Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany.
Phys Chem Chem Phys. 2011 Jan 21;13(3):966-77. doi: 10.1039/c0cp00078g. Epub 2010 Nov 12.
The conversion of cis-2-butene with deuterium over a well-defined Pd/Fe(3)O(4) model catalyst was studied by isothermal pulsed molecular beam (MB) experiments under ultra high vacuum conditions. This study focuses on the processes related to dissociative hydrogen adsorption and diffusion into the subsurface of Pd nanoparticles and their influence on the activity and selectivity toward competing cis-trans isomerization and hydrogenation pathways. The reactivity was studied both under steady state conditions and in the transient regime, in which the reaction takes place on a D-saturated catalyst, over a large range of reactant pressures and reaction temperatures. We show that large olefin coverages negatively affect the abundance of D species, as indicated by a reduction of both reaction rates under steady state conditions as compared to the transient reactivity on the catalyst pre-saturated with D(2). Limitations in D availability during the steady state lead to a very weak dependence of both reaction rates on the olefin pressure. In contrast, when the surface is initially saturated with D, the transient reaction rates of both pathways exhibit positive kinetic orders on the butene pressure. Cis-trans isomerization and hydrogenation show kinetic orders of +0.7 and +1.0 on the D(2) pressure, respectively. Increasing availability of D noticeably shifts the selectivity toward hydrogenation. These observations together with the analysis of the transient reaction behavior suggest that the activity and selectivity of the catalyst is strongly controlled by its ability to build up and maintain a sufficiently high concentration of D species under reaction conditions. The temperature dependence of the reaction rates indicates that higher activation energies are required for the hydrogenation pathway than for the cis-trans isomerization pathway, implying that different rate limiting steps are involved in the competing reactions.
在超真空条件下，通过等温脉冲分子束（MB）实验研究了顺式-2-丁烯在具有明确定义的 Pd/Fe(3)O(4)模型催化剂上与氘的转化。本研究重点关注与解离氢吸附和扩散到 Pd 纳米粒子的次表面有关的过程，以及它们对活性和对竞争的顺式-反式异构化和加氢途径的选择性的影响。在稳态和瞬态条件下研究了反应性，在瞬态条件下，反应在 D 饱和催化剂上进行，反应物压力和反应温度范围很大。我们表明，大烯烃覆盖率会降低 D 物种的丰度，这表现在与 D(2)预饱和催化剂上的瞬态反应性相比，稳态下的反应速率都降低了。在稳态下 D 的可用性有限会导致两种反应速率对烯烃压力的依赖性非常弱。相比之下，当表面最初被 D 饱和时，两种途径的瞬态反应速率都对丁烯压力表现出正动力学顺序。顺式-反式异构化和加氢分别对 D(2)压力表现出 +0.7 和 +1.0 的动力学顺序。D 的可用性增加会明显改变加氢的选择性。这些观察结果以及对瞬态反应行为的分析表明，催化剂的活性和选择性受其在反应条件下建立和维持足够高的 D 物种浓度的能力强烈控制。反应速率的温度依赖性表明，加氢途径比顺式-反式异构化途径需要更高的活化能，这意味着竞争反应涉及不同的速率限制步骤。
