Crystallographic dependence of CO2 hydrogenation pathways over HCP-Co and FCC-Co catalysts

Abstract

Efficient conversion of CO2 is of great significance for sustainable supply of chemicals and fuels. While Co-based catalysts are known to be effective for CO hydrogenation in Fischer-Tropsch synthesis, they work very differently in CO2 hydrogenation. This study reveals a crystallographic dependence of reaction pathways for CO2 hydrogenation on Co catalyst showing a new type of structure sensitivity and structure-activity-selectivity relationship for CO2 conversion to chemicals and fuels. The experimental work on CO2 conversion including steady-state isotopic transient kinetic analysis (SSITKA) using 13C-labeled CO2 shows a preferential CH4 formation over HCP-Co but dominant CO formation over FCC-Co. The density functional theory calculations indicate that CO2 does dissociate directly into chemisorbed CO* and O* on both HCP-Co and FCC-Co, but the CO* intermediates on HCP-Co prefer to be hydrogenated to form CH4 whereas the CO* on FCC-Co preferentially desorb to form CO. The significantly altered adsorption strength of CO* due to the presence of chemisorbed O* and CO2* species on the catalyst surface is responsible for the mechanistic disconnection in product selectivity between the CO2 and CO hydrogenation over Co catalysts. This study also shows that the addition of K to Co diminishes the direct impact of Co crystal structure, but improves the selectivity to C2+ hydrocarbons along with higher CO2 conversion. This seems to result from another pathway originating from HCOO* intermediate from bonding interaction of surface Co atoms with carbon in CO2, leading to the formation of CHx* whose coupling subsequently give rises to C2+ products. The present study sheds new light into the crystallographic structural sensitivity of CO2 hydrogenation towards the rational design of more selective catalysts for CO2 conversion.