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Adsorption of small molecules on silver clusters
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Image of FIG. 1.
FIG. 1.

A schema of the experimental setup: The silver is thermally evaporated (a) and aggregates in a helium atmosphere (b). The clusters thermalize with the helium in a thermalization tube at 77 K (c). A soft discharge ionizes the clusters during formation (a). Oxygen and nitrogen are injected by calibrated dosing valves into the buffer gas of the aggregation chamber (d). Finally, differential pumping separates the gases and the TOF analyses the products.

Image of FIG. 2.
FIG. 2.

The products of Ag4 + after passage through the thermalization and reaction chamber. (a) Intensity histograms of Ag4 +(N2) m over m at different nitrogen pressures. (b) The mean number 〈m〉 of N2 adsorbed on Ag4 + and on Ag4 +O2 is plotted in comparison as a function of nitrogen pressure p. The presence of chemisorbed O2 enforces the physisorption of N2. (c) The mean number of oxygen molecules on all Ag4 +(N2) m clusters depending on the mean number of nitrogen on Ag4 +. In spite of a constant oxygen pressure, the probability to find oxygen on the silver clusters more than doubles in co-adsorption with nitrogen. The pressure of oxygen is kept constant at 10−3 mbar in both data sets (b) and (c).

Image of FIG. 3.
FIG. 3.

Experimental intensity histograms of Ag6 +(N2) m and Ag7 +(N2) m over m at different nitrogen pressures. In both cases m = 2 appears as a magic number, indicating that there is a pronounced drop of binding energy from the 2nd to the 3rd site for nitrogen molecules. This corresponds to our calculations, where the two high-coordinated atoms that appear in the most stable configurations a7 and a6 (Fig. 5), interact strongest with the nitrogen, since they are close to the charge center.

Image of FIG. 4.
FIG. 4.

Structure of Ag n (n = 5–7) neutral clusters and cations. The structures (a) in the left column are the most stable ions. (a)-(d) indicate decreasing stability of the ions. Italic indicates the most stable neutral structures. The energetics and the IP potential are listed in Table I by two different DFT approximations.

Image of FIG. 5.
FIG. 5.

N2-adsorption on Ag n + (n = 3, 4, 6, and 7). Both PBE and HSE06 have been used and results are listed in Table II. The physisorption to sites at obtuse corners with high coordination number are found to be stronger than to those at acute ones.

Image of FIG. 6.
FIG. 6.

O2-adsorption on Agn + (n = 1–6) cations. Small spheres represent O atoms and large ones Ag. For n = 4–6, isosurfaces of charge difference (at the value of 0.02 e 3), are plotted for the ground state cluster. The blue (darker) color represents electron accumulation and yellow (lighter) deficiency. The adsorption energies and the Bader charge transfer are listed in Table III.

Image of FIG. 7.
FIG. 7.

The lowest energy states of Ag4O2 +-(N2) m and of Ag4 +-(N2) m with m = 2–5, respectively.


Generic image for table
Table I.

Total energy (TE), binding energy per atom (BE/n), and ionization potential (IP) obtained by using HSE06 (3rd and 4th columns) and PBE (5th and 6th columns), respectively. All energies are in the units of eV and shifted to zero, the lowest energy state. The letter a5-c7 denotes clusters size n = 5–7 and various isomer structures shown in Fig. 4; the superscript q indicate the charge state. Numbers in italic refer to the lowest isomer.

Generic image for table
Table II.

Total energy (TE) and adsorption energy per N2 molecule (AE/m) obtained by using HSE06 (3rd and 4th columns) and PBE (5th and 6th columns), respectively. All energies are in units of eV and shifted to zero the lowest energy state. The letters a-h refer to various isomer structures and the superscript q to charge states. Numbers in italics refer to the lowest isomer.

Generic image for table
Table III.

Adsorption energy and charge transfer to O2 molecules adsorbed on Ag n + cations. The lowest energy states are written in italic. To put the charge state into perspective, our test calculation shows that the Bader analysis gives a 0.8 e charge transfer for a NaCl unit. All energies are in eV and charge transfer in e .


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Adsorption of small molecules on silver clusters