Gas-phase Photocatalytic Oxidation Research Articles

TiO2 /Loofah-Halloysite Bio-Hybrid Composites as Efficient Systems for VOCs Removal.

Volatile organic compounds (VOCs), recognized as hazardous air contaminants, prompt the exploration of sustainable air purification methods. Solar photocatalytic oxidation emerges as a promising solution, utilizing semiconductor photocatalysts like titanium dioxide (TiO2 ). However, the raw material crisis necessitates reduced TiO2 usage, leading to investigations into TiO2 modification techniques. The study introduces a novel approach by employing natural fibers, specifically loofah sponge, as a TiO2 support. This method aims to maintain photocatalytic activity while minimizing TiO2 content. The article explores using halloysite, a natural clay mineral, as a supportive material, enhancing mechanical strength and adsorption properties. The resulting TiO2 /loofah-halloysite composites are evaluated for their efficacy in gas-phase photocatalytic oxidation of toluene and ethanol, chosen as representative VOCs. The conversion of toluene and ethanol on the composite was 88 % and 39 %, respectively, with high selectivity toward CO2 . In addition to its high performance, the bio-composite was stable for several conversion cycles, keeping the conversion activity unchanged. The study contributes to developing green hybrid materials for VOC removal, showcasing potential applications across industries.

Gas-phase Photocatalytic Oxidation of 1-Hexene Using Heterogeneous Semiconductor Materials

Photo-catalytic reactions have been carried out and proved to be conducted under mild conditions in contrast to several conventional industrial chemical processes, which involve the use of high amounts of temperature and pressure. In this work, an investigation of the photo-catalytic oxidation of 1-hexene was carried out using UVA-activated TiO2, and coupled TiO2/WO3 to drive organic reactions. Different mol % of WO3 ranging from 1-7mol% were used to monitor effect of coupling amount. The chosen photocatalyst was immobilized on glass beads using an innovative in-house procedure. The immobilized catalyst was then used in a vapour-phase flow reactor designed for alkene photo-oxidation studies. It was possible to establish oxidation rates of 0.14+-0.02 mmol/h for 1-hexene with TiO2/WO3 (1mol %). 1,2-epoxyhexane was successfully synthesized from 1-hexene achieving 15.9% conversion and 41% and 54% selectivity for the 1-hexene, 1,2-epoxyhexane and CO2 respectively when H2O/O2 was used as oxidant. Selectivity of 96% and 97% for CO2 were achieved with N2 and O2 respectively. No epoxide was formed with either N2 or O2 alone.

Singlet-oxygen-driven photocatalytic degradation of gaseous formaldehyde and its mechanistic study

Conventional studies on photocatalytic oxidation reaction have focused primarily on hydroxyl radical (•OH) as a major reactive oxygen species (ROS). This perspective may not be suitable for gaseous HCHO photodegradation due to the short lifetime and diffusion length of •OH. Identification of divergent ROS and understanding their characteristics for the gas-phase photocatalytic oxidation are further required. Here, we report a BiOI/TiO2 p-n junction photocatalyst, which uses mobile singlet oxygen (1O2) as the main ROS for gaseous HCHO photooxidation. The introduction of p-n heterojunctions causes a charge transfer that prefers mobile 1O2 generation rather than •OH due to an energetically favorable desorption free energy, accelerating HCHO oxidation. Finally, we demonstrated the photocatalytic filter performance of HCHO-to-CO2 conversion for 150 h with a conversion efficiency of 87 ± 1.3 % in a single-pass reactor. Our systematic study reveals the potential of the overlooked 1O2-driven gaseous HCHO photocatalytic degradation process.

Redox Dynamics of Pt and Cu Nanoparticles on TiO2 during the Photocatalytic Oxidation of Methanol under Aerobic and Anaerobic Conditions Studied by In Situ Modulated Excitation X-ray Absorption Spectroscopy

In situ modulated excitation X-ray absorption spectroscopy (ME-XAS) at the Pt L3- and Cu K-edges has been successfully used in this work to shed light on the controversial role that Pt and Cu nanoparticles (NPs) on TiO2 have in photocatalysis. TiO2 photocatalysts, either bare or modified by Pt and Cu NPs, synthesized in a single step by flame spray pyrolysis, were tested in the gas-phase photocatalytic oxidation of methanol to CO2 under both aerobic (CH3OH + 3/2O2 → CO2 + 2H2O) and anaerobic (CH3OH + H2O → CO2 + 3H2) conditions. Both Pt- and Cu-containing TiO2 showed lower activity than bare TiO2 under aerobic conditions, while higher H2 and CO2 production rates were attained under anaerobic conditions in the presence of the Pt co-catalyst, with copper-containing TiO2 having in this case an activity similar to that of bare titania. ME-XAS, coupled with phase-sensitive detection, proved in fact that Pt NPs efficiently capture the electrons photopromoted in the conduction band of TiO2 and are the active sites for H2 or H2O evolution under anaerobic or aerobic conditions, respectively. Much more complex is the role of Cu-NPs on titania, with ME-XAS revealing that copper is simultaneously present in different oxidations states, depending on the reaction conditions. Cu NPs are not the reaction active sites under anaerobic conditions, while a Cu(II)/Cu(I) redox switching occurs under chopped irradiation under aerobic conditions.

Effects of Anodic Aluminum Oxide Substrate Pore Geometry on the Gas-Phase Photocatalytic Activity of ZnO/Al2O3 Composites Prepared by Atomic Layer Deposition

We report on the photocatalytic activity of ZnO layers deposited by atomic layer deposition on a porous anodic aluminum oxide substrate with hexagonal pore symmetry and varied pore dimensions. ZnO/Al2O3 composites were prepared with pore diameters in the range 93–134 nm and interpore distance in the range 185–286 nm, and their photocatalytic activity was measured for gas-phase photocatalytic oxidation of acetaldehyde at varying UV illumination intensities (0.08–3.94 mW cm−2). The results show that substrates with narrower pore diameters (<115 nm, in the case of this study) have a detrimental effect on the photocatalyst performance, despite their higher effective surface. The results are explained on the basis of limited mass transfer inside the porous structure and can be used as a guideline in the purposeful design of photocatalysts with a nanoporous or nanotubular structure.

Model-Based Analysis of the Photocatalytic HCl Oxidation Kinetics over TiO2

The kinetic modeling of photocatalytic reactions is a powerful tool for process optimization. We applied a holistic kinetic model for the gas-phase photocatalytic oxidation of HCl to Cl2 to identif...

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.

Enhanced gas-phase photocatalytic removal of aromatics over direct Z-scheme-dictated H3PW12O40/g-C3N4 film-coated optical fibers

Environmentally benign photocatalytic oxidation is one of the most promising approaches to remove volatile, highly toxic and refractory aromatics, and the key points of gas-phase photocatalytic oxidation of aromatics are to develop efficient photocatalysts and design suitable photoreactors. Here, the custom-designed H3PW12O40/g-C3N4 film-coated optical fiber photoreactor is demonstrated for gas-phase simulated sunlight photocatalytic removal of aromatics including benzene, toluene and m-xylene. At relative humidity of 73% and air atmosphere, the H3PW12O40/g-C3N4 film-coated optical fibers with H3PW12O40 doping level of 3.2% exhibit remarkably higher photocatalytic removal efficiency of benzene, toluene and m-xylene than that of the g-C3N4 film, and apparent rate constant of the H3PW12O40/g-C3N4 film for benzene, toluene and m-xylene removal is 2.42, 1.75 and 3.67 times higher than that of g-C3N4 film. The enhanced photocatalytic activity of the H3PW12O40/g-C3N4 film is ascribed to direct Z-scheme-dictated charge carrier migration mechanism, imparting not only superior photogenerated charge carrier separation ability but also undiminished redox capability of the photogenerated electrons and holes; additionally, the increased contact area of catalyst film with the substrates and improved light harvesting ability give rise to the important contribution to gas-phase photocatalytic removal of aromatics. Moreover, the catalyst film exhibits excellent adhesion, stability and recyclability, and the activity loss is negligible after total thirty times’ catalytic runs.

Design and development of nanostructured filter media for VOCs abatement in closed environments

The presence of volatile organic compounds released at significantly high concentration levels from livelihood sources entails more and more the development of effective active filter media for their control and abatement within indoor environments. Nanostructured nonwoven mats, obtained by combination of electrodynamic technologies (electrospinning and electrospraying), were produced and characterized by SEM and TGA methods. TiO2 was the main photocatalyst, whereas Graphene and cyclodextrins were used as co-catalyst and sorption components, respectively. The photocatalytic activities of all the samples were tested in the gas-phase photocatalytic oxidation (PCO) reaction of acetaldehyde, methanol and n-hexane in a static reactor by the FT-IR in situ method. The influence of the production approach, the chemical structure of the pollutants and the testing unit layouts, were evaluated.Filtering media produced by performing alternatively electrospinning of polymeric support and electrospraying of photocatalyst nanoparticles, led a 96% acetaldehyde (C0 = 133 ppm) abatement after 30 min. with a half time of 9 min. with the PAN + TiO2. Further reduction of the half-time was achieved with a particular corrugated sheets configuration, wherein the testing unit was able to treat 6.4 m3/h of air. The proposed system showed good versatility in terms of photodegradation of different pollutants (methanol and n-hexane) both pure and in mixture.

Enhanced gas-phase photocatalytic oxidation of n-pentane using high visible-light-driven Fe-doped WO3 nanostructures

The main aim of this study was to investigate the photocatalytic potential of pure and Fe-doped WO3 nanoparticles for conversion of volatile organic compounds (VOCs). N-pentane, one of the n-alkanes VOCs, was selected as the pollutant model in gaseous phase. The used photocatalysts were synthesized by a facile method and characterized to determine the structural and optical properties. Results showed that the synthesized nanoparticles had small grain size with high crystallinity. The analyses of XRD, FTIR, and Raman exhibited that all doped and un-doped WO3 samples were in monoclinic form. The UV–vis and PL spectroscopies revealed that introducing Fe into WO3 led to narrowing band gap energy from 2.83 to 2.56 eV and decreasing the recombination rate of photo-generated electron-holes due to implanted defects and impurities in the host matrix. The photocatalytic experiments of n-pentane degradation were conducted in a self-design batch photo-reactor in the presence of O2 as oxidizing agent. The Fe-doped samples obviously exhibited the photocatalytic activity higher than the bare WO3. An almost full conversion of n-pentane, 99.91%, was provided without any catalyst deactivation by visible-light-irradiated 4.65 at.% Fe-doped WO3 which was 14.77% greater than that of the un-doped sample. Finally, a mechanism study was done based on the main intermediates detected using mass spectroscopy.

Novel photothermocatalytic synergetic effect leads to high catalytic activity and excellent durability of anatase TiO2 nanosheets with dominant {001} facets for benzene abatement

We studied the effect of reaction temperature on the photocatalytic activity of anatase TiO2 nanosheets with dominant {001} facets for the gas-phase photocatalytic oxidation of recalcitrant and carcinogenic benzene. It is found that the photocatalytic activity of the TiO2 nanosheets significantly enhances with increasing reaction temperature from room temperature to 240, and 290°C. Meanwhile, obvious thermocatalytic oxidation of benzene is observed on the TiO2 nanosheets in the dark when the reaction temperature increases above 240°C. The photothermocatalytic activity (CO2 production rate, rCO2-ptc) of the TiO2 nanosheets at 240 or 290°C under the irradiation of an Hg lamp is much higher than the summation of the photocatalytic activity (rCO2-pc) with the same UV light intensity at room temperature and thermocatalytic activity (rCO2-tc) at the same reaction temperature in the dark. This result indicates the presence of a photothermocatalytic synergetic effect between the photocatalysis and the thermocatalysis on the TiO2 nanosheets, thus significantly increasing the catalytic activity. The photothermocatalytic synergetic effect is further verified by in-situ FTIR spectra with CO as probe molecule: active species produced by photogenerated electrons and holes accelerate the thermocatalytic oxidation on the TiO2 nanosheets, thus significantly increasing the catalytic activity. Just by coating the sample of TiO2 nanosheets on the surface of the Hg lamp, we realized highly efficient photothermocatalytic oxidation of benzene with utilizing UV light and heating effect of infrared light from the He lamp without using additional heater. Under such photothermocatalytic condition, the photothermocatalytic activity (rCO2-ptc) of the TiO2 nanosheets increased by 42.3 times as compared to the photocatalytic activity (rCO2-pc) at room temperature with the same Hg lamp, and the TiO2 nanosheets exhibit excellent durability for benzene oxidation.

Gas-phase photocatalytic oxidation of refractory VOCs mixtures: Through the net of process limitations

Photocatalytic oxidation (PCO) of volatile organic compounds (VOCs) presents an effective means of indoors air treatment. Shortcomings, however, include relatively low oxidation rate, sometimes formation of toxic by-products and catalyst deactivation. These obstacles are to be surmounted via synthesis of active catalytic materials and efficient catalyst regeneration at properly chosen process parameters. The article describes the photocatalytic performance of a novel flame aerosol synthesised nano-structured titania F3 and the method of accelerated photocatalyst activity restoration using small amounts of ozone. The experimental research into PCO abating VOCs in mixtures was undertaken following the performance of F3 compared to the benchmarking commercially available photocatalyst P25. The mixture containing acrylic acid, acrylonitrile and toluene was degraded at F3 and P25 photocatalysts, whereas the mixture of acrylonitrile, styrene and toluene—at P25 with admixtures of ozone; ozone was also used for regeneration of P25. The contact time, VOCs and ozone concentration in mixtures, temperature, and air humidity varied. In the experimental conditions, the novel photocatalyst showed activity substantially surpassing the one of the benchmark: within the experimental conditions, F3, contrary to P25, showed no symptoms of deactivation, emitting only trace amounts of toxic HCN product. The novel photocatalyst thus confirmed its promising efficiency in abatement of VOCs mixtures. Ozone appeared to be effective aid in both PCO of VOCs and the photocatalyst activity restoration dramatically shortening the photocatalyst regeneration time.

Gas-phase photocatalytic activity of sputter-deposited anatase TiO2 films: Effect of 〈0 0 1〉 preferential orientation, surface temperature and humidity

We present a systematic in situ study of the environmental reaction conditions on the photocatalytic activity of sputter deposited anatase TiO2 films with controlled amounts of preferential 〈001〉 orientation. In particular, the effects of relative humidity (RH) and substrate temperature (Tsurf) are investigated. It is found that {001} facets, which are present in higher abundance on highly oriented samples, exhibit an order of magnitude higher reactivity for gas-phase photocatalytic oxidation of the indoor air pollutant acetaldehyde (CH3CHO) than {101} facets do, and a functional dependence of the reaction rate on facet orientation is determined. It is proposed that water adsorbed on the film contributes to two counteracting effects on the photocatalytic activity: (i) It provides hole acceptors to complete the photo-induced redox cycle and subsequent OH radical formation for pollutant degradation, and (ii) it creates a diffusion barrier between the catalyst interface and pollutant molecules adsorbed in the water layer. As a consequence, increasing Tsurf at high RH has the beneficial effect of removing excess water and reducing the diffusion barrier, thereby improving the photocatalytic activity. A comparison is also made with a commercial anatase TiO2 film, with less developed surface crystallinity and random facet distribution, where the improvement is even more pronounced. Films with a higher degree of orientation exhibit much more stable performance over a range of operating conditions, which suggests that it is possible to tune the effects of water and exposed facet orientation to achieve optimum activity and make TiO2 films amenable to a larger (RH, Tsurf) parameter space for practical applications.

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.

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.

How Pt nanoparticles affect TiO2-induced gas-phase photocatalytic oxidation reactions

The effect of Pt nanoparticles on the gas-phase photocatalytic oxidation activity of TiO2 is shown to be largely dependent on the molecular functionality of the substrate. We demonstrate that Pt nanoparticles decrease rates in photocatalytic oxidation of propane, whereas a strong beneficial effect of Pt was observed in oxidation of ethanol. On the basis of oxygen conversion, Pt nanoparticles result in an increase in rates of TiO2 from 1.55mmol O2/g/h to 4.65mmol O2/g/h, at a light intensity of 8mW/cm2 at 375nm. The latter value is comparable to obtained in propane oxidation in the absence of Pt and represents a photonic efficiency of approximately 2%. Besides an effect on oxygen conversion rate, we also observed significant effects of Pt nanoparticles on reaction selectivity. DRIFT analysis demonstrates that acetone is a rather abundant surface-bound intermediate when propane is oxidized in the presence of Pt nanoparticles, while this is barely observed in the absence of Pt nanoparticles. In ethanol oxidation, both surface-bound and gas-phase acetaldehydes are produced more significantly in the presence than in the absence of Pt. The activity data are discussed on the basis of adsorption affinity of the reactants toward TiO2, much higher for ethanol as compared to propane. The changes in (surface) selectivity are discussed on the basis of Pt-induced alterations in the rate-determining steps.

Photocatalytic oxidation of diethyl sulfide vapor over TiO2-based composite photocatalysts.

Composite TiO2/activated carbon (TiO2/AC) and TiO2/SiO2 photocatalysts with TiO2 contents in the 10 to 80 wt. % range were synthesized by the TiOSO4 thermal hydrolysis method and characterized by AES, BET, X-ray diffraction and FT-IR ATR methods. All TiO2 samples were in the anatase form, with a primary crystallite size of about 11 nm. The photocatalytic activities of the TiO2/AC and TiO2/SiO2 samples were tested in the gas-phase photocatalytic oxidation (PCO) reaction of diethyl sulfide (DES) vapor in a static reactor by the FT-IR in situ method. Acetaldehyde, formic acid, ethylene and SO2 were registered as the intermediate products which finally were completely oxidized to the final oxidation products – H2O, CO2, CO and SO42− ions. The influence of the support on the kinetics of DES PCO and on the TiO2/AC and TiO2/SiO2 samples’ stability during three long-term DES PCO cycles was investigated. The highest PCO rate was observed for TiO2/SiO2 photocatalysts. To evaluate the activity of photocatalysts the turnover frequency values (TOF) were calculated for three photocatalysts (TiO2, TiO2/AC and TiO2/SiO2) for the same amount of mineralized DES. It was demonstrated that the TOF value for composite TiO2/SiO2 photocatalysts was 3.5 times higher than for pure TiO2.

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.

Titanium Dioxide in gas-Phase Photocatalytic Oxidation of Aromatic and Heteroatom Organic Substances: Deactivation and Reactivation of Photocatalyst

Contemporary views about the photocatalytic oxidation of the vapor of organic substances on the surface of titanium oxide catalysts, leading to their deactivation, are summarized. The effect of external factors (humidity, concentration of the substrate vapor) on the deactivation of TiO2 during the oxidation of various types of organic substances and primarily aromatic and heteroatom substances is examined. Various methods of reactivation of titanium oxide catalysts are analyzed. Practical conclusions are reached about the use of TiO2 in the photooxidation of the vapor of aromatic and heteroatom organic substances and the working life of the photocatalysts.

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.