Structure sensitivity of gallium oxide catalyzed propane dehydrogenation reaction co-fed with hydrogen

Developing efficient propane dehydrogenation (PDH) processes without uses of expensive Pt- and toxic CrOx-based catalysts are of broad interest and great importance. Co-feeding of hydrogen is a widely-adopted strategy to improve catalytic performance of various catalysts in the PDH reaction. Herein, we systematically study the promotion effect of co-fed H2 on different oxide catalysts in the PDH reaction. It comes mainly from the alleviated poisoning effect on ZnO and additionally from H2 reduction-enhanced coordination-unsaturated Zr3+ active sites on ZrO2, while mainly from the formation of dynamically-cycles metastable gallium hydride species on Ga2O3 capable of activating the C–H bond activation of propane at significantly reduced barriers (C3H8+2Ga(III)Ga2O3-H*=C3H6+2H2+2Ga(III)Ga2O3). During the Ga2O3-catalyzed PDH reaction with co-fed H2, the Ga(III)Ga2O3-H* species initially forms by H2 dissociation, then reacts with C3H8 to produce C3H6 and H2, and finally gets recovered by H2 dissociation, leading to the dynamically-cycled hydride active site on Ga2O3 with greatly enhanced catalytic performance. Such a hydride catalysis is highly sensitive to the structure of Ga2O3, being more efficiently on Ga2O3{111} facets than on Ga2O3{100} facets. A fine Ga2O3 nanocatalyst with a high ratio of exposed {111} facets and a large specific surface area is fabricated to give a C3H6 space-time yield (STY) as high as 4.9 kgC3H6/(kgcatalyst h) with a C3H6 selectivity of 96.5% and a good recyclability in the PDH reaction with co-fed H2 at 550 ºC. These results provide an alternative strategy for developing none-Pt- and none-toxic CrOx-based PDH catalysts.开发无需使用昂贵铂基与有毒三氧化铬基催化剂的高效丙烷脱氢（propane dehydrogenation, PDH）工艺，是当前广受关注且具有重要研究价值的方向。氢气共进料是PDH反应中用于优化多种催化剂催化性能的通用策略。本研究中，我们系统探究了共进料氢气对PDH反应中不同氧化物催化剂的促进作用：该作用主要源于对氧化锌（ZnO）的中毒效应缓解，以及二氧化锆（ZrO₂）表面经氢气还原强化得到的配位不饱和Zr³+活性位点；而在三氧化二镓（Ga₂O₃）表面，促进作用主要来自动态循环亚稳态氢化镓物种的形成——这类物种可显著降低丙烷C-H键活化的能垒，其反应路径为：C₃H₈ + 2Ga(III)Ga₂O₃-H* = C₃H₆ + 2H₂ + 2Ga(III)Ga₂O₃。在氢气共进料的Ga₂O₃催化PDH反应中，Ga(III)Ga₂O₃-H*物种首先通过氢气解离生成，随后与丙烷（C₃H₈）反应生成丙烯（C₃H₆）与氢气，最终再经氢气解离得以再生，由此在Ga₂O₃表面形成动态循环的氢化物活性位点，大幅提升催化性能。这类氢化物催化对Ga₂O₃的晶面结构具有高度敏感性，在Ga₂O₃{111}晶面上的催化效率显著优于Ga₂O₃{100}晶面。本研究制备了一种暴露高比例{111}晶面且比表面积较大的优质Ga₂O₃纳米催化剂，在550℃、氢气共进料的PDH反应中，该催化剂的丙烯（C₃H₆）时空收率（space-time yield, STY）高达4.9 kgC₃H₆/(kg催化剂·h)，丙烯选择性达96.5%，且具备良好的循环稳定性。上述研究结果为开发无铂基、无毒三氧化铬基PDH催化剂提供了全新的可行策略。