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

Abstract

The heterogeneous interaction of isoprene with TiO 2 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 TiO 2 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 TiO 2 . 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 TiO 2 . The reactive steady-state uptake coefficient, γ ss , was independent of RH under most ambient relative humidity conditions (>50%). However, γ ss was strongly dependent on isoprene initial concentration according to the empirical expression: γ ss = (2.0 × 10 −4 ) × [isoprene] −n 0 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 CO 2 (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 Preparation of TiO 2 Samples. TiO 2 powder (Degussa, Aeroxide-P25) was used as purchased. The fractional content of anatase versus rutile has been determined in the literature to be ca. 85:15 [22]. Its specific surface area (SSA) was determined using a custom-built gas sorption analyzer (52 ± 6 m 2 g −1 ) and was in excellent agreement with that given by the supplier (50 ± 15 m 2 g −1 ). The mass of each sample was weighed before and after the photocatalytic experiments with a high accuracy mass balance (±0.1 mg), and the difference between the two measurements never exceeded 5% for masses above 30 mg. Nevertheless, at masses below

Uptake Mechanism of Acetic Acid onto Natural Gobi Dust

“…The experimental setup used in the present study to investigate both the gas and adsorbed phases during AcA uptake on Gobi dust consists of three main parts: (i) a gas flow preparation system, (ii) a U-shape flow-through reactor coupled with a transmission FTIR spectrophotometer for gas-phase analysis, and (iii) a DRIFT spectroscopy optical sample cell for surface monitoring, as illustrated in Figure . These experimental setups and the approaches used have been extensively deployed in the past to study the heterogeneous interactions/reactions of organic and inorganic compounds on the surface of dust and catalysts. ,,− …”

Geocatalytic Uptake of Ozone onto Natural Mineral Dust

“…This experimental set up has been used extensively in the past to investigate the heterogeneous interaction/reaction of volatile organic compounds (VOCs) and inorganic species with natural mineral dusts and proxies [36][37][38][39][40], and thus only a brief description is provided. relative abundances of mineral oxide phases in Gobi dust determined by XRD spectroscopy combined with ICP mass spectrometry are 55.0% SiO2, 17.8% CaCO3, 10.5% NaAlSi3O8, 6.9% Al2O3, 2.6% Fe3O4, 1% TiO2 and 6.2% of uncounted fraction [36].…”

Reactive uptake of NO2 on volcanic particles: A possible source of HONO in the atmosphere

“…The CWFT reactor was surrounded by 3 Ultraviolet A lamps (UVA, Philips lighting 18 Watt; 315-400 nm with maximum emission at 352 nm) (Romanias et al, 2017). In order to characterize the irradiance intensity in the flow tube, the NO 2 photolysis frequency was measured as a function of the number of lamps turned on.…”

“… OH, O 2 - ) on mineral dust surfaces that contain photocatalysts (Chen et al, 2012;Herrmann, 2005;Romanias et al, 2012;Schneider et al, 2014) and thus enhance the reactivity of the surface towards NO 2 (Herrmann, 2005;Romanias et al, 2012;Zein et al, 2014). On the other hand, adsorbed water may also block some sorptive and reactive sites and induce a decrease to γ ss,BET (Romanias et al, 2012(Romanias et al, , 2017Zein et al, 2014). It seems that under ambient RH conditions (RH > 30%), the effect of sites blocking by water is probably compensated by its role as a source of radical species on the surface of the dust.…”