Heterogeneous and Photochemical Reactions of Solid Benzophenone−Catechol Films with NO₂

Abstract

Solid−air interfaces are ubiquitous to the atmosphere, and heterogeneous reactions between gaseous oxidants and surface adsorbed organics on these interfaces can impact tropospheric chemistry. Solid benzophenone−catechol films serve as model photosensitizer−polyphenol compounds that we reacted with NO2 in the parts-per-billion range under dark and light conditions at 300 K and 20% relative humidity. Attenuated total reflectance infrared spectroscopy (ATR-FTIR) monitored chemical changes in the organic film during these reactions to directly identify condensed-phase products. Catechol, when mixed with benzophenone or dicyclohexyl ketone, reacted with NO2 under dark conditions, forming 4-nitrocatechol as the exclusive condensed-phase product; pure catechol films did not react. Kinetic isotope experiments found rate[C6H4(OH)2]/rate[C6H4(OD)2] = 3.3 ± 0.5, indicating that breaking an O−H bond was critical to the rate-determining step. A mechanism involving the ortho-semiquinone radical, possibly stabilized by hydrogen bonding to the coadsorbed ketone, is discussed. The reaction was also found to be second order (2.09 ± 0.18) with respect to NO2, suggesting a possible pre-equilibrium with N2O4. Although benzophenone is a well-known photosensitizer, the rate of 4-nitrocatechol formation was not enhanced by UV-A/visible radiation. This observation eliminates this pathway as a possible photoenhanced daytime source of HONO. However, ATR-FTIR detected additional photochemical products resulting from a photoinitiated reaction between benzophenone and 4-nitrocatechol. These results highlight the potential for heterogeneous chemistry involving surface adsorbed organics to form nitroaromatic compounds, which are of interest due to their phytotoxic and UV absorbing properties.

UVA/Vis-induced nitrous acid formation on polyphenolic films exposed to gaseous NO2

“…These are (I) photolysis of a nitroaromatic intermediate formed by a non-photochemical process also in the dark, (II) direct photo-oxidation, and (III) sensitized electron or hydrogen transfer as suggested before but also demonstrated for the visible irradiation here. These results in some way resolve the discrepancy between the previous George et al 29 and Nichols et al 33 studies, in that photoenhanced NO 2 to HONO conversion comes with formation of nitrophenols. Thus, processing of atmospherically relevant phenolic compounds with NO 2 may lead to in situ formation and accumulation of nitroaromatic compounds, similar to those identified in atmospheric aerosol samples.…”

“…The interaction of GA and TA coatings with gaseous NO 2 at 45% relative humidity was investigated first in the dark, to provide a link to recent studies with catechol films by Nichols et al 33 and Woodill et al 52 and because we found that first exposing such films to NO 2 in the dark significantly affected the photochemical processes later on.…”

“…58 The possible mechanism giving rise to these increased NO X concentrations is photolysis of non-volatile nitro compounds that formed in the heterogeneous reaction of NO 2 with soot 59 or polyphenolic material as suggested for the present system and reported in comparable studies. 33,52 Nitrophenols are known to strongly absorb UV light in the range 300-400 nm. 34-36 Nitrophenols having an OH group in ortho position to the nitro group and their methylated derivatives undergo photodissociation resulting in HONO elimination in the liquid phase 35,36 and in the gas phase.…”

“…The overall scheme of this light-induced NO 2 to HONO conversion is also suggested for humic acid coatings, where chromophoric centres would take over the role of the sensitizer, and the phenolic moieties that of the electron donor. 31, 32 Recently, Nichols et al 33 have shown that light did not apparently enhance the yield of other non-volatile products of the dark heterogeneous chemistry of NO 2 with phenols, e.g. aromatic nitro compounds.…”

Surface Water Structure and Hygroscopic Properties of Light Absorbing Secondary Organic Polymers of Atmospheric Relevance

“…54 The tailing of the OH broad band till 2500 cm –1 is characteristic of hydrogen bonding network on organic particles and films and is correlated with the presence of stronger hydrogen bonds relative to those in the liquid phase of water. 40,55,56…”