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Applied Catalysis B, Environmental (v.128, #)
Use of competitive kinetics for the understanding of deep hydrodesulfurization and sulfide catalysts behavior by Vrinat Michel; Laurenti Dorothée; Geantet Christophe (pp. 3-9).
Display Omitted► Competitive experiments allowed proposing a general mechanism for HDS of alkyldibenzothiophenes. ► Competitive reactions for understanding H2S inhibitor or promoter in the hydrogenation reaction. ► Importance of the metal–sulfur bond energy ( EM–S) on the electronic state of the sulfide. ► Kinetic proof of a “metallic like character” for ReS2 supported catalysts.Drastic regulations concerning sulfur content in fuels has lead to and still imply numerous researches on hydrotreating reactions and hydrodesulfurization catalysts. The objective of this paper was to illustrate how, besides physicochemical characterization techniques, the kinetics could be also a useful and precious tool for the understanding of the reactions involved and of the catalysts used in this process. That was illustrated by application of competitive kinetics to the comprehension of the transformation of sulfur refractory compounds, of the H2S partial pressure effect on the activity of transition metal sulfides, and to the understanding of support effect for rhenium sulfide based catalysts.
Keywords: Kinetics; Hydrodesulfurization; Sulfide catalysts; Support effect; Rhenium sulfide
Preferential CO oxidation over nanosized gold catalysts supported on ceria and amorphous ceria–alumina by Juliana Fonseca; Sébastien Royer; Nicolas Bion; Laurence Pirault-Roy; Maria do Carmo Rangel; Daniel Duprez; Florence Epron (pp. 10-20).
Display Omitted► CeO2–Al2O3 (Ce XAl) were prepared by Evaporation Induced Self Assembly. ► Some Au/Ce XAl present higher oxygen mobility and storage capacity than CeO2. ► The reducibility, oxygen mobility and storage capacity increase with Ce content. ► The CO conversion (without H2) increases with the cerium content of the support. ► Au/Ce XAl catalysts with medium Ce content are the most performant in CO-PrOx.1wt.% Au supported on CeO2–Al2O3 (Ce/Ce+Al: 0, 2, 5, 10, 15, and 20mol%) was prepared by impregnation in organic medium of high-surface area supports prepared by Evaporation Induced Self Assembly (EISA) route. For comparison, a 1wt.% Au/CeO2 was also prepared using a commercial support. All the prepared catalysts present a BET surface area roughly between 200 and 340m2g−1. The reducibility, the oxygen storage capacity (OSC) and the oxygen mobility determined by oxygen isotopic exchange (OIE) of the 1wt.% Au/CeO2–Al2O3 are enhanced by the presence of gold and increases with the cerium content in the support. The values of the OSC and the oxygen mobility in the Au/CeO2–Al2O3 catalysts with high cerium loading are higher than those determined on their Au/CeO2 counterpart. The activity in CO oxidation of the Au/CeO2–Al2O3 catalysts increases with the cerium content in the support, which could be linked to the better oxygen mobility of these systems, whereas in CO-PrOx the Au/CeO2–Al2O3 catalysts with medium cerium loading (Ce/Ce+Al=5 and 10mol%) present the highest CO conversion at 50–60°C. The catalysts are stable at 100°C in CO+O2+H2 mixtures. The addition of CO2 in the stream strongly deactivates Au/CeO2, whereas Au/CeO2–Al2O3 is less affected. Whatever the catalyst, the presence of steam has no significant effect on the catalytic performances.
Keywords: CO oxidation; Preferential CO oxidation; CO-PrOx; Gold; Ceria; CeO; 2; –Al; 2; O; 3
Preferential oxidation of CO (PROX) over CoO x/CeO2 in hydrogen-rich streams: Effect of cobalt loading by Preshit Gawade; Burcu Bayram; Anne-Marie C. Alexander; Umit S. Ozkan (pp. 21-30).
XANES studies showed the Co species in the CoO x/CeO2 catalyst to remain as a fully oxidized Co3O4 phase during PROX reaction.Display Omitted► Co catalysts supported on nano-particles of ceria are examined for PROX reaction. ► The role of Co loading on the activity and oxygen selectivity has been examined. ► In situ DRIFTS is used to examine surface species during reaction. ► In situ XAFS is used to examine the active phase under reaction conditions.PROX reaction over Co catalysts supported on ceria nano-particles was investigated with a special focus on the effect of Co loading. High surface area ceria nano-particles were prepared through a precipitation technique using cerium (III) nitrate hexahydrate and sodium hydroxide. Co incorporation was achieved using a wet impregnation method in aqueous media. Catalysts were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, temperature-programmed reduction, dispersion measurements, and in situ diffuse reflectance Fourier transform infrared spectroscopy and X-ray absorption spectroscopy. Activity and selectivity of CoO x/CeO2 catalysts with different Co loadings showed higher Co loadings to favor higher CO oxidation at the expense of oxygen selectivity. Higher activation energies for H2 oxidation compared to CO oxidation were observed, regardless of the Co loading, implying a higher temperature sensitivity for the H2 oxidation reaction. The Co phase was identified as Co3O4 and was stable in the reducing environments present in PROX reactions.
Keywords: PROX; Cobalt catalyst; Nano ceria; DRIFTS; XANES; XPS
Simple method to synthesize high surface area magnesium oxide and its use as a heterogeneous base catalyst by Jonathan K. Bartley; Chunli Xu; Rhys Lloyd; Dan I. Enache; David W. Knight; Graham J. Hutchings (pp. 31-38).
Display Omitted► Simple synthesis method for high surface area MgO catalysts. ► Exceptionally high activity in the Meerwein–Ponndorf–Verley test reaction. ► Simple methodology has widespread applicability as heterogeneous base catalysts.High surface area magnesium oxide (∼250–300m2g−1) was synthesized via the thermal decomposition of different precursors including (MgCO3)4Mg(OH)2, Mg(OH)2, MgCO3 and MgC2O4. The high surface area MgO displayed exceptionally high catalytic activity for the liquid phase Meerwein–Ponndorf–Verley (MPV) reaction of benzaldehyde with different alcohols. The effect of the calcination temperature and precursor source on the catalytic activity and morphology of MgO has been investigated in detail. It was found that the optimum calcination temperature was 450°C leading to a high surface area material containing no MgCO3 which is the key to obtaining high catalytic activity of MgO for the MPV reaction. The high area MgO catalyst could be reused without loss of catalyst activity. The simplicity of the synthesis method gives this methodology widespread applicability in countless applications that currently use homogeneous base catalysts or stoichiometric bases as reagents.
Keywords: Base catalysis; High surface area MgO; Meerwein–Ponndorf–Verley reaction
New insights into the synergistic effect in bimetallic-boron catalysts for hydrogen generation: The Co–Ru–B system as a case study by G.M. Arzac; T.C. Rojas; A. Fernández (pp. 39-47).
Display Omitted► Activity for hydrogen generation of Co–B catalysts is enhanced by small amounts of Ru. ► Nanostructure of Co–Ru–B catalysts is related to boron-metal chemistry. ► Co–Ru–B catalysts isostructural to Co–B. Nanostructural transition occurs for Ru–B. ► Cobalt segregation and Ru dispersion explain the synergistic effect. ► The effect is not universal but [OH−] dependent.Catalysed sodium borohydride hydrolysis is a high-potential method to produce hydrogen for portable applications. Co–B catalysts are the most chosen because they are easily prepared, cheap and efficient. The addition of small amounts of Ru produces a significant enhancement in catalytic activity.In the present work a series of Co–Ru–B catalysts with variable Ru content was prepared, isolated and characterized. The comprehension of the synergistic effect was achieved trough the incorporation of the nanostructural dimension to the study of surface and bulk chemical states of the involved atoms along the series. It was found that up to 70% (of total metal) atomic content of Ru the catalysts can be considered isostructural to the single Co–B catalyst in the nanoscale. A structural transition occurs in the case of the pure Ru–B material to produce a boron deficient material with higher nanoparticle size. This structural transition together with Co segregation and Ru dispersion play a key role when explaining a [OH−] dependent effect.The inexistence of borate layers in Ru rich catalysts is suggestive in the research for non deactivating catalysts.
Keywords: Sodium borohydride hydrolysis; Co–Ru–B catalysts; Synergistic effect; Nanostructural transition; Ru dispersion
Electrochemical promotion of methane oxidation on Pd catalyst-electrodes deposited on Y2O3-stabilized-ZrO2 by C. Jiménez-Borja; S. Brosda; F. Matei; M. Makri; B. Delgado; F. Sapountzi; D. Ciuparu; F. Dorado; J.L. Valverde; C.G. Vayenas (pp. 48-54).
Electrochemical promotion of methane oxidation on impregnated Pd/YSZ under oxidizing conditions and low operating temperatures. Transient response of the catalytic CO2 formation rate and of the catalyst potential upon application of (a) I=100μA at 350°C and (b) I=400μA at 400°C. Experimental conditions:PCH4=1.3kPa,PO2=4.5kPa, F=200ml/min.Display Omitted► Electrochemical promotion of Pd/YSZ catalyst achieved under oxidizing conditions. ► Ceria interlayer enhances the catalytic activity of Pd/YSZ catalysts. ► Oxidation state of working Pd catalyst monitored via conductivity measurements.The catalytic activity of Pd/YSZ and Pd-CeO2/YSZ catalysts prepared via organometallic paste calcination or via wet impregnation has been investigated under conditions of excess in oxygen and at temperatures from 250 to 450°C. The catalyst-electrodes were characterized via XRD, SEM, in situ electrical conductivity measurements, electrochemical current–potential curves and catalytic kinetic measurements. The impregnated Pd films were found to be more active with light-off temperatures of 270°C vs. 320°C for metal paste catalysts. Only the impregnated catalyst could be electropromoted at temperatures above 320°C. The Pd-CeO2/YSZ catalyst exhibited high catalytic activity but could not be electropromoted.Co-feeding of ethylene or pre treatment in oxidizing ethylene–oxygen gas mixtures enhances the rate of methane conversion in comparison to non-treated samples and allows electrochemical promotion of the methane oxidation at temperatures as low as 320°C. The reason for this behavior is discussed in view of detailed pre and post surface characterization and in situ electrochemical measurements.
Keywords: Methane oxidation; Electrochemical promotion; Pd catalyst-electrode; CeO; 2; interlayer; YSZ
Study by electrical conductivity measurements of semiconductive and redox properties of ceria and phosphated ceria catalysts by Ionel Popescu; Ioan-Teodor Trotuş; Ioan-Cezar Marcu (pp. 55-63).
.Display Omitted► Oxidation reaction mechanism over ceria and surface phosphated-ceria catalysts. ► Surface lattice O2− species involved in the isobutane reaction on ceria. ► At high P content, CePO4 phase controlled both the electric and catalytic properties. ► Surface lattice O−species involved in the isobutane reaction at high P content. ► At low P content, both O2−and O− species were involved.Pure ceria and two surface-phosphated ceria containing different amounts of phosphorus, 1.1P/CeO2 and 2.2P/CeO2, catalysts for isobutane oxydehydrogenation, were characterized by in situ electrical conductivity measurements. Their electrical conductivity was studied as a function of temperature and oxygen partial pressure and was followed with time during sequential exposures to air, nitrogen, isobutane–air mixture (reaction mixture) and isobutane–nitrogen mixture in conditions similar to those of catalysis. All the materials appeared to be n-type semiconductors under air with quasi-free electrons as the main charge carriers and their electrical conductivity increased following the order: CeO2<1.1P/CeO2<2.2P/CeO2. While CeO2 and 1.1P/CeO2 remained n-type semiconductors when contacted with isobutane–nitrogen or isobutane–air mixture, 2.2P/CeO2 became a p-type semiconductor with positive holes as the main charge carriers. For 2.2P/CeO2 catalyst the oxydehydrogenation reaction involves surface lattice O− species, whereas for CeO2 surface lattice O2− anions are involved. For 1.1P/CeO2 catalyst the reaction mechanism involves both surface lattice O2− anions and surface lattice O− species. These results explained the difference in catalytic performances encountered on these solids. In all cases the overall reaction mechanism can be assimilated to a Mars and van Krevelen mechanism.
Keywords: Electrical conductivity; Ceria; Phosphated ceria; Catalytic oxydehydrogenation; Isobutane
Selective removal of the ammonium-nitrogen in ammonium acetate aqueous solutions by catalytic wet air oxidation over supported Pt catalysts by M. Bernardi; M. Le Du; I. Dodouche; C. Descorme; S. Deleris; E. Blanchet; M. Besson (pp. 64-71).
Display Omitted► Pt/TiO2 catalysts were highly active in NH4+ removal from ammonium acetate effluent. ► The reaction was very selective to N2 (>97.5%). ► Mineralization of the acetate was moderate. ► The Pt catalysts were stable in continuous experiments.The selective removal of ammonia from industrial waste streams and the conversion of the organic matter to biodegradable compounds, without excessive mineralization of the carbon has been investigated over powder and pelletized titania- and zirconia-supported Pt catalysts in the wet air oxidation (WAO) of aqueous solutions at 200°C under 36bar of air, in batch and trickle-bed reactors.Ammonium acetate was chosen as a model compound (58mmolL−1, Total Organic Carbon, TOC=1392mgL−1, Total Nitrogen, TN=812mgL−1). In the absence of any catalyst, very little ammonium was removed. With addition of a Pt catalyst supported, NH4+ was completely eliminated from the reaction mixture and converted to dinitrogen with a selectivity higher than 97.5%. The treated effluent contained very low concentration of nitrates as a by-product. Interestingly, the platinum catalysts were very little active in the mineralization of the organic carbon, so that the effluent could be post-treated in a conventional biological plant. The catalysts showed the same performances over recycling experiments in a batch reactor. Moreover, the pelletized catalysts were shown to be stable upon preliminary continuous experiments in a trickle-bed reactor over a period of 350h. The 3%Pt/TiO2 catalysts were significantly more active than the 3%Pt/ZrO2 catalysts in batch and continuous experiments.
Keywords: Selective catalytic wet air oxidation; Ammonium acetate; Supported platinum catalysts; Batch reactor; Trickle-bed reactor
Preparation of calcium tantalum oxynitride from layered oxide precursors to improve photocatalytic activity for hydrogen evolution under visible light by Ryo Sasaki; Kazuhiko Maeda; Yoko Kako; Kazunari Domen (pp. 72-76).
Display Omitted► A new precursor route to prepare CaTaO2N photocatalyst was studied. ► Physicochemical properties of CaTaO2N depended on the precursor oxide employed. ► Pt-loaded CaNbO2N photocatalyzed water reduction under visible light ( λ>420nm). ► Layered RbCa2Ta3O10 was a better precursor than a bulk-type Ca-Ta oxide.A new precursor route to prepare calcium tantalum oxynitride (CaTaO2N) was explored in an attempt to improve the photocatalytic activity for H2 evolution from water under visible light. CaTaO2N catalysts were prepared using either a layered metal oxide (e.g., RbCa2Ta3O10 or HCa2Ta3O10) or a bulk-type Ca-Ta oxide prepared by the polymerized complex (PC) method. CaTaO2N samples prepared from the layered oxides had a less-aggregated morphology and a bluer absorption edge than an analog from a PC-derived precursor, in addition to a lower density of anionic defects. Samples prepared from the layered oxide also showed an enhanced photocatalytic activity for H2 evolution from an aqueous methanol solution under visible light ( λ>420nm), primarily because of their lower density of anionic defects and the enhanced driving force for surface redox reactions that resulted from their blue-shifted absorption edge.
Keywords: Layered oxide; Hydrogen production; Oxynitride; Photocatalyst; Visible light; Water splitting
Origin of visible-light activity of N-doped TiO2 photocatalyst: Behaviors of N and S atoms in a wet N-doping process by Taizo Sano; Nobuaki Mera; Yusuke Kanai; Chifumi Nishimoto; Sakiko Tsutsui; Tsutomu Hirakawa; Nobuaki Negishi (pp. 77-83).
Display Omitted► A strategy to develop a N-doped TiO2 with high visible-light activity on NO x removal was indicated. ► Behaviors of N and S in a wet N-doping process were clarified. ► NH3 coordinated to Ti4+ was related to the visible-light activity of N-TiO2. ► A substitutional doping of nitrogen anions into TiO2 was not essential for the activity. ► The activity was improved by removing SO42− by heat-treatment in CO.Behaviors of N and S atoms in formation of N-doped TiO2 with a wet N-doping process were analyzed to investigate the origin of photocatalytic activity under visible light. In a titanium oxyhydroxide precursor obtained by mixing titanium oxysulfate and ammonia solutions, two types of NH3 species were formed. One was directly bonded to Ti4+ (coordinated NH3, located at 400.0eV in X-ray photoelectron spectrum), which was related to photocatalytic activity on NO x removal under visible light ( λ>405nm). The other was NH3 in (NH4)2SO4 (at 401.8eV), which was unnecessary for producing the activity. During heat-treatment at 400°C, roughly half amount of the coordinated NH3 was left behind in TiO2 without changing the oxidation state of nitrogen, and the other half was released from the solid phase regardless of partial O2 pressure. A local unusual structure of TiO2, e.g. crystal defects and distortion, formed by the coordination of NH3 to Ti4+ and/or the release of NH3 seems to be an origin of the visible-light activity of N-doped TiO2. A substitutional doping of nitrogen anions (N3−) into TiO2 was not essential for the activity. The amount of SO42− in N-doped TiO2 was decreased by washing of the precursor or heat-treating the photocatalyst in CO. The activity was significantly improved by decreasing the amount of SO42− in N-TiO2.
Keywords: Visible light; Photocatalyst; Nitrogen doping; Oxygen vacancy; Titanium dioxide; Distortion
Deep UV photocatalytic activation of ethane on silica surfaces by Francesc Sastre; Avelino Corma; Hermenegildo García (pp. 84-90).
Display Omitted► Deep UV irradiation of silicas transforms ethane into ethanol. ► The presence of oxygen during the photolysis increases conversion of ethane. ► Porous silicas are more efficient photocatalysts than amorphous silica.Deep UV photolysis (165 or 185nm) of surface silanol groups leads to the homolytic OH bond breaking, generating silyloxyl radicals and hydrogen atoms. Silyloxyl radicals are able to activate ethane through hydrogen abstraction, whereby ethyl radicals are formed. Coupling of these ethyl radicals with silyloxyl radicals forms surface bound ethoxysilane that eventually will form ethanol. The product distribution of this radical process depends on the absence or presence of oxygen and may lead to the formation of ethanol together with light alkanes (methane, propane, butane and hexane) accompanied by C2 (acetaldehyde and acetic acid) and C1 (methanol, formaldehyde and formic acid) oxygenates. The presence of oxygen enhances ethane conversion and quenches the formation of alkanes by trapping alkyl radicals. It was found that micro and mesoporous silicas behave qualitatively similar with some differences in the product distribution. The most efficient material (higher conversion and higher percentage of products in the solid) was found to be Al-MCM 41. The energy consumption estimated based on a conversion of 6% on commercial beta zeolite was 2.0Gcal per mol of ethane converted that is about 3.6 times smaller than the energy consumed form methane activation through an analogous process.
Keywords: Photocatalysis; Ethane activation; Deep UV photolysis; Aluminosilicates as photocatalysts
The role of electron transfer in photocatalysis: Fact and fictions by Hanan H. Mohamed; Detlef W. Bahnemann (pp. 91-104).
.Display Omitted► Electron transfer plays a key role in photocatalysis. ► Electron transfer dynamics measured by flash photolysis, pulse radiolysis or stopped flow. ► Electron transfer efficiency depends on photocatalyst size, surface composition and morphology. ► The electron transfer rate was found to increase with increasing the driving force. ► New findings on the electron transfer reactions at TiO2 nanoparticles are reviewed.Interfacial electron transfer at semiconductor nanoparticles is a fundamental process that is relevant to many applications in photocatalysis such as wastewater cleaning, air cleaning and energy production. Fundamental understanding of the dynamic of the electron transfer is of crucial importance for the understanding of the fundamental concepts of photocatalytic processes and hence results in understanding and industrialization of photocatalytic reactions as well as a rational design of the photocatalytic systems.This review summarizes the progress in understanding electron transfer dynamics from semiconductor nanoparticles to the electron acceptor molecules. The approaches to improve the electron transfer efficiency will be also reviewed. Of particular focus will be the advancement of methodology as well as overview of some new highlights in electron transfer reactions at TiO2/liquid interface.
Keywords: Electron transfer; Photocatalysis; Electron dynamic techniques; Kinetic aspects; Mechanistic perspectives
Photocatalytic detoxification of aqueous organophosphorus by TiO2 immobilized silica gel by Nobuaki Negishi; Taizo Sano; Tsutomu Hirakawa; Fumiko Koiwa; Chamorn Chawengkijwanich; Nuttaporn Pimpha; Glory-Rose Mangat Echavia (pp. 105-118).
Display Omitted► Safe elimination of organophosphorus pesticides in an aqueous phase by TiO2 immobilized photocatalyst. ► Water-soluble organophosphoric compounds were effectively degraded. ► Phosphoric intermediates and H3PO4 with photocatalysis of organophosphorus compounds were adsorbed onto TiO2. ► TOC level with photocatalysis was increased than in the dark condition.Photocatalytic degradation of organophosphorus compounds including organophosphonic and organophosphinic acids by TiO2 immobilized silica gel in a water phase was carried out. Photocatalytic degradation of parent organophosphorus compounds and formation of intermediates were observed and the photocatalytic degradation scheme for each pesticide is proposed. Overall, the photocatalytic degradation of organophosphorus compounds in aqueous phase was estimated to form organophosphoric (organophosphonic and organophosphinic) acids as intermediates of the reaction. These organophosphoric acid intermediates were selectively adsorbed onto TiO2 surface and as a result, elution of these compounds into the aqueous phase was not observed. Similarly, after photocatalytic degradation, H3PO4 was either not observed or slightly observed as the final product. Likewise, rapid adsorption of the studied parent organophosphoric acids on TiO2 surface resulted in significant reduction in the concentration of these compounds even under dark condition. Upon UV irradiation, total organic carbon (TOC) level increased, and this is indicating the elution of some organic intermediates into the aqueous phase. However, these organic intermediates were eventually degraded with the UV irradiation.
Keywords: TiO; 2; immobilized photocatalyst; Water purification; Organophosphorus compounds; Organophosphoric acid; Pesticide
Optimization of mild solar TiO2 photocatalysis as a tertiary treatment for municipal wastewater treatment plant effluents by L. Prieto-Rodriguez; S. Miralles-Cuevas; I. Oller; P. Fernández-Ibáñez; A. Agüera; J. Blanco; S. Malato (pp. 119-125).
Effect of H2O2 dosing in photocatalytic reaction rate at low solar radiation intensity and working with low TiO2 photocatalyst concentration.Display Omitted► Solar photocatalytic treatment for eliminating emerging contaminants. ► We studied it in municipal wastewater treatment plant effluents. ► Hydrogen peroxide is highly recommendable with TiO2 at low concentrations. ► High photon flux must be avoided. ► Too low (less than 40mgL−1) TiO2 concentration is not recommendable.The appropriate catalyst concentration for industrial wastewater treatment is several hundred milligrams per liter in solar photoreactors. Nonetheless, when the purpose of eliminating emerging contaminants in municipal wastewater treatment plant effluents is for reuse of the water, and they are present at extremely low concentrations, a tertiary treatment with a much lower photocatalyst concentration might be possible. TiO2 concentrations of only tens of mgL−1 were selected to evaluate the influence of catalyst load, initial hydrogen peroxide dose and radiation intensity on the degradation rate of five emerging contaminants (100μgL−1 of ofloxacin, sulfamethoxazole, flumequine, carbamazepine, and 2-hydroxy-biphenyl) spiked in a real municipal wastewater treatment plant effluent. Response surface methodology based on a spherical central composite design was used to optimize the parameters to find the maximal degradation rate. The experiments were carried out using an Evonik P-25 titanium dioxide suspension in a Suntest solar simulator. It has been demonstrated that the use of hydrogen peroxide is highly recommendable for working with TiO2 at low concentrations and high photon flux must be avoided. It has also been demonstrated that too low (less than 40mgL−1) TiO2 concentration is not recommendable.
Keywords: Emerging contaminants; Photocatalysis; Solar photoreactors
Solar photocatalytic disinfection of water with immobilised titanium dioxide in re-circulating flow CPC reactors by D.M.A. Alrousan; M.I. Polo-López; P.S.M. Dunlop; P. Fernández-Ibáñez; J.A. Byrne (pp. 126-134).
Display Omitted► Solar photocatalytic disinfection of water was tested with different reactor configurations. ► Compound parabolic collectors improved the SODIS and SPC-DIS of water. ► The presence of photocatalyst gave additional enhancement over the solar disinfection. ► These enhancements should not add high cost to the treatment for deployment in developing countries.It is estimated that 780 million people lack access to improved water supplies and many more are forced to rely on sources that are microbiologically unsafe. While piped-in water supplies are the ultimate goal for the provision of water in developing countries, low cost point-of-use disinfection treatments could help to significantly reduce the incidence of water borne disease. Solar disinfection of water (SODIS) is a simple method of treating water where the UV and thermal energy of the sun act to inactivate pathogenic microorganisms in water; however, the recommended protocol is 6h under direct sunlight, the efficiency depends on environmental factors, and some pathogens are more resistant to solar disinfection.The use of compound parabolic collectors (CPC) and immobilised titanium dioxide for photocatalysis were investigated as enhancement technologies for solar disinfection. The reactors consisted of borosilicate glass tubes (1.5m in length) as either single tubes of diameter 50mm or two concentric tubes (inner tube of diameter 32mm) with and without CPC. For solar photocatalytic disinfection (SPC-DIS), the inside wall of the 50mm tube was dip coated with TiO2 (TiO2, Evonik P25) and/or the outside wall of the 32mm tube. SPC-DIS and SODIS were tested under flow conditions using different reactor configurations under real sun conditions. E. coli was used as the model microorganism for the disinfection studies in 0.9% saline solution and the total volume in each experiment was 7L. It was found that the use of CPCs improved the SODIS and SPC-DIS efficiency. The disinfection kinetics were observed to follow a log-linear with shoulder and/or tailing model. The kinetic parameters were determined using the UVA dose as the independent variable and the configurations were compared for E. coli inactivation efficiency based on the log of the residual concentration (Log N res) and the first order rate constant ( k). Three reactor configurations showed a residual bacterial count below the detection limit and they were compared based upon the first order rate constant. The concentric tube arrangement (a tube within a tube) with CPC was the most effective configuration. The following order was found for k where coated refers to TiO2 coating and the equals sign indicates no significant difference; uncoated external – coated internal≥double coated tube≥uncoated double tube. It is known that E. coli is inactivated by SODIS and it may be a ‘soft’ target for comparing the effectiveness of SODIS vs SPC-DIS. Nevertheless, photocatalysis presents advantages in terms of the non-recovery of inactivated organisms and the inactivation of SODIS resistance organisms.
Keywords: Solar disinfection of water; Compound parabolic collectors; Photocatalysis; Titanium dioxide; E. coli
Glycerol as a probe molecule to uncover oxidation mechanism in photocatalysis by Claudio Minero; Andrea Bedini; Valter Maurino (pp. 135-143).
Display Omitted► Glycerol is a probe to assess different aspect of photocatalytic reaction rates. ► Single site photocatalysis is ruled out. ► Reactivity in the chemisorbed state leads to CC bond cleavage. ► Reactivity in the physisorbed state leads to H-abstraction. ► Photocatalytic reaction mechanism and back reactions depend on surface chemistry.Glycerol is a good probe molecule to understand what type of photocatalytic mechanism could occur at the catalyst surface, as the analysis of the produced intermediates is able to discriminate between a direct electron transfer and a radical mediated oxidation mechanism. The mechanism of transformation of glycerol is explored by an extensive analysis of formed intermediates as a function of glycerol concentration on two different commercial TiO2 powders. Over TiO2 Degussa P25 the transformation rate of glycerol shows a sharp maximum as a function of the substrate concentration. The type of product detected at low glycerol concentration (glyceraldehyde and dihydroxyacetone) changes after the sharp maximum giving mainly formaldehyde and glycolaldehyde. On Merck TiO2, characterized by a lower density and more uniform population of hydroxyls at surface sites, mainly glyceraldehyde and dihydroxyacetone are observed. Because these products are formed through anOH-like mechanism, the products observed on P25 derive from a direct electron transfer. The results are rationalized invoking the surface complex of glycerol where an inner sphere electron transfer occurs, which leads to glycerol fragmentation, and where the substrate-mediated recombination plays the major role. A simplified kinetic approach confirms the observed rate dependence on concentration.The working hypothesis is also confirmed by addition of fluoride anions that compete for surface sites and impede glycerol complexation. Without the possibility of inner sphere electron transfer, on P25 only glyceraldehyde and dihydroxyacetone are observed. Thus different catalysts differ for the possibility of surface complexation.
Keywords: Glycerol; Photocatalysis; Fluorination; Shallow traps; OH radical
An overview of the methylene blue ISO test for assessing the activities of photocatalytic films by Andrew Mills (pp. 144-149).
Display Omitted► The main features of the ISO standard dealing with methylene blue and photocatalysis are described. ► Assumptions: dye purity, adsorption and pH, light source, diffusion, mechanism and kinetics. ► Errors arising from these assumptions are considered and improvements suggested. ► Implementation of improvements should improve the standard's repeatability and reproducibility.The main features are described of the ISO published standard: 10678:2010, namely the ‘Determination of photocatalytic activity of surfaces in an aqueous medium by degradation of methylene blue’. The main underlying assumptions of the standard are considered, namely: (i) dye purity, (ii) adsorption and pH, (iii) light source, (iv) stirring and diffusion, (v) reaction mechanism and (vi) kinetics. Possible sources of errors arising form these assumptions are identified and changes to the standard's protocol suggested. The major suggested changes are: (i) a source of MB+ of known and proven on site high purity (>90%) should be used to make up the standard test solution, which should have a referenced absorbance, 0.74, at 665nm; (ii) the conditioning solution should be the same concentration as the standard test solution, (iii) the initial pH of the reaction solution should lie in the range: 5.5–6.0; (iv) a BLB UVA light with a europium-doped strontium fluoroborate or borate phosphor should be used; (v) the reaction solution should be vigorously and continuously stirred if possible; (vi) the solution height should be the minimum recommended value of 2cm, and the photoreactor cylinder i.d., 4.7cm; (vii) after the 3h irradiation the reaction solution should be left stirring overnight to check that no dye photoreductive bleaching has occurred. Application of most of these suggestions should improve considerably the standard's reported current poor percentage errors for repeatability within a lab (9.2%) and reproducibility between labs (30.6%).
Keywords: Photocatalysis; Methylene blue; ISO; Titania
Selective photooxidation of alcohols as test reaction for photocatalytic activity by F.J. López-Tenllado; A. Marinas; F.J. Urbano; J.C. Colmenares; M.C. Hidalgo; J.M. Marinas; J.M. Moreno (pp. 150-158).
Display Omitted► Reactivity order found for the 25 tested catalysts in both test reactions follow a similar trend. ► Selectivity to crotonaldehyde is a function of the conversion level in all cases. ► Selectivity to acetone is higher than expected for platinum-containing systems. ► Trans/ cis isomerization of crotonaldehyde as the reaction proceeds.Twenty-four different titania-based systems synthesized through the sol–gel process varying the precursor (titanium isopropoxide or tetrachloride) and/or the ageing conditions (magnetic stirring, ultrasounds, microwave or reflux) were tested for liquid-phase selective photooxidation of 2-butenol (crotyl alcohol) to 2-butenal (crotonaldehyde) and gas-phase selective photooxidation of 2-propanol to acetone. To the best of our knowledge, the former process is suggested for the first time as test reaction for photocatalytic activity. Interestingly, both test reactions (despite having very different reactant/catalyst ratio and contact times) showed quite similar results in terms of influence of the precursor (titanium isopropoxide leading to better results than titanium tetrachloride) and the metals (the presence of iron, palladium or zinc being detrimental to activity whereas zirconium and especially gold improved the results as compared to pure titania). To our mind, these results give validity to both processes as test reactions for a fast screening of catalysts for photocatalytic tranformations. Finally, some gold-containing solids even improved photocatalytic activity of Degussa P25.
Keywords: Photocatalysis; Selective photooxidation; Test reactions; Titania-based catalysts; Metal-modified catalyst; Sonication; Microwave treatment; 2-Propanol (isopropanol); 2-Butenol (crotyl alcohol); Sol–gel synthesis
Key parameters influencing the performance of photocatalytic oxidation (PCO) air purification under realistic indoor conditions by Hugo Destaillats; Mohamad Sleiman; Douglas P. Sullivan; Catherine Jacquiod; Jean Sablayrolles; Laurent Molins (pp. 159-170).
Display Omitted► Role of key design and dimensioning parameters on PCO performance. ► 20-m3 room-sized chamber using TiO2-coated quartz felt in realistic indoor conditions. ► Efficient removal of VOC mixture that includes formaldehyde and acetaldehyde. ► Contact time between compounds and TiO2 surface is a critical parameter. ► Kinetic model identifies optimal operation conditions.Photocatalytic oxidation (PCO) air cleaning is a promising technology suitable for the elimination of a broad range of volatile organic compounds (VOCs). However, performance of poorly designed PCO systems may be affected by the formation of volatile aldehydes and other partially oxidized byproducts. This study explored the role of key design and dimensioning parameters that influence the effective removal of primary pollutants and can help reduce or eliminate the formation of secondary byproducts. A model pollutant mixture containing benzene, toluene, o-xylene, undecane, 1-butanol, formaldehyde and acetaldehyde was introduced at a constant rate in a 20-m3 environmental chamber operating at an air exchange rate of 1h−1. Individual pollutant concentrations were kept at realistically low levels, between 2 and 40μgm−3. A prototype air cleaner provided with flat or pleated PCO filtering media was operated in an external ductwork loop that recirculated chamber air at flow rates in the range 178–878m3h−1, corresponding to recycle ratios between 8.5 and 38. Air samples were collected upstream and downstream of the air cleaner and analyzed off-line to determine single-pass removal efficiency. The final-to-initial chamber concentration ratio was used to determine the global chamber removal efficiency for each pollutant. In the flat filter configuration, longer dwelling times of compounds on the TiO2 surface were attained by reducing the recirculation airflow by a factor of ∼5, leading to increasing total pollutant removal efficiency from 5% to 44%. Net acetaldehyde and formaldehyde removal was achieved, the later at airflow rates below 300m3h−1, illustrating the critical importance of controlling the contact time of primary and secondary pollutants with the TiO2 surface. The use of pleated media was shown to increase significantly the system performance by extending the dwelling time of pollutants on the irradiated surface of the PCO media, with a 70% degradation of target pollutants. With the pleated media, formaldehyde removal efficiency increased to 60%. Irradiation using either a UVC or a UVA lamp under identical flow conditions produced similar pollutant elimination. A simple correlation between the steady-state single pass removal efficiency and the global chamber removal efficiency was used to rationalize these experimental results and identify optimal operating conditions.
Keywords: Photocatalysis; Air cleaner; Indoor air quality; Aldehydes; HVAC
Characterization of a new photocatalytic textile for formaldehyde removal from indoor air by Pierre-Alexandre Bourgeois; Eric Puzenat; Laure Peruchon; France Simonet; Delphine Chevalier; Emmanuel Deflin; Cédric Brochier; Chantal Guillard (pp. 171-178).
Display Omitted► New photocatalytic textile integrating photocatalyst and light. ► Characterization of photocatalytic lighted textile by optic and electronic microscopy. ► Light emission modelling: quantification of diffuse and intense light. ► Efficiency of photocatalytic lighted textile for removing formaldehyde.The treatment of volatile organic compounds (VOCs) from different sources (industrial process, human activity, release of pollutants by materials, etc) in confined areas is a major challenge to prevent human health issues. Actually, one stays around 80% of a day in closed areas. The photocatalytic process is now recognized as an efficient method to remove organic pollutants present in gaseous phase. However, there are still some drawbacks with current reactors as for example lightutilization limitations due to absorption and/or scattering by the reaction medium or as restricted processing capacities due to mass transport limitations. The solution proposed in this work lies in the use of a lighted textile with optical fibers. The polymer optical fibers are treated to allow a radial leak of light through several dotted lights, allowing the carrying and the supply of UV light onto the overall textile area. Once coated using a suspension of TiO2, the textile becomes a photocatalytic materials for which UV light is carried into the bulk of the photocatalytic bed. The modifications of the fibers surface have been characterized by ESEM. The topology was characterized by optical microscopy and the radiant flux was measured by radiometry. The characteristics of the coating such as the photocatalyst content, its location and its adherence have been studied to understand the coating properties. The photocatalytic efficiency of samples coated with TiO2 was measured using formaldehyde as model molecule. The results obtained with a such new textile are very encouraging because this innovation allows to imagine a three dimensional light sources irradiate photocatalytic bed either its surface than its bulk. New perspectives in photocatalytic process are so opened.
Keywords: Photocatalysis; Lighted textile; Optical fiber; VOC; Irradiance modelling
Mechanism of photocatalytic bacterial inactivation on TiO2 films involving cell-wall damage and lysis by C. Pulgarin; J. Kiwi; V. Nadtochenko (pp. 179-183).
Display Omitted► The damage of E. coli cell-wall bilayers on the TiO2 film was followed by FTIR. ► The FTIR with AFM and cell viability data shows that E. coli photocatalysis leads to lysis. ► A small amount of damage of the bilayer functional groups leads to increased wall fluidity and lysis.This article addresses the cell wall damage of Escherichia coli (from now on E. coli) by TiO2 suspensions. The dynamics of TiO2 photocatalysis by thin films layers is described. The films were characterized by FTIR spectroscopy and atomic force microscopy (AFM). The E. coli complete inactivation is shown to be due to the partial damage of the cell-wall components (peroxidation). A small increase in the cell wall disorder concomitant with a decrease of the cell wall functional groups leads to higher cell wall fluidity as the precursor step preceding cell lysis.
Keywords: E. coli; inactivation; TiO; 2; photocatalysis; Lysis; AFM; FTIR; Cell-wall