Visualization of the Active Sites of Zinc-Chromium Oxides and the CO/H 2 Activation 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/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.