Department of Materials Science and Engineering, Stanford University , 496 Lomita Mall, Stanford, California 94305, United States.
Department of Chemical Engineering, Stanford University , 443 Via Ortega, Stanford, California 94305, United States.
J Am Chem Soc. 2016 Mar 23;138(11):3705-14. doi: 10.1021/jacs.5b12087. Epub 2016 Mar 9.
Synthesis gas (CO + H2) conversion is a promising route to converting coal, natural gas, or biomass into synthetic liquid fuels. Rhodium has long been studied as it is the only elemental catalyst that has demonstrated selectivity to ethanol and other C2+ oxygenates. However, the fundamentals of syngas conversion over rhodium are still debated. In this work a microkinetic model is developed for conversion of CO and H2 into methane, ethanol, and acetaldehyde on the Rh (211) and (111) surfaces, chosen to describe steps and close-packed facets on catalyst particles. The model is based on DFT calculations using the BEEF-vdW functional. The mean-field kinetic model includes lateral adsorbate-adsorbate interactions, and the BEEF-vdW error estimation ensemble is used to propagate error from the DFT calculations to the predicted rates. The model shows the Rh(211) surface to be ∼6 orders of magnitude more active than the Rh(111) surface, but highly selective toward methane, while the Rh(111) surface is intrinsically selective toward acetaldehyde. A variety of Rh/SiO2 catalysts are synthesized, tested for catalytic oxygenate production, and characterized using TEM. The experimental results indicate that the Rh(111) surface is intrinsically selective toward acetaldehyde, and a strong inverse correlation between catalytic activity and oxygenate selectivity is observed. Furthermore, iron impurities are shown to play a key role in modulating the selectivity of Rh/SiO2 catalysts toward ethanol. The experimental observations are consistent with the structure-sensitivity predicted from theory. This work provides an improved atomic-scale understanding and new insight into the mechanism, active site, and intrinsic selectivity of syngas conversion over rhodium catalysts and may also guide rational design of alloy catalysts made from more abundant elements.
合成气（CO+H2）转化是将煤、天然气或生物质转化为合成液体燃料的一种很有前途的途径。铑长期以来一直受到研究，因为它是唯一一种表现出对乙醇和其他 C2+含氧化合物选择性的元素催化剂。然而，铑上合成气转化的基本原理仍存在争议。在这项工作中，开发了一个用于在 Rh（211）和（111）表面上将 CO 和 H2 转化为甲烷、乙醇和乙醛的微动力学模型，选择这两种表面来描述催化剂颗粒上的步骤和密排面。该模型基于使用 BEEF-vdW 功能的 DFT 计算。均场动力学模型包括横向吸附物-吸附物相互作用，并且使用 BEEF-vdW 误差估计集合来从 DFT 计算传播到预测的速率的误差。该模型表明，Rh（211）表面的活性比 Rh（111）表面高约 6 个数量级，但对甲烷具有高度选择性，而 Rh（111）表面本质上对乙醛具有选择性。合成了各种 Rh/SiO2 催化剂，用于催化含氧化合物的生产，并使用 TEM 进行了表征。实验结果表明，Rh（111）表面对乙醛具有内在选择性，并且观察到催化活性和含氧化合物选择性之间存在强烈的反比关系。此外，铁杂质被证明在调节 Rh/SiO2 催化剂对乙醇的选择性方面起着关键作用。实验观察结果与理论预测的结构敏感性一致。这项工作提供了对铑催化剂上合成气转化的机制、活性位和内在选择性的改进的原子尺度理解和新的见解，并且可能还指导了由更丰富的元素制成的合金催化剂的合理设计。
