Increasing the activity and selectivity of Co-based FTS catalysts supported by carbon materials for direct synthesis of clean fuels by the addition of chromium

Abstract

Synthesis of clean transportation fuels from syngas (CO + H2) derived from coal, natural gas, and renewable biomass is an efficient method for reducing dependence on oil reservoirs, for which developing effective catalysts is significant. Co-based catalysts supported by carbon materials possess advantages such as longer lifetimes and easy handling of wasted catalysts, but increasing their activity and selectivity remains a challenge. Here we report using Cr as a promoter to enhance the CO hydrogenation process over activated carbon (AC)-supported Co-based catalysts, namely, CoxCr/AC catalysts. An almost twofold increase in activity (from 28.9% to 47.0%), accompanied by decreased CH4, olefin, and alcohol selectivity, was achieved by adding Cr, resulting in a significant increase in the yield of transportation fuel products. The results of chemisorption indicate that Cr facilitates H2 adsorption and suppresses the formation of cobalt carbide (Co2C), forming a relative H-rich and C-lean surface chemical environment, so that the CO hydrogenation process and C–C coupling step are enhanced. Structural characterizations show that Cr exists in the form of Cr2O3 and aggregates on the surfaces of the catalysts in a highly dispersed manner at relative low loadings (<2 wt%), and that Co nanoparticles are partly covered by Cr at high loadings. We suggest that the interaction between Cr3+ and adjacent metallic Co may play a pivotal role in the elevation of CO hydrogenation activity and C5+ selectivity.