Adsorption of NO2 on YSZ(111) and Oxygen-Enriched YSZ(111) Surfaces

The adsorption of NO2 onto yttrium-stabilized zirconia (YSZ) surfaces is rarely considered a necessary concern when designing devices or structures which rely on YSZ either as a high-temperature mechanical support or in its role as an oxide ion conducting electrolyte. However, in this work we show that the presence or absence of a subsurface oxygen ion vacancy can play a significant role in not only the type of adsorption which can occur but also the reaction mechanism of NO2 on the surface at different temperatures. In this work the binding energy, vibrational frequencies, density of states, magnetic moments, electron localization, and charge transfer, as determined using density functional theory calculations, are presented for all stable NO2 adsorption configurations on YSZ(111) and oxygen-enriched YSZ+O(111) surfaces. Additionally, ab initio molecular dynamics simulations performed at 298 and 773 K show how high temperatures influence the adsorption and desorption of NO2 onto YSZ(111) and YSZ+O(111) surfaces, with respect to their application as an oxide ion conducting electrolyte in high-temperature electrochemical gas sensors. It was found that, on an oxygen-enriched YSZ+O(111) surface, NO2 bonded with a surface oxygen atom, which could yield an adsorbed NO2, nitrate, or cis-peroxynitrite (OONO) molecule. Adsorbed NO2 was found to desorb from the YSZ(111) and YSZ+O(111) surfaces at 298 K. The adsorbed nitrate or cis-peroxynitrite species, however, are more stable, remaining on the surface at 298 K up to a simulation time of 3 ps. At 773 K, either NO3 desorbs from the surface or the OONO dissociates into O2 and NO, which can also desorb from the surface. Hence, the presence of subsurface O results in the oxidation of NO2, returning the surface to YSZ(111). A further oxide ion could then migrate to the vacancy, allowing reaction with further NO2 molecules. This work is of particular interest to those developing solid-state electrochemical NO x sensors and for understanding the adsorption and reaction of NO2 with YSZ surfaces.