How to Control the Selectivity of Palladium-based Catalysts in Hydrogenation Reactions: The Role of Subsurface Chemistry
How to Control the Selectivity of Palladium-based Catalysts in Hydrogenation Reactions: The Role of Subsurface Chemistry
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DOI：
10.1002/cctc.201200100
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发表时间：
2012-08-01
期刊：
影响因子：
4.5
通讯作者：
Wowsnick, Gregor
中科院分区：
文献类型：
--
作者：
Armbruester, Marc;Behrens, Malte;Wowsnick, Gregor
讨论了由多个合作小组通过联合多方法和多材料方法获得的用于炔烃选择性加氢的钯基催化剂的最新实验和理论结果。通过在氢化条件下 X=H、C 的原位光谱观察到通过将外来物质 X 掺入 Pd 金属的亚表面来对催化活性 Pd 表面进行关键修饰。在某些条件下（低氢气分压），炔烃裂解导致反应气体进料中形成 Pd2C 表面相。通过将大量氢从最外层活性表面层解耦，将 C 作为改性剂插入到亚表面中，大大提高了炔烃半氢化相对于 Pd 基催化剂的选择性。 DFT 计算证实 Pd?C 阻碍氢的扩散。它的形成取决于碳的化学势（反应物分压），并且当氢/炔压力比高时受到抑制，这导致原位形成的块状 Pd2H 发生相当非选择性的氢化。然而，改性剂物质 X 对选择性的有益影响也存在于 X=Ga 的金属间化合物中。这种 PdxGay 催化剂的一大优点是在原位条件下表现出更长的稳定性。使用冶金、清洁的样品来确定 PdGa 和 Pd3Ga7 的固有催化性能。对于高性能催化剂，负载的纳米结构金属间化合物是更优选的，并且在氢气中加热Pd/Ga2O3时，Ga2O3的部分还原被证明导致在中等温度下形成Pd2Ga金属间化合物。通过这种方式，Pd5Ga2 和 Pd2Ga 可以分别以负载纳米粒子、薄膜模型和真实粉末样品的形式获得。
Discussed are the recent experimental and theoretical results on palladium-based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi-method and multi-material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X into the sub-surface of Pd metal was observed by in situ spectroscopy for X=H, C under hydrogenation conditions. Under certain conditions (low H2 partial pressure) alkyne fragmentation leads to formation of a Pd?C surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub-surface increases considerably the selectivity of alkyne semi-hydrogenation over Pd-based catalysts through the decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that Pd?C hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high, which leads to rather unselective hydrogenation over in situ formed bulk Pd?H. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X=Ga. As a great advantage, such PdxGay catalysts show extended stability under in situ conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd3Ga7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga2O3, upon heating of Pd/Ga2O3 in hydrogen, was shown to lead to formation of Pd?Ga intermetallic compounds at moderate temperatures. In this way, Pd5Ga2 and Pd2Ga are accessible in the form of supported nanoparticles, in thin film models, and realistic powder samples, respectively.