Photocatalytic Oxidation Of Methanol Research Articles

Beta zeolite supported sol–gel TiO2 materials for gas phase photocatalytic applications

Beta zeolite supported sol–gel TiO2 photocatalytic materials were prepared according to a sol–gel route in which high specific surface area Beta zeolite powder was incorporated into the titanium isopropoxide sol during the course of the sol–gel process. This led to an intimate contact between the zeolite surface and the TiO2 precursors, and resulted in the anchorage of large amounts of dispersed TiO2 nanoparticles and in the stabilization of TiO2 in its anatase form, even for high TiO2 wt. contents and high calcination temperatures. Taking the UV-A photocatalytic oxidation of methanol as gas phase target reaction, high methanol conversions were obtained on the Beta zeolite supported TiO2 photocatalysts when compared to bulk sol–gel TiO2, despite lower amounts of TiO2 within the photoactive materials. The methanol conversion was optimum for about 40wt.% TiO2 loading and calcination temperatures of 500–600°C.

Photonic efficiency and mechanism of photocatalytic molecular hydrogen production over platinized titanium dioxide from aqueous methanol solutions

The photocatalytic molecular hydrogen (H 2) production from aqueous methanol solutions over Pt-loaded commercial (Evonik Aeroxide TiO 2 P25 and Sachtleben Hombikat UV100) and home made (TiO 2 P25HT) titanium dioxide photocatalysts has been studied. The photonic efficiencies were calculated by dividing the H 2 production rate by the photon flux. The effect of the employed light intensity on the H 2 production rate was investigated. The products of the photocatalytic methanol oxidation were quantitatively analyzed employing different test conditions, i.e., different illumination times, pH values, and methanol concentrations. The balance between the amount of evolved H 2 and the amount of photocatalytic methanol oxidation products was checked. Based on the obtained results, the difference in the photocatalytic activities between the investigated photocatalysts, the effect of the employed light intensities, and the description of the photocatalytic H 2 production from aqueous methanol solutions is discussed.

Electrochemically Assisted Photocatalytic Oxidation of Methanol on TiO 2 Nanotube Arrays

The influence of an externally applied bias on photocatalytic performance of crystallized TiO2/Ti nanotubular electrode formed by anodization in fluoride-based electrolyte were investigated and compared to the behavior of multiporous TiO2 electrode. The photoelectrocatalytic oxidation behavior of methanol over the nanotubular electrode has been studied by measuring photocurrent response, potentiodynamic polarization spectroscopy and using electrochemical impedance spectroscopy (EIS). It was found that the photoelectrocatalytic oxidation and the charge transfer rate constant of reaction on TiO2/Ti nanotubular electrode can significantly be increased by applying electrochemical bias. Moreover, based on the results, it was found that the nanotubular TiO2 electrodes have considerably better performance in comparison with porous samples in photoelectrocatalytic performance.

Photocatalytic selective oxidation of methanol to methyl formate in gas phase over titanium(IV) oxide in a flow-type reactor

Photocatalytic oxidation of methanol in air over titanium(IV) oxide particles having a large surface area of 120 m2 g−1 using a flow-type reactor at room temperature yielded methyl formate with a high level of selectivity (91 mol%) without catalyst deactivation due to deposition of intermediate(s). The conversion of methanol increased with elevation of the reaction temperature up to 523 K maintaining high selectivity.

A high-throughput reaction system to measure the gas-phase photocatalytic oxidation activity of TiO2 nanotubes

A sixteen-channel, high-throughput system was designed and built to test the activity of catalysts for gas-phase photocatalytic oxidation of methanol. The system utilizes granular catalyst films to model relevant applications and allow for rapid processing. It is capable of 48 catalyst tests per day using the procedure described herein. Several experiments were performed to minimize both the within-node and between-node variances of the system. Utilizing the high-throughput system, the significance of preparation methods on the photocatalytic activity of TiO2 nanotubes was investigated. A one-half fractional factorial experiment identified the factors that significantly impact catalyst activity as the following: precursor type (Degussa P-25, or nanotubes), platinum loading, the interaction between precursor and dope time, and the interaction between the precursor and calcination temperature. Based on experimental results, catalyst activity is optimized by doping TiO2 nanotubes directly (rather than doping P-25 prior to nanotube formation), a low platinum loading (0.01 wt %), and using a dope time of 30 min followed by calcination at 773 K. The optimum catalyst preparation conditions produced a catalyst that was three times more active than the starting P-25 material.

Gold Nanoparticles on Mesoporous Interparticle Networks of Titanium Dioxide Nanocrystals for Enhanced Photonic Efficiencies

Nanocomposites consisting of Au and TiO2 nanocrystals have been synthesized through simple one-step sol−gel reactions of titanium tetraisopropoxide with hydrogen tetrachloroaurate in the presence of an F127 triblock copolymer as the template to direct the formation of nanostructured photocatalysts. The thus formed Au/TiO2 network gels were calcined at 500 °C for 4 h, leading to Au/TiO2 nanocomposites with high interparticle mesoporosity, pore diameters of around 10 nm, and BET surface areas of about 100 m2/g. TEM investigations of 0.5 wt % Au/TiO2 reveal that the TiO2 particles are quite uniform in size and shape. The particle sizes of TiO2 and Au are in the range of 5−10 and 20−120 nm, respectively. The photocatalytic methanol oxidation to formaldehyde chosen as the test reaction to examine the photocatalytic activity of the Au/TiO2 was shown to be more effective as compared with pure TiO2 and with Degussa P25, respectively. The increased photocatalytic activity of Au/TiO2 with interparticle mesopores is...

Mechanism of Visible Light Photocatalytic Oxidation of Methanol in Aerated Aqueous Suspensions of Carbon-Doped TiO2

Visible photolysis of aerated carbon-doped TiO2 (C-TiO2) aqueous suspensions induces methanol oxidation to formaldehyde. The rate of HCHO formation increases with the concentration of CH3OH or C-TiO2 and is nearly doubled in the presence of catalase or excess of H2O2. The mechanism involves oxidation of CH3OH by surface trapped holes, although these holes have lower energy than those formed upon UV photolysis of undoped TiO2. The C-TiO2 electrons reduce O2 to H2O2. CH3OH oxidation via reduction of H2O2 by the C-TiO2 electrons was observed in the presence of added H2O2, where it competes efficiently with O2 for the C-TiO2 electrons. At [H2O2] > 0.1 mM, the yield of HCHO is about twice that in the absence of added H2O2. Light absorption measurements in an integrating sphere show that the limiting absorption fraction at high [C-TiO2] is 0.5 at 450 nm, the rest of the light being mostly scattered backward. The HCHO quantum yield depends on the square root of the absorbed light density and is only a few percent at the intensities used in this work. A parallel photocatalytic decomposition of H2O2 has been observed, which is not associated with HCHO formation.

Dependence of target–substrate distance on crystallographic and optical properties of WO 3 films prepared by reactive radio frequency magnetron sputtering

WO 3 films were deposited on glass substrates using radio frequency magnetron sputtering in a mixed gas of 80%Ar–20%O 2. From the X-ray diffraction patterns, the WO 3 films deposited at source to substrate distance ( D T–S) of 40 mm were polycrystalline, crystallizing in the monoclinic crystal structure, with highly preferred c-axis orientation perpendicular to the film plane. On the other hand, amorphous films were observed in the WO 3 films deposited at 70 mm. The crystallite sizes of the WO 3 films deposited at D T–S of 40 mm were larger for films deposited at lower working gas pressures ( P W). The optical absorption edge of the films shifted to shorter wavelength region with increase in P W irrespective of the D T–S. The oxygen deficient films are obtained when the films are deposited at D T–S of 40 mm and P W of 0.3 Pa and stoichiometric films are formed at higher P W. The photocatalytic oxidation of methanol proceeds via CO and formaldehyde as the intermediate species on WO 3/TiO 2 bilayer films and CO 2 is identified as the final product.

Photocatalytic oxidation of methanol using titania-based fluidized beds

Experiments determined methanol removal and catalyst elutriation rates during photocatalytic oxidation (PCO) of fluidized and packed beds of various titania-based catalysts. The study developed elutriation-resistant catalysts in which TiO2 was precipitated from solution (p-TiO2), or was coated on an Al2O3 support (TiO2/Al2O3) and compared them to Degussa P-25 TiO2. Although Degussa P-25 TiO2 oxidized methanol effectively, it elutriated at a rate that was two orders of magnitude greater than those of p-TiO2 and TiO2/Al2O3. In addition, TiO2/Al2O3 removed methanol at a significantly greater rate than did P-25, with p-TiO2 being the least active. Fluidized beds produced greater PCO rates than did packed beds of P-25 and TiO2/Al2O3. Fluidization enhancers, such as vibration and incorporation of a static mixer, improved the performance of the P-25 fluidized bed, but did not change the effectiveness of TiO2/Al2O3 or p-TiO2. The activity of TiO2/Al2O3 decreased with increasing calcination temperature (over the temperature range 673–873K). The optimum TiO2 loading for TiO2/Al2O3 was 30wt.%.

Removal of methanol from pulp and paper mills using combined activated carbon adsorption and photocatalytic regeneration

Methanol is one of the major hazardous air pollutants emitted from chemical pulp mills. Its collection and treatment is required by the Maximum Achievable Control Technology portion of the 1998 Cluster Rule. The objective of this study is to investigate the technical feasibility of combined adsorption and photocatalytic regeneration for the removal and destruction of methanol. To facilitate the regeneration, activated carbon (AC) was coated with commercially available photocatalyst by a spray desiccation method. Laboratory-scale experiments were conducted in a fixed-bed reactor equipped with an 8 W black light UV lamp (peak wavelength at 365 nm) at the center. The photocatalyst loaded onto AC had no significant impact on the adsorption capacity of the carbon. High humidity was found to greatly reduce the material’s capacity in the adsorption and simultaneous adsorption and photocatalytic oxidation of methanol. The photocatalytic regeneration process is limited by the desorption of the adsorbate. Increasing desorption rate by using purge air greatly increased the regeneration capacity. When the desorption rate was greater than the photocatalytic oxidation rate, however, part of the methanol was directly desorbed without degradation.

Vapor-Phase Photo-Oxidation of Methanol Over Nano-Size Titanium Dioxide Clusters Dispersed in MCM-41 Host Material Part 2: Catalytic Properties and Surface Transient Species

We report in this paper on the ultraviolet-assisted vapor-phase oxidation of methanol at room temperature, with the help of nano-size clusters of titanium dioxide dispersed in an MCM-41 silicate matrix. The surface species formed during the adsorption/oxidation of methanol and the transformation that they undergo as a result of ultraviolet irradiation were monitored using in-situ Fourier transform infrared and thermal desorption spectroscopy techniques. Parallel experiments conducted on TiO2/MCM, bulk titania, and pristine MCM-41 samples helped in identifying the individual role of titanium dioxide and host matrix in these processes. The photo-catalytic oxidation of methanol, at concentrations of 0.1 to 1.1 mol% in air, gave rise to formation of CO2 and H2O as products, for both the TiO2/MCM and bulk TiO2 samples. No such reaction occurred on titania-free MCM. Furthermore, the rate of reaction depended upon the TiO2 content of a sample and also on the concentration of methanol in reaction mixture. Thus, the rate of conversion increased progressively with the increase in TiO2 loading from 5 to 21 wt% in TiO2/MCM samples, particularly for the experiments with high concentration of methanol. For low methanol concentration (0.1 mol%) in air, the effect of titania content in MCM was very small. The specific activity (per g of titania) of a sample, on the other hand, showed an inverse relationship with the loading of titanium dioxide in a sample. Infrared and temperature-programmed desorption results revealed that the mode of CH3OH adsorption and the reactivity of the transient species formed during the oxidation process were independent of the size of dispersed titania particles. Thus, the particles of approximately 2-6 nm size, present in TiO2/MCM, exhibited a chemisorption behavior similar to that of the bulk titania. The results of the present study provide strong evidence that the hydroxyl groups, both on the host matrix and at the titania sites, participate independently in the formation of methoxyl groups and at the same time promote the heterogeneous photo-catalytic oxidation of methanol molecules via formation of transient formate groups. Our results also show that the effect of titania crystallite size in the photo-catalytic properties relate mainly to the larger surface area and hence to the enhanced number of chemisorption sites, rather than to the changes in electronic properties.

Quantum Yield of Formaldehyde Formation in the Presence of Colloidal TiO2-Based Photocatalysts: Effect of Intermittent Illumination, Platinization, and Deoxygenation

Colloidal TiO2 (2.4 nm average particle diameter) has been prepared and modified by photodeposition of Pt (PtTi-S1) and by mixing with Pt nanoparticles (PtTi-S2). Transmission electron microscopy reveals particle aggregation in colloidal TiO2 and large networks of agglomerated particles in the platinized samples. The photocatalytic activity of the samples (0.1 g L-1) has been investigated by measuring the quantum yield, φHCHO, of HCHO formed from aqueous methanol at pH 3.5 under different conditions. In CW photolysis of the oxygenated suspensions (300−400 nm UV light, 8 × 10-7 einstein L-1 s-1 photon absorption rate) the platinized photocatalysts (1 wt % Pt) enhance φHCHO by a factor of 1.5−1.7 with respect to neat colloidal TiO2 where φHCHO is 0.02. In addition to the action of Pt as an electron sink, the strong promotion of photocatalytic methanol oxidation by PtTi-S2 at the small Pt/TiO2 particle ratio of ca. 1:1060 is proposed to arise from transfer of excitation energy or of photogenerated charge carriers through the particle network. Repetitive laser-pulse illumination of the oxygenated samples (351 nm, 0.5 Hz repetition frequency) increases φHCHO by ca. 50% in comparison with CW illumination at the same average photon absorption rate of ca. 8 × 10-7 einstein L-1 s-1. As a tentative explanation an increase of the photocatalyst surface by laser-pulse stimulated deaggregation of colloidal TiO2 and fragmentation of the networks in the platinized samples is suggested. HCHO is also produced in the deoxygenated suspensions under repetitive laser-pulse illumination. PtTi-S1 and PtTi-S2 increase φHCHO by factors of 1.8 and 1.2, respectively, in comparison with neat colloidal TiO2 where φHCHO is 0.027. Possible mechanisms are discussed. The higher activity of PtTi-S1 is attributed to stronger electrocatalysis of H2 formation by highly dispersed Pt in PtTi-S1 as compared with the Pt particles in PtTi-S2.

A comparative study of nanometer sized Fe(iii)-doped TiO2photocatalysts: synthesis, characterization and activity

Fe(III)-doped TiO2 nanoparticles have been prepared in three different ways using inorganic or organic precursors, respectively. Transmission electron microscopic (TEM) structure characterization of the obtained particles reveals that TiO2 particles with and without the Fe(III) dopant possess the crystal structure of anatase. Cryo-TEM of vitrified samples confirms earlier assumptions that TiO2 nanoparticles tend to form three-dimensional networks in solution. A novel energy transfer mechanism is suggested employing these three-dimensional networks as antenna systems which lead to improved photocatalytic activities of the colloidal preparations. The performance of the nanostructured TiO2 with and without Fe-dopant for the photocatalytic oxidation of methanol producing HCHO is strongly dependent on the preparations and the Fe-content. Fe(III)-doped TiO2 nanoparticles prepared from organic precursors by the novel method (≤2.5 atom% Fe) exhibit a greatly enhanced photocatalytic activity (quantum yield of HCHO up to ca. 15%).

Photocatalytic oxidation of bacteria, bacterial and fungal spores, and model biofilm components to carbon dioxide on titanium dioxide-coated surfaces.

We report carbon mass balance and kinetic data for the total oxidation of cells, spores, and biomolecules deposited on illuminated titanium dioxide surfaces in contact with air. Carbon dioxide formation by photocatalytic oxidation of methanol, glucose, Escherichia coli, Micrococcus luteus, Bacillus subtilis (cells and spores), Aspergillus niger spores, phosphatidylethanolamine, bovine serum albumin, and gum xanthan was determined as a function of time. The quantitative data provide mass balance and rate information for removal of these materials from a photocatalytic surface. This kind of information is importantfor applications of photocatalytic chemistry in air and water purification and disinfection, self-cleaning surfaces, and the development of self-cleaning air filters.

Effect of Trichloroethylene on the Photocatalytic Oxidation of Methanol on TiO2

During photocatalytic oxidation (PCO), methanol reacts to formaldehyde, some of which desorbs from TiO2 at room temperature. Formaldehyde that remains on the surface oxidizes to adsorbed formate, which dehydrogenates to CO2 in a single step without forming any long-lived intermediates. Adding trichloroethylene (TCE) during PCO of methanol increases the rates of methanol ↠ formaldehyde and formic acid ↠ CO2. Trichloroethylene decreases the rate that formaldehyde oxidizes to formate, however. Adding TCE during PCO of methanol, formaldehyde, and formic acid does not produce any new surface species or reaction pathways. Chlorine radicals that are produced during PCO of TCE may be responsible for the increases in dehydrogenation rates of methanol and formic acid.

An electrochemical study of the photocatalytic oxidation of methanol on rutile

Photocatalytic oxidation of methanol and 2-propanol by oxygen on illuminated rutile catalysts was analyzed by electrochemical measurements, and the indication was obtained that the Wagner-Traud mechanism prevails in the photocatalytic oxidation of these alcohols; i.e., the oxidation reaction consists of the anodic oxidation of the alcohol and the cathodic reduction of either oxygen or a complex consisting of oxygen and an intermediate of oxidized alcohol.