Ethane Dehydrogenation on Single and Dual Centers of Ga-modified γ‑Al2O3

Density-functional theory calculations and microkinetic analysis are used to investigate the efficacy of Ga-modified γ-Al2O3 (110) surfaces for the catalytic dehydrogenation of ethane and elucidate the synergy between Ga and Al sites. The model surfaces are modified by either Ga grafting or doping. We consider and analyze numerous active sites and rank them using microkinetic analysis. The kinetic parameters obtained from microkinetic modeling are compared with experimental values for ethane dehydrogenation over Ga2O3–Al2O3 mixed oxides prepared by coprecipitation. The dominant reaction pathway proceeds via heterolytic C–H bond dissociation to a surface proton and a metal-carbanion intermediate that undergoes β-hydride elimination. We find that grafted Ga sites are catalytically inactive. In contrast, Ga-doped sites exhibit 5-fold enhancement in catalytic activity when compared to the sites on pristine Al2O3, owed to the synergy between neighboring AlIII and GaIV sites. Furthermore, we model and investigate the effect of surface hydroxylation, demonstrate how surface water interferes with the aforementioned synergy between AlIII and GaIV sites and discuss the implications for the catalytic activity of the modified surfaces. Increase in the partial pressure of H2O significantly increases the apparent activation energies of dehydrogenation and interestingly changes the most active site.

本研究采用密度泛函理论（Density-functional theory）计算与微观动力学分析，旨在探究镓（Ga）修饰的γ-氧化铝(110)晶面对乙烷催化脱氢的催化效能，并阐明镓与铝位点之间的协同效应。模型表面通过镓接枝或掺杂两种方式进行改性。我们系统考量并分析了多种活性位点，并借助微观动力学分析对其进行优先级排序。将微观动力学建模得到的动力学参数，与通过共沉淀法制备的氧化镓（Ga2O3）-氧化铝（Al2O3）复合氧化物上乙烷脱氢反应的实验值进行对比。主导反应路径为：C-H键异裂生成表面质子与金属碳负离子中间体，该中间体随后发生β-氢消除（β-hydride elimination）反应。研究结果表明，接枝型镓位点不具备催化活性。与之相对，与纯相氧化铝（Al2O3）表面的位点相比，掺杂型镓位点的催化活性提升5倍，这一现象源于相邻三价铝（AlIII）与四价镓（GaIV）位点间的协同效应。此外，我们对表面羟基化（surface hydroxylation）的影响进行建模与探究，阐明了表面水如何干扰上述AlIII与GaIV位点间的协同效应，并讨论了其对改性表面催化活性的影响。水蒸气分压（partial pressure of H2O）的升高会显著提升脱氢反应的表观活化能（apparent activation energies），且会有趣地改变体系中最具活性的位点。