Influence of carbon deposition on the hydrogen distribution in Pd nanoparticles and their reactivity in olefin hydrogenation.
Influence of carbon deposition on the hydrogen distribution in Pd nanoparticles and their reactivity in olefin hydrogenation.
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DOI：
10.1002/anie.200801923
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发表时间：
2008-11
影响因子：
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通讯作者：
M. Wilde;K. Fukutani;Wiebke Ludwig;Björn Brandt;Janina Fischer;S. Schauermann;H. Freund
中科院分区：
文献类型：
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作者：
M. Wilde;K. Fukutani;Wiebke Ludwig;Björn Brandt;Janina Fischer;S. Schauermann;H. Freund
由反应物的早期分解产生的碳质沉积物的积累被认为显著影响过渡金属促进的烃转化的活性和选择性。[1]碳诱导的反应性变化的潜在微观机制最近已经使用表面科学方法用于炔的选择性氢化[2]和烯烃的异构化和氢化。[3]特别是，我们已经表明，持续的活性对烯烃双键的氢化，可以实现只有在存在的共吸附的强烈脱氢的碳质沉积物。[3]然而，这些碳物种在促进持久氢化活性中发挥的确切作用仍然未知。通常，烯烃加氢的促进作用似乎与氢原子在金属催化剂上的吸附状态密切相关。Ceyer等人第一次质疑了只有表面氢物种参与氢化过程的传统观点[1]。[4]最近对负载型纳米颗粒的研究[5，6]揭示了弱结合体积吸收的氢物种在烯烃氢化中的关键作用。具体而言，在低压条件下观察到Pd簇而不是单晶的高氢化活性;这归因于小颗粒在有限体积中储存大量氢原子的独特能力。然而，在烯烃氢化中体积吸收的氢原子的参与仍然是一个有争议的问题，主要是因为在等温反应条件下检测存在的氢物种的实验困难。ag）用于氢深度剖析的12 C核反应分析（NRA），以分别监测表面吸附的和体积吸附的。吸附在模型氧化物膜上的Pd纳米颗粒中的氢物种。我们报道了第一次直接观察到大量的氢吸收在低压条件下的金属团簇的体积。[7]在本研究中，我们结合联合收割机的Pd纳米粒子的NRA氢的深度剖析与瞬态分子束反应性测量回答两个密切相关的问题：1）什么是不同的氢物种在氢化的烯烃双键的作用，和2）碳沉积物如何影响Pd纳米粒子中的氢分布，以促进持续的催化活性？因此，我们获得了第一个直接的实验证据表明，吸附在Pd颗粒体积中的H的存在下是烯烃加氢所需的。我们进一步表明，即使是亚单层覆盖的碳显着影响的Pd颗粒中的H深度分布。我们归因于持续的氢化活性，促进碳质沉积物，促进H扩散到颗粒体积的反应条件下，这导致在增强的能力的Pd簇，以补充所需的氢化体积吸收的H原子。我们的实验方法如图1所示。我们采用定义明确的Pd模型催化剂在超高真空条件下原位制备模型薄氧化膜。[8]在等温条件下通过分子束（MB）探测Pd簇合物在顺式-2-丁烯与D2反应中的催化活性（图1a）。在相同的条件下，利用掠入射离子束NRA进行了互补的高分辨率氢深度剖面测量，区分了表面吸附氢和氢原子。
The accumulation of carbonaceous deposits resulting from the early decomposition of the reactants is recognized to considerably affect the activity and the selectivity in hydrocarbon conversions promoted by transition metals.[1] The underlying microscopic mechanisms of carbon-induced changes in reactivity have recently been addressed using the surface science approach for the selective hydrogenation of alkynes [2] and the isomerization and hydrogenation of alkenes.[3] In particular, we have shown that sustained activity toward hydrogenation of the olefinic double bond can be achieved only in the presence of co-adsorbed strongly dehydrogenated carbonaceous deposits.[3] However, the exact role that these carbon species play in the promotion of the persistent hydrogenation activity remained unknown. Generally, promotion of olefin hydrogenation appears to be closely connected to the adsorption state of hydrogen atoms on the metal catalyst. The traditional opinion that only surface hydrogen species are involved in the hydrogenation process [1] was questioned for the first time by Ceyer et al.[4] More recent studies on supported nanoparticles [5, 6] revealed the crucial role of weakly bound volume-absorbed hydrogen species in alkene hydrogenation. Specifically, high hydrogenation activity under low-pressure conditions was observed on Pd clusters but not on single crystals; this was attributed to the unique ability of the small particles to store large amounts of hydrogen atoms in a confined volume. Still, the involvement of the volume-absorbed hydrogen atoms in the olefin hydrogenation remained a controversial issue, mainly because of the experimental difficulty in detecting the hydrogen species present under the isothermal reaction conditions.Recently, we overcame this limitation by applying 1H-(15N, ag) 12C nuclear reaction analysis (NRA) for hydrogen depth profiling to separately monitor the surface-adsorbed and volume-absorbed hydrogen species in Pd nanoparticles supported on a model oxide film. We reported the first direct observation of substantial hydrogen absorption in the volume of metal clusters under low pressure conditions.[7] In the present study, we combine NRA hydrogen depth profiling of Pd nanoparticles with transient molecular beam reactivity measurements to answer two closely related questions: 1) What is the role of the different hydrogen species in the hydrogenation of the olefinic double bond, and 2) how do the carbonaceous deposits affect the hydrogen distribution in the Pd nanoparticles to promote the sustained catalytic activity? As a result, we obtained the first direct experimental evidence that the presence of H absorbed in the Pd particle volume is required for olefin hydrogenation. We further show that even a submonolayer coverage of carbon significantly affects the H depth distribution in the Pd particles. We attribute the sustained hydrogenation activity, promoted by carbonaceous deposits, to the facilitation of H diffusion into the particle volume under the reaction conditions, which results in the enhanced ability of the Pd clusters to replenish the volume-absorbed H atoms required for hydrogenation. Our experimental approach is illustrated in Figure 1. We employed well-defined Pd model catalysts prepared in situ under ultra-high-vacuum conditions on model thin oxide films.[8] The catalytic activity of the Pd clusters in the reaction of cis-2-butene with D2 was probed by molecular beams (MB) under isothermal conditions (Figure1a). Under identical conditions, complementary high-resolution hydrogen depth profiling by grazing ion incidence NRA was performed, which distinguishes surface-adsorbed H from H …