Significant roles of oxygen and unbound •OH radical in phenol formation during photo-catalytic degradation of benzene on TiO2 suspension in aqueous system

Abstract

Studies on photo-catalytic degradation of benzene using TiO 2 photocatalyst as a suspension in water is reported. Degradation studies have been carried out using 350 nm UV light. Phenol, a photo-catalytic product of benzene, was monitored under varying experimental conditions such as amount of TiO 2 , concentration of benzene, photolysis time, ambient (air, O 2 , Ar, N 2 O and N 2 O-O 2 mixture), etc. The phenol yields in both aerated and O 2 -purged systems increased with the photolysis time. In contrast to oxygenated systems, the yields of phenol in deoxygenated (viz. Ar-purged and N 2 O-purged) systems were quite low (*30 lM) and remained steady. H 2 O 2 yields in all these systems were also monitored, and found lower than an order of magnitude as compared to phenol yields for the respective systems. The rate of phenol production in aerated 1 mM benzene solution containing 0.05% TiO 2 suspension was evaluated at 12.3 lM min -1 which is lower than the rate obtained in an O 2 -saturated system (22.4 lM min -1 ). The low yields of phenol in both Ar-and N 2 O-purged systems, and also the increasing trends in oxygenated systems, together reveal that, for the phenol formation with an enhanced rate, oxygen is essential. In the present study, it is implied that the photo-generated hole, which is mainly an • OH radical, is either freely available in the aqueous phase or migrates to the aqueous phase from the catalyst surface, to react with benzene to produce HO-adduct radical. Later, following reaction with oxygen, the adduct produces phenol. On the other hand, h ? and surface adsorbed • OH radical, being trapped/bonded due to rigid association with the catalyst surface, were not able to generate phenol under similar experimental conditions. The mechanism of phenol formation with TiO 2 photolysis in an aqueous system is rechecked, on the basis of present results on h ? /surface adsorbed • OH radical/unbound • OH radical scavenging by benzene, collectively with previous reports on various systems.

Variable products in dielectric-barrier discharge assisted benzene oxidation

“…Gas-phase benzene concentration was measured at 254.6 nm (ε = 210 M −1 cm −1 ) in a 10 cm path cell [22]. The DBD outlet gas was sampled and analyzed by GC-FID (gas chromatograph-flame ionization detector) and GC-MS (gas chromatograph-mass spectrometer) with BPX50 30 m × 0.25 mm ID capillary column, flow at 1.5 mL min −1 , temperature programming: 60 • C-r(10)-180, and in some cases by Folin-Ciocalteu's (FC) colorimetric analysis [23,24]. In addition, after each DBD cycle, a straw yellow colored deposited on each electrode surface were washed with 10 mL ethanol, and separately subjected to GC and GC-MS analyses.…”

Benzene transformation in two dielectric barrier discharge reactors having special arrangement

“…The phenol yields observed were less in all systems, except System A. Moreover, biphenyl generation was found to be more in Systems B and C and the decrease in phenol in Systems B and C indicated the significant contribution of moisture and H 2 O 2 in phenol formation through • OH‐initiated reactions . Furthermore, biphenyl formation took place possibly due to phenyl ( • C 6 H 5 ) dimerization, which was found more prominent in the systems containing less moisture.…”

Effects of Metal Ions on H2 Generation during Photolysis of Suspended TiO2 in Aqueous Systems

“…This was happened due to the presence of 2-propanol, a hole scavenger, which resists hole -e recombination (reverse of reaction 1 which is very fast 10 -100 ns [5,30]) by reacting with photo-generated hole. The hole has been considered as an • OH radical or surface attached • OH radical generated in TiO 2 photolysis (reaction 2) under aqueous matrix [1,11,31] ) [32]). The presence of 2propanol and its high concentration (0.5 M) together make the system more favorable towards reducing condition by rapid scavenging • OH/h + , subsequently allowing photo-generated e to react with H 3 + O (reaction 6) yielding more H 2 .…”