Visualization of the Active Sites of Zinc–Chromium Oxides and the CO/H2Activation Mechanism in Direct Syngas Conversion

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/H2activation 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 ZnCr2O4spinel surface with atomic ZnOxoverlayer 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 ZnCr2O4with aggregated ZnOxoverlayers, pure ZnO, and the stoichiometric ZnCr2O4exhibit 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 D2validates direct CO dissociation over ZnCrnoxides in CO, forming CH2and further to hydrocarbons if H2is present and CH2CO intermediates in syngas. The activity of CO dissociation and hydrogenation over ZnCrnoxides correlates well with the syngas-to-light-olefins activity of ZnCrn-SAPO-18 composite catalysts as a function of the Cr/Zn ratio.