High Surface Reactivity and Water Adsorption on NiFe2O4 (111) Surfaces

Transition metal-doped ferrites are attractive candidates for a wide range of applications including catalysis and electronic and magnetic devices. Although their bulk characteristics are well-understood, very little is known about their surface properties at the molecular level. Here, we demonstrate high reactivity of NiFe2O4 (111) surfaces, a Ni-doped ferrite, by elucidating the surface structure and water adsorption mechanism using density functional theory with on-site correction for Couloumb interaction (DFT + U). The surface reactivity of NiFe2O4 (111) surfaces (with 0.25 ML Fe-tet1 and 0.5 ML Feoct2-tet1 terminations) is shown to be significantly higher in comparison with the undoped Fe3O4 (111) surfaces. Dissociation of water is found to be highly favorable with an adsorption energy of -1.11 eV on the 0.25 ML Fe-tet1 terminated surface and -2.30 eV on the 0.5 ML Feoct2-tet1 terminated surface. In addition, we computed a low activation barrier of 0.18 eV for single water molecule dissociation on the 0.25 ML Fe-tet1 termination, while the corresponding dissociation reaction on the 0.5 ML Feoct2-tet1 termination proceeded without a barrier. The reactivity of NiFe2O4 surfaces toward water is understood based on strong interactions between the adsorbing OH radical molecular orbitals and the d orbitals of the surface Fe atom. In particular, the new bonding orbitals created due to the interaction of the OH 3 sigma orbital and the Fe d states are pushed deeper down the energy axis resulting in a greater energy gain and higher water adsorption strength in the case of 0.5 ML Feoct2-tet1 termination. Furthermore, transition-metal surface resonances (TMSR) are found to be good descriptors of the surface reactivity in the two ferrites investigated and is a useful measure to design ferrite-based catalytic systems. These findings have strong implications toward the use of NiFe2O4 as an effective metal-doped ferrite catalyst in a typical industrial process such as the water-gas shift (WGS) reaction and are of significance in fuel materials durability in nuclear reactors where ferrites are known to trap boron resulting in failure of the reactors.