Alexander V. Vorontsov

Vibrofluidized- and fixed-bed photocatalytic reactors: case of gaseous acetone photooxidation

Abstract Vibrofluidized- and multiple fixed-bed photoreactors were compared under identical operating conditions. The comparison was based on the quantum efficiency for the gas-phase photocatalytic oxidation of acetone using TiO 2 (Hombikat UV 100). Multiple fixed-bed reactor configurations were used, along with a vibrofluidized-bed configuration. Quantum efficiency decreased in the following order: vibrofluidized bed (8.7%)>fixed-bed granules (6.9%)>fixed film bed (5.9%) ∼fixed powdered bed (5.8%). The increased activity of the vibrofluidized-bed could not be attributed to enhanced external mass transport, as all reactor systems used in the present study demonstrated negligible external mass transfer resistances. Instead, the increased activity is most likely credited to the effect of periodic illumination phenomenon taking place because of the random motion of catalyst granules in the fluidized-bed and higher light absorption of scattered light. The enhanced activity observed for the granular fixed bed could be related to mechanical activation of TiO 2 during the preparation of granules, as well as to increased light absorption. A maximum of acetone oxidation rate with respect to humidity level was observed. Even at high humidity levels, ultrasound did not affect the rate of gas-phase acetone oxidation. A model has been developed to quantify the absorption of light in a fluidized-bed photoreactor. The model takes into account absorption and single-light scattering and accurately describes the dependence of acetone oxidation rate on the quantity of TiO 2 granules in the vibrofluidized bed.

Parametric studies of diethyl phosphoramidate photocatalytic decomposition over TiO2

The present study is focused on influences of parameters including pH, temperature, TiO(2) catalyst concentration, and reactant concentration on the rate of photocatalytic diethyl phosphoramidate (DEPA) decomposition with Hombikat UV 100 (HK) and Degussa P25 (P25) TiO(2). Total mineralization of DEPA is observed. Two regimes of pH, namely in acid and near-neutral environments were found where maximum total carbon (TC) decomposition was observed. The electrostatic effects on adsorption over the TiO(2) surface explain the above phenomena. The maximum rate is observed for P25 at DEPA concentration 1.3 mM whereas the rate grows continuously with DEPA concentration rise for HK. The temperature dependence of TC decomposition rate in the range of 15-63°C with both HK and P25 follows the Arrhenius equation. The activation energy for total carbon decomposition with HK and P25 are 29.5±1.0 and 24.3±3.1 kJ/mol, respectively. The decomposition rate of DEPA is larger over P25 than over HK. The rate over P25 increases faster than that with HK for each unit of the titania added when the TiO(2) concentration is less than 375 mg/l. The higher light absorption and particles aggregation of P25 are responsible for the decrease of reaction rate we observed at catalyst concentration above a certain level. In contrast, the rate over HK increases monotonically with the concentration of the photocatalyst used.

Benchmarking semiempirical and DFT methods for the interaction of thiophene and diethyl sulfide molecules with a Ti(OH) 4 (H 2 O) cluster

Semiempirical methods pm6 and pm7 as well as density functional theory functionals exchange LSDA, exchange-correlation PW91 and PBE, hybrid B3LYP1 and PBE0 were compared for energy and geometry of thiophene, diethyl sulfide (DES) molecules and their binding to a frozen Ti(OH)4(H2O) complex having one coordinatively unsaturated Ti5C site representing small fragment of TiO2 anatase (001) surface. PBE0/6-31G(d) with DFT-D3 dispersion correction was the best method for description of thiophene and DES molecules geometries as comparison with experimental data demonstrated. Semiempirical methods pm6 and pm7 resulted in only three of four possible binding configurations of thiophene with the Ti(OH)4(H2O) complex while pm7 described correctly the enthalpy and all configurations of DES binding with the Ti(OH)4(H2O) complex. SBKJC pseudopotential and LSDA with and without dispersion correction produced flawed results for many configurations. PBE0 and PBE with and without dispersion correction and PW91 with 6-31G(d) basis set systematically produced dependable results for thiophene and DES binding to the Ti(OH)4(H2O) complex. PBE0-D3/6-31G(d), B3LYP1-D3/6-31G(d), and PBE-D3/6-31G(d) gave best match of binding energy for thiophene while PBE0/6-31G(d) gave best match of DES binding energy as comparison with CCSD(T) energy demonstrated. On the basis of the superior results obtained with PBE0/6-31G(d), it is the recommended method for modeling of adsorption over TiO2 surfaces. Such a conclusion is in agreement with recent literature.

Photocatalytic Transformations of Sulfur-Based Organic Compounds

Organic sulfur compounds play significant roles in the life of modern humankind. There are many instances of the need to destroy or convert sulfur compounds into useful products. Photocatalytic reactions are capable of removing and transforming these compounds. High reactivity of sulfur compounds allows utilization of a variety of photocatalysts – heterogeneous like TiO2 or other species, homogeneous, heterogeneous, or heterogenized. The transformations are mediated by electron transfer with generation of sulfur radical cations, energy transfer, or hydrogen atom transfer. Further reactions majorly comprise S-oxidation, α-C–H deprotonation, C–S bond cleavage, and polymerization. Reactions with photogenerated OH˙ radicals are also possible.