Studies of the Chemical Properties of Size Selected Metal Clusters: Kinetics and Saturation

Abstract

Our studies of small (n=2−40) gas phase transition metal clusters (cations, neutrals and anions) have revealed a number of size-dependent chemical and physical properties. This paper will discuss results involving activated dissociative molecular chemisorption of hydrogen (deuterium) and small alkanes on cationic, neutral and anionic platinum and gold clusters. For example, we have yet to find any size gold cluster anion to exhibit measurable reactivity towards di-deuterium, but reactivity is observed on small (n<15) gold cations and very small (n<9) neutral gold clusters. Methane or ethane chemisorption occurs most readily only on small Pt and Au cluster cations and least rapidly, if at all, on cluster anions. The larger Pt and Au clusters are found to be non-reactive towards the alkanes under our experimental conditions.At the other extreme, at steady state, small clusters of Pt, Rh, Ni, Pd, V, Nb, and Ta are hydrogen “rich” exhibiting H/M stoichiometrics >> 1. These results are consistent with a limited data base on supported clsutes obtained via EXAFS measurements and have important implications for catalytic reactions involving hydrogen and light hydrocarbons.

Surface chemistry of catalysis by gold

“…Calculations indicate that gold above the oxygen vacancy is electron rich as a charge transfer is imagined to take place from the support to the gold particle. Such an active site would also be consistent with gas phase experiments which demonstrated that oxygen preferred to adsorb on negatively charged clusters (170). Since there is some evidence that oxygen adsorption (and dissociation) is the rate-limiting step (87,196,388), the presence of anionic clusters is perhaps crucial to the reaction's success.…”

“…Cox et al examined adsorption of D2, O2 and CH4 on various sized gold clusters (170,171). The authors found that for cationic gold clusters of less than 16 atoms, deuterium readily adsorbed.…”

“…Some have amended this idea with the corollary that the thickness of the gold particle is the critical parameter. The role of the support is merely to stabilize these small particles (192,196,323,388,392 ) species which results as a consequence of the interaction of a particle with a F center defect in the underlying oxide support and which allows for the adsorption (and possible dissociation) of oxygen as shown in gas phase experiments (170,184,185,220,247,374,(462)(463)(464)(465). 5 The active site for catalysis involves the interface between the particle and support.…”

Size‐Dependent Structural Evolution and Chemical Reactivity of Gold Clusters

“…Early studies have shown that the reactivities of small gas-phase gold clusters (Au n in the range of up to 30 atoms) depend on the number of atoms and the charge state of the clusters, with cluster anions with an even n showing enhanced reactivity. [23,24] These observations were confirmed and extended more recently, [25,26] including observations of oddeven oscillations in the O 2 adsorption probability, which correlate with those measured for vertical detachment energies (vDEs). [27] It has been shown theoretically that the reactivity toward oxygen of gold cluster anions originates from their propensity to transfer electronic charge into the antibonding 2p* orbital of molecular oxygen, thus weakening the OÀO bond which becomes activated into a superoxo-or peroxo-like state; a similar charging effect occurs upon bonding of CO to charged (gas-phase or supported) gold clusters.…”

Nanocatalysis

“…In addition, the reactive clusters adsorb just one O 2 molecule. The relative reactivities correlate well with measured electron affinities of neutral clusters, suggesting a mechanism involving the adsorption of O 2 as a one-electron acceptor [36,37]. The reactivity of gold cluster anions with CO also shows a distinct size-selectivity, although less pronounced than that with O 2 [38].…”