Transmission FT-IR and Knudsen cell study of the heterogeneous reactivity of gaseous nitrogen dioxide on mineral oxide particles
- Univ. of Iowa, Iowa City, IA (United States). Dept. of Chemistry
Nitrogen oxides are a major component of air pollution, and their chemical reactivity is of great importance in atmospheric chemistry. Heterogeneous reactions that can alter NO{sub x} (NO{sub x} = NO + NO{sub 2}) concentrations have important implications for tropospheric ozone formation as ozone is not directly emitted into the troposphere, but is formed by a complex series of photochemical reactions involving nonlinear interactions among NO{sub x}, volatile organic compounds (VOCs), and carbon monoxide. The heterogeneous reactivity of gaseous nitrogen dioxide on mineral oxide particles was investigated. In particular, spectroscopic and kinetic measurements have been made to investigate surface reactions of NO{sub 2} on Al{sub 2}O{sub 3}, Fe{sub 2}O{sub 3}, and TiO{sub 2} at 298 K. Both gas-phase and surface-bound products are formed from the reaction of NO{sub 2} with these mineral oxide particles. At low coverages, FT-IR spectra of the mineral oxide surface exposed to gaseous NO{sub 2} show absorptions due to surface nitrate, specifically a chelating nitro species. As the coverage increases, the surface becomes populated with surface nitrate bonded in several different bonding coordinations (monodentate, bidentate, and bridging). A Knudsen cell reactor coupled to a quadrupole mass spectrometer was used to measure the uptake coefficient, {gamma}, for NO{sub 2} on these oxide particles and to characterize gas-phase product formation. The Knudsen cell data showed NO to be the major gas-phase product with a delay in the onset of NO production. There was little production of N{sub 2}O and no gas-phase HONO or HNO{sub 3} was detected. By monitoring the reaction until completion, the ratio of NO{sub 2} reacted to NO produced was determined to be {approximately}2:1. These results complement the FT-IR data and suggest a two-step mechanism in which NO{sub 2}(g) is initially adsorbed as a nitrite species which subsequently reacts with additional NO{sub 2} to form surface nitrate and gas-phase NO. Finally, the initial uptake coefficient was determined from the Knudsen cell data for NO{sub 2} on Al{sub 2}O{sub 3}, Fe{sub 2}O{sub 3}, and TiO{sub 2}. Because NO{sub 2} can diffuse into the underlying layers of these oxide particles, the use of a geometric area does not give accurate values of the uptake coefficient. Gas diffusion must be taken into account to more accurately determine the uptake coefficient.
- OSTI ID:
- 682150
- Journal Information:
- Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory, Vol. 103, Issue 31; Other Information: PBD: 5 Aug 1999
- Country of Publication:
- United States
- Language:
- English
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