Simultaneous spectroscopic detection of adsorbed and gas-phase species during atmospherically relevant heterogeneous reactions

Abstract

The application of a recently developed ATR-IR flow cell coupled with an FTIR gas phase detection system to simultaneously monitor the gas-phase and surface composition during the well-known reaction of nitrogen oxide gases NO₂ and N₂O₅ with NaCl powder is reported. Infrared absorption band assignments for the major products are presented for species in the gas and solid phase, and correlations between the appearance of gas phase and surface species are used to follow the reactions qualtitatively in real time. The reaction of NO₂ with NaCl produces the expected surface NO₃⁻ and gaseous ClNO products, but is also observed to produce in a minor reaction channel a second surface nitrate species correlated with surface water loss as a result of heterogeneous hydrolysis. The reaction of N₂O₅ similarly produces ClNO₂ in the gas phase and a surface nitrate in one reaction channel, with simultaneous production of a significant amount of gas-phase HNO₃ in a second heterogeneous hydrolysis reaction channel. These observations confirm the usefullness of the combined surface and gas phase analytical methodology in following the mechanistic and kinetic detail of atmospherically relevant reactions.