Intermediate Transfer Rates and Solid-State Ion Exchange are Key Factors Determining the Bifunctionality of a Tandem CO2 Hydrogenation Catalyst
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abstract
Probing the interaction between different active sites and transfer of reaction intermediates in bifunctional catalysts for tandem hydrogenation of CO2 is crucial for optimal catalyst design that maximize synergy to achieve high rates and product selectivity. Herein, thermocatalytic conversion of CO2 to hydrocarbon (HC) via a methanol (CH3OH) intermediate was investigated by modulating the placement of In2O3 and HZSM-5 in bifunctional admixtures at temperatures between 350 to 450 C and 500 psig, to probe the key factors that drive synergy in these bifunctional systems. Analysis of the intermediate CH3OH transfer rates showed that although a millimeter scale placement of In2O3 and HZSM-5 yields a simple tandem reaction with a total HC and methanol CH3OH space-time yield of 810-6 molCgcat-1min-1, a microscale placement exhibits a ten-fold increase in catalytic activity with a total HC and CH3OH space-time yield of 810-5 molCgcat-1min-1 (at 400 C) due to a faster advective and diffusive transfer rate of CH3OH. A combination of reactivity, spectroscopy with Raman, X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD) patterns, microscopy with scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and control experiments on methanol to hydrocarbons (MTH) revealed that further enhancing the reaction intermediate transfer at a nanoscale placement was counteracted by solid-state ion exchange (SSIE) between Brnsted acid sites (H+) of the HZSM-5 with the In+ ions from In2O3, and that the formation of CH4 at the nanoscale placement was likely through CH3OH hydrogenolysis and not CO2 methanation at these intimate distances. Overall, our data showed the interconnected and subtle ways through which bifunctionality of catalysts could be regulated and paves the way for the development of design principles for designing more effective bifunctional catalysts for tandem CO2 hydrogenation.
