Insight into the Formation of Co@Co2C Catalysts for Direct Synthesis of Higher Alcohols and Olefins from Syngas

Cobalt carbide (Co2C) has recently been reported to be efficient for the conversion of syngas (CO+H-2) to lower olefins (C-2-C-4) and higher alcohols (C2+, alcohols); however, its properties and formation conditions remain ambiguous. On the basis of our previous investigations concerning the formation of Co2C, the work herein was aimed at defining the mechanism by which the manganese promoter functions in the Co-based catalysts supported on activated carbon (CoxMn/AC). Experimental studies validated that Mn facilitates the dissociation and disproportionation of CO on the surface of catalyst and prohibits H-2 adsorption to some extent, creating a relative C-rich and H-lean surface chemical environment. We advocate that the surface conditions result in the transformation from metallic Co to Co2C phase under realistic reaction conditions to form Co@Co2C nanoparticles, in which residual small Co-0 ensembles (<6 nm) distribute on the surface of Co2C nanoparticles (similar to 20 nm). Compared with the Co/AC catalyst, where the active site is composed of Co2C phase on the surface of Co-0 nanoparticles (Co2C@Co), the Mn-promoted catalysts (Co@Co2C) displayed much higher olefin selectivity (10% versus 40%), while the selectivity to alcohols over the two catalysts are similar (similar to 20%). The rationale behind the strong structure performance relationship is twofold. On the one hand, Co Co2C interfaces exist universally in the catalysts, where synergistic effects between metallic Co and Co2C phase occur and are responsible for the formation of alcohols. On the other hand, the relative C-rich environment created by Mn on the Co@Co2C catalysts facilitates the formation of olefins.