The effect of metal-oxide support on the activity of ethylene hydrogenation catalyzed by size-selected platinum clusters (8–20 atoms per cluster) is investigated. Size-selected clusters have been shown to possess unique catalytic properties which change as a function of the precise cluster size. The temperature-programmed reaction and isothermal pulsed molecular beam data both demonstrate a size-dependent reactivity on each support. However, predicted trends based on the acidic (SiO2) or basic (MgO) properties of the support are more subtle, with the influence of the support also being a function of the cluster size. Only the average hydrogenation activity of all sizes measured, as well as the CO stretching frequency on clean clusters, were observed to follow predicted trends, but atomic-level resolution demonstrates a much more complicated picture. Pt13 had the lowest hydrogenation activity of all sizes on amorphous SiO2, while being the most active size on MgO, and trends between the same cluster sizes were observed to exhibit opposing behavior between the two supports. This demonstrates the potential of cluster materials for designing catalytic systems and also the difficulties encountered when formulating general concepts for reactivity in the nonscalable size regime.Keywords: clusters; ethylene hydrogenation; MgO; model catalysis; platinum; SiO2; support effects;

In this ultrahigh vacuum study, temperature-programmed desorption, Auger electron spectroscopy, and ex-situ atomic force microscopy are used to evaluate the surface chemistry of ethanol on the GaN(0001) surface. Ethanol undergoes dehydration and dehydrogenation reactions on the GaN(0001) surface to a larger extend than on the TiO2(110) surface. This enhanced reactivity is attributed to a higher amount of metal-bound ethoxy. In addition, molecular H2 has been identified as a byproduct of the ethanol dehydrogenation to acetaldehyde. We attribute the reactivity, including the formation of molecular hydrogen, to the combination of wurtzite structure and nitride chemistry, since surface amines are considered to be less stable than surface hydroxyls on other model oxides.

In this work we present a stoichiometric reaction mechanism for the photocatalytic ethanol oxidation on TiO2(110). The reaction products are analyzed either under reaction conditions or after irradiation at lower temperatures. Water is identified as a quantitative by-product, which resides in a defect site. These water molecules cause a blocking of the defect sites which results in poisoning of the catalyst. By different preparation techniques of the TiO2(110) surface, the role of surface defects is further elucidated and the role of molecular oxygen is investigated. Based on the investigation, a complete photochemical reaction mechanism is given, which provides insights into general photon driven oxidation mechanisms on TiO2.

Employing rationally designed model systems with precise atom-by-atom particle size control, we demonstrate by means of combining noninvasive in situ indirect nanoplasmonic sensing and ex situ scanning transmission electron microscopy that monomodal size-selected platinum cluster catalysts on different supports exhibit remarkable intrinsic sintering resistance even under reaction conditions. The observed stability is related to suppression of Ostwald ripening by elimination of its main driving force via size-selection. This study thus constitutes a general blueprint for the rational design of sintering resistant catalyst systems and for efficient experimental strategies to determine sintering mechanisms. Moreover, this is the first systematic experimental investigation of sintering processes in nanoparticle systems with an initially perfectly monomodal size distribution under ambient conditions.

Well defined thin molecular films of 2,2′-dihydroxy-1,1′binaphthyl (binol) molecules at coverages between 5 × 1015 molecules per cm2 and 1017 molecules per cm2 on thin glass (BK7) substrates were investigated under ultra-high-vacuum (UHV) conditions. Second-Harmonic-Generation Optical-Rotatory-Dispersion measurements (SHG-ORD) were performed using a dedicated spectroscopic setup which allows for the determination of the rotation angle of the SH-signal of two enantiomers. Rotation angles of up to 38 degrees were measured. The chirality of the two enantiomers has been studied at 674 nm (337 nm resonance wavelength) in the transmission mode. Coverage dependent orientation evolution of binol molecular films was revealed by precise monitoring of the surface coverage while performing SHG-ORD experiments. We show that the molecules reach their final orientation at a surface coverage of 5 × 1016 molecules per cm2. From the obtained experimental data the ratio of chiral and achiral susceptibility components could be calculated and was observed to change with coverage.

Recent research in heterogeneous catalysis, especially on size-selected model systems under UHV conditions and also in realistic catalytic environments, has proved that it is necessary to think in terms of the exact number of atoms when it comes to catalyst design. This is of utmost importance if the amount of noble metal, gold in particular, is to be reduced for practical reactions like CO oxidation. Here it is shown that on TiO2 only Au6 and Au7 clusters are active for CO oxidation which holds for the single crystal, thin films, and titania clusters deposited on HOPG. Size-selected cluster deposition and TPD methods have been employed to investigate the CO oxidation activity of Aun/TiO2 systems which are compared to recent results reported by Lee et al. to form a consistent picture in which only two species can be regarded as “active”. The efficiency of investigated Aun/(TiO2)93/HOPG composite materials is attributed to carbon-assisted oxygen spillover from gold to support particles and across grain boundaries.

The photocatalytic water reduction reaction on CdS nanorods was studied as function of Pt cluster size. Maximum H2 production is found for Pt46. This effect is attributed to the size dependent electronic properties (e.g., LUMO) of the clusters with respect to the band edges of the semiconductor. This observation may be applicable for the study and interpretation of other systems and reactions, e.g. H2O oxidation or CO2 reduction.

We introduce size-selected subnanometer cluster catalysts deposited on thin films of colloidal semiconductor nanocrystals as a novel platform to obtain atomic scale insight into photocatalytic generation of solar fuels. Using Pt-cluster-decorated CdS nanorod films for photocatalytic hydrogen generation as an example, we determine the minimum amount of catalyst necessary to obtain maximum quantum efficiency of hydrogen generation. Further, we provide evidence for tuning photocatalytic activities by precisely controlling the cluster catalyst size.

An experimental setup consisting of three highly sensitive spectroscopic methods for the investigation of the optical properties of surface adsorbates at low coverages is presented. The three spectroscopies cover a large wavelength range from ultraviolet to infrared and can be applied from ambient conditions down to ultrahigh vacuum. For the IR and the visible range, surface-sensitive cavity ringdown spectroscopy is used; the UV region is covered by surface second harmonic generation spectroscopy, a nonlinear technique. The potential of the setup is revealed by measurements of the laser dye Rhodamine 110 that is spin coated on a substrate. The reliability of the methods is verified by comparing obtained spectra with those obtained using standard techniques. Finally, the sensitivity of all three methods is estimated and shown to be sensitive enough to perform measurements in the sub-monolayer limit (in the best case down to <0.1% of a monolayer).

In this work we present a stoichiometric reaction mechanism for the photocatalytic ethanol oxidation on TiO2(110). The reaction products are analyzed either under reaction conditions or after irradiation at lower temperatures. Water is identified as a quantitative by-product, which resides in a defect site. These water molecules cause a blocking of the defect sites which results in poisoning of the catalyst. By different preparation techniques of the TiO2(110) surface, the role of surface defects is further elucidated and the role of molecular oxygen is investigated. Based on the investigation, a complete photochemical reaction mechanism is given, which provides insights into general photon driven oxidation mechanisms on TiO2.