The oxidizing pretreatment-mediated autoreduction behaviour of cobalt nanoparticles supported on ordered mesoporous carbon for Fischer–Tropsch synthesis

Abstract

Ordered mesoporous carbons (OMCs) were employed to support cobalt species for insight into the autoreduction behaviour of cobalt oxide on carbon substrates and their catalytic performance during the Fischer–Tropsch synthesis (FTS). The as-synthesized samples were subjected to the characterization of N₂-physisorption, XPS, in situ XRD, TPR, TG-MS, HAADF-STEM and H₂-chemisorption. The results show that the oxidizing pretreatment of OMC substantially improves the dispersion of cobalt species. Consequently, more uniform cobalt particles are observed on the pretreated supports in comparison with their counterparts on pristine OMCs, which enables the autoreduction of cobalt oxide on the carbon supports to occur at a lower temperature owing to the weak metal–support interaction. As compared to the hydrogen-reduced samples, the morphology of cobalt particles suffers from a significant change after the autoreduction and a mechanism with respect to the anti-oriented diffusion of oxygen atoms is proposed to account for the formation of ellipsoidal or quasi-ellipsoidal particles. During the evaluation of FTS, the autoreduced catalysts exhibit higher activity than the hydrogen-reduced ones owing to more cobalt atoms being exposed on the surface. Dictated by the decreasing cobalt particles, the cobalt-specific activity of pretreated samples is ca. 2.2 times higher than its analog on the pristine OMCs under optimum conditions without any influence on the TOF or at the expense of the C₅₊ hydrocarbon selectivity.