Morphological Evolution of Co Nanoparticles Induced by MnO Adsorption: A Density Functional Theory Study

The crystal structure and nanoparticle morphology of cobalt (Co)-based materials directly determine their performance in Fischer–Tropsch synthesis, catalytic conversion, and related fields. MnO, as an additive, can optimize morphology by regulating the surface energy and exposed crystal planes of Co. However, there is currently a lack of systematic theoretical studies on the differences in surface energy between FCC and HCP Co phases and the corresponding response patterns to MnO adsorption. In this study, density functional theory (DFT) was employed to calculate the surface energies of key exposed crystal planes for FCC and HCP Co, and the equilibrium morphologies of both phases were constructed using Wulff theory. Furthermore, the study investigated the surface-energy reconstruction of each crystal plane upon MnO adsorption and analyzed the evolution characteristics of Wulff morphologies. The results indicate that the addition of MnO can enhance the stability of the nanoparticle surfaces. In particular, we found that, as the MnO/Co ratio increased, the exposure of the (311) and (110) crystal planes in FCC Co, which are highly active for the Fischer–Tropsch reaction and rich in B5 and 5F active sites, increased significantly. In HCP Co, the exposure fraction of the (10_11) facet decreases, whereas that of the (10_12) facet increases. Therefore, by regulating the MnO loading, it is possible to control the exposure of cobalt crystal facets, thereby enhancing the Fischer–Tropsch synthesis rate. This study elucidates the surface-energy differences between the two cobalt crystal structures and the mechanism by which MnO adsorption modulates them. By employing Wulff theory, a quantitative correlation model of “crystal structure–surface energy–morphology” is established, providing theoretical support for the selection of cobalt-based crystal forms, precise morphological control, and optimization of promoter dosage.