Identify Zr Promotion Effects in Atomic Scale for Co-Based Catalysts in Fischer–Tropsch Synthesis

Abstract

Introducing promoters on cobalt-based catalysts for Fischer–Tropsch synthesis (FTS) have been found efficient for adjusting their performance in converting syngas into long-chain hydrocarbons. Details of the promotion mechanism established on atomic precise identification upon the active sites structure as well as the electronic status is seldomly reported yet. In the present work, we report the direct identification of valence status and coordination configuration of ZrO2 promoter additive over the cobalt-based FTS catalysts under a wide range of Zr/Co molar ratios from 0.12 to 1.5. Evidences from multiple technologies, including in situ/ex situ atomic resolution STEM imaging, EDS elemental mapping, electron energy loss spectroscopy (EELS), as well as physicochemical analyses (in situ XRD, CO chemisorption, CO temperature-programmed surface reaction, etc.) disclose that the ZrO2 promoter presents as single-site dispersion on the surface of Co nanoparticles (NPs) and the SiC support at low content, plays as the real active site to promote CO dissociation at the Co-ZrO2 interface. While at high ZrO2 content (Zr/Co molar ratio of 1.0), Zr species on the SiC support turns to nucleate to form an amorphous coating, whereas those on the Co NPs surface maintain monodispersion. When further increasing the ZrO2 content (Zr/Co molar ratio up to 1.5), cobalt NPs start to be encapsulated by ZrO2 coating, leading to the decline of FTS activity. The in situ EELS analysis and density functional theory calculations disclose that the Zr atom tends to bind at the Co NPs surface rather than embed into the lattice meanwhile a charge transfer from Zr species to Co NPs occurs, which facilitates the stronger interaction between Zr and cobalt and thus enhances the adsorption with the H2 molecule as well as the CO dissociation. It thus offers enhanced catalytic activity and long-chain hydrocarbon selectivity during the FTS reaction.