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 Fetet1 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 Fetet1 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 Fetet1 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 3σ 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.