Formation of carbonate and oxalate species on a Cobalt-modified Fe3O4(111) surface: Comparison of DFT+U, hybrid functionals, and the random phase approximation

Abstract

We discuss the formation of carbonate, CO32-, and the thermodynamically competing formation of a planar oxalate, C2O42-, species in a Co-modified (111) surface of magnetite. The structure corresponds to a Cooct2-Fetet1-terminated (111)-(1 × 1) surface of Fe3O4. It appears that the conversion of CO2 into oxalate involves host-guest-type of interactions due to the cup-like morphology of the surface. As our system of interest is known to be challenging for electronic structure methods in terms of accuracy, we compare GGA-PBE(+U) results with results obtained using the computationally more costly and supposedly more accurate HSE hybrid functional, as well as the random phase approximation (RPA). While PBE+U and HSE predict thermodynamically competing species, RPA predicts C2O42- to be substantially more stable than two individual carbonate species on the surface. We discuss infra-red vibrational bands and suggest scanning tunneling microscopy to directly prove the formation of this surface oxalate species, because of characteristic and readily recognizable butterfly-like images.