Structural evolution of cobalt for the production of long-chain paraffins by CO2 hydrogenation

Abstract

A new role of alkali metals in the direct hydrogenation of CO to C species over Co catalysts was proposed. During the CO hydrogenation, Na-promotion encouraged a new structural evolution (i.e., a thin oxygen vacancy (O)-rich CoO/CoC shell and a Co core). This facilitated the migration of CHO and CO species produced at the shell to the adjacent CoC shell and Co core, where they undergo further chain growth. In contrast, Li- and K-promotion resulted in the generation of a thick and O-poor CoO shell without CoC, while in the absence of alkali metal promotion, Co was exposed as the outmost surface; in both the cases, methanation dominated. The ability of Na-promotion to remove –OH* from the Co surface helped maintain the thickness and valance state of the Co oxide shell. Thus, the Na-promotion developed the chain growth and CO producing core–shell structure, rather than any electronic promotional effects.