Role of Surface Oxygen Vacancies in Intermediate Formation on Mullite-type Oxides upon NO Adsorption

Identifying the nature and reactivity of surface intermediate species is critical to understanding the fundamental reaction pathways of NO oxidation on mullite-type oxide catalysts. Using in situ Fourier transform infrared spectroscopy and density functional theory calculations, we examine the intermediates formed during NO adsorption on SmMn₂O₅ with different surface conditions (as-made, oxidized, and reduced surfaces). Nitrite (NO₂^–) species form during initial NO exposure regardless of surface pretreatment, but its density and stability depend on the surface oxidation conditions and whether there is oxygen in the environment. The formation of NO₂^– arises when NO adsorbs with the nitrogen end down on a lattice oxygen and then rotates so that the oxygen fills the adjacent oxygen vacancy site. With the release of N₂, the surface becomes reoxidized. With continued NO exposure in the presence of lattice oxygen or O₂ gas, nitrate (NO₃^–) species form as the stable intermediate. Our results shed light on the role of oxygen vacancies in catalyzed NO oxidation by mullite-type oxides.