Oxidation and Storage Mechanisms for Nitrogen Oxides on Variously Terminated (001) Surfaces of SrFeO3-δ and Sr3Fe2O7-δ Perovskites.

Abstract

The Ruddlesden-Popper (RP)-type layered perovskite is a candidate material for a new nitrogen oxide (NOx) storage catalyst. Here, we investigate the adsorption and oxidation of NOx on the (001) surfaces of RP-type oxide Sr3Fe2O7-δ for all of the terminations by comparing to those of simple perovskite SrFeO3-δ by the density functional theory (DFT) calculations. The possible (001) cleavages of Sr3Fe2O7 generate two FeO2- and three SrO-terminated surfaces, and the calculated surface energies indicated that the SrO-terminated surface generated by the cleavage at the rock salt layer is the most stable one. The oxygen of the FeO2-terminated surfaces could be removed with significantly low energy because the process involves the favorable reduction of the Fe4+ site. Consequently, the surface oxygen at the FeO2 site could easily oxidize adsorbed NO to NO2 by the Mars-van Krevelen mechanism. The resulting oxygen vacancy in the surface would be filled easily with lattice oxygen in bulk. The oxidation of NO with adsorbed molecular O2 was unfavorable by both the Langmuir-Hinshelwood and Eley-Rideal mechanisms because this process does not involve the reduction of the Fe4+ site. The oxygen of the SrO-terminated surfaces was tightly bound and acted as the adsorption site of NO and NO2. An electron transfer strengthened the NOx binding to the surface by forming nitrite (NO2-) or nitrate (NO3-) species. The DFT calculations revealed that the RP-type structure promoted NOx oxidation and storage properties by forming active oxygen due to the Jahn-Teller distortion and by exposing SrO-terminated surfaces due to the cleavage at the rock salt layer.