International Tomography Center, 3A Institutskaya St., Novosibirsk 630090, Russia.
Phys Chem Chem Phys. 2012 Aug 21;14(31):11008-14. doi: 10.1039/c2cp40690j. Epub 2012 Jul 5.
In this work, the contribution of the pairwise H(2) addition to the overall reaction mechanism was studied under the systematic variation of both the Pd particle size and the properties of the catalyst support using the hydrogenation of propene and propyne over supported Pd catalysts as representative examples. For Pd supported on alumina, silica and zirconia, only propene formed upon hydrogenation of propyne with para-H(2) exhibits hyperpolarization. In contrast, propane formed in hydrogenation of propyne or propene is not hyperpolarized. This demonstrates the existence of different routes of H(2) addition to double and triple bonds on supported Pd catalysts. The unique ability of Pd/TiO(2) catalysts to add H(2) in a pairwise manner not only to the triple but also to the double bond is demonstrated. This finding indicates that the Pd-support interaction is of primary importance in determining not only the magnitude of the hyperpolarization of the NMR lines of the reaction products but even the involvement of the pairwise H(2) addition and hence the mechanism of heterogeneous hydrogenation. The comparative analysis of the selectivities toward pairwise H(2) addition suggested the existence of different surface active sites responsible for all three reaction routes: the direct total hydrogenation of propyne into propane, its selective hydrogenation into propene, and hydrogenation of propene into propane. A reaction scheme which accounts for the formation of the observed hyperpolarized and non-polarized reaction products in propyne and propene hydrogenation with para-H(2) over supported Pd catalysts is suggested. For the first time, application of the PHIP technique allowed us to demonstrate that hydrogenation of propene does not take place in the presence of propyne on supported Pd catalysts.
在这项工作中，通过系统改变钯颗粒大小和催化剂载体的性质，研究了对向体系中添加氢气（H2）在整体反应机制中的作用，选用丙烯和丙炔在负载钯催化剂上的加氢反应作为典型实例。对于负载在氧化铝、二氧化硅和氧化锆上的钯，在用仲氢（para-H2）进行丙炔加氢时，只有丙烯形成，表现出超极化。相反，在丙炔或丙烯加氢时形成的丙烷没有超极化。这表明在负载钯催化剂上，H2 对双键和叁键的加成存在不同的途径。Pd/TiO2 催化剂具有以对向方式加成 H2 到叁键和双键的独特能力。这一发现表明，Pd-载体相互作用不仅对反应产物的 NMR 谱线超极化的幅度，而且对加成 H2 的对向方式的参与，从而对多相氢化的机制具有决定性作用。对向 H2 加成选择性的比较分析表明，存在不同的表面活性位，负责所有三种反应途径：丙炔直接完全加氢成丙烷、选择性加氢成丙烯，以及丙烯加氢成丙烷。提出了一个能够解释在负载钯催化剂上用仲氢进行丙炔和丙烯加氢时观察到的超极化和非极化反应产物形成的反应方案。首次应用 PHIP 技术证明了在负载钯催化剂上，丙炔的存在不会导致丙烯加氢。
