Phase Photocatalytic Oxidation Research Articles

Enhanced photocatalytic performance and moisture tolerance of nano-sized Me/TiO2–zeolite Y (Me=Au, Pd) for gaseous toluene removal: activity and mechanistic investigation

This research developed a new type of photocatalyst for the gas phase photocatalytic oxidation of toluene under highly humid conditions to overcome the disadvantages of the traditional TiO2 photocatalyst. By doping noble metal nanoparticles (Au, Pd) on the TiO2 and adding the zeolite Y as an adsorbent, the adsorption-enhanced nanosized Me/TiO2–zeolite Y (Me=Au, Pd) shown significant improvements in both photocatalytic activity and stability for toluene photocatalytic oxidation in a continuous annular reactor. The catalysts were analyzed using advanced techniques for understanding the structure and morphology of the materials such as X-ray diffraction (XRD), scanning electro-microscopy (SEM), transmission electro-microscopy (TEM) and N2 adsorption at low temperature (77 K) for surface area analysis. The photocatalytic activity of the catalyst has been investigated under various reaction conditions such as different initial toluene concentrations in the range of 300–1300 ppmv, water concentration in the range 5.91–23.65 mg/L at two reaction temperatures (39 °C and 50 °C). The maximum photocatalytic efficiency of toluene removal was stable at 3.1 mmol gTiO2−1 h−1 under highly humid condition. Among four kinetic models that have been tested, the Langmuir–Hinshelwood kinetic model in which toluene and water vapor adsorb on separated sites ( $$ -r=k\times \frac{K_A{C}_{Tol}}{1+{K}_A{C}_{Tol}}\times \frac{K_W{C}_W}{1+{K}_W{C}_W}\Big) $$ was the most fitted to the experimental data and a reaction mechanism was discussed and proposed for the photocatalytic oxidation of toluene under humid condition using the nanosized Me/TiO2–zeolite Y catalyst. The parameters for the rate equation at 39 °C were k = 2.847 × 10−3 mmol L−1 g_TiO2−1 h−1, KA = 0.429 × 10−2 ppmv−1, and Kw = 0.194 × 10−3 ppmv−1.

Photocatalytic activity of TiO2 films prepared by surfactant-mediated sol–gel methods over commercial polymer substrates

Commercially available transparent polymers are evaluated as support for photoactive TiO2 and tested for their application in gas phase photocatalytic oxidation processes. The polymers were selected on the basis of their optical properties and the TiO2 film adhesion. Two low-temperature surfactant-mediated sol–gel synthesis methods of TiO2–SiO2 nanocomposites are investigated to provide acid TiO2–basic SiO2 interactions for the formation of photocatalytic films of higher quality. These procedures are expected to improve photocatalytic properties compared to those obtained by direct coating of TiO2 on the polymers. One method consists in modifying the polymer surface with a layer of poly(diallyldimethylammonium) chloride, which provides a positively charged surface for the fixation of the alkaline SiO2 sol. The other method is based on reducing the surface tension of the SiO2 sol using perfluorobutane sulfonates. Both methods are compared to direct deposition in terms of homogeneity of the layers and photocatalytic activity for trichloroethylene oxidation. Among the commercial polymers employed poly(methyl methacrylate) processed by sheet moulding showed the best optical properties, TiO2 adhesion, and photocatalytic activity, which was promoted by the deposition of the silica interlayer by both preparation methods.

Dispersion of colloidal TiO2 nanoparticles on mesoporous materials targeting photocatalysis applications

Pre-formed titanium dioxide nanoparticles were deposited onto hexagonal mesoporous supports in order to enhance the efficiency of these photocatalysts in oxidation reactions. To do so, well-calibrated TiO2 nanoparticles synthesized by an acid sol–gel process (ACSG) were introduced using either incipient wetness impregnation method or a “one-pot” procedure. The structure and texture of the resulting materials were characterized by powder X-ray diffraction (XRD), nitrogen adsorption–desorption measurements and TEM imaging. Detailed information on the nature and morphology of the TiO2 nanoparticles were obtained by diffuse reflectance UV–visible spectroscopy, high angle XRD and TEM imaging. Most of the materials prepared displayed well-structured porosities with a good dispersion of TiO2 nanoparticles with average diameters varying between 5 and 7nm.The liquid phase photocatalytic oxidation of methanol into formaldehyde was selected in order to test the activity of the obtained materials. Newly prepared solids described here, especially the one based on the introduction of TiO2 colloids in the SBA-15 synthesis gel, showed a higher catalytic activity per titanium atom compared to the fresh TiO2 colloids or the commercially available P-25 photocatalyst.

The effect of active sites’ nature on the photo-catalytic performance of Cr-TUD-1 in the oxidation of C1–C3 hydrocarbons

Chromium-incorporating mesoporous material denoted as Cr-TUD-1 was synthesized by using a surfactant-free one-pot synthesis procedure. Samples with different chromium loading were prepared and extensively characterized by means of X-ray diffraction (XRD), UV–vis and Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), 29Si MAS NMR, high-resolution transition electron microscopy (HR-TEM), nitrogen physisorption and elemental analysis. Characterization results show that as a function of Cr loading, Cr exists in different morphological structures such as isolated tetrahedrally coordinated Cr6+, nano-particulates, and finally bulky crystalline Cr2O3. The catalytic performance of the Cr-TUD-1 samples was investigated in the gas phase photo-catalytic oxidation of a mixture of light hydrocarbons i.e. methane, ethylene, ethane, propylene, and propane by molecular oxygen. Isolated Cr6+ species were found to be more active than Cr3+. The sample with 5wt.% Cr loading which contains a mixture of Cr6+ and Cr3+ moieties was found to be the most active sample. A mechanism is proposed to show the synergy between Cr6+ and Cr3+. Cr-TUD-1 was found to be stable under dry conditions, the photocatalytic oxidation rates of the different hydrocarbons were almost the same up to six runs, however, the presence of water vapour in the reaction mixture causes serious deactivation to the catalyst. The activity of Cr-TUD-1 was superior to other metals-incorporated TUD-1 (e.g. Ti, V, and Mo) or other Cr-containing mesoporous materials (e.g. Cr-MCM-41 and Cr–SiO2) under the same reaction conditions.

Influence of operating parameters on gas phase photocatalytic oxidation of methyl-ethyl-ketone in a light emitting diode (LED)-fluidized bed reactor

The effect of operating parameters on gas phase photocatalytic oxidation of methyl ethyl ketone (MEK) has been investigated over P25 and PC500 photocatalysts. The parameters were gas flow rate, relative humidity (RH), reaction temperature, and ultraviolet intensity. The photoreactor was composed of two LEDs matrices and a fluidized bed reactor (LED-FBR). No mass transfer limitation phenomena occurred at flow rates in the range 30–70 NL/h. The oxidation of MEK over the two photocatalysts did not highly depend on the presence or absence of humidity in the air stream. Temperature was determined as an important parameter in conversion of MEK over 60 to 120 °C, influencing also the selectivity to carbon dioxide and carbon monoxide. The simultaneous influence of RH and temperature was observed in the reaction at different temperatures and RHs, evidencing the decrease in the selectivity towards CO with increase of RH. With the increase of ultraviolet intensity, the rate of MEK oxidation increased over both catalysts, with the better activity of PC500 than P25 particularly at higher light intensities. The main effect of temperature was both to promote the MEK conversion and to limit the competitive water adsorption in the PCO reaction over the tested titanias.

Gas phase photocatalytic oxidation of decane at ppb levels: Removal kinetics, reaction intermediates and carbon mass balance

Abstract This study focuses on the photocatalytic oxidation of decane at ppb levels, close to indoor air conditions. Although these concentrations are typical of indoor air volatile organic compound (VOC) levels, such conditions have been poorly investigated to date. Decane conversion rates higher than 90% were reached within 15 h for the highest initial concentration tested in the operating conditions used. Despite this high decane conversion, 18 reaction intermediates were detected in the gas phase. The main compounds, in terms of concentration level in the gas phase, were formaldehyde, acetaldehyde and propanal. The amounts of these compounds in the gas phase were linearly dependent on the initial decane concentration. A reaction pathway is proposed, based on reaction intermediate temporal profiles and the literature. It consists of six main steps describing the oxidation process from decane to CO and CO2. The influence of the relative humidity level on the diversity and amounts of reaction intermediates was also studied. It was shown that moisture tends to shift the equilibrium of intermediates adsorption toward desorption, resulting in a relative increase in intermediate quantities in the gas phase. However, the monitoring of CO and CO2 formation highlighted that, at the end of the reaction, a high mineralization rate could be obtained. Finally, an overview of the photocatalytic reaction is given through the carbon mass balance determined for various reaction advancements. This approach gives an overall evaluation of the photocatalytic process performance, from primary compound removal to reaction intermediate and mineralization.

Enhanced performances of grafted VOx on titania/silica for the selective photocatalytic oxidation of ethanol to acetaldehyde

In this work gas phase photocatalytic selective oxidation of ethanol to acetaldehyde was studied with directly fluidizable photocatalysts made of grafted VOx on titania/silica in presence of phosphors as light carriers to test their performances in the photocatalytic system. The presence of VOx species anchored on TiO2/SiO2 enhanced ethanol conversion up to 66%, with acetaldehyde selectivity higher than 99%. The performances of V-based sample at 11wt% of TiO2 supported on fluidizable support were higher than those evaluated when nano-TiO2 powder is used as support. Moreover, the photocatalytic conversion of ethanol was further increased in presence of phosphors as light carriers. From the obtained results, it is concluded that by preparing VOx based catalyst through grafting method allows a dramatic enhance in the productivity of acetaldehyde, even when the concentration level of alcohol at photoreactor inlet is increased.

Ordered mesoporous TiO2with exposed (001) facets and enhanced activity in photocatalytic selective oxidation of alcohols

Single-crystal TiO2 with 2D and 3D ordered mesoporous structures and exposed (001) facets were synthesized by SBA-15 or KIT-6 templated alcoholysis of TiOSO4 under solvothermal conditions. In the liquid phase photocatalytic selective oxidation of alcohols to aldehydes under UV-light irradiation, the single-crystal TiO2 exhibited much higher activity than the polycrystalline TiO2, since the high crystallization degree favored the transfer of photoelectrons, which might reduce their rate of recombination with holes. Meanwhile, the TiO2 with ordered mesopores showed higher activity than that with disordered mesopores, and the 3D ordered mesoporous channels were more favorable for photocatalytic oxidation than the 2D ordered mesoporous channels, which could be attributed to both the enhanced light harvesting resulting from the multiple light reflections in the pore channels, and the facilitated diffusion and adsorption of the reactant molecules. Also, the exposed (001) facets favored the photocatalytic oxidation due to its high surface energy and increased oxygen vacancies, which reduced the photocharge recombination rate by capturing photoelectrons.

Factors driving the activity of commercial titanium dioxide powders towards gas phase photocatalytic oxidation of acetaldehyde

The photocatalytic activity of two commercial titanium dioxide powders (Cristal Global, Millennium PC500 and Evonik, P25) is compared towards acetaldehyde degradation in the gas phase. In contrast to the extensive literature available, we found a higher activity for the PC500 than for the P25 coating. Here, we present a comprehensive characterization of the bulk and surface properties of both powders. Our comparison shows that the material properties that dominate the overall photocatalytic activity in gas phase differ from those required for the photodegradation of water-borne pollutants.

Conventional versus lattice photocatalysed reactions: Implications of the lattice oxygen participation in the liquid phase photocatalytic oxidation with nanostructured ZnO thin films on reaction products and mechanism at both 254 nm and 340 nm

This study aimed to investigate the relationship between surface morphologies of nanostructured ZnO thin films, the reaction intermediates and products produced, and the liquid phase and solid phase reaction mechanisms under both oxygen rich and oxygen limited conditions at two different wavelengths (254nm and 340nm). Four different ZnO morphologies were prepared by hydrothermal deposition on two different substrates – clean glass (CG) and direct current magnetron sputtered ZnO glass (MS). The two films grown on the MS templates were highly vertically aligned columnar structures and those grown without templates (CG) had little or no structural alignment. Methylene blue (MB) was used as the reactant, since its reaction intermediates and products are well defined and measurable, allowing predictable comparison between these catalysts.Results showed that there was a significant difference in MB degradation rates as well as reaction intermediate formation and destruction rates correlated to the morphologies and crystallinity at both UV wavelengths, with the highest reaction rates at 340nm. Reaction analysis indicates that there may be a competition between two different photocatalytic mechanisms: conventional photocatalysed radical oxidation and lattice oxygen driven oxidation. The dominant reaction mechanism present depended on the thin film morphology, crystallinity, availability of oxidant and the wavelength of the incident UV. The surface photocatalysed radical formation is thought to be predominant for more highly aligned and more crystalline morphologies, where there was plentiful oxygen and UV irradiation at 340nm. Lattice oxygen photodegradation is thought to be predominant for less aligned more amorphous morphologies and UV irradiation at 254nm. Based on these results a new MB ZnO photocatalysed oxidation mechanism is proposed.

Photocatalytic removal of 2,4,6-trichlorophenol from water exploiting commercial ZnO powder

2,4,6-trichlorophenol is an important water pollutant owing to the severity of its toxicity. The aqueous phase photocatalytic oxidation of 2,4,6-trichlorophenol over ZnO was investigated as a potential method for the abatement of this pollutant. The effects of operating parameters such as initial ZnO doses and substrate concentration on the removal of 2,4,6-trichlorophenol were studied and optimised at 0.75 g L − 1 and 50 mg L − 1 , respectively. The photocatalytic system afforded the highest degradation efficiency at neutral pH. The decomposition of 2,4,6-trichlorophenol by the photoprocess agreed satisfactorily with pseudo zero-order kinetic model. The effect of the presence of SO 4 2−, S 2O 8 2−, HPO 4 2− and Cl − on the 2,4,6-trichlorophenol removal rate was for the first time revealed. Some hitherto unreported pathway intermediates of ZnO-assisted 2,4,6-trichlorophenol degradation were recorded using gas chromatography–mass spectrometry (GC–MS) and high performance liquid chromatography (HPLC). A tentative reaction mechanism for the formation of these intermediates was proposed.

Photocatalytic Properties of One-Dimensional Nanostructured Titanates

One-dimensional titanate nanostructures were synthesized by a hydrothermal route with use of the anatase and rutile phases of titania as precursors in 10 M NaOH. Hollow, multilayer trititanate (H2Ti3O7) nanotubes were formed at an autoclave temperature of 100 °C, while solid trititanate nanorods were formed at 200 °C. A further calcination at 300 °C leads to the formation of thicker nanorods of the TiO2 (B) phase. These high aspect ratio titania-based nanostructures were found to show enhanced catalytic activity in the gas phase photocatalytic oxidation of ethylene, which to the best of our knowledge is attempted for the first time for these types of protonic titanates. Among the various nanostructures studied, the H2Ti3O7 nanotubes show the highest photoactivity and factors affecting its activity are studied as a function of calcination temperature of these catalysts so as to arrive at the structure activity correlation.

Gas phase photocatalytic oxidation of toluene using highly active Pt doped TiO 2

Platinum doped TiO 2 materials were studied for the gas phase photocatalytic degradation of toluene. Platinum deposition was achieved by photodeposition method over TiO 2 prepared by means of a sol–gel route. The effect of sulphuric acid pretreatment on the further platinisation process has been extensively studied. From the wide structural and surface analysis of the catalysts an interesting synergetic effect has been demonstrated. The previous sulphate treatment over TiO 2 leads to improved dispersion of the Pt which presents a lower aggregation and homogeneous cluster size. This fact, together with the adequate control of anatase structural and surface parameter due to the sulphate treatment, renders a good photocatalytic performance for toluene oxidation reaction. The highest reaction rates and CO 2 selectivities have been obtained for Pt–S–TiO 2 samples.

Products distribution during the gas phase photocatalytic oxidation of ammonia over the various titania based photocatalysts

The adsorption and photooxidation of gaseous ammonia have been investigated by the FTIR spectroscopy method in a static reactor using TiO2 and modified TiO2 as photocatalysts. Two types of modified TiO2 were tested: TiO2 impregnated with transition metal oxides (NiO, V2O5, MoO3, and Fe2O3) and TiO2 doped with noble metals like Ag, Au, Pt, Pd by the chemical and photochemical deposition methods. Molecular nitrogen, NO3−, N2O and water were found as the major final products of ammonia photocatalytic oxidation (PCO). One of the purposes of the current study was to minimize the N2O formation during PCO. The pure unmodified TiO2 (anatase, SBET∼350m2/g) turned out to be the best photocatalyst for this purpose giving only 7% of initial NH3 quantity converted into N2O. Long-term experiments were carried out to determine the photocatalysts stability in PCO of NH3. All the catalysts exhibited stable performance; the rate of deactivation was low due to the transformation of NH3 nitrogen mainly into N2. Only ≈30% of initial NH3 quantity was oxidized into HNO3 which was adsorbed on the catalysts surface in all cases.

Reactivity and antimicrobial properties of nanostructured titanium dioxide

The photo-induced reactivity of three nanostructured titanium dioxides including TiO 2 nanoparticles (i.e., nano-TiO 2), titania–silica aerogel and nanotextured TiO 2 film, were investigated for gas phase photocatalytic oxidation of trichloroethylene and inactivation of Bacillus subtilis and Escherichia coli microbial cells. A correlation was observed between photoreactivity for TCE conversion and bactericidal activity within TiO 2 of similar nanostructures and composition (i.e., elemental and phase). This indicates that the photogenerated radical species from UV-irradiated TiO 2 responsible for TCE photo-oxidation also play a major role in the inactivation of microbial cells. The superb bactericidal activities of titania–silica aerogels (i.e., up to 6log reductions in viable B. subtilis) and nanotextured TiO 2 film (3.4log reduction) compared to commercial Degussa P25 TiO 2 (0.64log reduction) and optimum nano-TiO 2 (1.3log reduction) far exceeds the observed activity enhancement for TCE photo-oxidation. This implies that nanoscale chemical and structural environment also contribute towards the bactericidal activities.

기-액 복합 광반응기에서의 악취성 암모니아 제거를 위한 촉매개발과 반응시스템의 최적조건 색출 연구

Ammonia is a major compound of odor in livestock house. To enhance the performance of ammonia oxidation (decomposition). the gas-liquid, two phase photocatalytic oxidation system was designed and prepared in this study. Commercial P-25 as catalyst was used for ammonia decomposition. V/P-25 catalyst prepared by sol gel method was also used for the removal of by-producted in oxidation reaction. When was used as a photocatalyst, the conversion to in ammonia decomposition reached above 90% until 200hr (The air flow rate of 4L/min with the ammonia concentration up to 25ppm.). However, considerable amounts of NO and were formed as a result of oxidation (as a by-product). Therefore, we added Vanadia impregnated (P-25) catalyst for the removal of at the end of reaction trail. The results of a pilot-scale operation were successful to achieve the simultaneous removal of about 81 and 87%, respectively.

Photocatalytic Oxidation of Tabun Simulant-Diethyl Cyanophosphate: FTIR in Situ Investigation

Gas phase photocatalytic oxidation of diethyl cyanophosphate vapor in a static reactor using TiO2 and modified TiO2 as the photocatalyst was studied with the FTIR in situ method. The transition metals Pt, Au, and Ag were used for TiO2 modification by the chemical and photochemical deposition methods as well as the mechanical mixture of TiO2 with manganese oxide to improve its adsorption and catalytic activity. Photocatalytic oxidation of diethyl cyanophosphate in a static reactor results in its complete mineralization with carbon dioxide, phosphoric and nitric acids, and water as the major final products. HCN was demonstrated to be the only toxic gaseous intermediate of diethyl cyanophosphate photocatalytic oxidation, formed as a result of diethyl cyanophosphate hydrolysis. Diethylphosphate and acetic and formic acids were registered as the surface intermediates. It was found that cyanhydric acid is oxidized slowly with the use of unmodified TiO2. The formation of surface cyanide complexes with Ag and Au ions could be responsible for the fast removal of HCN from the gas phase and its further photooxidation in the case of using TiO2 with deposited Au and Ag.

Influence of mesoporous and platinum-modified titanium dioxide preparation methods on photocatalytic activity in liquid and gas phase

Mesoporous titanium dioxide photocatalysts have been synthesized by template method using widely used precursors and templates with the purpose of choosing the best method for liquid and gas phase photocatalytic oxidation. Titanyl sulphate (TS) and tetrabutyl titanate (TBT) were precursors and cetyl trimethylammonium bromide (CTAB) and dodecylamine (DDA) were templates. Hydrothermal treatment temperature was optimized for the best obtained photocatalyst. The catalysts were characterized by N2 adsorption–desorption measurements, XRD and UV–vis diffuse reflectance spectroscopy. Samples obtained from TBT contained significant amounts of brookite. Activity was measured in liquid phase for dimethyl methylphosphonate (DMMP) and in gas phase for acetone complete oxidation. The highest activity in liquid phase was observed for catalyst prepared from TS and DDA and aged at 160°C. Activity was higher than that of TiO2 Degussa P25 in the low concentration range. The highest activity in gas phase was observed for catalyst prepared from TBT and CTAB. Activity of mesoporous catalysts correlates with their surface area and pore volume for gas phase reaction, whereas in the case of liquid phase DMMP oxidation, there is no sufficient correlation. The effect of DMMP initial concentration on initial oxidation rate is well fitted by Langmuir–Hinshelwood competitive adsorption model. Mesoporous catalysts possess higher adsorption constant but lower reaction rate constant than TiO2 P25. Platinum was deposited by impregnation and soft chemical reduction (SCR) or photodeposition methods (PD). Catalysts obtained by PD were 50% more active than by SCR and 4.5-fold more active than TiO2 P25 in aqueous phase. The reasons for extraordinarily high activity of this mesoporous catalyst are discussed.

An investigation of trichloroethylene photocatalytic oxidation on mesoporous titania-silica aerogel catalysts

Freestanding blocks of mesoporous, titania-silica aerogel catalyst were prepared and tested for gas phase photocatalytic oxidation of trichloroethylene (TCE) in a new photoreactor where the reactant gases flow through the aerogel block. The intimate contact between the reactants and catalyst resulted in a higher conversion. TS01 aerogel was prepared by ethanol supercritical drying, and contained nanometer-sized crystalline domains of anatase TiO2 (i.e., TiOTi sites) within an amorphous SiOTi and SiOSi network. The TS02 aerogel prepared by CO2 supercritical drying remained amorphous even after high temperature treatment, and analysis indicated that the TiOSi is the predominant site on this catalyst. The reaction results indicated that only the TS01 aerogel containing anatase TiO2 was active, but the formation of dichloroacetaldehyde on the amorphous SiOSi sites led to a rapid catalyst deactivation. However, a stable catalyst was obtained after these sites were passivated by ozone pretreatment under ultraviolet (UV) irradiation. The TS01 aerogel catalysts exhibited an average conversion of 30% and a quantum yield of 0.27 at a TCE concentration of 56ppm, feed flow rate of 40sccm and UV irradiation of 710μWcm−2.

Opposite effect of gas phase H2O2 on photocatalytic oxidation of acetone and benzene vapors

Abstract The effect of additions of gas phase H 2 O 2 was measured for gas phase photocatalytic oxidation of organic vapors. Photocatalytic oxidation of benzene vapor over TiO 2 in a flow reactor resulted in a quick catalyst deactivation. Additions of gas phase H 2 O 2 into the reactor feed provided enhanced and sustained oxidation of benzene vapor. The increase of inlet H 2 O 2 vapor concentration from 0 to about 1000 ppm led to the one order of magnitude growth of benzene vapor complete oxidation rate. The highest rate of 1.1 nmol/s was observed at C 6 H 6 concentration 124 ppm and H 2 O 2 concentration 1000 ppm. In the case of acetone vapor photocatalytic oxidation, the rate of complete oxidation in the flow reactor decreased with an increase of gas phase H 2 O 2 inlet concentration. TiO 2 Degussa P25 provided higher oxidation rate in the presence of H 2 O 2 than pure anatase TiO 2 .