Cobalt carbide (Co2C) has recently been reported to be efficient for the conversion of syngas (CO+H2) to lower olefins (C2–C4) 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 H2 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 Co0 ensembles (<6 nm) distribute on the surface of Co2C nanoparticles (∼20 nm). Compared with the Co/AC catalyst, where the active site is composed of Co2C phase on the surface of Co0 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 (∼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 and H-lean surface chemical environment created by Mn on the Co@Co2C catalysts facilitates the formation of olefins.
Unsupported Co–Co2C catalyst and active carbon supported Co–Co2C (Co–Co2C/AC) catalysts were prepared and have been first proven to be highly active for 1-hexene hydroformylation under low pressure (P = 3.0 MPa and T = 453 K). It is found that the catalytic performances over the Co–Co2C and Co–Co2C/AC catalysts were strongly dependent on the ratio of Co2C to Co. Highly catalytic performances were achieved with the XRD intensity ratio of Co2C to Co ranging from 0.7 to 1.2. Co–Co2C/AC catalyst with carburization for 20 h has a highly catalytic stability for 1-hexene hydroformylation with a time stream of 140 h, indicating that no dissolved cobalt carbonyl species were formed and thus led to no cobalt elusion during hydroformylation under reaction conditions. Density functional theory (DFT) calculations have been conducted to understand the nature of the catalytic performance. We found that the interface between Co and Co2C plays a significant role in ethylene hydroformylation. Metallic Co sites are used for olefin adsorption and activation to form surface carbonaceous species, while Co2C sites, for CO molecular adsorption, activation, and insertion. Our results have provided a strategy for designing highly active bifunctional non-noble metal catalysts.
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