Structural variation of size-selected metal clusters in chemical reactions

This thesis is comprised of studies in the characterisation of monolayer-protected and metal cluster of the structural response of size-selected (bare) clusters to chemical reactions. The technique employed is high-angle annular dark field (HAADF) aberration-corrected scanning transmission electron microscopy (AC-STEM). The effect of chemical reactions on size-selected metal clusters was investigated. The clusters under investigation were imaged with AC-STEM and their structure was assigned by comparing the atomic resolution images with a set of multi-slice STEM image simulation atlases. The effect of vapour-phase 1-pentyne hydrogenation on size-selected Aux (x=923 and 2057) cluster was studied and it was found that the gold nanoclusters demonstrate high stability in both size distribution and structure under the reaction. On the contrary, size-selected Pdx (x=923 and 2057) clusters tended to transform from amorphous to high symmetry structures under the same reaction condition. The gas-phase CO oxidation reaction on size-selected Aux (x=561, 923 and 2057) cluster was studied with regard to cluster size distribution and atomic structure. It was found that under the same conditions of the CO oxidation reaction, two different kind of ripening modes could be identified depended on the cluster size. Smoluchowski ripening, in which clusters diffuse intact and coalescence, is found to occur for Au2057 in the CO oxidation reaction. Ostwald ripening, in which larger clusters grow at the expense of smaller ones, was found to occur for Au561 and Au923 clusters, due to the extra energy generated from catalytic CO oxidation reaction

Composition-Tuned Pt-Skinned PtNi Bimetallic Clusters as Highly Efficient Methanol Dehydrogenation Catalysts

Platinum is the most active anode and cathode catalyst in next-generation fuel cells using methanol as liquid source of hydrogen. Its catalytic activity can be significantly improved by alloying with 3d metals, although a precise tuning of its surface architecture is still required. Herein, we report the design of a highly active low-temperature (below 0 °C) methanol dehydrogenation anode catalyst with reduced CO poisoning based on ultralow amount of precisely defined PtxNi1–x (x = 0 to 1) bimetallic clusters (BCs) deposited on inert flat oxides by cluster beam deposition. These BCs feature clear composition-dependent atomic arrangements and electronic structures stemming from their nucleation mechanism, which are responsible for a volcano-type activity trend peaking at the Pt0.7Ni0.3 composition. Our calculations reveal that at this composition, a cluster skin of Pt atoms with d-band centers downshifted by subsurface Ni atoms weakens the CO interaction that in turn triggers a significant increase in the methanol dehydrogenation activity

Unravelling the nucleation mechanism of bimetallic nanoparticles with composition-tunable core–shell arrangement

The structure and atomic ordering of Au–Ag nanoparticles grown in the gas phase are determined by a combination of HAADF-STEM, XPS and Refl-XAFS techniques as a function of composition. It is shown consistently from all the techniques that an inversion of chemical ordering takes place by going from Au-rich to Ag-rich compositions, with the minority element always occupying the nanoparticle core, and the majority element enriching the shell. With the aid of DFT calculations, this composition-tunable chemical arrangement is rationalized in terms of a four-step growth process in which the very first stage of cluster nucleation plays a crucial role. The four-step growth mechanism is based on mechanisms of a general character, likely to be applicable to a variety of binary systems besides Au–Ag

Hybrid atomic structure of the Schmid cluster Au₅₅(PPh₃)₁₂Cl₆ resolved by aberration-corrected STEM

We have investigated the atomic structure of the Au55(PPh3)12Cl6 Schmid cluster by using aberration-corrected scanning transmission electron microscopy (STEM) combined with multislice simulation of STEM images.</p

Au₄₀(SR)₂₄ Cluster as a Chiral Dimer of 8-Electron Superatoms: Structure and Optical Properties

We predict and analyze density-functional theory (DFT) -based structures for the recently isolated Au40(SR)24 cluster. Combining structural information extracted from ligand-exchange reactions, circular dichroism and transmission electron microscopy leads us to propose two families of low-energy structures that have a chiral Au-S framework on the surface. These families have a common geometrical motif where a non-chiral Au26 bi-icosahedral cluster core is protected by 6 RS-Au-SR and 4 RS-Au-SR-Au-SR oligomeric units, analogously to the “Divide and Protect” motif of known clusters Au25(SR)18-/0, Au38(SR)24 and Au102(SR)44. The strongly prolate shape of the proposed Au26 core is supported by transmission electron microscopy. Density-of-state-analysis shows that the electronic structure of Au40(SR)24 can be interpreted in terms of a dimer of two 8-electron superatoms, where the 8 shell electrons are localized at the two icosahedral halves of the metal core. The calculated optical and chiroptical characteristics of the optimal chiral structure are in a fair agreement with the reported data for Au40(SR)24

Atomic Resolution Observation of a Size-Dependent Change in the Ripening Modes of Mass-Selected Au Nanoclusters Involved in CO Oxidation

Identifying the ripening modes of supported metal nanoparticles used in heterogeneous catalysis can provide important insights into the mechanisms that lead to sintering. We report the observation of a crossover from Smoluchowski to Ostwald ripening, under realistic reaction conditions, for monomodal populations of precisely defined gold particles in the nanometer size range, as a function of decreasing particle size. We study the effects of the CO oxidation reaction on the size distributions and atomic structures of mass-selected Au_561±13, Au_923±20 and Au_2057±45 clusters supported on amorphous carbon films. Under the same conditions, Au_561±13 and Au_923±20 clusters are found to exhibit Ostwald ripening, whereas Au_2057±45 ripens through cluster diffusion and coalescence only (Smoluchowski ripening). The Ostwald ripening is not activated by thermal annealing or heating in O₂ alone