Wang Peng, Senftle Thomas P
Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX 77005, USA.
Phys Chem Chem Phys. 2021 Jan 21;23(2):1401-1413. doi: 10.1039/d0cp04669h.
Identifying catalysts for non-oxidative propane dehydrogenation has become increasingly important due to the increasing demand for propylene coupled to decreasing propylene production from steam cracking as we shift to lighter hydrocarbon feedstocks. Commercialized propane dehydrogenation (PDH) catalysts are based on Pt or Cr, which are expensive or toxic, respectively. Recent experimental work has demonstrated that earth-abundant and environmentally-benign metals, such as iron, form in situ carbide phases that exhibit good activity and high selectivity for PDH. In this work, we used density functional theory (DFT) to better understand why the PDH reaction is highly selective on Fe3C surfaces. We use ab initio thermodynamics to identify stable Fe3C surface terminations as a function of reaction conditions, which then serve as our models for investigating rate-determining and selectivity-determining kinetic barriers during PDH. We find that carbon-rich surfaces show much higher selectivity for propylene production over competing cracking reactions compared to iron-rich surfaces, which is determined by comparing the propylene desorption barrier to the C-H scission barrier for dehydrogenation steps beyond propylene. Electronic structure analyses of the d-band center and the crystal orbital Hamilton population (COHP) of the carbides demonstrate that the high selectivity of carbon-rich surfaces originates from the disruption of surface Fe ensembles via carbon. Finally, we investigated the role of phosphate in suppressing coke formation and found that the electron-withdrawing character of phosphate destabilizes surface carbon.
随着对丙烯的需求不断增加，而随着我们转向更轻质的烃类原料，蒸汽裂解制丙烯的产量却在下降，因此确定非氧化丙烷脱氢的催化剂变得越来越重要。商业化的丙烷脱氢（PDH）催化剂基于Pt或Cr，而它们分别价格昂贵或有毒。最近的实验工作表明，诸如铁等储量丰富且对环境无害的金属会形成原位碳化物相，这些相表现出良好的PDH活性和高选择性。在这项工作中，我们使用密度泛函理论（DFT）来更好地理解为什么PDH反应在Fe3C表面上具有高度选择性。我们使用从头算热力学来确定稳定的Fe3C表面终止结构作为反应条件的函数，然后将其用作我们研究PDH过程中速率决定和选择性决定动力学势垒的模型。我们发现，与富铁表面相比，富碳表面在丙烯生成方面相对于竞争性裂解反应具有更高的选择性，这是通过比较丙烯脱附势垒与丙烯之后脱氢步骤的C-H断裂势垒来确定的。对碳化物的d带中心和晶体轨道哈密顿布居（COHP）进行电子结构分析表明，富碳表面的高选择性源于碳对表面Fe原子簇的破坏。最后，我们研究了磷酸盐在抑制焦炭形成中的作用，发现磷酸盐的吸电子特性会使表面碳不稳定。
