Hydrogen Adsorption and Site-Selective Reduction of the Fe3O4(001) Surface: Insights From First Principles

Density functional theory calculations including an on-site Hubbard term are used to explore hydrogen adsorption on the surface of Fe3O4(001). The adsorption energy exhibits a minimum for two hydrogen atoms per (root 2 x root 2)R45 degrees surface unit cell and gets less favorable with increasing hydrogen coverage due to OH-OH repulsion. Terminations with two and four hydrogen atoms per surface unit cell are stable for moderate to high partial pressures of O and H. The strong tilt of the OH bond parallel to the surface facilitates hydrogen bonding to neighboring oxygen and hopping of the protons between surface oxygen sites. Furthermore, the formation of surface OH groups leads to a monotonic reduction of work function with increasing H coverage. The analysis of the electronic properties reveals selective switching of neighboring surface and subsurface Fe from Fe3+ to Fe2+ upon hydrogen adsorption. This provides a promising way to tune the catalytic activity of the Fe3O4(001) surface.