Interaction of NO₂ with TiO₂ surface under UV irradiation: measurements of the uptake coefficient

• Main Authors: Y. Bedjanian, A. El Zein
• Format: Article
• Language: English
• Published: Copernicus Publications 2012-01-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/12/1013/2012/acp-12-1013-2012.pdf

The interaction of NO₂ with TiO₂ solid films was studied under UV irradiation using a low pressure flow reactor (1–10 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The NO₂ to TiO₂ reactive uptake coefficient was measured from the kinetics of NO₂ loss on TiO₂ coated Pyrex rods as a function of NO₂ concentration, irradiance intensity (<i>J</i>_NO₂ = 0.002–0.012 s⁻¹), relative humidity (RH = 0.06–69 %), temperature (<i>T</i> = 275–320 K) and partial pressure of oxygen (0.001–3 Torr). TiO₂ surface deactivation upon exposure to NO₂ was observed. The initial uptake coefficient of NO₂ on illuminated TiO₂ surface (with 90 ppb of NO₂ and <i>J</i>_NO₂&cong;0.006 s⁻¹) was found to be &gamma;₀ = (1.2±0.4) &times;10^&minus;4 (calculated using BET surface area) under dry conditions at <i>T</i> = 300 K. The steady state uptake, &gamma;, was several tens of times lower than the initial one, independent of relative humidity, and was found to decrease in the presence of molecular oxygen. In addition, it was shown that γ is not linearly dependent on the photon flux and seems to level off under atmospheric conditions. Finally, the following expression for γ was derived, γ = 2.3×10⁻³ exp(&minus;1910/<i>T</i>)/(1 + <i>P</i>^0.36) (where <i>P</i> is O₂ pressure in Torr), and recommended for atmospheric applications (for any RH, near 90 ppb of NO₂ and <i>J</i>_NO₂ = 0.006 s⁻¹).