Showing papers in "Applied Catalysis B-environmental in 2004"

TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations A review

Abstract: The photocatalytic degradation of azo dyes containing different functionalities has been reviewed using TiO2 as photocatalyst in aqueous solution under solar and UV irradiation. The mechanism of the photodegradation depends on the radiation used. Charge injection mechanism takes place under visible radiation whereas charge separation occurred under UV light radiation. The process is monitored by following either the decolorization rate and the formation of its end-products. Kinetic analyses indicate that the photodegradation rates of azo dyes can usually be approximated as pseudo-first-order kinetics for both degradation mechanisms, according to the Langmuir–Hinshelwood model. The degradation of dyes depend on several parameters such as pH, catalyst concentration, substrate concentration and the presence of electron acceptors such as hydrogen peroxide and ammonium persulphate besides molecular oxygen. The presence of other substances such as inorganic ions, humic acids and solvents commonly found in textile effluents is also discussed. The photocatalyzed degradation of pesticides does not occur instantaneously to form carbon dioxide, but through the formation of long-lived intermediate species. Thus, the study focuses also on the determination of the nature of the principal organic intermediates and the evolution of the mineralization as well as on the degradation pathways followed during the process. Major identified intermediates are hydroxylated derivatives, aromatic amines, naphthoquinone, phenolic compounds and several organic acids. By-products evaluation and toxicity measurements are the key-actions in order to assess the overall process.

Degradation of chlorophenols by means of advanced oxidation processes: a general review

Abstract: Advanced oxidation processes (AOPs) constitute a promising technology for the treatment of wastewaters containing non-easily removable organic compounds. Chlorophenols (CPs) are a group of special interest due to their high toxicity and low biodegradability. Data concerning the degradation of CPs by means of AOPs reported during the period 1995–2002 are evaluated in this work. Among the AOPs, the following techniques are studied: processes based on hydrogen peroxide (H2O2+UV, Fenton, photo-Fenton and Fenton-like processes), photolysis, photocatalysis and processes based on ozone (O3, O3+UV and O3+catalyst). Half-life times and kinetic constants for CP degradation are reviewed and the different mechanistic degradation pathways are taken into account.

MnOx-CeO2 mixed oxides prepared by co-precipitation for selective catalytic reduction of NO with NH3 at low temperatures

Abstract: A series of manganese-cerium oxide catalysts were prepared by co-precipitation method and used for low temperature selective catalytic reduction (SCR) of NO x with ammonia in the presence of excess O 2 . These catalysts were characterized by X-ray diffraction (XRD), surface area measurement and FTIR. The experimental results showed that the best Mn-Ce mixed-oxide catalyst yielded 95% NO conversion at 150 °C at a space velocity of 42,000 h −1 . As the manganese content was increased from 0 to 40% (i.e. the molar ratio of Mn/(Mn+Ce)), NO conversion increased significantly, but decreased at higher manganese contents. The most active catalyst was obtained with a molar Mn/(Mn+Ce) ratio of 0.4. Only N 2 rather than N 2 O was found in the product when the temperature was below 150 °C. At higher temperatures, trace amounts of N 2 O were detected. A mechanistic pathway for this reaction was proposed based on earlier findings and FTIR results obtained in this work. The initial step was the adsorption of NH 3 on Lewis acid sites of catalyst, followed by reaction with nitrite species to produce N 2 and H 2 O. Possible intermediates are proposed and all the intermediates could transform into NH 2 NO, which could further react to produce N 2 and H 2 O.

Visible light-induced photocatalytic degradation of Acid Orange 7 in aqueous TiO2 suspensions

Abstract: The photocatalytic degradation of a model azo-dye (Acid Orange, AO7) in aerated aqueous TiO2 dispersion has been studied under visible light (λ>400 nm) irradiation. The presence and role of oxidative species, such as singlet oxygen ( 1 O 2 ), superoxide (O2− ) and hydroperoxy (HO2 ) radicals was examined with the use of appropriate quenchers of these species. The reaction pathway of dye degradation was also investigated by monitoring the temporal evolution of intermediates and final products on both the photocatalyst surface and in solution, with the use of a variety of techniques, including GC–MS, FTIR and UV-Vis spectroscopies. It has been found that complete decolorization of the solution may be achieved, accompanied by a substantial decrease of the chemical oxygen demand (COD) of the solution. Evidence is presented that the main oxidative species is O2− (or HO2 ), while singlet oxygen, when formed, is also active. The adsorbed dye molecule is initially cleaved in the vicinity of the azo-bond and the resulting fragments are oxidized toward compounds of progressively lower molecular weight and, eventually, to CO2 and inorganic ions. However, when the solution is bleached, formation of active oxidative species does not take place, oxidation reactions cease and the concentrations of the dye intermediates remain practically stable upon further exposure to visible light irradiation. Formation of photoinduced hydrogen peroxide, which is also generated under the present conditions, also stops when the dye concentration in solution drops to very low levels. This behavior has been explained evoking the photosensitization mechanism of wide band-gap semiconductors, according to which the reaction is triggered by excitation of the dye molecule by visible light photons, followed by charge injection to the conduction band of the semiconductor and subsequent production of active oxygen radicals. Formation of the latter oxidizing species is possible only in the presence of visible light-absorbing compounds and cannot take place after fragmentation of the parent AO7 molecule in the vicinity of the azo-bond and decolorization.

Transition metal/UV-based advanced oxidation technologies for water decontamination

Abstract: This study explores the effect of ultraviolet (UV) light radiation and/or transition metals (M) for the activation of common oxidants (Ox) with the objective of treating recalcitrant organic contaminants in water. Hydrogen peroxide, potassium peroxymonosulfate and potassium persulfate were combined with iron, cobalt and silver, respectively, and/or with UV light (254 nm) and were tested for the treatment of 2,4-dichlorophenol (2,4-DCP). Results from our previous studies indicated that these particular transition metals are the best catalysts for the activation of the respective oxidants [G.P. Anipsitakis, D.D. Dionysiou, Environ. Sci. Technol. 37 (2003) 4790; G.P. Anipsitakis, D.D. Dionysiou, Environ. Sci. Technol. 38 (2004) 3705]. From the combined use of UV, the oxidants and the transition metals, four general categories of advanced oxidation technologies were evaluated and compared for the degradation and mineralization of 2,4-DCP. Those were (i) the dark conjunction of each oxidant with its favorable metal activator (M/Ox), (ii) the use of UV alone, (iii) the combination of UV with each oxidant (UV/Ox) and (iv) the use of UV combined with each metal/oxidant systems (UV/M/Ox). In particular, the systems UV/KHSO5, UV/Co(II)/KHSO5 and UV/Ag(I)/K2S2O8 and the sulfate radicals generated thereby have never been tested before for water decontamination, as opposed to the extensively investigated hydroxyl radicals generated by UV/H2O2 and the photo-Fenton. The comparison of the results with respect to the transformation of 2,4-DCP and the extent of organic carbon removal led to the construction of the following order of efficiencies: UV/K2S2O8 > UV/KHSO5 > UV/H2O2 for the UV/Ox processes and UV/Fe(III)/H2O2 > UV/Fe(II)/H2O2 > UV/Co(II)/KHSO5 > UV/Ag(I)/K2S2O8 for the UV/M/Ox processes tested here. All experiments were homogeneous and conducted at ambient room temperature. The relative absorbance of the species participating in the reactions supports the former order of efficiency, since persulfate followed by peroxymonosulfate were proven more photosensitive than hydrogen peroxide. Among the metals tested, only iron species such as Fe(OH)2+ were found to absorb strongly at 254 nm and to this is attributed the higher efficiencies obtained with the photo-Fenton reagents.

Effect of pH, inorganic ions, organic matter and H2O2 on E. coli K12 photocatalytic inactivation by TiO2: Implications in solar water disinfection

Abstract: The effect of different chemical parameters on photocatalytic inactivation of E. coli K12 is discussed. Illumination was produced by a solar lamp and suspended TiO2 P-25 Degussa was used as catalyst. Modifications of initial pH between 4.0 and 9.0 do not affect the inactivation rate in the absence or presence of the catalyst. Addition of H2O2 affects positively the E. coli inactivation rate of both photolytic (only light) and photocatalytic (light Plus TiO2) disinfection processes. Addition of some inorganic ions (0.2 mmol/l) like HCO3-, HPO42-, Cl-. NO3- and SO42- to the suspension affects the sensitivity of bacteria to sunlight in the presence and in absence of TiO2. Addition of HCO3- and HPO42- resulted in a meaningful decrease in photocatalytic bactericidal effect while it was noted a weak influence of Cl-, SO42- and NO3-. The effect of counter ion (Na+ and K-) is not negligible and can modify the photocatalytic process as the anions. Bacteria inactivation was affected even at low concentrations (0.2 mmol/l) of SO42- and HCO3- but the same concentration does not affect the resorcinol photodegradation, suggesting that disinfection is more sensitive to the presence of natural anions than photocatalytic degradation of organic compounds. The presence of organic substances naturally present in water like dihydroxybenzenes isomers shows a negative effect on photocatalytic disinfection. The effect of a mixture of chemical substances on photocatalytic disinfection was also studied by adding to the bacterial suspension nutrient broth, phosphate buffer and tap water. (C) 2004 Elsevier B.V. All rights reserved.

Photocatalytic decolorization of Remazol Red RR in aqueous ZnO suspensions

Abstract: The photocatalytic decolorization of aqueous solutions of Remazol Red RR, a commercial azo-reactive textile dye, in the presence of various semiconductor powder suspensions has been investigated in a quartz batch reactor with the use of artificial light sources (UV-C). ZnO and TiO2 have been found the most active photocatalysts; however ZnO indicated slightly higher efficiency. The effects of various process variables on decolorization performance of the process have been investigated. The results showed that the decolorization efficiency increases with increase in pH, attaining maximum value at pH 10 for ZnO. The zero-point charge for ZnO is 9.0 above which ZnO surface is negatively charged by adsorbed OH− ions, favoring the formation of strong oxidant OH radicals. The efficiency is inversely related to the dye concentration; increasing dye concentration enhances dye adsorption on the active sites of the catalyst surface, and consequently hinders OH− adsorption on the same sites, this results with a decreasing OH formation rate.

Solar photocatalytic degradation of dyes: high activity of combustion synthesized nano TiO2

Abstract: The solar photocatalytic degradation of various dyes such as methylene blue (MB), remazol brill blue R (RBBR) and orange G (OG) has been studied over combustion synthesized nano TiO2 and the activity was compared with that of commercial Degussa P-25 TiO2 under similar conditions. The effect of catalyst loading, initial concentrations of the dyes and the deactivation studies of the catalysts were also investigated. The catalyst was characterized by techniques like X-ray diffraction (XRD), BET, gravimetric-differential thermal analysis (TG-DTA), IR and UV absorption. The initial degradation rates with combustion synthesized nano TiO2 was 20 times higher for RBBR, 4 times higher for MB and 1.6 times higher for OG, compared to Degussa P-25 TiO2. The enhanced photocatalytic activity of combustion synthesized catalyst is attributed to the crystallinity, nano-size, large amount of surface hydroxyl species and reduced band-gap.

Bactericidal action of illuminated TiO2 on pure Escherichia coli and natural bacterial consortia: post-irradiation events in the dark and assessment of the effective disinfection time

Abstract: Bactericidal action of illuminated TiO2 on pure culture of Escherichia coli K 12 and bacterial consortium was studied. Photocatalytic E. coli inactivation rate was dependent on the biological parameters such as: physiological state, generation and initial concentration of bacteria. The behavior of the bacterial suspension during the subsequent dark period was discussed in order to estimate the potential of using the photocatalytic treatment process in a real water disinfection Situation. The effective disinfection time (EDT) was defined as the time required for total inactivation of bacteria without re-growth in a subsequent dark period referenced at 48 It. An increase of E. coli concentration was observed after illumination of bacteria without TiO2 (named here as photolysis). In the presence of TiO2 (photocatalysis), the decrease of bacteria continues in the dark, and no regrowth was observed within the following 60 h. The extent) applied during the illumination period. Thus, of this "residual disinfecting effect" was dependent on the light intensity (400 or 1000 W/m(2)) EDT was reached in photocatalytic but not in photolytic treatment. Bacterial consortia present in two different wastewaters were phototreated with and without TiO2 and a significant difference in bacterial sensitivity to both phototreatments was observed. Enterococcus sp. appear to be less sensitive than coliforms and other Gram-negative bacteria. Considerable differences in photoreactivity were observed for both samples taken at the same place but at different date. The first sample named as wastewater 1, exhibited a bacteriostatic but no bactericidal effect after both phototreatments; the addition of TiO2 accelerated the solar disinfection. In the second sample, wastewater 2, a bactericidal effect was observed in photolytic and photocatalytic experiments and the EDT was attained in both cases, even if the bacterial inactivation rate was the lowest in the presence of the catalyst. The chemical oxygen demand (COD) decreases during photocatalytic treatment, but not during the photolytic one; whereas the Dissolved organic carbon (DOC) increases in both cases. COD/DOC ratio decreases during phototreatrnents. (C) 2003 Elsevier B.V. All rights reserved.

Noble metal catalysts for the preferential oxidation of carbon monoxide in the presence of hydrogen (PROX)

Abstract: Oxide-supported noble metal catalysts were tested in the preferential oxidation of carbon monoxide (PROX) reaction in the temperature range between 50 and 300 °C. Both the influence of the noble metal nature (Pt, Ir, Pd), the support physical and chemical properties (redox, acidity, basicity) and the reaction conditions (oxygen stoichiometry) on the catalyst activity and selectivity was evaluated. Platinum and iridium were shown to be the most active and selective catalysts in the whole temperature range compared with palladium. Furthermore, noble metals supported over ceria-based oxides were shown to be active and selective, especially at low temperature. Additionally, it was observed that the higher the molar fraction in ceria in the oxide, the higher the activity and the selectivity in the PROX reaction. Ceria, with the highest oxygen mobility at the oxide surface, was shown to be the best support. Accordingly, on simple oxides (CeO2, SiO2–Al2O3, Al2O3, SiO2, La2O3 and MgO), the induced mobility of the oxygen atoms at the surface of the support determined elsewhere, well correlated with the basicity of the support, was shown to be one key parameter for the performances of the catalysts in the PROX reaction. Finally, the formation of water (hydrogen oxidation) at high temperature and high oxygen excess was shown to be responsible for the increasing activity of the catalysts in the conversion of CO to CO2 via the water gas shift reaction (WGSR).

Evaluating the activities of immobilized TiO2 powder films for the photocatalytic degradation of organic contaminants in water

Abstract: In a previous paper [J. Mater. Sci. 38 (2003) 823] we have described the preparation and characterization of conventional alkoxide sol–gel derived TiO2 films [J. Mater. Sci. 23 (1988) 2259] and of TiO2 powder enriched alkoxide sol–gel derived films. The powder films were prepared on flat stainless steel substrates and on glass beads. These films were characterized for various parameters like particle size, crystal phase, pore size, thickness and mechanical properties. In our current study, the photocatalytic activities of these sol–gel derived TiO2 films were studied utilizing a quartz batch reactor. The quartz batch reactor was characterized for parameters like mixing, recycle, aeration and UV radiation flux, and the TiO2 coated substrates were used as the photocatalyst. The activities of the catalyst films were evaluated by measuring the degradation rate of 4-chlorobenzoic acid used as a model organic pollutant. Immobilized TiO2 powder films on stainless steel containing a mixture of anatase and rutile phases were found to be more effective than films that were substantially composed of anatase phase particles. The activity of glass beads coated with TiO2 powder was compared to the activity of commercially available TiO2 catalyst beads. While the activity of the commercially available TiO2 catalyst beads was higher there was significant attrition of the TiO2 catalyst film. The films on the glass beads possessed better mechanical properties than the commercial catalyst beads and their activity can be significantly improved by optimizing the film synthesis process parameters.

The role of NO2 and O2 in the accelerated combustion of soot in diesel exhaust gases

Abstract: The role of NO2 and O2 in the combustion of soot was investigated by analysing the soot combustion characteristic and the DRIFT spectra of surface oxygen complexes (SOCs) as soot oxidation intermediates. It is found that in NO2/O2-soot, like in the CRT system, the oxidation of soot is initiated by the NO2-soot reaction to create SOCs as intermediates. O2 reacts with SOCs, eventually yielding CO and/or CO2. This mechanism results in a higher oxidation rate when NO2-soot reaction occurs in the presence of O2. The potential of using Printex-U, a flame soot, in exploratory diesel soot combustion studies is discussed. It is concluded that in general this is the case except for the fact that real soot can contain adsorbed hydrocarbons that will increase the reactivity of the soot in the absence of NO2.

Photocatalytic degradation of 2-mercaptobenzothiazole in aqueous La3+–TiO2 suspension for odor control

Abstract: The La3+–TiO2 photocatalysts were prepared by doping lanthanum ion into TiO2 structure in a sol–gel process. The catalyst samples were then characterized by XRD and XPS analyses. The analytical results demonstrated that the lanthanum doping could inhibit the phase transformation of TiO2 and enhance the thermal stability of TiO2 structure. In addition, it was found that the lanthanum doping could reduce the crystallite size and increase the Ti3+ content on the surface of the catalysts with the increase of lanthanum doping. With a purpose of odor control, 2-mercaptobenzothiazole (MBT) was used in this study as a model chemical and both the adsorption isotherm and photocatalytic activity of the La3+–TiO2 catalysts were evaluated based on the MBT photodegradation in aqueous solution. The experimental results showed that both the adsorption capacity and adsorption equilibrium constants of the La3+–TiO2 catalysts increased with an increase of lanthanum doping. The kinetics of the MBT photodegradation using different La3+–TiO2 catalysts was also studied. The experiments demonstrated that an optimum doping of lanthanum ion at 1.2% achieved the highest MBT photodegradation rate. It was concluded that the enhancement of MBT photodegradation using the La3+–TiO2 catalysts mainly involved in both the improvement of the organic substrate adsorption in catalysts suspension and the enhancement of the separation of electron-hole pairs owing to the presence of Ti3+.

Photocatalytic degradation of the alimentary azo dye amaranth: Mineralization of the azo group to nitrogen

Abstract: The photocatalytic degradation of amaranth, an alimentary dye, was investigated in an irradiated titanium dioxide aqueous suspension. FT-IR spectroscopy of amaranth adsorbed on TiO 2 powder provided an insight to the mode of its adsorption on TiO 2 via the sulfonate group located in the ortho -position with respect to the OH group. The photodisappearance of amaranth was followed by monitoring the degradation of the dye either by UV-Vis spectroscopy or by HPLC/UV-Vis analysis while the total mineralization was monitored by DCO, TOC, GC/catharometer and HPLC ion-chromatography. A prompt removal of the colors was observed which was, however, slightly less rapid than the disappearance of amaranth itself; this indicates the presence of other colored molecules, such as hydroxylated or desulfonated amaranth as suggested by the initial changes in DCO, TOC and sulfate concentrations. In addition to the decolorization, TiO 2 /UV-based photocatalysis was simultaneously able to fully oxidize the dye, with a complete mineralization of organic carbon into CO 2 . Sulfur heteroatoms were converted into SO 4 2− ions, which remain partially adsorbed on TiO 2 , while the central –N=N– azo group was 100% selectively transformed into gaseous dinitrogen this represents. An ideal issue for the elimination of a nitrogen-containing pollutants. A TiO 2 -mediated photodegradation mechanism for the evolution of aliphatic organic acids during the amaranth photocatalytic degradation was proposed. The formation of lactic and formic acids at the beginning of irradiation-time is indicative of a the fast and easy naphtalenic ring breaking.

Photocatalytic degradation of atrazine using suspended and supported TiO2

Abstract: The degradation of 6-chloro- N 2 -ethyl- N 4 -isopropyl-1,3,5-triazine-2,4-diamine (atrazine) by heterogeneous photocatalysis using suspended and supported TiO 2 is reported. The influence of reaction parameters such as catalyst and atrazine concentrations, pH, light intensity and temperature has been investigated and the optimal conditions for the abatement of atrazine have been identified. The primary degradation and the total mineralization of atrazine have been monitored by high performance liquid chromatography (HPLC) and total organic carbon (TOC), respectively. Atrazine got destroyed in both suspended and supported systems, but its complete mineralization is not observed, probably due to the formation of cyanuric acid as a stable product. The activation energy for the degradation reaction of atrazine is determined as 2.6 kcal mol −1 . The efficiencies of photodegradation in both suspended and supported systems (the latter in a 150 ml flow reactor) are compared and the results indicate that the catalytic activity of TiO 2 is not affected when it is immobilized using a functionalized polymer. It is observed that the durability of the supported catalyst activity is reduced after six or seven runs, but it could be fully re-activated by a simple heat treatment at 480 °C for 1 h. The findings clearly suggest that immobilized photocatalysts could indeed be developed for photocatalysis, without compromising on the activity of the primary catalyst particles.

Catalytic ozonation of refractory organic model compounds in aqueous solution by aluminum oxide

Abstract: Batch experiments on catalytic ozonation in buffered and non-buffered solution of refractory organic compounds like oxalic, acetic, salicylic and succinic acids are presented. The concentration of dissolved organic carbon (DOC) in each test was 60 mg/l and the applied batch procedure allowed a clear distinction between adsorptive and reaction processes. Results have confirmed that Al 2 O 3 can be an effective catalyst for the conversion of dissolved organic compounds into CO 2 , but strength and performance of the catalysis are directly depending on the applied model compound and the matrix of solution. Catalytic effects were mostly pronounced in non-buffered solution and succinic acid exhibited the highest affinity for the reaction with an applied γ-Al 2 O 3 . Semi-continuous trails in buffered and non-buffered solution of succinic acid confirmed the catalytic activity of Al 2 O 3 for oxidation and, moreover, showed increasing catalytic effectiveness of the alumina with rising ozone dosage. The performance was comparable or superior to classical perozone treatment depending on the applied dosage of ozone. Different reaction mechanisms of the heterogeneous system are discussed and a suggestion of possibly involved reaction paths is presented. Higher oxidation performance might be explained by an interaction between ozone and OH surface groups of the alumina.

Catalytic oxidation of nitrogen monoxide over Pt/SiO2

Abstract: The catalytic oxidation of NO was studied on a catalyst consisting of platinum supported on SiO2. The kinetic behavior over Pt/SiO2 with a platinum loading of 2.5 wt.% was investigated in a feed containing 5% water and various concentrations of oxygen, nitrogen monoxide and nitrogen dioxide. The conversion of NO to NO2 increases when the oxygen concentration is increased from 0.1 to 10%, but levels off at higher concentrations. Increasing feed concentrations of NO lead to a decrease in the conversion to NO2. The formation of NO2 is also depressed by the addition of NO2 to the feed. Both observations suggest that the oxidation of NO on Pt/SiO2 is autoinhibited by the reaction product NO2. Further experiments have shown that the inhibition caused by NO2 is mostly persistent, i.e. a deactivation of the catalyst occurs. A pretreatment at 250 °C in a feed containing 500 ppm NO2 causes a very strong decrease in activity. However, the initial activity can be restored either by a thermal regeneration at 650 °C in air or by a regeneration under reducing conditions at 250 °C, e.g. in a feed containing NH3. This suggests that the deactivation by NO2 is due to the formation of a thin layer of platinum oxide covering the platinum surface at least partially.

Low temperature oxidation of carbon monoxide: the influence of water and oxygen on the reactivity of a Co3O4 powder surface

Abstract: In this paper, the effect of the presence of humidity and molecular oxygen on the low temperature oxidation of carbon monoxide on a Co3O4 powder surface was studied. The interaction between the probe molecules and the Co3O4 surface was investigated by means of diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and quadrupolar mass spectrometry (QMS). Carbon monoxide interacts with the Co3O4 surface and is converted to carbon dioxide at rather low temperature (T≥323 K); the formation of carbonate species is also evident. The reactivity of Co3O4 is strongly depressed by humidity: as a matter of fact, in steam conditions carbon monoxide oxidation is significant at temperatures higher than 523 K. Moreover, the formation of carbonate species is more evident than in dry conditions. The Co3O4 powder surface interacts with oxygen molecules at T≥323 K to originate several activated surface oxygen species. The activation with oxygen does not appear to facilitate significantly the formation of carbon dioxide.

Photocatalytic properties of titania powders prepared by hydrothermal method

Abstract: Nanocrystalline titania with particle size ∼20–50 nm and specific surface area ∼20–80 m 2 g −1 is prepared by hydrothermal treatment of aqueous TiOSO 4 , H 2 TiO(C 2 O 4 ) 2 , and TiO(NO 3 ) 2 solutions. The photocatalytic behavior of synthesized TiO 2 powders is studied in the reaction of phenol photodegradation in water and optimal characteristics of these materials are explored. The best photocatalytic activity was observed for a mixture of rutile (15%) and anatase (85%) prepared by high-temperature hydrolysis of aqueous TiOSO 4 solution.

CeO2 catalysed soot oxidation: The role of active oxygen to accelerate the oxidation conversion

Abstract: The influence of CeO2 in the acceleration of NOx-assisted soot oxidation has been studied in flow-reactor equipment by comparing two catalyst configurations, namely: (1) Pt upstream of soot and (2) Pt upstream of CeO2-soot. The role of CeO2 has been elucidated by means of DRIFT spectroscopy coupled with mass spectrometry and TAP reactor experiments. It was found that CeO2 has the potential to accelerate the oxidation rate of soot due to its active oxygen storage. The formation of active oxygen is initiated by NO2 in the gas phase. A synergetic effect is observed as a result of surface nitrate decomposition, which results in gas phase NO2 and desorption of active oxygen. Stored oxygen is postulated to exist in the form of surface peroxide or super oxide. Active oxygen is likely to play a role on the acceleration of soot oxidation and to contribute more than desorbed NO2 or NO2 from surface nitrate decomposition.

Nano Ru/CNTs: a highly active and stable catalyst for the generation of COx-free hydrogen in ammonia decomposition

Abstract: Very small Ru nanoparticles on carbon nanotubes (CNTs) were prepared by means of impregnation and H2-reduction. The CNTs-supported Ru nanoparticles are highly active and stable in catalyzing the decomposition of ammonia for the generation of COx-free hydrogen. The modification of Ru/CNTs with potassium ions leads to a remarkable improvement in the activity. The excellent catalytic performances of Ru/CNTs and K–Ru/CNTs are related to the high dispersion of Ru, and to the high graphitization and purity of CNTs.

Photodegradation of formaldehyde by photocatalyst TiO2 : effects on the presences of NO, SO2 and VOCs

Abstract: Formaldehyde is a common indoor air pollutant and its removal by photocatalysis is investigated. Similar to other pollutants, however, formaldehyde is not the only pollutant which exists in the atmosphere but no study has yet reported the effects of the presence of other pollutants on the photodegradation of formaldehyde. In this study, common indoor air pollutants, namely nitrogen monoxide (NO), sulfur dioxide (SO 2 ) and volatile organic compounds (VOCs) were selected to concurrently photodegradate with formaldehyde to identify their effects on formaldehyde. Formic acid was identified as the intermediate from the photodegradation of formaldehyde. The conversion of formaldehyde and the yield of formic acid decreased with increasing humidity levels. The presence of NO promoted the conversion of formaldehyde. The yield of formic acid was also higher under the presence of NO. Sulfate ion formed from the presence of SO 2 inhibited the conversion of formaldehyde and the yield of formic acid. Similar inhibition effect is observed from the concurrent photodegradation of VOCs and formaldehyde. Intermediates generated from the photodegradation of VOCs inhibited the conversion of formaldehyde.

Photocatalytic property of perovskite bismuth titanate

Abstract: Bismuth titanate Bi 4 Ti 3 O 12 was prepared by using the chemical solution decomposition (CSD) method. The obtained powders were characterized by X-ray diffraction (XRD), UV-Vis spectroscopy. Photocatalytic property of Bi 4 Ti 3 O 12 crystals was evaluated by using methyl orange as a model organic compound. The factors affecting the photocatalytic activity, such as calcination condition for the photocatalyst, the pH value of the reaction suspension, the strong oxidant species additives H 2 O 2 and the addition of NaCl, NaNO 3 and K 2 SO 4 into the suspension, have been studied. The band gap energy value for Bi 4 Ti 3 O 12 compound is 3.08 eV observed by a UV-Vis recording spectrophotometer. Bi 4 Ti 3 O 12 exhibits photocatalytic activities to photodegrade methyl orange. The bond angle of TiOTi in Bi 4 Ti 3 O 12 and the formed intra-electric field between the (Bi 2 O 2 ) 2+ layer and the (Bi 2 Ti 3 O 10 ) 2− layer are assumed to motivate the separation of the photogenerated electrons–holes pairs and then improve the photocatalytic activity of the catalysts.

Metolachlor photocatalytic degradation using TiO2 photocatalysts

Abstract: The photocatalytic degradation of the herbicide metolachlor is studied, using TiO 2 photocatalysts. Apart from the titania aqueous dispersions, immobilized semiconductor thin films were used in order to evaluate their effectiveness. The films exhibit a rough, nanocrystalline configuration, ideal for heterogeneous processes. Additionally the thin films were silver modified to determine if noble metal nanoclusters affect the photocatalytic efficiency. The kinetics obtained demonstrated that powder titania ( t 1/2 =21 min) was more efficient for the photocatalytic decomposition of metolachlor compared to pure titania thin film catalysts ( t 1/2 =52 min), while the presence of silver ions in TiO 2 particles modification did not prove to promote the solar photocatalytic process. Intermediate products from the slurry system were isolated and identified by means of solid-phase extraction (SPE) coupled to gas chromatography–mass spectroscopy techniques (GC–MS). A proposed degradation pathway of metolachlor is presented, involving mainly hydroxylation and dealkylation reactions. In addition, toxicity studies based on the luminescence of the bacteria Vibrio fischeri before and after the photocatalytic treatment were performed. The results demonstrated that the transient intermediates formed were more toxic compared to parent compound. At low photocatalyst loads, the toxicity of the irradiated solution remains relatively high even though 88% of metolachlor is decomposed.

Hydrogen production by ethanol steam reforming over a commercial Pd/γ-Al2O3 catalyst

Abstract: In the present work the reaction of biomass-derived ethanol steam reforming for hydrogen rich gas streams production, over a commercial alumina supported palladium catalyst was investigated. In particular, the dependence of the catalytic activity and selectivity on reaction temperature, H2O/EtOH molar ratio and contact time was studied. In order to evaluate the catalytic stability long-term experiments were also performed. It was found that hydrogen selectivity was proportional to the H 2O/EtOH molar ratio and ethanol was completely converted even at relatively low temperature values. Hydrogen selectivities up to 95% were obtained at temperature values close to 650 ◦ C. It was also observed that for the examined H2O/EtOH molar ratios, carbon monoxide concentration exhibits for thermodynamic reasons a minimum at a temperature value close to 450 ◦ C. Furthermore, carbon formation was found to be negligible even for H2O/EtOH molar ratio equal to the stoichiometric one. On the contrary, as water to ethanol ratio in the feed stream was decreased below the stoichiometric, carbon rate formation was increased resulting in catalyst deactivation. © 2003 Elsevier B.V. All rights reserved.

Single and combined deactivating effect of alkali metals and HCl on commercial SCR catalysts

Abstract: Two commercial SCR catalysts, with a nominal W content of about 9 wt.% and a V nominal content of 0.55 and 1.8 wt.%, respectively, were contacted with different amounts of Na and K and with HCl vapours in order to simulate poisoning by species more specifically contained in exhaust gases from MSW combustion. Catalysts were characterised using XRD analysis, SEM/EDX analysis, BET and pore size distribution measurements, NH 3 TPD, TG analysis. Poisoning agents do not cause loss of surface area nor pore occlusion. A significant loss of surface acidity was observed upon alkali metals poisoning whereas a decreasing of vanadium content was observed for the more concentrated catalysts upon HCl poisoning. Catalysts deactivation is proportional to the number of acid sites neutralised by alkali metals adsorbing ammonia in the temperature range typical of SCR process. HCl promotes the formation of new acid sites showing a lower activity compared to the original one.

Development of novel TiO2 sol–gel-derived composite and its photocatalytic activities for trichloroethylene oxidation

Abstract: A novel method for the processing of sol–gel-derived titanium dioxide composites has been developed and investigated for the purpose of producing thick films and self-supported photocatalysts. The processing of the composite sol–gel (CSG) photocatalysts, prepared as “coating” and self-supported “bulk” bodies involved utilizing precalcined commercial titania (Degussa P-25 and Aremco) as filler mixed with the sol and then, heat treatment at relatively low temperature (up to 700 °C). The resulting photocatalyst exhibited relatively high surface area and enhanced mechanical stability and integrity. Scratch adhesion tests performed on the CSG photocatalysts showed very good mechanical integrity and scratch resistance of about 5–10 N, far greater than the 0–0.2 N observed for P-25 and Aremco photocatalysts. Photocatalytic activities of the developed composites were evaluated through the oxidation of a known and standard air contaminant trichloroethylene (TCE), which has been widely investigated by many researchers in the field of photocatalysis. The CSG photocatalysts coatings provided TCE removals ranging up to 90% of that provided by P-25. However, the better mechanical integrity offered by the CSG photocatalysts make them viable alternatives for commercial applications that require long term stability and good mechanical properties.

Adsorptive removal of sulfur and nitrogen species from a straight run gas oil over activated carbons for its deep hydrodesulfurization

Abstract: As a pre-treatment step for ultra deep hydrodesulfurization (UD-HDS), sulfur and nitrogen species were adsorptively removed from the straight run gas oil (SRGO) over activated carbon materials. An activated carbon having the largest surface area and highest surface polarity showed the highest total capacity for adsorption of sulfur and nitrogen species as 0.098 g sulfur and 0.039 g nitrogen per 1 g of the activated carbon, respectively, among the carbon materials examined. Removal of both nitrogen and sulfur species was found very effective to achieve UD-HDS under conventional HDS conditions. SRGO treated over activated carbon contained only 11 ppm of sulfur, while non-treated SRGO contained still 193 ppm of sulfur in its HDS product.

Fe-exchanged Y zeolite as catalyst for wet peroxide oxidation of reactive azo dye Procion Marine H-EXL

Abstract: This paper evaluates the degradation of a reactive azo dye, Procion Marine H-EXL, by catalytic wet hydrogen peroxide oxidation (CWHPO). The catalyst was prepared by ion-exchange, starting from a commercially available ultrastable Y zeolite. All experiments were performed on a laboratory scale set-up. The effects of different reaction parameters such as initial pH, catalyst and hydrogen peroxide concentrations on the oxidation of the dye aqueous solution were assessed. Apart from the conventional parameters, the toxic potential of the dye’s degradation products was investigated using the bioluminescence test. HPICE analysis was also performed to obtain detailed information on the resulting oxidation products (organic and inorganic anions). The results indicate that after only 10 min at 50 °C, 20 mmol/l H2O2 and 1g/l FeY11.5 the color removal was as high as of 97% at pH=3 and 53% at pH=5. More than 96% removal of the dye could be attained in 30 min at pH=5, t=50 °C, 20 mmol/l H2O2 and 1 g/l FeY11.5 which corresponds to about 76% reduction of the initial COD and 37% removal of the initial TOC. A preliminary study of catalytic oxidation with hydrogen peroxide of the synthetic textile wastewater containing the specific dye is also presented. Leaching tests indicate that the activity of the catalyst is not due to leached iron ions, although an amount of 0.1–4.0 ppm of iron ions was found in aqueous solution. The catalyst allows almost total elimination of the dye and a significant removal of COD and TOC without the significant leaching of Fe ions. It was also observed that by using this catalyst, it is possible to extend the range of pH values for which Fenton-type oxidation can occur and no iron hydroxide sludge is formed.

Preparation, characterization and investigation of catalytic activity for NO+CO reaction of LaMnO3 and LaFeO3 perovskites prepared via microemulsion method

Abstract: Two typical perovskites LaMnO3 and LaFeO3 have been successfully prepared via microemulsion method both in the reverse and bicontinuous state. Pure perovskite crystal phases are achieved at final calcinations temperature of 800 °C. The specific surface areas (Sp) of the obtained materials were 24 and 12 m2 g−1 for the LaMnO3 prepared via reverse and bicontinuous phase and while the corresponding Sp for LaFeO3 were 30 and 14 m2 g−1. SEM images have shown that perovskites developed via reverse micelles have smaller particle size than those developed via the bicontinuous phase. The perovskites were tested for NO+CO reaction and exhibited higher catalytic activity than similar materials prepared via the ceramic method. The reactants NO and CO react in a 2:1 ratio at low temperature but they convert in a 1:1 ratio at high temperatures. From the kinetic analysis the heat of NO adsorption is estimated. In the case of manganese-containing materials except for the Sp, the percentage of Mn3+OMn4+ pairs seems to be another critical factor which influences the catalytic activity.