中国科学院大连化物所包信和团队报道了直接合成气转化中锌-铬氧化物活性位点和CO/H2活化机制的可视化。相关研究成果于2024年1月11日发表在《美国化学会杂志》。

尽管广泛的研究证明了金属氧化物沸石（OXZEO）催化剂概念在应对合成气化学中的选择性挑战方面的多功能性，但金属氧化物的活性位点和CO/H₂活化机制仍有待阐明。

该文中，研究人员通过实验证明了Cr在锌-铬氧化物中的作用，并首次使用扫描透射电子显微镜电子能量损失谱以表面碳物种的体积密度为描述符，直观地揭示了CO活化的活性位点。具有原子ZnOx覆盖层的ZnCr₂O₄尖晶石表面是C–O键离解的最活跃位点，特别是在限制在（311）和（111）晶面之间的窄ZnCr₂O₃（110）晶面处，其次是Cr掺杂的纤锌矿ZnO表面。相比之下，具有聚集的ZnOx覆盖层、纯ZnO和化学计量的ZnCr₂O₄的ZnCr₂O₃的表面表现出显著较低的活性。

基于同步加速器的真空紫外光电离质谱研究对C¹⁸O、¹³CO和D₂同位素的不同程序升温表面反应进行了研究，验证了CO在CO中的ZnCrn氧化物上直接离解，形成CH₂，如果合成气中存在H₂和CH₂CO中间体，则进一步离解为碳氢化合物。CO在ZnCrn氧化物上的离解和加氢活性与ZnCrn-SAPO-18复合催化剂的合成气，对轻烯烃的活性很好地相关，作为Cr/Zn比的函数。

附：英文原文

Title: Visualization of the Active Sites of Zinc–Chromium Oxides and the CO/H2 Activation Mechanism in Direct Syngas Conversion

Author: Yuxiang Chen, Shaobo Han, Xiulian Pan, Feng Jiao, Wei Liu, Yang Pan, Xinhe Bao

Issue&Volume: January 11, 2024

Abstract: Despite wide studies demonstrating the versatility of the metal oxide-zeolite (OXZEO) catalyst concept to tackle the selectivity challenge in syngas chemistry, the active sites of metal oxides and the mechanism of CO/H2 activation remain to be elucidated. Herein, we demonstrate experimentally the role of Cr in zinc–chromium oxides and unveil visually, for the first time, the active sites for CO activation employing scanning transmission electron microscopy-electron energy loss spectroscopy using the volumetric density of surface carbon species as a descriptor. The ZnCr2O4 spinel surface with atomic ZnOx overlayer is the most active site for C–O bond dissociation, particularly at the narrow ZnCr2O4(110) facets constrained between the (311) and (111) facets, followed by the Cr-doped wurtzite ZnO surface. In comparison, the surfaces of ZnCr2O4 with aggregated ZnOx overlayers, pure ZnO, and the stoichiometric ZnCr2O4 exhibit a significantly lower activity. In situ synchrotron-based vacuum ultraviolet photoionization mass spectrometric study on different temperature programmed surface reactions with isotopes of C18O, 13CO, and D2 validates direct CO dissociation over ZnCrn oxides in CO, forming CH2 and further to hydrocarbons if H2 is present and CH2CO intermediates in syngas. The activity of CO dissociation and hydrogenation over ZnCrn oxides correlates well with the syngas-to-light-olefins activity of ZnCrn-SAPO-18 composite catalysts as a function of the Cr/Zn ratio.

DOI: 10.1021/jacs.3c07332

Source: https://pubs.acs.org/doi/abs/10.1021/jacs.3c07332