Excited state nonadiabatic dynamics of bare and hydrated anionic gold clusters Au3(-)[H2O]n (n = 0-2).

We present a joint theoretical and experimental study of excited state dynamics in pure and hydrated anionic gold clusters Au3(-)[H2O]n (n = 0-2). We employ mixed quantum-classical dynamics combined with femtosecond time-resolved photoelectron spectroscopy in order to investigate the influence of hydration on excited state lifetimes and photo-dissociation dynamics. A gradual decrease of the excited state lifetime with the number of adsorbed water molecules as well as gold cluster fragmentation quenching by two or more water molecules are observed both in experiment and in simulations. Non-radiative relaxation and dissociation in excited states are found to be responsible for the excited state population depletion. Time constants of these two processes strongly depend on the number of water molecules leading to the possibility to modulate excited state dynamics and fragmentation of the anionic cluster by adsorption of water molecules.

Size-dependent electronic structure controls activity for ethanol electro-oxidation at Ptn/indium tin oxide (n = 1 to 14).

Understanding the factors that control electrochemical catalysis is essential to improving performance. We report a study of electrocatalytic ethanol oxidation - a process important for direct ethanol fuel cells - over size-selected Pt centers ranging from single atoms to Pt14. Model electrodes were prepared by soft-landing of mass-selected Ptn(+) on indium tin oxide (ITO) supports in ultrahigh vacuum, and transferred to an in situ electrochemical cell without exposure to air. Each electrode had identical Pt coverage, and differed only in the size of Pt clusters deposited. The small Ptn have activities that vary strongly, and non-monotonically with deposited size. Activity per gram Pt ranges up to ten times higher than that of 5 to 10 nm Pt particles dispersed on ITO. Activity is anti-correlated with the Pt 4d core orbital binding energy, indicating that electron rich clusters are essential for high activity.

Size-selected gold clusters on porous titania as the most "gold-efficient" heterogeneous catalysts.

Research on homogeneous and heterogeneous catalysis is indeed convergent and finds subnanometric particles to be at the heart of catalytically active species. Here, monodisperse gold clusters are deposited from the gas phase onto porous titania generating well-defined model systems and the resulting composite materials exhibit a sharp size-dependency on the number of gold atoms per cluster and exceptionally high-turnovers toward the bromination of 1,4-dimethoxybenzene are observed. This indicates that the deliberate generation of active centres is of utmost importance for the creation of the most "gold-efficient" catalysts.

Parallel deposition of size-selected clusters: a novel technique for studying size-selectivity on the atomic scale.

A new size-selected cluster deposition technique referred to as "parallel-deposition" is presented. An ion beam of multi-sized Aun clusters was spatially separated into individual cluster sizes by utilizing a Wien filter and the clusters spatially separated based on their atomic sizes were simultaneously deposited on a SiO2/Si(100) substrate. Parallel-deposited Aun clusters (n = 6, 7, and 8) on the SiO2/Si(100) substrate showed even-odd oxidation behaviour upon exposure to an atomic oxygen atmosphere, demonstrating the potential of this new technique to study the size-dependent properties of deposited clusters in various research fields.

Cage-Like Nanoclusters of ZnO Probed by Time-Resolved Photoelectron Spectroscopy and Theory.

Zinc oxide nanoclusters have been predicted as promising building blocks for cluster-assembled materials with unprecedented properties. Here, for the first time these clusters are probed by time-resolved photoelectron spectroscopy and characterized in detail by density functional theory. Their validity as building blocks for cluster-assembled materials is confirmed via rigid cage-like structures facilitating three-dimensional aggregation in combination with large band gaps that are nevertheless significantly lower than any known ZnO polymorph. In addition, electron-hole pair localization in the excited state of the cluster anions combined with their structural rigidity leads to extraordinary long-lived states above the band gap virtually independent of the cluster size, defying the rule "every atom counts".

Hydrogen mimicking the properties of coinage metal atoms in Cu and Ag monohydride clusters.

A systematic study of the electronic structure and equilibrium geometries of Cun, Cun-1H, Agn, and Agn-1H; n = 2-5 clusters is carried out using photoelectron spectroscopy (PES) experiments and density functional theory based calculations. Our objective is to see if the substitution of a coinage metal atom by hydrogen would retain the electronic structure of the parent metal cluster since both systems are isoelectronic. For clusters with n ≥ 3, we find that the measured PES and vertical detachment energies (VDEs) (i.e. energies necessary to remove an electron from the anionic Mn(-) (M = Cu, Ag) clusters without changing their geometries) are close to those of Mn-1H(-) clusters, suggesting that substitution of a metal atom with hydrogen does not perturb the electronic structure of the parent cluster anion significantly. Calculated VDEs agree very well with experiment validating the theoretical methods used as well as the geometries of the neutral and anionic clusters.

Strong effects of cluster size and air exposure on oxygen reduction and carbon oxidation electrocatalysis by size-selected Pt(n) (n ≤ 11) on glassy carbon electrodes.

Model Pt(n)/glassy carbon electrodes (Pt(n)/GCE) were prepared by deposition of mass-selected Pt(n)(+) (n ≤ 11) on GCE substrates in ultrahigh vacuum. Electrocatalysis under conditions appropriate for the oxygen reduction reaction (ORR) was studied, for samples both in situ with no exposure to laboratory air and with air exposure prior to electrochemical measurements. Of the small clusters, only a few cluster sizes show the expected ORR activity, and in those cases, the activity per Pt atom is similar to that seen under identical conditions with a conventionally prepared electrode with Pt nanoparticles grown on a GCE. For other small Pt(n) on GCE, any ORR signal is overwhelmed by large oxidative currents attributed to catalysis of carbon oxidation by water. If the samples are exposed to air prior to electrochemistry, both ORR and carbon oxidation signals are absent, and instead only small capacitive currents or currents attributed to redox chemistry of adventitious organic adsorbates are observed, indicating that air exposure results in passivation of the small Pt clusters.

Composites of metal nanoparticles and TiO2 immobilized in spherical polyelectrolyte brushes.

The synthesis and the catalytic activity of nanocomposites consisting of metal nanoparticles (Au, Pt, Pd) and nanoparticles of TiO(2) (anatase) is presented. These composite particles have been synthesized by reduction of the respective metal ions adsorbed on the surface of as-prepared TiO(2) nanoparticles that are immobilized on spherical polyelectrolyte brush particles (SPB) as carrier system. The SPB particles consist of a polystyrene core from which long chains of poly(styrene sodium sulfonate) are grafted. We demonstrate that the metal nanoparticles (such as Au, Pt, and Pd) are only generated on the surface of the anatase particles having a size of ca. 10 nm. These metal NP/TiO(2)@SPB composite particles exhibit a high colloidal stability. They are excellent heterogeneous photocatalysts for the degradation of the dye Rhodamine B under UV irradiation. The photocatalytic activity of the composite particles is 2-5 times higher than that of the pure TiO(2) particles. This finding is traced back to an enhanced adsorption of the dye on the metal@TiO(2) composites.

Pt@MOF-177: synthesis, room-temperature hydrogen storage and oxidation catalysis.

The gas-phase loading of [Zn(4)O(btb)(2)](8) (MOF-177; H(3)btb=1,3,5-benzenetribenzoic acid) with the volatile platinum precursor [Me(3)PtCp'] (Cp'=methylcyclopentadienyl) was confirmed by solid state (13)C magic angle spinning (MAS)-NMR spectroscopy. Subsequent reduction of the inclusion compound [Me(3)PtCp'](4)@MOF-177 by hydrogen at 100 bar and 100 degrees C for 24 h was carried out and gave rise to the formation of platinum nanoparticles in a size regime of 2-5 nm embedded in the unchanged MOF-177 host lattice as confirmed by transmission electron microscopy (TEM) micrographs and powder X-ray diffraction (PXRD). The room-temperature hydrogen adsorption of Pt@MOF-177 has been followed in a gravimetric fashion (magnetic suspension balance) and shows almost 2.5 wt % in the first cycle, but is decreased down to 0.5 wt % in consecutive cycles. The catalytic activity of Pt@MOF-177 towards the solvent- and base-free room temperature oxidation of alcohols in air has been tested and shows Pt@MOF-177 to be an efficient catalyst in the oxidation of alcohols.

Group 10 metal aminopyridinato complexes: synthesis, structure, and application as aryl-Cl activation and hydrosilane polymerization catalysts.

(4-Methyl-pyridin-2-yl)(trimethylsilanyl)amine (ApSi-H) and tert-butyl(4-methyl-pyridin-2-yl)amine (AptBu-H) were synthesized via salt metathesis and aryl amination reactions, respectively. Lithiation of these two aminopyridines using n-BuLi and the reactions with [(dme)NiCl2] (dme = dimethoxyethane) or [(cod)PdCl2] (cod = cyclooctadiene) in THF at low temperature gave rise--after workup in hexane--to group 10 amido compounds, [(ApSi)4Ni2], [(AptBu)2Pd], [(AptBu-H)(AptBu)2Ni], [(AptBu)3(C2H5O)3Ni3OLi(thf)], and [(AptBu)2Ni(tBupy)2] (tBupy = 4-tert-butylpyridine). The aminopyridinato complexes were characterized by X-ray crystal structure analysis. The highly strained binding situation of the aminopyridinato ligands suggested that these compounds might be efficiently converted into catalytically active species. The applications of some of the synthesized complexes as Suzuki cross-coupling catalysts (activation of aryl chlorides) are described and [(ApSi)4Ni2] is a rare example of a "phosphine-free" catalyst system. A number of late transition metal complexes were found to successfully catalyze polymerization of MeH2SiSiH2Me toward soluble, linear poly(methylsilane). Remarkable activity was observed for [(ApSi)2Pd].