Visualization of the Active Sites of Zinc–Chromium Oxides and the CO/H<sub>2</sub> Activation Mechanism in Direct Syngas Conversion
Yuxiang Chen, Shaobo Han, Xiulian Pan, Feng Jiao, Wei Liu, Yang Pan, Xinhe Bao
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/H 2 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 ZnCr 2 O 4 spinel surface with atomic ZnO x overlayer is the most active site for C–O bond dissociation, particularly at the narrow ZnCr 2 O 4 (110) facets constrained between the (311) and (111) facets, followed by the Cr-doped wurtzite ZnO surface. In comparison, the surfaces of ZnCr 2 O 4 with aggregated ZnO x overlayers, pure ZnO, and the stoichiometric ZnCr 2 O 4 exhibit a significantly lower activity. In situ synchrotron-based vacuum ultraviolet photoionization mass spectrometric study on different temperature programmed surface reactions with isotopes of C 18 O, 13 CO, and D 2 validates direct CO dissociation over ZnCr n oxides in CO, forming CH 2 and further to hydrocarbons if H 2 is present and CH 2 CO intermediates in syngas. The activity of CO dissociation and hydrogenation over ZnCr n oxides correlates well with the syngas-to-light-olefins activity of ZnCr n -SAPO-18 composite catalysts as a function of the Cr/Zn ratio.