Synthesis of monodispersed model catalysts using softlanding cluster deposition

In nanocatalysis, clusters deposited on solid, well-defined surfaces play an important role. For the detection of size effects it is, however, important to prepare samples consisting of deposited clusters of a single size, as their chemical properties change with the exact number of atoms in the cluster. In this paper, the experimental tools are presented to prepare such model systems. The existence of monodispersed clusters is confirmed by various experimental findings. First, the carbonyl formation of deposited Nin clusters shows no change in the nuclearity when comparing the size of the deposited clusters with one of the formed carbonyls. Second, scanning tunneling microscopy (STM) studies show that fragmentation of Sin clusters upon deposition can be excluded. In addition, the adsorption behavior of CO on deposited Pd atoms points to the existence of single atoms on the surface. Furthermore, CO oxidation results on Aun clusters confirm the existence of monodispersed clusters trapped on well-defined adsorption sites. Finally, we use Monte-Carlo simulations to define the range of clusters and defect densities, for which monodispersed clusters can be expecte

Cu+(H2O)n の赤外スペクトルと溶媒和構造

2005年分子構造総合討論会, 2005年9月27日-30日, タワーホール船堀(東京), 1P03

赤外光解離分光による[Al(NH3)n]+の溶媒和構造に関する研究

2005年分子構造総合討論会, 2005年9月27日-30日, タワーホール船堀(東京), 3P03

Low-Temperature Cluster Catalysis

Free and supported metal clusters reveal unique chem. and phys. properties, which vary as a function of size as each cluster possesses a characteristic electron confinement. Several previous exptl. results showed that the outcome of a given chem. reaction can be controlled by tuning the cluster size. However, none of the examples indicate that clusters prepd. in the gas phase and then deposited on a support material are indeed catalytically active over several reaction cycles nor that their catalytic properties remain const. during such a catalytic process. In this work we report turn-over frequencies (TOF) for Pdn

Interaction of Ag, Rh, and Pd Atoms with MgO thin films studied by the CO probe molecule

The interaction of carbon monoxide with Ag, Rh, and Pd atoms deposited on MgO thin films has been studied by thermal desorption and Fourier transform infrared spectroscopies. To obtain an atomistic view of the surface complexes, we performed density functional calculations on cluster models of the MgO(100) surface. The combined experimental−theoretical information leads to the following picture. Ag atoms interact weakly with the surface and do not form stable complexes with CO. Pd atoms become trapped at oxygen vacancies (F centers) already at low temperature; at these sites Pd(CO)2 complexes form after exposure to CO. The CO molecules desorb from these sites at T = 230 K. At higher temperature, 370 K, we observe formation of small Pd aggregates with bridge-bonded CO molecules, indicating diffusion of Pd atoms or Pd(CO) units. Rh atoms bind quite strongly to the oxide anions at steps where they form relatively stable Rh(CO)2 or even Rh(CO)3 complexes; a minority of the Rh atoms populates the F centers. A first CO desorption occurs at 180 K and leaves on the surface rather mobile, very stable Rh(CO) units which diffuse until they become trapped at F centers; at 390 K a second CO desorption occurs leaving on the surface isolated Rh atoms, with no indication of formation of small Rh aggregates

Acetylene trimerization on Ag, Pd and Rh atoms deposited on MgO thin films

The acetylene trimerization on the group VIII transition metal atoms, Rh and Pd, as well as on Ag atoms supported on MgO thin films has been studied exptl. and theor. The three metal atoms with the at. configurations 4d85s1 (Rh), 4d10s0 (Pd) and 4d105s1 (Ag) behave distinctly differently. The coinage metal atom silver is basically inert for this reaction, whereas Pd is active at 220 and 320 K, and Rh produces benzene in a rather broad temp. range from 350 to ca. 430 K. The origins of these differences are not only the different electronic configurations, leading to a weak interaction of acetylene with silver due to strong Pauli repulsion with the 5s electron but also the different stability and dynamics of the three atoms on the MgO surface. In particular, Rh and Pd atoms interact differently with surface defects like the oxygen vacancies (F centers) and the step edges. Pd atoms migrate already at low temp. exclusively to F centers where the cyclotrimerization is efficiently promoted. The Rh atoms on the other hand are not only trapped on F centers but also at step edges up to about 300 K. Interestingly, only Rh atoms on F centers catalyze the trimerization reaction whereas they are turned inert on the step edges due to strong steric effects. [on SciFinder(R)

Turn-over frequencies of catalytic reactions on nanocatalysts measured by pulsed molecular beams and quantitative mass spectrometry

We present an exptl. scheme for measuring turn-over frequencies (TOFs) and obtaining mechanistic details of catalytic processes on small clusters on surfaces with high sensitivity. The scheme uses a newly designed pulsed, piezo-elec. driven valve with extremely high pulse-to-pulse stability in combination with quant. mass spectrometry. The exptl. scheme is applied for studying the oxidn. of CO on mass-distributed palladium clusters deposited from a mol. beam onto well-characterized oxide surfaces. The obtained results are compared to the ones of palladium single crystals and supported palladium particles on oxide surfaces. The measured TOF is 47±8 CO2 mols. per deposited atom per s at 350 K and the obsd. mechanistic details can be explained by different active centers on the clusters and by the competitive adsorption of the two reactant mols. The interpretation is in agreement with earlier studies on the oxidn. of CO on palladium model catalysts. [on SciFinder(R)

Acetylene polymerization on supported transition metal clusters

A review. The polymn. of acetylene, studied exptl. and theor. on nano catalysts consisting of nanoscale clusters of different size and elemental compn., is reviewed. As on bulk systems palladium is the most active transition metal for this reaction. More important, however, is the changing selectivity as function of size and elemental compn. As an example, palladium atoms, dimers, and trimers, as well as nanoscale copper clusters are highly selective for the cyclotrimerization reaction. In the case of palladium, the π-bonding of acetylene and a charge transfer from the substrate to the atom/cluster are responsible for the high selectivity. In addn. calcns. revealed the whole reaction path of this reaction on palladium atoms and it could be shown that in contrast to bulk systems the rate-detg. step is the formation of benzene from the Pd(C4H4)C2H2 complex. [on SciFinder(R)

Low-Temperature Production of Genuinely Amorphous Carbon from Highly Reactive Nanoacetylide Precursors

Copper acetylide is a well-known explosive compound. However, when the size of it crystals is reduced to the nanoscale, its explosive nature is lost, owing to a much lower thermal conductance that inhibits explosive chain reactions. This less explosive character can be exploited for the production of new carbon materials. Generally, amorphous carbon is prepared by carbonization of organic compounds exposed to high temperature, which can induce partial crystallization in graphite. In this work, we present a new method in which the carbonization reaction can proceed at a lower annealing temperature (under 150°C) owing to the highly reactive nature of copper acetylide, thus avoiding crystallization processes and enabling the production of genuinely amorphous carbon materials