Surface reactions of metal clusters. II. Reactivity surveys with D2, N2, and CO

Abstract

Reactions on the surface of a variety of transition metal clusters have been studied in the gas phase at near room temperature using a newly developed fast-flow reaction device. Initial examples of the use of this device are provided by survey studies of the reactivity of iron, cobalt, nickel, copper, and niobium clusters in contact with low concentrations of D2, N2 and CO. Dissociative chemisorption of D2 is found to occur with dramatic sensitivity to cluster size in the cases of iron, cobalt, and niobium clusters, the detailed pattern of reactivity differing markedly for each metal. The corresponding reaction is also observed with nickel clusters, but here the reactivity shows only a slow, steady increase with cluster size. Copper clusters are found to be completely unreactive to H2 chemisorption under these conditions. Molecular nitrogen is found to chemisorb readily to clusters of cobalt and niobium, with a reactivity pattern very similar to that observed with D2. Iron clusters are found to show slight reactivity with N2; only a small amount of chemisorption is observed on the most reactive clusters at high N2 concentration, but the pattern of this reactivity with cluster size is consistent with that observed in D2 chemisorption. In contrast to these highly structured reactivity patterns of D2 and N2, carbon monoxide is found to show only a slow, monotonic increase in reactivity with cluster size. It is suggested that these dramatic reactivity patterns for chemisorption on metal clusters provide stringent tests for future theories as to the nature of chemisorption on metal surfaces at a detailed, molecular level.

Cluster Size Selectivity in the Product Distribution of Ethene Dehydrogenation on Niobium Clusters

“…In the presence of excess He bath gas the clusters equilibrate to flow tube temperature and pressure conditions before reaching the reaction zone. 11,31 Flow tube pressure is varied in the range 0.5 to 2.2 Torr by partially closing the gate valve to the vacuum (600 Ls -1 Edwards EH2600 Rootes blower backed by an Edwards E1M275 mechanical pump). Data were collected at 300 K and at constant pressures ranging from 0.4 to 1.5 Torr.…”

“…Early work by Morse et al 11 showed that Nb clusters exhibit significant cluster-size selectivity, with particularly low reactivity noted for Nb 8 ,N b 10 , and Nb 16 with D 2 and N 2 . This result has been confirmed more recently by absolute rate coefficient measurement by Berces et al 14 Zakin et al 12 showed that these cluster masses, and Nb 12 , showed low reactivity as cationic, neutral or anionic clusters with D 2 .…”

Association Complex Formation in Gas-Phase Ta Cluster Reactions with Simple Alkanes: Probing the Role of Entropy in Rate Determination for Barrierless Adsorption Processes

“…Indeed, reactions of CO with several transition metal clusters are known to show nonsize selective chemistry reminiscent of the alkane rate coefficient data presented here. 13 Small molecules and those not normally thought of as strong coordinating ligands, such as H 2 , and N 2 , for which selectivity is commonly observed, 1,[6][7][8][9][10][11][12][13][14][15][16] are expected to form much weaker association complexes, involving less dative bond formation in the precursor complex. This should be augmented by lower physisorption energies associated with their decreased polarizability.…”

“…1,13,16 The absence of any significant potential energy barrier to reaction is thought to distinguish the CO and O 2 reactions from those of other diatomics. The temperature independence of the Nb n -isobutane rate coefficients exemplifies the absence of a significant activation energy barrier in metal cluster-hydrocarbon reactions.…”

“…This approach has been used to establish structure-dependent reactivity as a paradigm in the field of cluster science. 1,[6][7][8][9][10][11][12][13][14][15][16] However, for classes of cluster reactions that show little size dependence in the rate of cluster removal, the cluster/flow system approach affords an opportunity to systematically vary cluster mass and corresponding number of constituent atoms, to determine the effect of these parameters on the reaction rate. In addition, working with a family of chemically similar molecules differing principally in terms of their size and entropy offers an opportunity to directly probe the role of entropy in determining reaction rate for barrierless cluster reactions.…”