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