Surface water enhances the uptake and photoreactivity of gaseous catechol on solid iron(III) chloride

Uptake and photoreactivity of catechol-Fe complexes are investigated at the gas/solid interface under humid and dry conditions, along with the nature of the hydrogen-bonding network of adsorbed water. Catechol was chosen as a simple model for organics in aerosols. Iron chloride was used to distinguish ionic mobility from binding to coordinated iron(III) in hematite. Studies were conducted using diffuse reflectance infrared Fourier transform spectroscopy as a function of irradiation time. Results show that adsorbed water at 30% relative humidity (RH), not light, increases the concentration of adsorbed catechol by a factor of 3 over 60 min relative to dry conditions. Also, our data show that, at 30% RH and under light and dark conditions, growth factors describing the concentration of adsorbed catechol are very similar suggesting that light does not significantly enhance the uptake of catechol vapor on FeCl3. Surface water also enhances the initial photodecay kinetics of catechol-Fe complexes at 30% RH by a factor of 10 relative to control experiments (RH < 1%, or no FeCl3 under humid conditions). Absorptions assigned to carbonyl groups were not observed with irradiation time, which was explained by the dominance of FeCl(2+) species relative to FeOH(2+) in the highly acidic "quasi-liquid" phase at 30% RH. Clear differences in the hydrogen-bonding network upon gaseous catechol uptake are observed in the dark and light and during the photodecay of adsorbed catechol. The implications of these results on our understanding of interfacial processes in aged iron-containing surfaces are discussed.