Comparison of alkene hydrogenation in carbon nanoreactors of different diameters: probing the effects of nanoscale confinement on ruthenium nanoparticle catalysis
Comparison of alkene hydrogenation in carbon nanoreactors of different diameters: probing the effects of nanoscale confinement on ruthenium nanoparticle catalysis
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DOI：
10.1039/c7ta03691d
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发表时间：
2017-10-28
影响因子：
11.9
通讯作者：
Chamberlain, Thomas W.
中科院分区：
文献类型：
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作者：
Aygun, Mehtap;Stoppiello, Craig T.;Chamberlain, Thomas W.
使用探索性烯烃氢化反应评估了不同直径的碳纳米反应器（单壁碳纳米管（SWNT，空腔宽度 1.5 nm）和空心石墨化纳米纤维（GNF，空腔宽度 50-70 nm））负载的钌纳米颗粒（RuNP）的催化性能，并与吸附在碳纳米反应器上的 RuNP 进行了比较。 SWNT 表面或沉积在市售 Ru/C 中的炭黑上。研究表明，超临界 CO2 对于将反应物有效传输至催化 RuNP 至关重要，特别是对于非常窄的 RuNP@SWNT 纳米反应器。尽管观察到单壁碳纳米管中的 RuNP 非常活跃，但它们同时将非常窄的单壁碳纳米管的可接触体积减少了 30-40%，导致总体周转数 (TON) 降低。相比之下，与 SWNT 或炭黑外表面未限制的 RuNP 相比，限制在更广泛的 GNF 中的 RuNP 是完全可接近的，并且表现出显着的活性。对催化 RuNP 周围的纳米级环境的控制显着增强了催化剂的稳定性，并影响了靠近 RuNP 的反应物分子的局部浓度，这说明了催化剂中金属负载和 NP 尺寸的限制具有相当的重要性。有趣的是，极端空间限制似乎也不是控制降冰片烯和苯并降冰片二烯竞争反应中氢化选择性的最佳策略，更宽的RuNP@GNF纳米反应器对含芳香基团的烯烃（苯并降冰片二烯）的氢化选择性增强。这归因于 GNF 中纳米级石墨阶梯边缘的存在，使其成为制备催化中极窄 SWNT 纳米反应器的有吸引力的替代品。
The catalytic properties of ruthenium nanoparticles (RuNPs) supported in carbon nanoreactors of different diameters - single walled carbon nanotubes (SWNTs, width of cavity 1.5 nm) and hollow graphitised nanofibers (GNFs, width of cavity 50-70 nm) - were evaluated using exploratory alkene hydrogenation reactions and compared to RuNPs adsorbed on the surface of SWNT or deposited on carbon black in commercially available Ru/C. Supercritical CO2 is shown to be essential to enable efficient transport of reactants to the catalytic RuNPs, particularly for the very narrow RuNP@SWNT nanoreactors. Though the RuNPs in SWNT are observed to be highly active, they simultaneously reduce the accessible volume of very narrow SWNTs by 30-40% resulting in lower overall turnover numbers (TONs). In contrast, RuNPs confined in wider GNFs were completely accessible and demonstrated remarkable activity compared to unconfined RuNPs on the outer surface of SWNTs or carbon black. Control of the nanoscale environment around the catalytic RuNPs significantly enhances the stability of the catalyst and influences the local concentration of reactant molecules in close proximity to the RuNPs, illustrating the comparable importance of confinement to that of metal loading and size of NPs in the catalyst. Interestingly, extreme spatial confinement also appeared not to be the best strategy for controlling the selectivity of hydrogenations in a competitive reaction of norbornene and benzonorbornadiene, with wider RuNP@GNF nanoreactors displaying enhanced selectivity for the hydrogenation of the aromatic group containing alkene (benzonorbornadiene). This is attributed to the presence of nanoscale graphitic step-edges within the GNF making them an attractive alternative to the extremely narrow SWNT nanoreactors for preparative catalysis.