Cluster size selectivity in the product distribution of ethene dehydrogenation on niobium clusters
ABSTRACT 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 Nb8 and Nb10, 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 Nb10, yielding either Nb(n)C2H2 or Nb(n)C2. Over the range Nb-Nb6, 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 C2H4, C2H2, or C2. 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 calculations of structures of Nb clusters up to Nb(6), and the reaction products Nb(n)C2H2 and Nb(n)C2 (n = 1...6) are presented. Investigation of the thermochemistry for the dehydrogenation of ethene to form molecular hydrogen, for the Nb atom and clusters up to Nb6, demonstrates that the exergonicity of the formation of Nb(n)C2 species increases with cluster size over this range, which supports the proposal that the extent of dehydrogenation is determined primarily by thermodynamic constraints. Analysis of the structural variations present in the cluster species studied shows an increase in C-H bond lengths with cluster size that closely correlates with the increased thermodynamic drive to full dehydrogenation. This correlation strongly suggests that all steps in the reaction are barrierless, and that weakening of the C-H bonds is directly reflected in the thermodynamics of the overall dehydrogenation process. It is also demonstrated that reaction exergonicity in the initial partial dehydrogenation step must be carried through as excess internal energy into the second dehydrogenation step.
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ABSTRACT: Far-infrared absorption spectra of small neutral and cationic niobium clusters containing five to nine Nb atoms have been obtained by multiple photon dissociation spectroscopy of their argon complexes. The experimental far-IR spectra are recorded in the 85–600 cm−1 region and cover the range of the structure-specific vibrational fundamentals, i.e., the finger-print range, for these metal clusters. The experiments are accompanied by quantum chemical calculations employing the density-functional theory. A comparison of the experimental and calculated far-IR spectra allows to identify the cluster structures. Although the experimental spectra for clusters containing five, six, eight, and nine Nb atoms are very different for cationic and neutral clusters, the comparison with theory reveals that, nevertheless, the overall geometries for cations and neutrals are very similar, except for Nb60/+.The Journal of Chemical Physics 12/2007; 127(23):234306-234306-8. DOI:10.1063/1.2806176 · 3.12 Impact Factor
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ABSTRACT: We report a joint experimental and theoretical study of the gas-phase reactivity of Al cluster anions with ethanethiol (EtSH) in a fast-flow tube reactor. Nearly all Aln- clusters observed are reactive at the presence of sufficient EtSH molecules with minor exceptions. Sulfide species AlnSm- dominate the observed reaction products, indicating C-S bond activation of EtSH. By using DFT calculations we provide an in-depth analysis on the interesting cluster reactivity of Aln- with EtSH. It is demonstrated that the desulfurization leading to AlnSm- products is associated with the dehydrogen processes successively initiated by S-H bond cleavage and C-H bond cracking.Chemical Physics Letters 12/2013; 590:63-68. DOI:10.1016/j.cplett.2013.10.074 · 1.99 Impact Factor
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ABSTRACT: The main purpose of this work is to study metal−molecule interactions that can lead to the production of molecular hydrogen. Two systems were chosen for this analysis: yttrium atom and clusters interacting with the simple electron donor ammonia (NH3) and copper atoms and ions with imidazole. For yttrium with ammonia as well as for copper with imidazole there is a charge-transfer process from the metal to the molecule that promotes the dissociation of the hydrogen atoms.Journal of Chemical Theory and Computation 02/2007; 3(3). DOI:10.1021/ct6003615 · 5.31 Impact Factor