Conversion Of Trichloroethylene Research Articles

The importance of proper pH adjustment and control to achieve complete in situ enhanced reductive dechlorination.

In situ bioremediation of chlorinated compounds such as perchloroethylene (PCE) and trichloroethylene (TCE) through enhanced reductive dechlorination (ERD) requires appropriate growth conditions for organohalide-respiring bacteria (OHRB). One of the most important factors controlling OHRB metabolism is groundwater pH. Dehalococcoides spp. (DHC) growth may be inhibited when pH is lower than 6.0, which can lead to the accumulation of toxic daughter compounds including cis-dichloroethylene (cDCE) and vinyl chloride (VC). Aquifer pH may decline as HCl is released during reductive dechlorination and from substrate fermentation to fatty acids and carbonic acid. In this article, we demonstrate that using proper pH adjustment and control in situ is an appropriate strategy to achieve complete ERD (i.e., complete conversion of PCE and TCE to nontoxic ethylene) in remediation sites with inherently low pH values and/or low buffering capacity. To analyze the effectiveness of this approach, field monitoring results are presented for a challenging site containing high concentrations of PCE and TCE (>10 000 µg/L and >1000 µg/L, respectively) and low aquifer pH (~4.9). Addition of a bioaugmentation culture, emulsified vegetable oil (EVO), and a colloidal buffer (CoBupHTM ) to increase pH, stimulated rapid conversion of PCE and TCE to cDCE and VC. However, further conversion of cDCE and VC was very limited. To stimulate complete conversion to ethylene, additional CoBupHTM and nutrients were injected, resulting in a rapid increase in metabolic rates, and maintained the aquifer pH at ~6.5 for more than five years, thus demonstrating that complete ERD can be achieved in sites with similar characteristics. Proper pH adjustment and control is needed to limit the accumulation of toxic intermediates, maintaining in situ bioremediation as an efficient, affordable, and environmentally friendly option to treat chlorinated compounds. Integr Environ Assess Manag 2023;19:943-948. © 2022 SETAC.

Enhanced Performance of Supported Ternary Metal Catalysts for the Oxidation of Toluene in the Presence of Trichloroethylene

Chlorinated volatile organic compounds (CVOCs), even in small quantities, can cause Pt-based catalyst poisoning. Improving the low-temperature chlorine resistance of catalysts is of vital importance for industrial application, although it remains challenging. Considering actual industrial production, a TiO2-supported ternary metal catalyst was prepared in this work to study the catalytic oxidation of multicomponent VOCs (toluene and trichloroethylene (TCE)). Among all of the samples, PtWRu/TiO2 and PtWCr/TiO2 exhibited the best catalytic performance for toluene oxidation. In the mixed VOC oxidation, the PtWCr/TiO2 sample showed the best catalytic activity for toluene combustion (a toluene conversion of 90% was achieved at 258 °C and a space velocity of 40,000 mL g−1 h−1, and the specific reaction rate and turnover frequency at 215 °C were 44.9 × 10−6 mol gPt−1 s−1 and 26.2 × 10−5 s−1). The PtWRu/TiO2 sample showed the best catalytic activity for TCE combustion (a TCE conversion of 90% was achieved at 305 °C and a space velocity of 40,000 mL g−1 h−1, and the specific reaction rate and turnover frequency at 270 °C were 9.0 × 10−6 mol gPt−1 s−1 and 7.3 × 10–5 s−1). We concluded that the ternary metal catalysts could greatly improve chlorine desorption by increasing the active lattice oxygen mobility and surface acidity, thus reducing chlorinated byproducts and other serious environmental pollutants. This work may serve as a reasonable design reference for solving more practical industrial production emissions of multicomponent VOCs.

Catalytic performance and intermediates identification of trichloroethylene deep oxidation over Ru/3DOM SnO2 catalysts

SnO2 is widely used in catalysis owing to its oxygen vacancies, reducible oxygen species, and acidity. In this work, Ru nanoparticles (NPs) supported on three-dimensionally ordered macroporous (3DOM) SnO2 (xRu/3DOM SnO2, Ru loading (x) = 0.09–1.65 wt%) were fabricated via the polyvinyl alcohol-protected reduction routes, and their catalytic activities were determined for trichloroethylene (TCE) and/or toluene oxidation. It was shown that the 0.98Ru/3DOM SnO2 sample exhibited the best activity and good thermal stability, with the temperature (T90%) and apparent activation energy at a TCE (1000 ppmv) conversion of 90% being 300 °C and 44 kJ/mol at space velocity = 40,000 mL/(g h), respectively. Trichloromethane, carbon tetrachloride, tetrachloroethylene, and trichloroacetaldehyde hydrate were the main intermediates in TCE oxidation. The high catalytic performance of 0.98Ru/3DOM SnO2 was attributed to the presence of hydroxyl groups adsorbed at the oxygen vacancies, strong O2 adsorption ability, good low-temperature reducibility, and strong interaction between Ru NPs and 3DOM SnO2. In addition, a catalytic mechanism over xRu/3DOM SnO2 was proposed under dry and humid conditions. The introduction of H2O could block some of the reaction pathways from inhibiting the production of C2Cl4, reduce formation of some by-products (e.g., CH2Cl2, CH3COOH, and C2H2Cl2), promote removal of the chlorine species, and generate a greater amount of HCl and a smaller amount of Cl2. Furthermore, introducing toluene into the TCE stream could generate smaller amounts of organic chlorine by-products and greater amounts of inorganic chlorine products (HCl and Cl2). We are sure that this work can provide a guideline for the industrial application of supported Ru catalysts in the removal of Cl-containing volatile organic compounds.

The Use of Electronegative Additives as a Means to Improve Current Discharge Efficiency in Conversion of Volatile Organic Compounds

The effect that introduction of electronegative additives to air containing toxic impurities has on the processes of conversion of these impurities in plasma are studied. Taking as an example the conversion of trichloroethylene (C2HCl3) in air by nanosecond pulsed corona discharges, we show that additions of carbon tetrachloride (CCl4) to the model air mixture lead to both a decrease in discharge current and enhancement of the energy efficiency of conversion. Processes in the different zones of corona discharge are investigated.

Oxidative Degradation of Trichloroethylene over Fe2O3-doped Mayenite: Chlorine Poisoning Mitigation and Improved Catalytic Performance

Mayenite was recently successfully employed as an active catalyst for trichloroethylene (TCE) oxidation. It was effective in promoting the conversion of TCE in less harmful products (CO2 and HCl) with high activity and selectivity. However, there is a potential limitation to the use of mayenite in the industrial degradation of chlorinated compounds—its limited operating lifespan owing to chlorine poisoning of the catalyst. To overcome this problem, in this work, mayenite-based catalysts loaded with iron (Fe/mayenite) were prepared and tested for TCE oxidation in a gaseous phase. The catalysts were characterized using different physico-chemical techniques, including XRD, ICP, N2-sorption (BET), H2-TPR analysis, SEM-EDX, XPS FESEM-EDS, and Raman. Fe/mayenite was found to be more active and stable than the pure material for TCE oxidation, maintaining the same selectivity. This result was interpreted as the synergistic effect of the metal and the oxo-anionic species present in the mayenite framework, thus promoting TCE oxidation, while avoiding catalyst deactivation.

Alloying of gold with palladium: An effective strategy to improve catalytic stability and chlorine-tolerance of the 3DOM CeO2-supported catalysts in trichloroethylene combustion

The three-dimensionally ordered macroporous (3DOM) CeO2-supported Au–Pd alloys (xAuPdy/3DOM CeO2, x is the total loading (wt%) of Au and Pd, and y is the Pd/Au molar ratio) were synthesized using the polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction methods. The samples were characterized by a number of analytical techniques, and their catalytic performance was evaluated for the combustion of trichloroethylene (TCE). It is found that the xAuPdy/3DOM CeO2 samples displayed a good-quality 3DOM architecture, and the noble metal nanoparticles (NPs) with a size of 3–4 nm were uniformly dispersed on the skeleton surface of 3DOM CeO2. Among all of the samples, 2.85AuPd1.87/3DOM CeO2 exhibited the highest catalytic activity, with the temperature at a TCE conversion of 90% (T90%) being 415 °C at a space velocity of 20,000 mL/(g h). Furthermore, the 2.85AuPd1.87/3DOM CeO2 sample possessed the lowest apparent activation energy (33 kJ/mol), excellent catalytic stability, and good moisture- and chlorine-tolerant behaviors. Alloying of Au with Pd changed the pathway of TCE oxidation and reduced formation of perchloroethylene. We conclude that the excellent catalytic performance for TCE combustion of 2.85AuPd1.87/3DOM CeO2 was associated with the highly dispersed AuPd1.87 alloy NPs, high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between AuPd1.87 NPs and 3DOM CeO2 as well as the high-quality 3DOM structure and high surface acidity.

Plasma assisted Cu-Mn mixed oxide catalysts for trichloroethylene abatement in moist air.

The removal of dilute trichloroethylene (TCE) in moist air by post-plasma catalysis (PPC) using Cu-Mn mixed oxides heated at 150 °C was investigated. Cu-Mn mixed oxides were prepared by redox- and co-precipitation method. In comparison to the catalytic oxidation and non-thermal plasma (NTP) process, PPC was found to be the best process to convert TCE into CO2, in particular when Cu-Mn oxide was synthetized by redox precipitation method. The highest TCE conversion efficiency of more than 80% was obtained at the energy density of 60 J.L-1 using the catalyst prepared by redox-precipitation process in PPC configuration. The performance of Cu-Mn oxide prepared by redox-precipitation method did not show increase in TCE conversion with energy density which is attributed to the changes on the catalyst surface (such as reduction in SBET, chlorine poisoning and Mn enrichment). Although, Cu-Mn oxide prepared by co-precipitation method showed a lower TCE conversion, it exhibited a better stability in the PPC process for TCE abatement.

Three-dimensionally ordered macroporous Cr2O3−CeO2: High-performance catalysts for the oxidative removal of trichloroethylene

Abstract Three-dimensionally ordered macroporous (3DOM) CeO2, 3DOM Cr2O3, 3DOM xCr2O3−CeO2 (x (the weight percentage of Cr2O3) = 3.5, 5.5, and 8.0 wt%), and 5.5 wt% Cr2O3/3DOM CeO2 samples were prepared using the polymethyl methacrylate (PMMA)-templating and incipient wetness impregnation methods, respectively. A number of techniques were used to characterize physicochemical properties of the materials, and their catalytic activities were evaluated for the oxidation of trichloroethylene (TCE). These samples possessed a good-quality 3DOM structure and a surface area of 35−47 m2/g. The 3DOM 5.5Cr2O3−CeO2 sample performed the best (the temperature at TCE conversion = 90% = 255 °C at a space velocity of 20,000 mL/(g h)). Effects of water vapor and carbon dioxide on activity of the 5.5Cr2O3−CeO2 sample were also examined. It is observed that partial deactivation induced by H2O introduction of the 5.5Cr2O3−CeO2 sample was reversible, while that induced by CO2 addition was irreversible. Based on the activity data and characterization results, it is concluded that the good catalytic activity and thermal stability of 3DOM 5.5Cr2O3−CeO2 was associated with its high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between Cr2O3 and CeO2. We believe that the 3DOM 5.5Cr2O3−CeO2 catalyst is promising in the application for oxidative removal of chlorinated volatile organic compounds.

Influence of the synthesis method on the catalytic activity of mayenite for the oxidation of gas-phase trichloroethylene

Catalytic oxidation of trichloroethylene (TCE) in heterogeneous phase (gas-solid) is an effective strategy for the conversion of this pollutant in less harmful compounds, namely CO2, CO and HCl. In this work, we have studied the use of mayenite, a cost-effective material, as an active catalyst for the TCE conversion. In particular, we have assessed the influence of the mayenite synthesis method (hydrothermal, sol-gel and ceramic) on the reaction performance. The materials have been characterized by different techniques, such as XRD, N2-sorption (BET), TPR, Raman spectroscopy, FESEM-EDX and TEM. The analysis of the light-off curves and product distribution, has shown that the use of the hydrothermal method for the mayenite synthesis results in the most active and selective catalyst. This has been related with a higher surface area and with a higher concentration of oxygen anions in the mayenite prepared by this method. It has been found that the presence of water in the stream do not influence the catalytic performance of the material. A mechanism for the reaction and for the partial deactivation of the catalyst has been proposed.

AuPd/3DOM TiO2 Catalysts: Good Activity and Stability for the Oxidation of Trichloroethylene

Three-dimensionally ordered macroporous (3DOM) TiO2-supported AuPd alloy (xAuyPd/3DOM TiO2 (x = 0.87–0.91 wt%; y = 0.51–1.86)) catalysts for trichloroethylene (TCE) oxidation were prepared using the polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction methods. The as-prepared materials possessed a good-quality 3DOM structure and a surface area of 49–53 m2/g. The noble metal nanoparticles (NPs) with a size of 3–4 nm were uniformly dispersed on the surface of 3DOM TiO2. The 0.91Au0.51Pd/3DOM TiO2 sample showed the highest catalytic activity with the temperature at a TCE conversion of 90% being 400 °C at a space velocity of 20,000 mL/(g h). Furthermore, the 0.91Au0.51Pd/3DOM TiO2 sample possessed better catalytic stability and moisture-resistant ability than the supported Au or Pd sample. The partial deactivation induced by H2O introduction of 0.91Au0.51Pd/3DOM TiO2 was reversible, while that induced by CO2 addition was irreversible. No significant influence on TCE conversion was observed after introduction of 100 ppm HCl to the reaction system over 0.91Au0.51Pd/3DOM TiO2. The lowest apparent activation energy (51.7 kJ/mol) was obtained over the 0.91Au0.51Pd/3DOM TiO2 sample. The doping of Au to Pd changed the TCE oxidation pathway, thus reducing formation of perchloroethylene. It is concluded that the high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between AuPd NPs and 3DOM TiO2 as well as more amount of strong acid sites were responsible for the good catalytic activity, stability, and water- and HCl-resistant ability of 0.91Au0.51Pd/3DOM TiO2. We believe that 0.91Au0.51Pd/3DOM TiO2 may be a promising catalyst for the oxidative elimination of chlorine-containing volatile organics.

Elucidating the Photoredox Nature of Isolated Iron Active Sites on MCM-41

Photocatalytic performance is highly dependent on the nature and dispersion of the active sites, playing a crucial role in the optoelectronic and charge-transfer processes. Here, we report stabilized isolated iron on MCM-41 as a highly active catalyst for a photoredox reaction. The unique nature of the single-atom centers exhibit a trichloroethylene conversion per iron site that is almost 5 times higher than that of TiO2. Advanced characterization and theoretical calculations indicate the generation of hydroxyl radicals, through a photoinduced ligand-to-metal charge-transfer mechanism, which act as hole scavengers that lead to the formation of intermediate oxo–iron species (Fe═O). This intermediate species is the key step in promoting the photocatalytic reactions. Understanding the mechanistic photoredox pathway in isolated active site materials is imperative for developing highly efficient nonprecious photocatalysts for environmental or energy applications.

Hydrodechlorination of trichloroethylene over Pd supported on swellable organically-modified silica (SOMS)

The catalytic activity and resistance to poisoning of Pd catalysts supported on swellable organically-modified silica (SOMS) were investigated for hydrodechlorination (HDC) of trichloroethylene (TCE). The promising catalytic activity of 1% Pd/SOMS sample was attributed to the high affinity of SOMS for organics and its high hydrophobicity. While latter characteristic repels water, the adsorptive capacity for organics allows TCE dissolved in aqueous media to concentrate inside the pores, in the vicinity of the active sites, thus helping the kinetics. In the liquid phase, using a continuous flow reactor, higher TCE conversion was obtained over the 1% Pd/SOMS compared to the commercial 1% Pd/Al2O3 catalyst. When the pores of 1% Pd/SOMS sample were fully opened by pre-treating it with ethanol prior to the reaction, HDC activity was seen to significantly increase. In the gas phase, the extent of adsorption was less, reducing the concentration of reactants near the active sites. As a result, 1% Pd/SOMS was less active than 1% Pd/Al2O3 for HDC of TCE. To determine their resistance to poisoning, 1% Pd/SOMS and 1% Pd/Al2O3 catalysts were poisoned ex-situ with Li2S. The ex-situ poisoned Pd/SOMS sample maintained its catalytic activity for HDC of TCE. However, a significant loss in catalytic activity of the Pd/Al2O3 catalyst was observed after poisoning. Protection from aqueous phase sulfide poisoning was attributed to the hydrophobicity of the Pd/SOMS, which would exclude anionic species from the embedded Pd particles. The XPS, STEM and ICP-OES results indicated that when Pd/Al2O3 and Pd/SOMS were treated with 1M HCl, most of the Pd metal was leached from the Pd/Al2O3 catalyst in contrast to Pd/SOMS, which had negligible leaching. Overall, due to hydrophobicity and high affinity for organics, SOMS has potential as a catalyst scaffold for different reactions in groundwater remediation applications.

Post plasma-catalysis for total oxidation of trichloroethylene over Ce–Mn based oxides synthesized by a modified “redox-precipitation route”

Two CeMnxOy (CeMnx) catalysts (x=1, CeMn; x=4; CeMn4) were synthesized via a modified “redox-precipitation” route followed by calcination at 500°C. XRD analysis showed only the presence of ceria. Raman spectroscopy gave a featureless spectrum for CeMn while peaks characteristic of a birnessite-type structure were observed for CeMn4 oxide. A large surface area of 165m2/g was obtained for CeMn which decreased to 95m2/g with x=4. Furthermore XPS characterizations showed homogeneous distribution of the active phase. Trichloroethylene (TCE) abatement was investigated at 150°C with catalyst alone, non-thermal plasma (NTP) alone at ambient conditions and with a post plasma-catalytic (PPC) system using these cerium–manganese oxides as well as a reference Mn oxide as catalysts positioned downstream of the plasma. At 150°C, without NTP, MnOy converted specifically 5% of TCE into CO2 while volatile organic compound (VOC) adsorption mainly occurred over the other catalysts. With NTP alone, 87% TCE removal was achieved at 240J/L but formation of unwanted polychlorinated by-products, such as phosgene, dichloroacetylchloride and trichloroacetaldehyde occurred resulting in a low COx (x=1, 2) selectivity of about 27%. In comparison, a significant improvement was observed in TCE abatement performance when using the PPC configuration. Indeed, TCE conversion up to about 100% as well as COx selectivity up to 56% were achieved thanks to the high efficiency of the catalysts to completely decompose O3 leading to the production of active oxygen species which are able to oxidize the by-products. The best plasma-assisted catalyst performance observed for CeMn4 may be linked to the presence of residual potassium allowing to avoid the poisoning of catalytic sites with chlorinated organic species as well as slowing down the formation of chlorinated mineral phases.

Highly active Fe2O3-doped TiO2 photocatalyst for degradation of trichloroethylene in air under UV and visible light irradiation: Experimental and computational studies

0.0–5.0wt% Fe2O3-doped TiO2 photocatalysts were synthesized by an ultrasonic-assisted co-precipitation method and used for degradation of 500ppm trichloroethylene (TCE) in air. The photocatalysts were treated at 100–400°C and characterized by high resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), BET surface area, and Raman, UV–vis, and photoluminescence (PL) spectroscopies. For the samples treated at 100°C, XRD patterns show no significant peaks, while HRTEM micrographs indicate crystalline structures and diffraction patterns and Raman spectra reveal wide and weak peaks of anatase TiO2. As 0.1wt% Fe2O3 is added to TiO2, the surface area increases by a factor of 1.87 and reach to 448m2/g. UV–vis analyses indicate that, as the Fe2O3 content of the samples increases, the band gap energies decrease from 3.22eV for the pure TiO2 to 2.60eV for 5.0wt% Fe2O3-doped TiO2. Density functional theory simulations show that, when Fe3+ is doped into or FeO, Fe2O3 or Fe4O6 is adsorbed on anatase TiO2, the bad gap reduces and the iron oxide–TiO2 show photocatalytic activity in visible light region. The synthesized TiO2 showed about two times higher photocatalytic activity for TCE removal than that of Degussa P25. Moreover, 0.1wt% Fe2O3-doped TiO2 sample dried at 100°C showed a maximum photocatalytic TCE conversion of about 97% with minor amounts of CO as a hazardous byproduct. As the treatment temperature of the samples increase, the photocatalytic activity declines. Significant photocatalytic degradation of TCE was also observed for the Fe2O3-doped TiO2 samples, under visible light irradiation.

Catalytic conversion of trichloroethylene on nickel containing beta zeolites into value added products

Nickel containing BEA zeolites, NiSiBEA and NiHAlBEA, with 2wt% of Ni were prepared by a two-step postsynthesis method and conventional wet impregnation. The calcination of NiSiBEA and NiHAlBEA at 773K for 3h in air led to the formation of C-NiSiBEA and C-NiHAlBEA. After reduction at 873K for 3h in flowing 10% H2/Ar, red-C-NiSiBEA and red-C-NiHAlBEA were obtained and investigated as the catalysts in hydrodechlorination of trichloroethylene at 503K. Comparative study performed on red-C-NiSiBEA and red-C-NiHAlBEA showed very high stability of the catalyst prepared by a two-step postsynthesis method and strong deactivation of the catalyst prepared by conventional wet impregnation during ~17h of reaction. Application of both catalysts resulted in high selectivity to unsaturated hydrocarbons that are the most desired products in HDC processes. TPH of the catalysts after kinetic run have shown that the main cause of deactivation of NiHAlBEA is the formation of carbonaceous species during hydrodechlorination of trichloroethylene.

Development of an attached-growth process for the on-site bioremediation of an aquifer polluted by chlorinated solvents

A procedure for the design of an aerobic cometabolic process for the on-site degradation of chlorinated solvents in a packed bed reactor was developed using groundwater from an aquifer contaminated by trichloroethylene (TCE) and 1,1,2,2-tetrachloroethane (TeCA). The work led to the selection of butane among five tested growth substrates, and to the development and characterization from the site's indigenous biomass of a suspended-cell consortium capable to degrade TCE (first order constant: 96 L gprotein(-1) day(-1) at 30 °C and 4.3 L gprotein(-1) day(-1) at 15 °C) with a 90 % mineralization of the organic chlorine. The consortium immobilization had strong effects on the butane and TCE degradation rates. The microbial community structure was slightly changed by a temperature shift from 30 to 15 °C, but remarkably affected by biomass adhesion. Given the higher TCE normalized degradation rate (0.59 day(-1) at 15 °C) and attached biomass concentration (0.13 gprotein Lbioreactor(-1) at 15 °C) attained, the porous ceramic carrier Biomax was selected as the best option for the packed bed reactor process. The low TeCA degradation rate exhibited by the developed consortium suggested the inclusion of a chemical pre-treatment based on the TeCA to TCE conversion via β-elimination, a very fast reaction at alkaline pH. To the best of the authors' knowledge, this represents the first attempt to develop a procedure for the development of a packed bed reactor process for the aerobic cometabolism of chlorinated solvents.

The effects of excess manganese in nano-size lanthanum manganite perovskite on enhancement of trichloroethylene oxidation activity

The effects of excess manganese in LaMn1+xO3+δ perovskite on enhancement of trichloroethylene (TCE) oxidation, as a chlorinated volatile organic compound (CVOC), in air are studied. Various LaMn1+xO3+δ (x=0.0–0.4) nanocatalysts were synthesized by a microwave-assisted “gel combustion” method and calcined at 600°C in air. XRD patterns of the samples confirm formation of perovskite structure and SEM micrographs exhibit spongy agglomerated nanoparticles with a wide size distribution and presence of large pores in their structure. TEM micrographs show the presence of 12–30nm nanoparticles of possibly highly dispersed amorphous and/or fine crystallites of MnOx, which are not detectable in XRD patterns. TPR results indicate that the oxygen overstoichiometry of LaMn1+xO3+δ catalysts increases with excess Mn. Enhancement of BET surface area of the catalyst samples as much as 2.5-fold and the maximum TCE oxidation activity are observed for the LaMn1.2O3+δ catalyst. As compared to LaMnO3+δ, T50 and T90 of TCE oxidation on LaMn1.2O3+δ are decreased by 150 and 174°C, respectively. Loss of BET surface areas of the catalysts after exposure to reaction conditions may be responsible for the observed initial deactivation. XPS spectra show the formation of LaOCl during the initial deactivation. Simple zero-order kinetics expression with respect to TCE, provides fairly good fits for the evolution of the TCE conversion on LaMn1+xO3+δ catalysts.

Deactivation of H-zeolites during catalytic oxidation of trichloroethylene

This paper reports the stability of protonic zeolites (H-ZSM-5, H-MOR and H-BEA) in the catalytic oxidation of trichloroethylene (TCE), focused on determining the causes of the deactivation. The study was conducted by relating the evolution of TCE conversion (with and without water vapor) with the physicochemical properties of the zeolites as a function of time on stream (TOS). The oxidation reactions were carried out in a fixed-bed reactor under 1000ppmv of TCE in air. We found out that both coke formation and chlorine attack were the causes of H-zeolites’ rapid deactivation. Coke was formed from the TCE or the surface intermediates already formed before the TCE oxidation slower step. Chlorine atoms of TCE also caused irreversible deactivation by attacking to Brönsted acid sites, which lead to structural changes. Under wet conditions, H-zeolites were strongly hydrated restoring hydroxyl groups to keep TCE conversion invariably overtime.

SiO2/TiO2 Antireflective Coatings With Photocatalytic Properties Prepared by Sol–Gel for Solar Glass Covers

The glass covers of solar systems are usually coated with antireflective (AR) coatings on both sides that allow to increase the efficiency of the whole system. At the same time, the accumulation of dust and dirt particles on the surface of the AR coated glass decreases the transmittance of the covers, even to values lower than the uncoated glass. This decrease in the cover transmittance reduces the solar radiation that reaches the absorber and, in this way, a decrease in the solar plant efficiency would be obtained. So, the use of films that combine high transmittance and self-cleaning capacity seems to be a promising development. This paper reports the preparation of sol–gel porous TiO2 and SiO2 bilayers on borosilicate glass. The porosity and thickness of both layers have been optimized in order to obtain the optimal photocatalytic and optical properties. Solar transmittance values of 0.964 were obtained for the TiO2/SiO2 coated glass. The highest value of transmittance reached is 0.993 and it is placed at 600 nm. The self-cleaning properties were evaluated using methylene blue and trichloroethylene (TCE) as model organic molecules using UV-A light. The results showed that the SiO2/TiO2 samples degraded the colorant faster than the SiO2 sample. Moreover, the degradation of TCE in air in a continuous fix bed photoreactor for samples with different TiO2 film thicknesses was also performed. The TCE conversion was found dependent on the TiO2 thickness. Nevertheless, the increase in the thickness of this layer reduced transmittance properties. The study demonstrates that it is possible to combine antireflective and self-cleaning properties in the same material.

Highly selective one-dimensional TiO2-based nanostructures for air treatment applications

The aim of the present research is to determine the optimum hydrothermal synthesis procedure for using TiO2-based one-dimensional materials as photocatalysts for air treatment applications, in order to enhance their mineralization capacity. The prepared samples were thoroughly characterized by means of TEM, N2 adsorption–desorption isotherms, XRD and Raman spectroscopy. Trichloroethylene (TCE) was selected as a convenient model volatile organic compound (VOC) due to its elevated reactivity. In this study, it is described how the selection of the synthesis parameters (TiO2 precursor, hydrothermal treatment conditions, washing procedure or calcination temperature) allows modulating the nanotubes physicochemical characteristics and, thus, photoactivity. The optimization process led to nanotubes with remarkable photocatalytic performance, with high TCE conversion and selectivity to CO2, which can be favourably compared to that of the benchmark photocatalyst Degussa P25.