Theoretical exploration of intrinsic facet-dependent propene combustion catalysis of normal spinel ZnFe2O4.

Intrinsic facet effects on spinel catalysis for alkene combustion remain poorly understood, limiting morphology-guided catalyst design. Herein, DFT+U calculations compare propene combustion on the dominant low-index facets of normal-spinel ZnFe2O4 and benchmark ZnFe2O4(111) against ZnCr2O4(111). Wulff construction indicates that equilibrium ZnFe2O4 crystallites primarily expose (100) facets (85.4%) with a minority of (111) facets (14.6%). On ZnFe2O4(111), the activation barriers for the key C-H activation and subsequent oxygen-vacancy (OV) formation are lower than on ZnCr2O4(111) (e.g., the key C-H activation barrier decreases from 1.63 to 1.34 eV), indicating the presence of higher intrinsic activity of ZnFe2O4(111). In contrast, ZnFe2O4(100) binds propene weakly and requires high barriers to afford adsorbed acrolein (1.88-1.97 eV); acrolein also adsorbs weakly, suggesting limited completed combustion on this facet. Bader charge, projected density of states (PDOS), and interaction/deformation energy analyses attribute the (111) advantage to more favorable charge acceptance by lattice oxygen and iron, whereas the (100) suffers larger deformation costs and less favorable charge acceptance by lattice oxygen. Overall, the present calculations elucidate that the catalytic performance of spinel in alkene combustion can be controlled by facet engineering and support the prediction that octahedral ZnFe2O4, mainly exposing (111), is a promising non-toxic catalyst for alkene combustion with the potential to replace noble-metal-based catalysts.