Isoprene Heterogeneous Uptake and Reactivity on TiO2 : A Kinetic and Product Study

Abstract

The heterogeneous interaction of isoprene with TiO2 surfaces was studied under dark and UV light irradiation conditions. The experiments were conducted at room temperature, using zero air as bath gas, in a flow reactor coupled with a SIFT-MS (selected-ion flow-tube mass spectrometer) and a FTIR spectrometer for the gas-phase monitoring of reactants and products. The steady-state uptake coefficient and the yields of the products formed were measured as a function of TiO2 mass (9–120 mg), light intensity (37–112 W m−2), isoprene concentration (36–12000 ppb), and relative humidity (0.01–90% of RH). Under dark and dry conditions, isoprene was efficiently and reversibly adsorbed on TiO2. In contrast, under humid conditions, isoprene uptake was diminished, pointing to competitive adsorption with water molecules. In the presence of UV light irradiation, isoprene reacted on the surface of TiO2. The reactive steady-state uptake coefficient, γss, was independent of RH under most ambient relative humidity conditions (>50%). However, γss was strongly dependent on isoprene initial concentration according to the empirical expression: γss = (2.0 × 10−4) × [isoprene]0−n with n = 0.35 and 0.28 for 37 and 112 W m−2 irradiation conditions, respectively. In addition to the kinetics, a detailed product study was performed. The gas-phase oxidation products were mostly CO2 (ca. 90% of the carbon mass balance) and a large variety of carbonyl compounds (methyl vinyl ketone, acetone, methacrolein, formaldehyde, acetaldehyde, propanal, traces of butanal, and pentanal), the distribution of which was investigated as a function of mineral oxide mass, isoprene concentration, and RH. Furthermore, the surface-adsorbed products were determined employing off-line HPLC chromatography; their concentrations were inversely dependent on RH and decreased to background levels at RH greater than 30%. Finally, the reaction mechanism and possible implications of isoprene reaction on TiO2 are briefly discussed.