Roger C. Baetzold

Calculated Properties of Metal Aggregates. II. Silver and Palladium

Extended Huckel and CNDO calculations have been made for Ag and Pd aggregates. Bond energy, ionization potential (I.P.), and electron affinity (E.A.) are reported for these particles up to 30 atoms in size. Linear chains are the most stable geometric form of small Ag aggregates, but a transition above 30 atoms is expected at which fcc is more stable. The three‐dimensional symmetric geometry is the most stable form of small Pd aggregates. The I.P. and E.A. in both types of geometry converge to a common value at 20 atoms in Ag and at 4 atoms in Pd. These converged values depend upon aggregate geometry, but do approach the work function of bulk metal. Palladium particles are found to have ≈ 0.35 d‐band hole per atom, which is close to values reported for bulk metal.

Catalysis by Small Metal Clusters

Experimental and theoretical studies of small clusters of metal atoms are aimed at revealing how properties change in the ultrafinely divided state. Thermodynamic, electronic, and structural features can be size-dependent and may be involved in determining the activity of small-particle catalysts. Correlations between these properties give indications of the features important in catalysis.

Electronic properties of metal clusters

Extended Huckel calculations are reported for clusters of Pd, Ag, Cu, Au, Pd−Ag, and Ni−Cu containing up to 55 atoms. Electronic properties important in catalysis are calculated and include the width and position of conduction−band states. Silver particles have a conduction band that shows increasing spread with increasing size. It is 5 eV wide at 55 atoms and the d states are 5 eV below the Fermi level. Palladium clusters have a high density of d states at the Fermi level, which is −7.9 eV for Pd19. Fewer d holes are present on surface than interior Pd atoms in Pd19. Clusters of Cu/Ni and Ag/Pd show d holes on the Group VIII metal atom for compositions up to and greater than 60% Ib metal, in contrast to the rigid band model prediction.

ESCA and molecular orbital studies of small silver particles

The results of x‐ray photoelectron spectroscopy studies of evaporated silver clusters on carbon are reported. The samples were prepared and the particle‐size distribution was determined. Spectra have been recorded at coverages from 2.7×1013 to 4×1016 atoms cm−2. The very low coverages yield particle distributions dominated by single‐atom centers, whereas the high coverages represent bulk metallic silver. Low‐coverage spectra show low binding energy states that are thought to result either from interactions with the carbon substrate or reaction with sulfur contamination. CNDO calculations show low binding energy states to exist for Ag4 on a C16 model substrate. At higher coverages the Ag 4d states broaden and split, and the threshold moves 2.5 eV to lower binding energy. The splitting of the Ag 4d band is attributed to spin–orbit and crystal field effects. The change in threshold can be attributed to the filling and spreading of the Ag 5s states to form the s–p conduction band of the bulk metal. The measured threshold shift of 2.5 eV (Fermi‐level reference) is to be compared with 1.5 eV (vacuum‐level reference) predicted by extended Huckel calculations. Small shifts in the Ag 3d core levels are also observed and follow the trends predicted by CNDO charge calculations.The results of x‐ray photoelectron spectroscopy studies of evaporated silver clusters on carbon are reported. The samples were prepared and the particle‐size distribution was determined. Spectra have been recorded at coverages from 2.7×1013 to 4×1016 atoms cm−2. The very low coverages yield particle distributions dominated by single‐atom centers, whereas the high coverages represent bulk metallic silver. Low‐coverage spectra show low binding energy states that are thought to result either from interactions with the carbon substrate or reaction with sulfur contamination. CNDO calculations show low binding energy states to exist for Ag4 on a C16 model substrate. At higher coverages the Ag 4d states broaden and split, and the threshold moves 2.5 eV to lower binding energy. The splitting of the Ag 4d band is attributed to spin–orbit and crystal field effects. The change in threshold can be attributed to the filling and spreading of the Ag 5s states to form the s–p conduction band of the bulk metal. The measur...

Calculated Properties of Metal Aggregates. I. Diatomic Molecules

The extended Huckel and a modified CNDO method are described and applied to the calculation of electronic properties of Ag2, Au2, Cu2, Pd2, Na2, Ca2, and Cd2, molecules. The results indicate that these calculation procedures may be parameterized to describe the electronic properties of metallic diatomic molecules. The methods of calculation are compared and the origin of bonding in each molecule is described. The ordering of molecular energy levels calculated by EH and CNDO is the same, but their dependence on internuclear distance is different.

Molecular orbital description of catalysis by metal clusters

Abstract Extended Huckel and complete neglect of differential overlap (CNDO) molecular orbital calculations were performed for clusters of metallic atoms to determine possible catalytic properties. Data for pure clusters of Ag, Pd, Cd, Cu, and Ni are reported, as well as for alloy systems of PdNi and CuNi. The results indicate the presence of d -band holes in Ni and Pd clusters with a greater amount in Ni. Clusters of Ag and Cd have as low energy form the linear geometry in preference to other three-dimensional geometries. Clusters have a low bond energy (BE) and a different electronic configuration than their bulk metals. The effect of substrates on metal clusters is examined and is shown to be capable of altering electron occupancy of the cluster.

Molecular orbital description of silver clusters: Electronic structure

Electronic properties derived from calculations of Ag clusters up to 39 atoms in size are compared with bulk properties calculated within the same theoretical framework. Density of states profiles (DOS) determined by extended Huckel theory show a broadening with increasing atoms in this range of sizes. The bulk periodic DOS compare favorably with other calculations. Oscillations in electron affinity, binding energy, and ionization potential for open and closed shells of electrons are observed for Ag clusters using the CNDO method.

Ionic conductivity of AgBr films

Ionic conductivity in {200} and {111} silver bromide films deposited on cleaved mica or cleaved alkali halide crystals in an oil-free system is reported. Interstitial silver ions are the majority carrier in each film, as indicated by the effect of Cd++ on conductivity. These films have a conductivity at least an order of magnitude greater than that of single crystals and a lower temperature coefficient (0.41 eV for {111}, 0.40–0.53 eV for {200}). The substrate for preparation of {200} films has a large effect on AgBr conductivity, which increases with increasing misfit between AgBr and substrate lattice. Values for the free energy of defect formation and potential difference between bulk and surface are reported.

Toward A Coherent Theory of Chemisorption

Studies of chemisorption phenomena, the cornerstone of heterogeneous catalysis, have become the central part of contemporary surface science. As a result of the great variety of the available experimental techniques, a backlog of information, some of which conflicts with current theoretical constructs, has accumulated. New models that combine analytical and computational facets have now begun to appear, revealing intrinsic relations among seemingly disparate chemisorption phenomena. Among the major findings are (i) the crucial role of antibonding adsorbate orbitals in bond activation and in the heat of chemisorption, (ii) adsorbate-induced surface polarization leading to a decrease of the metal work function and to an increase of the surface core binding energy, and (iii) important differences between atomic and molecular adsorbate modes of bonding and surface migration.

Determination of the particle size required for bulk metallic properties

Small transition‐metal clusters have been prepared by vacuum deposition and characterized by transmission electron microscopy. The valence bandwidth has been measured by x‐ray photoelectron spectroscopy (XPS) and compared with the theoretical prediction of extended Huckel and self‐consistent‐field Xα scattered‐wave molecular‐orbital calculations. The semiempirical extended Huckel calculations agree much better with the experimentally determined values.