Deposition Of Mass-selected Clusters Research Articles

Preparation of Size- and Composition-Controlled PtnSnx/SiO2 (n = 4, 7, 24) Bimetallic Model Catalysts with Atomic Layer Deposition

Model catalysts prepared by mass-selected cluster deposition are useful in fundamental studies, but for bimetallic clusters, a precise selection of both size and composition can be difficult except for very small clusters. We discuss an alternative method for preparing bimetallic PtSn model catalysts, in which size-selected Pt4, Pt7, and Pt24 clusters, deposited on SiO2 and then hydrogenated, are used to seed selective addition of Sn by a self-limiting reaction with SnCl4. Catalysts are characterized using X-ray photoelectron spectroscopy, ion scattering spectroscopy, and CO temperature-programmed desorption, and structures are calculated by plane-wave density functional theory. It is found that Sn deposition saturates at a relatively low SnCl4 exposure, indicating that the reaction is self-limiting, and that nonselective Sn deposition on the SiO2 support is ∼40 times less efficient than Sn deposition on hydrogenated Pt cluster sites. The resulting PtnSnx stoichiometry is near 1:1 for Pt4 and Pt7, cluster...

Copper Cluster Size Effect in Methanol Synthesis from CO2

Size-selected Cun catalysts (n = 3, 4, 20) were synthesized on Al2O3 thin films using mass-selected cluster deposition. A systematic study of size and support effects was carried out for CO2 hydrogenation at atmospheric pressure using a combination of in situ grazing incidence X-ray absorption spectroscopy, catalytic activity measurement, and first-principles calculations. The catalytic activity for methanol synthesis is found to strongly vary as a function of the cluster size; the Cu4/Al2O3 catalyst shows the highest turnover rate for CH3OH production. With only one atom less than Cu4, Cu3 showed less than 50% activity. Density functional theory calculations predict that the activities of the gas-phase Cu clusters increase as the cluster size decreases; however, the stronger charge transfer interaction with Al2O3 support for Cu3 than for Cu4 leads to remarkably reduced binding strength between the adsorbed intermediates and supported Cu3, which subsequently results in a less favorable energetic pathway t...

(Keynote) Effects of Catalytic Site Size on Activity of Model Electrodes Prepared By Mass-Selected Cluster Deposition

Model size-selected electrodes were prepared by deposition of mass-selected metal cluster ions on electrode substrates, cleaned and characterized in ultra-high vacuum (UHV). After analysis by X-ray photoelectron spectroscopy (XPS), the electrodes were transferred and mated to a small electrochemical cell without air exposure, and then studied by cyclic voltammetry. Electrodes with small Ptn (n = 1 - 16) on glassy carbon and indium tin oxide substrates were studied. The ethanol oxidation reaction (EOR), the oxygen reduction reaction (ORR), and carbon oxidation by water were all found to depend strongly on cluster size, with peak currents varying by factors of up to 1000. For oxidation reactions, the activity was found to be anticorrelated with Pt core level binding energies measured by XPS, i.e., the dominant factor appears to be the electronic properties of the metal centers. For ORR, the dominant factor appears to be the physical size of the active site for O2 binding.

An experimental setup combining a highly sensitive detector for reaction products with a mass-selected cluster source and a low-temperature STM for advanced nanocatalysis measurements

We report on a home-built detector for catalytic reaction measurements offering good gas isolation from the surrounding ultrahigh vacuum components, high sensitivity for reaction products and a fast response time of 10 ms enabling dynamic studies correlated to reactant gas pulses. The device is mounted in ultrahigh vacuum and combined with a low-temperature scanning tunneling microscope and a source for the deposition of mass-selected clusters. This combination allows for a direct correlation between surface morphology and catalytic properties of model catalysts. The performances of the new detector are illustrated by measurements on two model systems. Thermal desorption spectroscopy of CO carried out on morphologically well characterized Pt on TiO2(110)-(1×1) reveals several desorption features, which can be attributed to different surface sites. Catalytic CO oxidation performed by alternatingly pulsing isotopic CO and O2 on a Pt film on yttria stabilized zirconia reveals the CO or O rich temperature regimes. The CO2 production rate correlated with either one of the reactants can perfectly be reproduced by a kinetic reaction model giving access to the respective adsorption energies.

Characterization and Reactivity of the Mo4S6+Cluster Deposited on Au(111)

Mass-selected cluster deposition was used to investigate the chemical and thermal properties of the Mo 4S 6 cluster deposited onto a Au(111) substrate. Auger spectroscopy and (13)CO thermal desorption measurements demonstrate that the clusters behave independently up to coverages of ∼0.15 ML, while at higher coverages, cluster crowding or island formation results in no net increase in Mo-atom adsorption sites. DFT calculations show that CO binding on the Mo-atom top site is strongly preferred over the side sites, with scaled binding energies in reasonable agreement with the experimentally derived binding energy of 0.7 eV. DFT calculations predict that the total adsorption energy for sequential addition of two CO molecules (top and side site) is nearly additive, whereas the addition of a third CO to another empty Mo side site is less stable. The latter is attributed to repulsive intercluster interactions and is consistent with the experimentally estimated sticking coefficient of 0.4 ± 0.1. In contrast to C...

Construction and Application of a UHV Compatible Cluster Deposition System

The design and operation of a new UHV-compatible atomic cluster deposition system is described. The design is optimised for high cluster fluxes and for the production of cluster-assembled nano-devices. One key feature of the system is a high degree of flexibility, including interchangeable sputtering and inert gas aggregation sources, and two kinds of mass spectrometer, which allow both characterisation of the cluster size distribution and deposition of mass-selected clusters. Another key feature is that clusters are deposited onto electrically contacted lithographically defined devices mounted on an UHV-compatible cryostat cold finger, allowing deposition at room temperature as well as cryogenic and elevated temperatures. In-situ electrical characterisation of cluster-assembled devices can then be performed in the temperature range 1.2--75 K.

Mass selected cluster deposition in strongly or weakly interacting media

Mass selected cluster deposition is discussed with respect to three questions: what is the integrity of the clusters after deposition; are they uniform in terms of size and isomerisation; and finally what is the influence of the support on the clusters? The answer to these questions will depend on the strength of cluster media interaction and we have chosen two extreme cases the inert rare gases on one hand and metal surfaces on the other. Three examples are given: Raman studies as well as low-temperature STM give structural information on the deposited clusters, while other optical spectroscopies (absorption and fluorescence) yield information on the electronic structure of the clusters. To cite this article: W. Harbich, C. Félix, C. R. Physique 3 (2002) 289–300.

An experimental setup for nondestructive deposition of size-selected clusters

An experimental setup for the deposition of mass-selected clusters using a laser vaporization source and a magnetic field mass selector is presented. Nondestructive deposition and a coverage of 1% of a monolayer within 5 h are achieved for mass-selected metallofullerene clusters as demonstrated for deposited Ce@C60 on highly oriented pyrolytic graphite.

Deposition of mass-selected clusters studied by thermal energy atom scattering and low-temperature scanning tunneling microscopy: An experimental setup

We present an experimental setup for the investigation of the processes occurring during the deposition of mass-selected clusters on a well-defined surface. The sample is analyzed in situ by two complementary methods: thermal energy atom scattering (TEAS) and scanning tunneling microscopy (STM). TEAS is used to study the dynamical processes during the deposition and to gather statistical information about the resulting structures on the surface. Subsequent STM measurements allow us to investigate the collision outcome on an atomic scale. The setup is highly versatile and guarantees ultra-high-vacuum conditions and cryogenic temperatures (≈30 K) of the sample at all times even during sample transfer. Clusters are produced in a CORDIS-type cluster source. A new compact multichannel effusive He source in combination with a new Wien-filter-based He detector are used for TEAS measurements. The new low-temperature STM allows measurements in a temperature range between 8 and 450 K. Atomic resolution on the Pt(111) surface is regularly observed at Tsample=8 K. The performances of the setup are illustrated by STM images obtained after the deposition of Ag7+ clusters with Ekin=95 and 1000 eV on bare Pt(111) and by measurements made of the deposition of Ag7+ clusters with Ekin=20 eV in a Xe–rare-gas matrix adsorbed on Pt(111).

‘Soft landing’ of size-selected clusters in chemically inert substrates

Mass-selected cluster deposition in inert rare-gas substrates is reviewed. The problem of soft landing, that is deposition without fragmentation, is briefly addressed. Matrix isolation spectroscopy...

Novel metal cluster complexes synthesized by matrix deposition of mass-selected clusters

Metal complexes produced by depositing size se- lected Fe and Ag cluster cations in N2 and O2 matrices respectively are studied by infrared spectroscopy. Unknown species such as Fe(N2)x ,F e 3(N2)x and Ag3(O2)x are ob- served. The IR spectra of Ag + ,A g + and Ag + in excess O2 indicate that no complexes involving molecular oxygen are formed. However, the strong silver cluster UV-visible absorptions detected in Ar matrices disappear in the oxy- gen matrices, suggesting that silver-oxygen complexes are formed with dissociated oxygen.

Direct Synthesis of Metal Cluster Complexes by Deposition of Mass-Selected Clusters with Ligand: Iron with CO

A new technique for preparing metal cluster complexes of a desired nuclearity by co-depositing ligand with metal clusters mass-selected from a cluster beam is illustrated by the deposition of Fe, Fe2, and Fe3 with excess CO. The matrices formed were studied by Fourier transform infrared spectroscopy. Unexpectedly, deposition of Fe2 produced the CO-unbridged Fe2(CO)8 isomer, known primarily from matrix photochemistry, rather than the well-known diiron nonacarbonyl.