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

Abstract

Cobalt carbide (Co 2 C) 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 (C 2+ alcohols); however, its properties and formation conditions remain ambiguous. On the basis of our previous investigations concerning the formation of Co 2 C, 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 Co 2 C phase under realistic reaction conditions to form Co@Co 2 C nanoparticles, in which residual small Co 0 ensembles (<6 nm) distribute on the surface of Co 2 C nanoparticles (∼20 nm). Compared with the Co/AC catalyst, where the active site is composed of Co 2 C phase on the surface of Co 0 nanoparticles (Co 2 C@Co), the Mn-promoted catalysts (Co@Co 2 C) displayed much higher olefin selectivity (10% versus 40%), while the selectivity to alcohols over the two catalysts are similar (∼20%). The rationale behind the strong structure–performance relationship is twofold. On the one hand, Co–Co 2 C interfaces exist universally in the catalysts, where synergistic effects between metallic Co and Co 2 C phase occur and are responsible for the formation of alcohols. On the other hand, the relative C-rich and H-lean surface chemical environment created by Mn on the Co@Co 2 C catalysts facilitates the formation of olefins.