Parnis J Mark, Escobar-Cabrera Eric, Thompson Matthew G K, Jacula J Paul, Lafleur Rick D, Guevara-García Alfredo, Martínez Ana, Rayner David M
Department of Chemistry, Trent University, Peterborough, Ontario, Canada K9J 7B8.
J Phys Chem A. 2005 Aug 18;109(32):7046-56. doi: 10.1021/jp0506944.
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.
在快速流动的金属团簇反应器中研究了乙烯与铌原子以及含有多达25个组成原子的团簇的反应。这些团簇与乙烯的反应速率约为气体动力学碰撞速率，这表明在团簇去除步骤中存在无障碍缔合过程。例外的是Nb8和Nb10，观察到它们的反应速率显著降低，这反映了一些团簇尺寸选择性。对实验初级产物质量的分析表明，除了Nb10之外，所有团簇都会使乙烯脱氢，生成Nb(n)C2H2或Nb(n)C2。在Nb - Nb6范围内，脱氢程度随团簇尺寸增加，然后对于更大的团簇则降低。对于许多团簇，还观察到了二级和三级产物质量，显示出对应于净添加C2H4、C2H2或C2的不同程度的脱氢。对于铌原子和几个小团簇，观察到至少六个乙烯分子的形式加成，这表明聚合过程可能是活跃的。对铌原子和几个Nb(n)团簇与乙烯反应的动力学分析表明，该过程与乙烯单元的顺序加成一致，加成速率大致对应于几个连续反应的乙烯分子的气体动力学碰撞频率。发现乙烯摄取速率存在一些变化，这可能反映了与各个机理步骤相关的小能量障碍或空间位阻。给出了高达Nb(6)的铌团簇以及反应产物Nb(n)C2H2和Nb(n)C2（n = 1...6）的密度泛函结构计算。对铌原子和高达Nb6的团簇，乙烯脱氢形成分子氢的热化学研究表明，在此范围内，Nb(n)C2物种形成的放能性随团簇尺寸增加，这支持了脱氢程度主要由热力学限制决定的提议。对所研究的团簇物种中存在的结构变化的分析表明，C - H键长度随团簇尺寸增加，这与完全脱氢的热力学驱动力增加密切相关。这种相关性强烈表明反应的所有步骤都是无障碍的，并且C - H键的弱化直接反映在整体脱氢过程的热力学中。还证明了初始部分脱氢步骤中的反应放能性必须作为过量内能传递到第二个脱氢步骤中。
