Thermal and photochemical reactions of NO2 on chromium(iii) oxide surfaces at atmospheric pressure

Abstract

While many studies of heterogeneous chemistry on Cr₂O₃ surfaces have focused on its catalytic activity, less is known about chemistry on this surface under atmospheric conditions. We report here studies of the thermal and photochemical reactions of NO₂ on Cr₂O₃ at one atm in air. In order to follow surface species, the interaction of 16–120 ppm NO₂ with a 15 nm Cr₂O₃ thin film deposited on a germanium crystal was monitored in a flow system using attenuated total reflectance (ATR) coupled to a Fourier transform infrared (FTIR) spectrometer. Gas phase products were monitored in the effluent of an ∼285 ppm NO₂–air mixture that had passed over Cr₂O₃ powder in a flow system. A chemiluminescence NO_y analyzer, a photometric O₃ analyzer and a long-path FTIR spectrometer were used to probe the gaseous products. In the absence of added water vapor, NO₂ formed nitrate (NO₃⁻) ions coordinated to Cr³⁺. These surface coordinated NO₃⁻ were reversibly solvated by water under humid conditions. In both dry and humid cases, nitrate ions decreased during irradiation of the surface at 302 nm, and NO and NO₂ were generated in the gas phase. Under dry conditions, NO was the major gaseous product while NO₂ was the dominant species in the presence of water vapor. Heating of the surface after exposure to NO₂ led to the generation of both NO₂ and NO under dry conditions, but only NO₂ in the presence of water vapor. Elemental chromium incorporated into metal alloys such as stainless steel is readily oxidized in contact with ambient air, forming a chromium-rich metal oxide surface layer. The results of these studies suggest that active photo- and thermal chemistry will occur when boundary layer materials containing chromium(III) or chromium oxide such as stainless steel, roofs, automobile bumpers etc. are exposed to NO₂ under tropospheric conditions.