Session: Basic Research in Colloids, Surfactants and Interfaces:

NO₂is known to be an abundant atmospheric pollutant, involved in numerous heterogeneous reactions for which the mechanisms and rates still need to be unravelled and quantified. Under atmospheric conditions, NO₂ is in equilibrium with its dimers, N₂O₄, which can exist in the form of constitutional isomers and stereoisomers, and whose vibrational signatures and relative stabilities are still being debated. Experimental limitations facing the spectroscopic characterization of the isomers of N₂O₄ prevent us from determining their relative contributions to reaction mechanisms and rates, as well as their role, either as reagents and/or intermediates. Therefore, most of the work reported in the literature approximates NO₂ as the main precursor reagent, overlooking contributions from specific dimers along the reaction path precluding their roles in reaction mechanisms to be established, and resulting in discrepancies in the reported reaction orders and rates.
Using Reflection-Absorption IR Spectroscopy (RAIRS), molecular beam deposition and matrix isolation techniques, it will be shown that the NO₂/N₂O₄ ratio can be controlled by heating or cooling the deposited gas. Furthermore, comparing spectra acquired for films prepared using molecular beam and background deposition techniques demonstrates how the N₂O₄ isomers distribution is sensitive to the experimental conditions and sample preparation protocols. This can be explained by possible interactions and dimerization/isomerization of NO₂/N₂O₄ upon adsorption onto/desorption from experimental setup surfaces by which the deposited gas transits before being deposited onto the substrate. These observations not only provide a better understanding of a possible source for the disagreements found in the literature, but also a methodology to control the relative abundance of NO₂ and the N₂O₄isomers distribution. Systematic experiments with various NO₂/N₂O₄ ratios and deposition techniques can therefore be elaborated to determine whether dimerization/isomerization is a limiting step in heterogeneous reaction mechanisms, improving our description of the kinetics of these important atmospheric processes.

Pat Ayotte

Professeur-chercheur, Universit&eacute; de Sherbrooke

Appearances

Professeur-chercheur, Université de Sherbrooke