Surface Organometallic Chemistry on Metals: Formation of a Stable ⋮Sn(n-C4H9) Fragment as a Precursor of Surface Alloy Obtained by Stepwise Hydrogenolysis of Sn(n-C4H9)4 on a Platinum Particle Supported on Silica
Surface Organometallic Chemistry on Metals: Formation of a Stable ⋮Sn(n-C4H9) Fragment as a Precursor of Surface Alloy Obtained by Stepwise Hydrogenolysis of Sn(n-C4H9)4 on a Platinum Particle Supported on Silica
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DOI：
10.1021/ja964405o
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发表时间：
1998-01
影响因子：
15
通讯作者：
F. Humblot;D. Didillon;F. Lepeltier;J. Candy;J. Corker;O. Clause;F. Bayard;J. Basset
中科院分区：
文献类型：
--
作者：
F. Humblot;D. Didillon;F. Lepeltier;J. Candy;J. Corker;O. Clause;F. Bayard;J. Basset
在Pt/SiO2催化剂上进行了Sn(n-C4H9)4在不同温度和金属表面覆盖率下的选择性氢解，通过表面有机金属化学制备了一类明确的双金属催化剂。在化学计量学和动力学之后，对试剂和产物进行了仔细的分析，包括提取未反应的试剂和对样品进行元素分析。通过电子显微镜（CTEM）、电子显微镜（TEM）、电子能谱（EDAX）和EXAFS分析对所形成的各种表面物质进行了表征。利用分子模型分析了表面有机金属碎片的可能结构。在50℃时，铂表面选择性地发生氢解反应，只析出正丁烷。首先在铂粒子上接枝Sn(n-C4H9)3碎片，铂粒子经过两个锡碳u键的逐步断裂，形成稳定的Pt-Sn（n-C4H9）碎片。无论反应时间、表面覆盖度或负载如何，每个铂接枝丁基片段的数量都不大于1，即当形成Sn(n-C4H9)3时，锡对铂的覆盖率为0.3，而当形成Sn（n-C4H9）时，铂的覆盖率接近1。因此，表面组成是由在表面“紧密堆积”的烷基链的体积决定的。在100°C时，反应在铂和二氧化硅表面同时发生。在铂表面鉴定出了相同的碎片Sn(n-C4H9)3、Sn(n-C4H9)2和Sn(n-C4H9)，但同时在二氧化硅表面也形成了描述良好的tsisn (n-C4H9)3。在Pts-Sn（n-C4H9）的氢作用下热处理，Sn K边缘的EXAFS （Pt-Sn距离为2.75 A，配位数约为4）证明了无烷基锡原子位于颗粒的外围。即使有机锡片段接枝时覆盖了整体，在300℃下完全氢解后，仍有大约40%的铂可被H2化学吸附。这可以用粒子直径的增加（+0.5 A）来解释，这阻止了锡原子在粒子周围的紧密堆积，而使一些铂原子仍然可以接近氢。在较高温度（通常为500°C）下处理催化剂后，由于锡原子迁移到颗粒的第一单层，因此催化剂的结构略有变化，这可以通过EXAFS测定的锡配位数（约4.4-5.6）显着增加来证明。在分子模拟的基础上，提出了被各种表面有机锡碎片覆盖的铂颗粒的假设表面结构。
Selective hydrogenolysis of Sn(n-C4H9)4 on a Pt/SiO2 catalyst has been carried out at various temperatures and coverages of the metallic surface to prepare via surface organometallic chemistry a well- defined class of bimetallic catalysts. The stoichiometry and kinetics of the reaction was followed by the careful analysis of reagents and products, including extraction of unreacted reagents, and elemental analysis of the samples. The various surface species formed were characterized by electron microscopy (CTEM and TEM EDAX) and EXAFS analysis. Possible structures of the surface organometallic fragments were considered using molecular modeling. At 50 °C, the hydrogenolysis reaction occurs selectively on the platinum surface with exclusive evolution of n-butane. There is first formation of a Sn(n-C4H9)3 fragment grafted on the platinum particle which undergoes a stepwise cleavage of two tin-carbon U-bonds to form a stable Pt-Sn(n-C4H9) fragment. Regardless of the reaction time, surface coverage, or loading, the number of grafted butyl fragments per platinum is never greater than unity, that is to say that when Sn(n-C4H9)3 is formed the platinum coverage by tin is 0.3 whereas when Sn(n-C4H9) is formed the platinum coverage is closer to 1. It is therefore suggested that the surface composition is governed by the bulkiness of the alkyl chains which are "close packed" on the surface. At 100 °C, the reaction takes place both on the platinum and the silica surface. On the platinum surface, the same fragments (namely Sn(n-C4H9)3, Sn(n-C4H9)2, and Sn(n-C4H9)) were identified, but simultaneously on the silica surface, the well-described tSiOSn(n-C4H9)3 species was also formed. Thermal treatment under hydrogen of Pts-Sn(n-C4H9) lead to alkyl-free tin atoms which are located at the periphery of the particle as evidenced by Sn K edge EXAFS (Pt-Sn distance of 2.75 A with a coordination number of ca. 4). Even if the organotin fragments are grafted with a coverage of unity, after their complete hydrogenolysis at 300 °C, about 40% of the platinum is still accessible to H2 chemisorption. This could be explained by the increase of the particle diameter (+0.5 A) which prevents a close packing of the tin atoms around the particle and leaves some platinum atoms still accessible to the hydrogen. After treatment of the catalyst at higher temperatures, typically 500 °C, the structure of the catalyst is slightly changed since the tin atoms migrate into the first monolayer of the particle, as evidenced by a significant increase of the tin coordination number (ca. 4.4-5.6) as determined by EXAFS. Hypothetical surface structures have been proposed on the basis of molecular modeling of platinum particles covered by various surface organotin fragments.