The geometrical structure of small nickel clusters is probed via molecular adsorption of nitrogen on their surfaces. Nitrogen uptake patterns can be rationalized with the proposed structures if it is assumed that N2 binds to every exposed nickel atom, that the binding energies decrease with increasing metal—metal coordination, and that atoms that are four or less coordinate can bind two nitrogen molecules. In some cases nitrogen adsorption causes a change in cluster structure, usually to one that can accommodate more nitrogen molecules. Cluster structures are proposed for all clusters (bare and nitrogenated) in the 3–15-atom size range except Ni4 and Ni11. The nitrogen uptake for Ni4 is consistent with virtually any structure, and the data for Ni11 could not be interpreted in terms of a specific structure. In general, nickel cluster structures are different from those found for rare gas clusters as well as those derived from bulk packing. A comparison of the experimental results with existing theoretical calculations is presented.

Methane activation by nickel cluster cations, Ni n + (n=2-16): Reaction mechanisms and thermochemistry of cluster-CH x (x=0-3) complexesThe molecular adsorption of nitrogen on nickel clusters is used to probe the clusters' geometrical structures. The application of nitrogen binding rules derived from earlier studies of both larger and smaller nickel clusters allows a determination of structure from nitrogen uptake patterns. In the 16and 28-atom size region cluster structure is dominated by local pentagonal symmetry, a consequence of a preference for close packing of atoms on clusters with curved surfaces. In most cases, the structures that result can be derived from the 13-atom icosahedron, the polyicosahedral 19-, 23-, and 26-atom clusters, and the 55-atom icosahedron, by adding or removing atoms. Icosahedral and polyicosahedral clusters often have substantial surface strain, which in some cases is relieved by deviations from the ideal geometry. Structures are proposed for all clusters in the Ni 16 to Ni 28 size range, with the exception of Ni 27 . Generally, there is no evidence for structural changes as a consequence of nitrogen binding, so that the proposed structures are those of the bare as well as the nitrogenated clusters. Where possible, comparison with existing theoretical calculations of nickel cluster structure is made.

Articles you may be interested inTheoretical studies of the structure and dynamics of metal/hydrogen systems: Diffusion and path integral Monte Carlo investigations of nickel and palladium clusters Tworeagent reactions of iron clusters with ammonia and deuterium: Saturated compositions and the kinetics of reactions of deuterium with ammoniated clusters J. Chem. Phys. 90, 1526 (1989); 10.1063/1.456095Chemical probes of metal cluster structure: Reactions of iron clusters with hydrogen, ammonia, and waterThe kinetics of the gas phase reactions of hydrogen and deuterium with iron clusters in the range Fe 6 to Fe 68 have been investigated. It is found that reaction rate constants are a strong function of cluster size, varying by more than four orders of magnitude in this size range. The largest rate constants correspond to approximately 3 % of a hard sphere cross section. Abrupt changes in the rate constant from one cluster to the next are seen. Qualitative temperature dependencies of cluster reactivity have been determined. The more reactive clusters show decreased reactivity with increased temperature, while the least reactive clusters become more reactive. Strong isotope effects are seen only in the FelO to Fe!4 size range. Mechanisms for the reactions ofH 2 and O 2 with iron clusters are discussed in light of these observations.

Copper clusters in the 50- to 100-atom size range are found to exhibit electronic shell structure as well as icosahedral geometry. Clusters corresponding to filled shells have minimum intensity in near-threshold photoionization mass spectra, implying that they have locally higher ionization potentials than other cluster sizes. The chemical stability of these clusters is illustrated by a reduced reactivity towards O2. Cluster geometry is probed via the equilibrium reactions with H2O: Clusters having one copper atom more than closed icosahedral subshells show an enhanced binding of water. The relative importance of electronic and geometrical structure in determining cluster chemical properties is discussed.

The reactions of nickel clusters with ammonia and with water are used to probe cluster geometrical structure. Ammonia uptake experiments allow the determination of the number of preferred binding sites on cluster surfaces. This number shows pronounced minima in the 50- to 116- atom size range for many of the cluster sizes that appear as magic numbers in mass spectra of rare gas clusters. Since these magic numbers arise from closings of shells and subshells of the Mackay icosahedra, the correlation suggests that ammoniated nickel clusters in this size region also have icosahedral structure. Similar structure is found for ammoniated clusters smaller than ∼30 atoms, but is not seen for room temperature clusters in the vicinity of the third shell closing at 147 atoms. Icosahedral features do appear for the larger clusters at elevated temperatures. For many clusters above 50 atoms, prolonged exposure to ammonia causes a conversion from the icosahedral structure to some other structure that binds more ammonia molecules, and often the two structures are seen together. The equilibrium reaction of a single water molecule with the bare clusters probes the strength of the cluster–water bond. Enhanced water adsorption is often seen for clusters one atom larger than those showing minima in ammonia uptake, suggesting that these bare clusters likewise have icosahedral structure. The reasons for minima in ammonia uptake and maxima in water binding are discussed.