Tuning the size of TiO₂-supported Co nanoparticle Fischer-Tropsch catalysts using Mn additions

Modifying traditional Co/TiO₂ based Fischer Tropsch (FT) catalysts with Mn promoters induces a selectivity shift from long-chain paraffins towards commercially desirable alcohols and olefins. In this work we use in situ gas cell scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS) elemental mapping, and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to illustrate how the elemental dispersion and chemical structure of the as-calcined materials evolves during the H2 activation heat treatment required for industrial CoMn/TiO₂ FT catalysts. We find Mn additions reduce both the mean Co particle diameter and the size distribution but that the Mn remains dispersed on the support after the activation step. Density functional theory calculations show that the slower surface diffusion of Mn is likely due to the lower number of energetically accessible sites for the Mn on the titania support, and that favourable Co-Mn interactions likely cause the greater dispersion and slower sintering of Co in the Mn promoted catalyst. These mechanistic insights into how the introduction of Mn tunes the Co nanoparticle size can be applied to inform the design of future supported nanoparticle catalysts for FT and other heterogenous catalytic processes.