G. C. Nieman

Chemical probes of metal cluster structure: Reactions of iron clusters with hydrogen, ammonia, and water

Evidence is presented for structural changes in iron clusters in the Fe13 to Fe23 size range. Abrupt changes with cluster size are found for several chemical properties, including reactivity with hydrogen and binding energies of ammonia and water. These changes often come at the same cluster sizes, pointing to a common origin—fundamental changes in the structure of the bare iron clusters. In addition, changes in structure as a consequence of adsorbate binding are suggested. The experimental observations leading to these conclusions are detailed, and possible structures for clusters in this size range are proposed.

The reaction of iron clusters with ammonia. I. Compositions of the ammoniated products and their implications for cluster structure

Studies are described of the chemisorption of ammonia on isolated neutral iron clusters Fen for 2≤n≤165. Clusters are generated by laser vaporization in a continuous‐flow‐tube reactor, and reaction products are detected by laser‐ionization time‐of‐flight mass spectrometry. Ammonia is found to chemisorb nondissociatively on cluster surfaces on the 1 ms time scale of these experiments. Measurements of ammonia uptake provide information on adsorption kinetics and on the number and nature of the binding sites. The ammonia binding energy is found to decrease with increasing cluster coverage. For chemically saturated clusters, the ratio of adsorbed NH3 molecules to surface iron atoms is found to decrease with increasing cluster size, going from >1/3 for small clusters to 100. Ammonia chemisorption is accompanied by a large decrease in cluster ionization potentials, as much as 2 eV for saturated clusters. At sufficiently high exposure the beginning of the formation of a second, physisorbed layer of mo...

REACTIONS OF NI38 WITH N2, H2, AND CO: CLUSTER STRUCTURE AND ADSORBATE BINDING SITES

The gas-phase reactions of nitrogen, hydrogen, and carbon monoxide with Ni38 are studied as a function of reagent pressure at several reaction temperatures. Saturation coverage of the cluster is found at Ni38(N2)24, Ni38H36, and Ni38(CO)36. These saturation levels are consistent with the metal core of the ligated cluster having the structure of a truncated octahedron in each case. An alternate fcc structure derived from a 40-atom truncated tetrahedron is consistent with the nitrogen data, but not with the hydrogen or carbon monoxide results. In addition, the nitrogen uptake data indicate that the bare Ni38 cluster also has the structure of a truncated octahedron or possibly a deformed truncated octahedron. There is no indication that Ni38 has an icosahedral or polyicosahedral structure. The nature of the binding of the three reagents to the cluster is discussed. Evidence is presented that CO initially binds to atop sites, but following saturation of these sites a local rearrangement to bridge sites occurs...

Metal‐deficient iron oxide clusters formed in the gas phase

Molecular beams of oxidized iron clusters are produced by pulsed laser evaporation of iron followed by reaction of the resulting iron clusters with O2 in a continuous flow reactor. Nonstoichiometry of the iron oxide clusters is found, the composition rapidly approaching Fe0.9O as cluster size increases. The relation of these results to the similar nonstoichiometries found in bulk FeO is discussed.

The kinetics of reactions of nickel clusters with hydrogen and deuterium

The kinetics of reactions of nickel clusters with hydrogen and deuterium are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. The abslute rate constants for the addition of the first H2 (D2) molecule to nickel clusters Nin (n=7→36 for H2 andn=7→60 for D2) have been measured. Rate constants are found to be only weakly dependent onn forn≧14, showing a gradual increase with size that scales approximately withn(2/3), i.e., the cluster geometrical cross section. Reaction probabilities for clusters in this size range are approximately 0.6 for H2 and 0.3 for D2. Belown=14, there is a stronger dependence of reactivity on size, with Ni9 being far less reactive than any other cluster studied. These results are compared to bulk nickel studies, and a discussion of possible correlation of reactivity to cluster structure is presented.

The uptake of ammonia by iron clusters: A new procedure for the study of metal cluster chemistry

A new procedure is described for the study of chemical reactions of metal clusters with simple gas phase molecules. It consists of determining the average number of reagent molecules taken up by the clusters as a function of reagent pressure over a very wide range of pressures. When the results of such experiments are appropriately analyzed, they provide information on cluster reaction kinetics, thermodynamics, and the composition of reaction products. An illustration of this procedure for the reaction of iron clusters with ammonia is presented.

The thermodynamics of nitrogen adsorption on nickel clusters: Ni19–Ni71

Equilibrium constants for the chemisorption reactions of molecular nitrogen with nickel clusters Nin have been determined as a function of temperature for n=19 to 71. Van’t Hoff analysis of the data yields standard-state changes in reaction enthalpy and entropy. These changes are related to what is known about nickel cluster structure and the nature of the cluster–N2 interaction. In general, the adsorption energy is highest for the smallest clusters studied, reaching values twice those for N2 adsorption on bulk nickel surfaces. In many cases, there is a correlation between enthalpy and entropy: high adsorption energy is accompanied by a large change in entropy, and vice versa. These effects are discussed in terms of the configurational entropy of reaction and the frequencies of the frustrated translational and rotational motions of the adsorbed N2 molecules.

Binding of deuterium to icosahedral nickel and cobalt clusters

The binding of deuterium atoms to icosahedral nickel and cobalt clusters in the 55 to 147 atom size region is studied at near room temperature conditions. The icosahedral clusters have closed shells at 55 and 147 atoms and closed subshells at many intermediate sizes. Deuterium saturation levels are determined for many of the closed shell and subshell clusters in this size region. From the series of saturation levels a set of binding rules (appropriate to room temperature conditions) have been determined that allow a prediction of the observed coverages for the entire series of closed shell and subshell clusters. The configurations of D-atom binding sites for clusters containing 55, 71, 83, 92, 101, 116, and 147 atoms are given. In general, the ratio of D atoms to surface metal atoms is near unity.

The reaction of manganese clusters and manganese cluster carbides with hydrogen: The Mn–CH3 bond energy

Manganese clusters are formed by laser vaporization in an inert gas condensation source cooled to −160 °C. A pure manganese target is used as well as a manganese target containing 2 mol % carbon. The clusters are reacted with hydrogen both in the region of cluster growth and in a flow‐tube reactor (FTR) downstream of the cluster source. The reactions, both with hydrogen atoms in the cluster growth region and dissociative chemisorption of H2 in the FTR, result in hydrogen atoms bound to the clusters, except that for Mn15 and smaller clusters the H atoms are unstable against H2 desorption. Above Mn15 stable hydrides are formed, but the rate of reaction in the FTR varies considerably with cluster size. This abrupt change in the ability to bind hydrogen may reflect a significant change in the character of the bonding within the cluster, perhaps from van der Waals to metallic. MnnC clusters readily react with hydrogen for n≳6 to form MnnCH2. Further reactivity generally follows the pattern of the bare clusters...

Investigation of structural changes in Ni19 and Ni23 induced by adsorption of hydrogen/deuterium and ammonia

Two-reagent reactions of Ni19 and Ni23 with hydrogen/deuterium and ammonia are studied to probe adsorbate-induced cluster structural changes. H/D and NH3 bind to the clusters noncompetitively, i.e., to different sites, so simultaneous saturation by the two reagents can be achieved. It is found that H/D adsorption causes a change in Ni19 from the double icosahedral structure to a face-centered cubic (fcc) or hexagonal close-packed (hcp) one, but that subsequent saturation with NH3 converts the cluster back to the double icosahedron. No such structural changes are seen for the triple icosahedral Ni23. The results for Ni19 are interpreted in terms of the electron withdrawing or donating character of the ligands and the consequent effect on the extent of d-orbital bonding in the cluster. Possible configurations of the H/D atom binding sites on the icosahedral Ni19 and Ni23 clusters and on the fcc/hcp Ni19 cluster are presented.