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

Abstract

/npsi/ctrl?action=rtdoc&an=12327971&lang=en http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=12327971&lang=fr READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/jsp/nparc_cp.jsp?lang=en Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. Questions? Contact the NRC Publications Archive team atPublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information. NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. For the publisher's version, please access the DOI link below./ Pour consulter la version de l'éditeur, utilisez le lien DOI ci-dessous.http://doi.org/10.1021/jp0506944The Journal of Physical Chemistry A, 109, 32, pp. 7046-7056, 2005-07- Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, Ontario, Canada K1A 0R6 ReceiVed: February 8, 2005; In Final Form: May 13, 2005 Ethene reactions with niobium atoms and clusters containing up to 25 constituent atoms have been studied in a fast-flow metal cluster reactor. The clusters react with ethene at about the gas-kinetic collision rate, indicating a barrierless association process as the cluster removal step. Exceptions are Nb 8 and Nb 10 , for which a significantly diminished rate is observed, reflecting some cluster size selectivity. Analysis of the experimental primary product masses indicates dehydrogenation of ethene for all clusters save Nb 10 , yielding either Nb n C 2 H 2 or Nb n C 2 . Over the range Nb-Nb 6 , the extent of dehydrogenation increases with cluster size, then decreases for larger clusters. For many clusters, secondary and tertiary product masses are also observed, showing varying degrees of dehydrogenation corresponding to net addition of C 2 H 4 ,C 2 H 2 ,o rC 2 . With Nb atoms and several small clusters, formal addition of at least six ethene molecules is observed, suggesting a polymerization process may be active. Kinetic analysis of the Nb atom and several Nb n cluster reactions with ethene shows that the process is consistent with sequential addition of ethene units at rates corresponding approximately to the gas-kinetic collision frequency for several consecutive reacting ethene molecules. Some variation in the rate of ethene pick up is found, which likely reflects small energy barriers or steric constraints associated with individual mechanistic steps. Density functional calcu...

Reaction of niobium and tantalum neutral clusters with low pressure, unsaturated hydrocarbons in a pickup cell: From dehydrogenation to Met-Car formation

“…In general, considerations of the preceding subsection on benzene reactions with Nb n can be employed to explain experimental results for the dehydrogenation reaction of various unsaturated hydrocarbons on Nb n and Ta n neutral clusters. Since ethylene dehydrogenation on niobium neutral and ionic clusters is also extensively studied by fast flow ͑high pressure͒ and FT-ICR ͑low pressure͒ experiments, 14,16,19 a comparison between the present pickup cell results and those previous studies is emphasized in this subsection.…”

“…The reaction of Nb n with C 2 H 4 has been suggested to be barrierless for both the association and dehydrogenation steps, and the extent of dehydrogenation is primarily determined by thermodynamic constraints. 14 We reexamine these suggestions and ideas in the light of the new pickup cell reaction experiment reported below.…”

“…Pierre et al also studied Nb n reactivity toward several cyclic hydrocarbons; 8 no product was observed with cyclohexane as a reactant but physisorbed and chemisorbed products are readily detected with unsaturated molecules ͑cyclohexene and 1,3-cyclohexadiene͒. Nb n clusters have also been reacted with isobutane 13 and ethylene 14 in a fast flow reactor. Although products were observed in both experiments, the bare metal cluster depletion rate for ethylene is generally two orders of magnitude larger than the depletion rate for the saturated molecule, isobutane.…”

Onset of carbon–carbon bonding in the Nb5Cy(y = 0–6) clusters: a threshold photo-ionisation and density functional theory study

“…34 They found the lowest energy isomer to have the two C atoms bound to separate Nb faces of the trigonal bipyramid Nb 5 cluster. In the present study the lowest energy isomer III A [ 2 B 1 , C 2v ] has the C atoms bound across the two Nb butterfly motifs of the oblate trigonal bipyramid Nb 5 cluster.…”

The far-infrared spectra of neutral and cationic niobium clusters: Nb5∕+ to Nb9∕+

“…13,[26][27][28][29][30][31][32][33][34][35] The reaction rate towards H 2 and N 2 is found to correlate with the electronic properties as expressed, e.g., by an ͑effective͒ ionization potential of the cluster. 13,34 The presence of isomers for several cationic and neutral clusters in the size range between 9-20 atoms has been observed in reactivity studies [30][31][32][33][34][35] but also spectroscopically.…”