R. O. Brickman

Gold clusters: reactions and deuterium uptake

We have examined the reactivity and saturation of small gold clusters (cations, neutrals and anions) towards several molecules and find that specific small gold clusters exhibit a pronounced variation in their reactivity towards hydrogen, methane and oxygen. The reactivity not only depends strongly on cluster size but also on the cluster charge state. For example, small (n<15) gold cations react readily with D2, but no evidence of reaction is observed for the anions under our experimental conditions. Similar behavior is seen for methane. With oxygen only even atom (odd electron) anions are reactive, and Au10+ is the only cation which exhibits evidence of reaction. The global features (small cluster cations reactive towards H2, CH4, but large ones not reactive, odd electron anions reactive towards O2) are qualitatively explained by appealing to a simple frontier orbital picture. The uptake of deuterium and methane on gold clusters also exhibits a pronounced size dependence with D/Au varying from a high of 3 for the dimer to zero for clusters containing more than 15 Au atoms. Comparison of the methane and deuterium saturation behavior leads us to suggest that methane is dissociated and bound as CH3 and H.

Dependence of metal cluster reaction kinetics on charge state. I. Reaction of neutral (Nbx) and ionic (Nb+x, Nb−x) niobium clusters with D2

The effect of charge state on niobium cluster chemisorption kinetics is explored via measurement of the relative rates of D2 activation by Nb−x, Nbx, and Nb+x containing up to 28 atoms. The presence of the + or − charge is found to have only a minor effect on rate for the majority of the clusters, with the reactivity of the ions being generally within a factor of 2.5 of the corresponding neutrals. The excess charge does, however, have a profund influence on reaction rate for a number of clusters in the 7≤x≤16 size range, which may be indicative of the importance of cluster electronic structure in the chemisorption process. Kinetic data for Nb9, Nb12, and Nb+12 are found to deviate significantly from the expected pseudo‐first‐order behavior, suggesting the existence of structural isomers for these species. The anomalous behavior for Nb9 and Nb12 was not observed in previous neutral Nbx chemisorption studies. The maximum uptake of D2 by niobium clusters is found to be essentially independent of charge state...

Dependence of metal cluster reaction kinetics on charge state. II. Chemisorption of hydrogen by neutral and positively charged iron clusters

The kinetics of D2 chemisorption by gas‐phase neutral (Fex ) and positively charged (Fe+x) iron clusters has been studied under identical reactor conditions. Similar to the neutrals, the reaction rate for the ions displays a nonmonotonic dependence on x, varying by more than four orders of magnitude between x = 1 and x = 31. The presence of the positive charge is found to have a substantial influence on reaction rate for the majority of clusters in the 1–31 atom size range. Clusters containing 4–6 and >17 atoms experience an enhancement in rate while in contrast those containing 3 and 9–14 atoms experience a decrease in rate. Further, studies of H2 chemisorption onto Fe+x (x = 4–22) indicate an enhanced, cluster size‐dependent isotope effect for the cluster ions which is quite similar to that previously observed for Fex. The effect of the positive charge on D2 chemisorption reactivity is explained within the framework of the frontier orbital model of activated chemisorption by invoking an ‘‘effective’’ ac...

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

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 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.

Abnormally large deuterium uptake on small transition metal clusters

Deuterium uptake experiments on gas phase transition metal cluster cations of Ni, Pt and Rh show that the small (< 10 Å dia.) clusters can bind many (up to 8) deuterium atoms per metal atom in the cluster, in contast to (H(D)/M)max ratios near unity typically reported for single crystal metal surfaces and in previous uptake experiments on nickel and iron clusters [11]. Abnormally large (H(D)/M)max ratios appear to be the rule rather than the exception for small transition metal clusters, an effect which has strong implications in chemical and catalytic processes involving hydrogen chemisorption.

Size‐selective dehydrogenation of benzene by gas‐phase niobium cluster ions (Nb+x)

The gas‐phase reaction of positively charged niobium cluster ions Nb+x with normal and perdeuterobenzene has been studied using a fast‐flow reactor and direct ion sampling techniques. Chemisorption of benzene onto the ion is facile, with the rate being essentially independent of x. However, both the resulting product distributions, consisting of species NbxC6H+m and NbxC6D+m with m=0,2,4,6, and the relative product yields not only display a dramatic dependence on cluster size, but are also remarkably similar to those previously observed for neutral Nbx using gentle, low‐fluence, single‐photon 6.42 eV photoionization. Direct observation of products with m≤6 indicates that both chemical dehydrogenation of benzene and desorption of hydrogen from the cluster ions must occur in the fast‐flow reactor. Further, the relative yield of dehydrogenated products is found to increase substantially with increase in cluster ‘‘temperature.’’ Complete dehydrogenation of benzene to m=0 is also found to occur for niobium clu...

IR laser-induced decomposition of volatile uranyl complexes II. New double resonance results

Abstract New IRIR double resonance measurements are presented for the 10.6 μm CO 2 -laser-induced dissociation of UO 2 [(CF 3 CO) 2 CH] 2 ·(CH 3 O) 3 PO (UO 2 L 2 TMP). The results confirm that loss of base, TMP, is preceded by isomerisation of the parent molecule to a species with a red-shifted UO 2 absorption profile; but the red shift is much less than previously deduced. A red shift is also reported for product UO 2 L 2 , and is attributed, like that of the isomer, to partial detachment of one of the bidentate ligands. Isomerisation and dissociation yields are lower than deduced from the previous double resonance measurements, but agree satisfactorily with results from a refined analysis of molecular beam experiments (paper I), demonstrating that the uranyl absorption profile must have a significant inhomogeneous component. The present isomerisation yields also disagree profoundly with laser-induced fluorescence depletion measurements, and it is argued that the latter measure a photophysical, rather than a photochemical, process.