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

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 CH_x* 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.