparticles and clusters catalyze a variety of important reactions, such as CO/CO2 hydrogenation to methanol. Recent studies demonstrate that also sub-nanometer clusters consisting of only a few atoms can actively catalyze chemical reactions. In this study, we investigate the interaction between Cu4 clusters and silica-surfaces, considering the de-hydroxylated and the fully hydroxylated α-quartz surfaces. We also considered various dopants such as Ti- and Nb-ions substitutional to Si, respectively, in order to see if an electronic change of the support has an effect on the reaction of the supported cluster. We find that hydroxyl groups can enhance the adsorption energy of the cluster, whereas the dopants have only little effects on the adsorption mode of the Cu cluster. On the fully hydroxylated surface, the cluster may react with the hydroxyl groups via reverse hydrogen spillover. Finally, we explore the reactivity of the silica-supported Cu4 cluster in terms of acetylene trimerization, for which extended Cu surfaces have shown catalytic activity. We find that this reaction should occur with activation barriers below 0.8 eV; Nb-doping of the support does not seem to produce any direct effect on the reactivity of the Cu tetramer.

Empirical-potential global minima and DFT local minima of trimetallic AglAumPtn (l + m + n = 13, 19, 33, 38) clusters

Using a basin-hopping global optimization method, an exhaustive global search of the potential energy surface (PES) of trimetallic Ag-Au-Pt clusters is performed at the empirical potential (Gupta) level. The clusters contain a total of N=13, 19, 33, 38 atoms while the Ag, Au, Pt content range from 0% to 100%. A broad region of the PES of these systems is sampled, finding a menagerie of geometries, which are catalogued into structural families according to geometrical considerations. The structural families so defined contain a fixed number of Pt atoms while the loading of Ag and Au atoms varies, and indicate that selective Pt doping can stabilize Ag-Au subnanometer clusters. Structural stability trends are identified thus obtaining the optimal composition for each cluster size. Segregation is analyzed through a chemical order parameter, and found to exhibit a size-dependent behavior, with a transition from mixed to segregated structures clearly evidenced at 38-atom cluster size. Optimal composition structures of the Gupta global minima and a few isomers are subjected to DFT reoptimization, which is found to predict qualitatively similar atomic arrangements with a certain degree of deformation, especially for 38-atom clusters, and to introduce changes in the energy ordering among structures with respect to the Gupta potential predictions, although the stabilizing effect of Pt and a cross-over character of 38-atom size seem confirmed.

Subnanometric Hybrid Pd-M(OH)2, M = Ni, Co, Clusters in Zeolites as Highly Efficient Nanocatalysts for Hydrogen Generation

Summary Hybrid multi-metallic nanocatalysts have attracted increasing attention because of the synergistic effect of metal species and considerably improved catalytic performance, but they often suffer from severe sintering and poor stability. Here, we show a facile strategy for preparing subnanometric hybrid bimetallic clusters Pd-M(OH) 2 (M = Ni, Co) within silicalite-1 (S-1) zeolite via a hydrothermal synthesis method. The hybrid bimetallic nanocatalysts exhibit excellent shape-selective catalytic performance and superior thermal stability. The incorporation of secondary Ni(OH) 2 species in S-1 can considerably increase the catalytic activity of the Pd nanoclusters for the dehydrogenation of formic acid (FA) as a result of the electron-enriched Pd surface and bimetallic interfacial effect. Notably, the 0.8Pd0.2Ni(OH) 2 @S-1 catalyst affords the highest initial turnover frequency value, up to 5,803 hr −1 toward complete FA decomposition without any additives at 60°C. The superior catalytic properties and excellent stability of the subnanometric hybrid bimetallic clusters confined in zeolites create new prospects for their practical high-performance catalytic application.

Synthesis and Demonstration of Subnanometric Iridium Oxide as Highly Efficient and Robust Water Oxidation Catalyst

Development of a highly efficient and robust water oxidation catalyst (WOC) with reduced usage of noble metals is extremely crucial for water splitting and CO2 reduction by photocatalysis or electrolysis. Herein, we synthesized subnanometric iridium dioxide clusters supported on multiwalled carbon nanotubes (MWCNTs) by a chemical vapor deposition method (nominated as IrO2/CNT). Benefiting from a mild oxidation process in air at 303 K, the deposited iridium clusters can be controlled to have a narrow size distribution from several atoms to 2 nm, having an average size of ca. 1.1 nm. The subnanometric iridium-containing sample is demonstrated to be highly efficient and robust for water oxidation. The optimal turnover frequency (TOF) of chemical water oxidation on the as-obtained sample can reach 11.2 s–1, and the overpotential of electrochemical water oxidation is 249, and 293 mV at 10 mA cm–2 in 1.0 M KOH (pH: 13.6), and 0.5 M H2SO4 (pH: 0), respectively. On the basis of the structural characterizations an...

More active Ir subnanometer clusters than single‐atoms for catalytic oxidation of CO at low temperature

This work reported the adsorption and reaction performance of FeOx supported subnanometer cluster and single‐atom Ir catalysts for the oxidation of CO at low temperature. By varying the pretreatment temperature and Ir loading, the single‐atom and subnanometer cluster Ir catalysts were obtained. The Ir subnanometer clusters exhibited higher activity for the oxidation of CO with or without the presence of H2 than the single‐atom counterpart. By using adsorption microcalorimetry and in situ infrared spectroscopy measurements, it was found that the Ir subnanometer clusters not only promoted the adsorption and reaction of CO and O2 but also facilitated the formation of OH species from reaction between H2 and O2, thus opening a new reaction pathway between CO and OH species to produce CO2 compared with that between CO and O species on the single‐atom counterpart. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4003–4012, 2017

Electrochemical Activation of Single-Walled Carbon Nanotubes with Pseudo-Atomic-Scale Platinum for the Hydrogen Evolution Reaction

The development of effective and inexpensive hydrogen evolution reaction (HER) electrocatalysts for future renewable energy systems is highly desired. Platinum-based materials are the most active electrocatalysts for catalyzing HER, but reducing the use of Pt is required because of the high price and scarcity of Pt. Here, we achieve pseudo-atomic-scale dispersion of Pt, i.e. individual atoms or subnanometer clusters, on the sidewalls of single-walled carbon nanotubes (SWNTs) with a simple and readily upscalable electroplating deposition method. These SWNTs activated with an ultralow amount of Pt exhibit activity similar to that of commercial Pt/C with a notably higher (∼66–333-fold) Pt loading for catalyzing the HER under the acidic conditions required in proton exchange membrane technology. These catalysts resemble pseudo-atomic-scale Pt systems which are mainly composed of a few to tens of Pt atoms dispersed on the sidewalls of the SWNTs. The Pt loading is only 0.19–0.75 atom % at the electrode surface,...

Ethylene Hydrogenation over Pt/TiO2: A Charge-Sensitive Reaction

Controlled charge transfer between a support and small metal particles provides unique opportunities to tune the activity of supported metal catalysts, as first proposed by Schwab [G. M. Schwab et al., Angew. Chem. 1959, 71, 101–104]. By controlling the thickness of polycrystalline anatase TiO2 films, the TiO2 carrier concentration can be manipulated by an order of magnitude. When 1 nm Pt particles are deposited on these TiO2 films, the variation in the charge transfer between the TiO2 support and the Pt particles is found to dramatically increase the ethylene hydrogenation activity. The sensitivity of ethylene hydrogenation to charge transfer was anticipated from the large effect of the Pt charge on the ethylene and ethylidyne adsorption energy, e.g., compared to CO and H. Our results demonstrate that the controllable Schwab effect provides a powerful tool to tune catalytic activity. An even larger effect can be expected for supported sub-nanometer clusters, and for the selectivity of hydrogenation reactions.

Coupling Sub-Nanometric Copper Clusters with Quasi-Amorphous Cobalt Sulfide Yields Efficient and Robust Electrocatalysts for Water Splitting Reaction.

Superefficient water-splitting materials comprising sub-nanometric copper clusters and quasi-amorphous cobalt sulfide supported on copper foam are reported. While working together at both the anode and cathode sides of an alkaline electrolyzer, this material gives a catalytic output of overall water splitting comparable with the Pt/C-IrO2 -coupled electrolyzer.

Crystal Plane Effect of Ceria on Supported Copper Oxide Cluster Catalyst for CO Oxidation: Importance of Metal–Support Interaction

Copper–ceria as one of the very active catalysts for oxidation reactions has been widely investigated in heterogeneous catalysis. In this work, copper oxide (1 wt % Cu loading) deposited on both ceria nanospheres with a {111}/{100}-terminated surface (1CuCe-NS) and with nanorod exposed {110}/{100} faces (1CuCe-NR) have been prepared for the investigation of crystal plane effects on CO oxidation. Various structural characterizations, especially including aberration-corrected scanning transmission electron microscopy (Cs-STEM), X-ray absorption fine structure (XAFS) technique, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), were used to precisely determine the structure and status of the catalysts. It is found that the copper oxides were formed as subnanometer clusters and were uniformly dispersed on the surface of the ceria support. The results from XAFS combined with the temperature-programmed reduction technique (H2-TPR) reveal that more reducible CuOx clusters w...

Sub-nanometre metal clusters for catalytic carbon-carbon and carbon-heteroatom cross-coupling reactions.

Catalytic cross-coupling reactions are fundamental transformations in modern organic synthesis. Traditionally based on single-atom transition metal complex catalysts, the use of sub-nanometre metal clusters with enhanced redox and coordinating properties may lead to more efficient catalysts. Recent developments and potential new directions of catalytic sub-nanometre metal clusters for cross-coupling reactions are briefly discussed here.

The wet synthesis and quantification of ligand-free sub-nanometric Au clusters in solid matrices

The synthesis of ligand-free sub-nanometric metal clusters on a large scale suffers typically from very low yields (<5% yield) and needs very high dilutions. Here we show that Au clusters can be prepared with ethylene-vinyl alcohol copolymers (EVOH), charcoal, and different metal oxides (CeO2, Al2O3, TiO2 and ZnO) in >15% yields, as unambiguously determined using a very simple and extremely sensitive analytical reaction test.

Addressing the characterisation challenge to understand catalysis in MOFs: the case of nanoscale Cu supported in NU-1000.

We explore the dynamic structure and reactivity of Cu species supported on NU-1000. By combining pair distribution function (PDF) analysis and difference envelope density (DED) analysis of in situ synchrotron-based X-ray scattering data, we simultaneously probe the local structure of supported Cu-species, their distribution within NU-1000 and distortions of the NU-1000 lattice under conditions relevant to catalysis and catalyst activation. These analyses show that atomic layer deposition (ALD) of Cu in NU-1000 (Cu-AIM) leads to the formation of Cu-oxo clusters within the small pores that connect the triangular and hexagonal channels. Exposure of Cu-AIM to a reducing atmosphere at 200 °C produces metallic Cu0 of two distinct particle sizes: ∼4 nm nanoparticles and small sub-nanometer clusters. The size of these nanoparticles appears to be constrained by NU-1000 pore dimensions, with evidence of the sub-nanometer clusters being bound within the triangular channels flanked by pyrene rings. This supported Cu0-NU-1000 system is catalytically active for gas-phase ethylene hydrogenation. Exposure of the catalyst to oxidative atmosphere re-oxidises the Cu species to a Cu2O cuprite phase. The dynamic restructuring of the system in different chemical environments underscores the importance of probing these systems in situ.

Towards highly active Pd/CeO2 for alkene hydrogenation by tuning Pd dispersion and surface properties of the catalysts.

Extensive applications of noble metals as heterogeneous catalysts are limited by their global reserve scarcity and exorbitant price. Identifying the intrinsic active nature of a catalyst benefits the designing of catalysts with trace amounts of noble metals; these catalysts display better or comparable overall catalytic efficiency than their heavily loaded counterparts. Herein, systematic studies on Pd dispersion and surface properties of a series of Pd/CeO2 catalysts for styrene hydrogenation showed that high Pd dispersion and surface abundant defects of the catalysts are essential to realize superior activity. Highly dispersed subnanometric Pd clusters on porous nanorods of ceria with a large surface Ce3+ fraction of 27.4%, high Pd dispersion of 73.6%, and low Pd loading of 0.081 wt% delivered a very large turnover frequency of 103 233 h-1 based on each exposed Pd atom for styrene hydrogenation at 1.0 MPa H2 and 30 °C. Experimental data, kinetic analysis, and density functional theory calculations revealed that the highly dispersed Pd shows a low affinity for styrene and provides more exposed Pd sites for hydrogen activation. The surface abundant defects (oxygen vacancy) of Pd/CeO2 catalysts can enrich the electron density of Pd, improve its capability for H2 dissociation and lower the affinity of styrene for Pd.

Synthesis of a ceria-supported iron–ruthenium oxide catalyst and its structural transformation from subnanometer clusters to single atoms during the Fischer–Tropsch synthesis reaction

The transformation from subnanometer clusters to single atoms during Fischer–Tropsch synthesis was revealed by X-ray absorption fine structure analysis of a ceria-supported iron–ruthenium oxide catalyst.