Gas-phase niobium cluster reactivity with isobutane

Abstract

A laser ablation fast-flow reactor apparatus, equipped with a laser ionization time-of-flight mass spectrometer detector, has been used to investigate the reactivity of gas-phase Nb n with isobutane. Absolute second-order rate coefficients k (2) for the removal of Nb n by isobutane were measured at 279, 300, and 371 K in a continuous flow of He carrier gas held at 1 or 2 Torr total flow tube pressure. The rate coefficients for a given cluster species were found to be pressure independent, and became larger with increasing temperature. A gradual increase in k (2) with increasing cluster size has been observed. This increase is simply explained by a direct correlation of k (2) with the hard-sphere collision cross section. Pseudo-first-order kinetic plots for all cluster reactions with isobutane were linear with no bimodal character evident, indicating an insensitivity to structural forms of Nb n . Activation energies have been estimated, and their values were found to be close to zero for all Nb n cluster reactions with isobutane. The small magnitudes of k (2) relative to k hs have been attributed to a reaction mechanism involving a ''tight'' transition-state complex. Product masses, after the reaction of a bare Nb cluster with one or more isobutane molecules, clearly show dehydrogenation to be an active process. A pseudo-first-order kinetic model for a consecutive bimolecular reaction scheme has been solved for the Nb 2 /isobutane system, and is in good agreement with the experimental data.

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

“…Later ICR work by Elkind et al 10 confirmed these conclusions. Note that all such examples of cluster size selectivity in cluster reactivity rate are in contrast with the observed behavior of Nb clusters with CO 11 and isobutane, 13 for which only gradual increases in reaction rate coefficient with cluster size are observed.…”

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

“…9 Lafleur et al observed smooth increases in rate coefficients for Nb cluster reactions with isobutane. 3 Nearmonotonic, modest increases in rate coefficients with increasing cluster size therefore appear to be characteristic of alkane reactions with early transition-metal clusters. This reactivity characteristic contrasts sharply with the observation that such clusters may exhibit marked cluster size selectivity in the extent to which they undergo subsequent chemical transformations, as is demonstrated by the variation in degree of dehydrogenation in the Nb cluster/isobutane work.…”

“…This fact is reflected in the significant variations in the extent of dehydrogenation with cluster size previously reported for Nb cluster reactions with isobutane. 3 Such an observation implies that sensitivity of the product distributions to the cluster size may be observed that is not reflected correspondingly in the rate of cluster removal. This is a key observation in terms of understanding the anomalous absence of selectivity in the kinetics of transition metal cluster reactions with larger molecules.…”

“…Rate coefficients for the reactions of Nb clusters with isobutane have been modeled in this way. 3 The success of the model indicates that the physical size of clusters, or some other extensive cluster property, plays a major role in determining rates of reactions in nonactivated systems. In more recent work, we have discussed the implications that cluster polarizability has on reactivity in determining the collision rate and the stability of physisorbed precursor states, which serve as intermediates in many cluster reactions.…”

The influence of polarizability in metal cluster reactions as seen in the reactions of gas-phase W clusters with cyclopropane

“…11 The rate coefficients for the reaction of W clusters with cyclopropane, however, cannot be modeled strictly in terms of the hard-sphere model as most of the coefficients exceed the hard-sphere collision rates. Using 2.2ϫ10 Ϫ10 m as the effective radius of cyclopropane 12 and a spherical-drop model of the W cluster radius, 11 hard-sphere collision rates can be calculated. A comparison of these calculated collision rates with the experimentally determined W n /cyclopropane rate coefficients is shown in Fig.…”