Oxygen Adsorption, Diffusion And Incorporation On Ru(0001) Surface:a First Principle Study

Ruthenium based surfaces have excellent catalytic activity in a variety of organic reactions, NH3 synthesis and CO oxidation. Meanwhile, as a platinum group metal it can form various oxidation states which often have unique properties. Fox example, RuO2 turned out to be an excellent oxidation catalyst in heterogeneous catalysis and electrocatalysis. However, there are still many questions unsolved for Ru based surface oxidation which is closely related to these catalytic properties. State of the art DFT calculations of the O-Ru(0001) system that has attracted some general interest over the past year. One open question is how the atomic oxygen penetrates from on-surface into subsurface. In this paper, we performed the first principle calculations on oxygen adsorption, diffusion and incorporation on Ru(0001). The main research contents and conclusions of this paper are as follows:1. Firstly, three types of geometries including pure on-surface adsorption, pure subsurface adsorption, and co-adsorption with oxygen on surface and in subsurface at a same time for Ru(0001) surface are disscused. Averaged binding energies, total binding energies, and the formation energies of oxygen adsorption for above three occupation structures are presented. As for pure on-surface oxygen adsorption, the hep site is energetically favorable relative to the fcc site. The binding energy difference between oxygen in hcp- and fcc-hollow sites decreases with increasing oxygen coverage. Upon the coverage increasing,dRu-Ru decreases significantly and meanwhile A12 increases which implies the weakness in the first interlayer coupling. For pure subsurface oxygen adsorption, the binding energy increases with coverage. For co-adsorption system, the fcc/tetra-I geometry is always the most stable one at all considered coverages with mixed occupation, but its binding energy is still obviously less than that for pure on-surface adsorption. Basing on the total binding energy calculation, the obtained incorporation barrier should not less than 1.70 eV without considering the intermediate state.2. Oxygen diffusion on surface and in subsurface, as well as oxygen incorporation from surface into subsurface are researched. Starting from pure hcp or fcc on-surface adsorption, the on-surface oxygen diffusion barrier between hcp and fcc site is in the range of 0.77 to 0.95 eV. The diffusion barrier for oxygen in subsurface is more complicated with the change of coverage and on-surface oxygen adsorption is not necessarily beneficial to oxygen diffusion in subsurface. The indirect path, by which atomic oxygen will diffuse on surface at first and then penetrate into subsurface, is more effective for the first oxygen incorporation into subsurface. At low oxygen coverage, the penetration barrier is much more than the on-surface and in-subsurface diffusion barrier. Before the last oxygen incorporation, the on-surface diffusion barrier increases and the penetration barrier decreases with increasing on-surface oxygen coverage. The minimal penetration barrier is obtained to be 1.81 eV by path starting from mixed on-surface hcp and fcc sites occupation at oxygen coverage of 0.75 ML which should be recognized as close to 1.0 ML on larger supercells. From the first oxygen incorporation to the formation of 1 ML subsurface oxygen occupation, both the diffusion barrier and the penetration barrier decrease obviously with increasing subsurface oxygen coverage. Hence, subsequent oxygen incorporation is much easier than the first one.3. Changes in deformation energy of substrate and interaction energies among various parts within adsorption system during oxygen penetration into subsurface are discussed as a function of subsurface oxygen coverage. In the case of penetration process, deformation energy and interaction energy between penetrating atomic oxygen and the topmost Ru layer, two incompatible items in penetration barrier, play decisive roles only for the first oxygen penetration but not for subsequent ones. The amplitude in height fluctuation of the topmost Ru layer regulates the penetration barrier strictly by enhancing the validity of substrate deformation, which could indicate the important role of steps or defects for oxygen incorporation and CO oxidation as proposed by experiments.