Trummer David, Searles Keith, Algasov Alexander, Guda Sergey A, Soldatov Alexander V, Ramanantoanina Harry, Safonova Olga V, Guda Alexander A, Copéret Christophe
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 2, CH-8093 Zürich, Switzerland.
The Smart Materials Research Institute, Southern Federal University, Sladkova 178/24, Rostov-on-Don, Russia, 344090.
J Am Chem Soc. 2021 May 19;143(19):7326-7341. doi: 10.1021/jacs.0c10791. Epub 2021 May 11.
Unveiling the nature and the distribution of surface sites in heterogeneous catalysts, and for the Phillips catalyst (CrO/SiO) in particular, is still a grand challenge despite more than 60 years of research. Commonly used references in Cr K-edge XANES spectral analysis rely on bulk materials (Cr-foil, CrO) or molecules (CrCl) that significantly differ from actual surface sites. In this work, we built a library of Cr K-edge XANES spectra for a series of tailored molecular Cr complexes, varying in oxidation state, local coordination environment, and ligand strength. Quantitative analysis of the pre-edge region revealed the origin of the pre-edge shape and intensity distribution. In particular, the characteristic pre-edge splitting observed for Cr(III) and Cr(IV) molecular complexes is directly related to the electronic exchange interactions in the frontier orbitals (spin-up and -down transitions). The series of experimental references was extended by theoretical spectra for potential active site structures and used for training the Extra Trees machine learning algorithm. The most informative features of the spectra (descriptors) were selected for the prediction of Cr oxidation states, mean interatomic distances in the first coordination sphere, and type of ligands. This set of descriptors was applied to uncover the site distribution in the Phillips catalyst at three different stages of the process. The freshly calcined catalyst consists of mainly Cr(VI) sites. The CO-exposed catalyst contains mainly Cr(II) silicates with a minor fraction of Cr(III) sites. The Phillips catalyst exposed to ethylene contains mainly highly coordinated Cr(III) silicates along with unreduced Cr(VI) sites.
揭示多相催化剂表面位点的性质和分布，尤其是菲利普斯催化剂（CrO/SiO）的表面位点性质和分布，尽管已经进行了60多年的研究，但仍然是一个巨大的挑战。Cr K边XANES光谱分析中常用的参考物依赖于与实际表面位点有显著差异的块状材料（Cr箔、CrO）或分子（CrCl）。在这项工作中，我们构建了一系列经过定制的分子Cr配合物的Cr K边XANES光谱库，这些配合物在氧化态、局部配位环境和配体强度方面各不相同。对前缘区域的定量分析揭示了前缘形状和强度分布的起源。特别是，在Cr(III)和Cr(IV)分子配合物中观察到的特征性前缘分裂与前沿轨道中的电子交换相互作用（自旋向上和向下跃迁）直接相关。通过对潜在活性位点结构的理论光谱扩展了一系列实验参考光谱，并用于训练Extra Trees机器学习算法。选择光谱中最具信息性的特征（描述符）来预测Cr的氧化态、第一配位球中的平均原子间距离和配体类型。这组描述符被应用于揭示菲利普斯催化剂在该过程三个不同阶段的位点分布。新鲜煅烧的催化剂主要由Cr(VI)位点组成。CO暴露的催化剂主要包含Cr(II)硅酸盐，还有少量Cr(III)位点。暴露于乙烯的菲利普斯催化剂主要包含高度配位的Cr(III)硅酸盐以及未还原的Cr(VI)位点。
