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Applied Catalysis B, Environmental (v.58, #3-4)
Atrazine removal by catalytic oxidation processes with or without UV irradiation by K.H. Chan; W. Chu (pp. 157-163).
In this study, the photo-catalytic oxidation process (UVCOP) for the removal of atrazine (ATZ) was successfully explored and the design charts for the optimization of the process were developed. The UVCOP was analyzed, mainly by using kinetic information to assist with the reaction mechanism and intermediates that were identified in this study. It was found that UVCOP greatly improves the removal of ATZ compared to sole-UV and dark-COP. In addition, a ‘compound effect’ is observed in the UVCOP, which supersedes the sum of the sole-UV and dark-COP. This compound effect was therefore investigated through the examination of the kinetic data of all of the three processes, and quantified in terms of the UV intensity and dosages of the reagent used. In addition, the enhancement index was introduced and verified. This is useful for verifying the effectiveness of the compound effect and the design of the UVCOP.
Keywords: Atrazine; Catalyzed; Kinetic; Quantification; UV
Atrazine removal by catalytic oxidation processes with or without UV irradiation by K.H. Chan; W. Chu (pp. 165-174).
The transformation mechanisms of atrazine (ATZ) degradation by sole-UV, dark-catalytic oxidation process (dark-COP), and UV-assisted catalytic oxidation process (UVCOP) were examined and compared. Depending on the selection of the processes, one to fourteen ATZ derivatives were detected by liquid chromatography electrospray tandem mass spectrometry (LC/ESI–MS/MS). For sole-UV, a single dechlorinated product (OIET) was identified due to the homolytic or heterolytic cleavage of ATZ. On the other hand, in dark-COP, seven chlorinated (CDET, CMIT, CDIT, CEAT, CIAT, CDAT, and CAAT) and three dechlorinated (OIET, ODIT, and OAAT) intermediates were identified. However, the dechlorinated derivatives, in dark-COP, appeared in low concentrations. In UVCOP, four more dechlorinated derivatives (ODET, OEAT, OIAT, and ODAT) were found. It was suggested that alkylic-oxidation, dealkylation, and dechlorination–hydroxylation (minor in dark-COP) are the leading pathways in dark-COP and UVCOP. The final products of sole-UV, dark-COP, and UVCOP were found to be OIET, CAAT, and OAAT, respectively. In addition, the ratio of dechlorinated versus chlorinated derivatives as well as OAAT to CAAT were used to quantify the detoxification effect of dark-COP and UVCOP, where the UVCOP was suggested as a more environmentally friendly process.
Keywords: Atrazine; Oxidation; Pathway; Toxicity; UV
Supported base metal catalysts for the preferential oxidation of carbon monoxide in the presence of excess hydrogen (PROX) by Fernando Mariño; Claude Descorme; Daniel Duprez (pp. 175-183).
Supported base metal catalysts were tested for the preferential oxidation of CO (CO PROX). The catalysts we investigated covered a wide range of transition metals (Co, Cr, Cu, Ni, Zn) supported on oxides with very different acidic, basic and redox properties (MgO, La2O3, SiO2–Al2O3, CeO2, Ce0.63Zr0.37O2). The influence of the metal loading (Cu), the support properties (acidity, basicity, redox, surface area) and the reaction conditions (reaction temperature, feed composition) on the catalyst activity and selectivity was evaluated. The activity of ceria and ceria–zirconia supported copper catalysts was comparable to the performances of noble metal samples classically used for the PROX reaction. In addition, Cu–CeO2 catalysts showed a practically constant and high selectivity towards CO oxidation in the temperature range of 50–150°C. Due to the strong synergetic effect between copper and ceria, only a small amount of copper (0.3wt.%) was necessary to get an active catalyst. The best catalytic performances were obtained for the samples containing 1–3wt.% copper. The presence of small copper particles in close interaction with the ceria support was shown to be responsible for the enhanced activity. Except for the hydrogen oxidation, no parallel reactions (CO or CO2 methanation reactions, coking, RWGS) could be detected over these catalysts. Classically, an increase of the oxygen excess led to an increased CO conversion with a simultaneous loss of selectivity towards CO2. Finally, the presence of CO2 in the feed negatively affected the catalytic activity. This effect was attributed to the adsorption of CO2 on the copper sites, probably as CO.
Keywords: Preferential oxidation of CO; PROX; Base metals; Copper; Ceria; Supported catalysts; Redox; Oxygen mobility; Hydrogen purification; Fuel cells
Photoactivity of anatase–rutile TiO2 nanocrystalline mixtures obtained by heat treatment of homogeneously precipitated anatase by Snejana Bakardjieva; Jan Šubrt; Václav Štengl; Maria Jesus Dianez; Maria Jesus Sayagues (pp. 193-202).
Nanosized titanium dioxide photocatalysts with varying amount of anatase and rutile phases have been synthesized. Homogeneous precipitation of aqueous solutions containing TiOSO4 with urea was used to prepare porous spherical clusters of anatase TiO2. Photoactive titania powders with variable amount of anatase and rutile phases were prepared by heating of pure anatase in the temperatutre range 800–1150°C. The structure evolution during heating of the starting anatase powders was studied by XRD analysis in overall temperature range of phase transformation. The morphology and microstucture characteristics were also obtained by HRTEM, BET and BJH. The spherical particle morphology of TiO2 mixtures determined by SEM was stable in air up to 900°C. The photocatalytic activity of the sample titania TIT85/825 heated to 825°C in air, contained 77.4% anatase and 22.6% rutile was higher than that nanocrystalline anatase powder. Titania sample TIT85/825 reveals the highest catalytic activity during the photocatalyzed degradation of 4-chlorophenol in aqueous suspension.
Keywords: Microstructure; Anatase; Rutile; Nanocrystals; Photoactivity; Photodegradation; Kinetics
Obtaining CeO2–ZrO2 mixed oxides by coprecipitation: role of preparation conditions by Sonia Letichevsky; Claudio A. Tellez; Roberto R. de Avillez; Maria Isabel P. da Silva; Marco A. Fraga; Lucia G. Appel (pp. 203-210).
The preparation of CeO2–ZrO2 mixed oxides preparation was studied by evaluating the influence of several conditions. Coprecipitation was taken as the standard method and the effects brought about by the cerium salt precursor ((NH4)2Ce(NO3)6 or Ce(NO3)3), the introduction of drying and aging steps as well as pH controlling upon precipitation were analyzed. The samples were characterized by X-ray diffraction, Raman spectroscopy, temperature-programmed reduction, infrared spectroscopy, oxygen storage capacity and surface area. The use of Ce(NO3)3 leads to the formation of c-CeO2 and t-ZrO2 mixed oxide whereas a solid solution is achieved by using (NH4)2Ce(NO3)6. It was observed that the cerium precursor is the most significant parameter of preparation procedure since it defines the crystalline phases and consequently the reducibility behavior of the CeO2–ZrO2 system.
Keywords: Cerium; Zirconium; Reducibility; Oxygen storage; Solid solution.
Photocatalytic degradation kinetics of di- and tri-substituted phenolic compounds in aqueous solution by TiO2/UV by Erdal Kusvuran; Ali Samil; Osman Malik Atanur; Oktay Erbatur (pp. 211-216).
In this study, photocatalytic degradation of 2,4,6-trimethylphenol (TMP), 2,4,6-trichlorophenol (TCP), 2,4,6-tribromophenol (TBP), 2,4-dimethylphenol (DMP), 2,4-dichlorophenol (DCP) and 2,4-dibromophenol (DBP) has been studied by TiO2/UV. Although degraded phenolic compound concentration increased by increasing initial concentration photocatalytic decomposition rates of di- and tri-substituted phenols at 0.1–0.5mM initial concentrations decreased when the initial concentration increased. The fastest degradation observed for TCP and the slowest for TMP. Photodegradation kinetics of the compounds has been explained in terms of Langmuir–Hinshelwood kinetics model. Degradation rate constants have been observed to be extremely depended on electronegativity of the substituents on phenolic ring. Degradation rate constant and adsorption equilibrium constant of TCP were calculated as k 0.0083mMmin−1 and K 9.03mM−1. For TBP and TMP the values of k and K were obtained as 0.0040mMmin−1, 19.20mM−1, and 0.0017mMmin−1, 51.68mM−1, respectively. Degradation rate constant of DBP was similar as DCP (0.0029mMmin−1 for DBP and 0.0031mMmin−1 for DCP) whereas adsorption equilibrium constants differed (48.40mM−1 for DBP and 30.52mM−1 for DCP). K and k of DMP found as 83.68mM−1 and 0.0019mMmin−1, respectively. The adsorption equilibrium constants in the dark were ranged between 1.11 and 3.28mM−1 which are lower than those obtained in kinetics. Adsorption constants have inversely proportion with degradation rate constants for all phenolic compounds studied.
Keywords: Titanium dioxide; Photocatalytic degradation; Substituted phenols; Kinetics
Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds by U. Roland; F. Holzer; F.-D. Kopinke (pp. 217-226).
The role of ozone was studied for two different configurations combining non-thermal plasma (NTP) and heterogeneous catalysis, namely the use of a gas phase plasma with subsequent exposure of the effluent to a catalyst in a packed-bed reactor (post-plasma treatment) and the placement of the catalyst directly in the discharge zone (in-plasma catalysis). Non-porous and porous alumina and silica were deployed as model catalysts. The oxidation of immobilised hydrocarbons, toluene as a volatile organic compound and CO as an inorganic pollutant were studied in both operational modes.While conversion and selectivity of hydrocarbon oxidation in the case of catalytic post-plasma treatment can be fully explained by the catalytic decomposition of O3 on γ-Al2O3, the conversion processes for in-plasma catalysis are more complex and significant oxidation was also measured for the other three materials (α-Al2O3, quartz and silica gel). It became obvious that additional synergetic effects can be utilised in the case of in-plasma catalysis due to short-lived species formed in the NTP.The capability of porous alumina for ozone decomposition was found to be correlated with its activity for oxidation of carbon-containing agents. It could be clearly shown that the reaction product CO2 poisons the catalytic sites at the γ-Al2O3 surface. The catalytic activity for O3 decomposition can be partially re-established by NTP treatment. However, for practical purposes the additional reaction pathways provided by in-plasma catalytic processes are essential for satisfactory conversion and selectivity.
Keywords: Non-thermal plasma; Post-plasma treatment; γ-Al; 2; O; 3
Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds by U. Roland; F. Holzer; A. Pöppl; F.-D. Kopinke (pp. 227-234).
The effect of plasma processes inside the intra-particle volume of porous materials (especially Al2O3) was studied in order to evaluate the potential of the combination of non-thermal plasma (NTP) and in situ heterogeneous catalysis (plasma catalysis), for the improvement of efficiency and selectivity towards total oxidation of organic pollutants in gas cleaning applications. Electron paramagnetic resonance (EPR) spectroscopy was applied as an appropriate method to detect both the formation of radical species by the NTP as well as the initiation of structural changes to the catalyst.The presence of paramagnetic oxygen or hydroxyl species (O−, O2− or OH) could not be detected by EPR spectroscopy. The observed signal was not significantly influenced either by the type of atmosphere present during NTP treatment or by applying reducing agents to the sample after plasma treatment.However, by using non-porous and porous alumina (α- and γ-Al2O3) as model catalysts, the effect of NTP modifying the surface structure in the interior of a porous material could be clearly demonstrated. A paramagnetic species probably related to an AlOO aluminium peroxyl group was formed by NTP processes independently of the oxygen content of the gas atmosphere. It was not formed when the alumina sample was positioned in the off-gas flow of a plasma reactor, i.e. used in the post-treatment mode.The structure of the paramagnetic site was investigated by employing several spectroscopic tools ( X- and Q-band EPR, electron spin echo envelope modulation [ESEEM] and EPR measurements after pre-deuteration).
Keywords: Plasma; Electron paramagnetic resonance (EPR); Alumina
Selective oxidation of ammonia to nitrogen on transition metal containing mixed metal oxides by Lucjan Chmielarz; Piotr KuÅ›trowski; Alicja Rafalska-Å?asocha; Roman Dziembaj (pp. 235-244).
The selective catalytic oxidation of ammonia to nitrogen (NH3-SCO) has been studied over hydrotalcite derived mixed metal oxides containing Cu, Co, Fe or Ni. XRD, BET, NH3-TPD and TPR techniques were used for catalysts characterization. Results of NH3-SCO were compared with those of selective catalytic reduction of NO with NH3 (NO-SCR). Reaction mechanism was studied by temperature-programmed surface reaction (TPSR) and activity tests with a various contact time. Catalytic performance of the studied samples depends on both kind and loading of transition metals in the mixed metal oxide system. The Cu-containing samples have been found to be the most active catalysts of the NH3-SCO process. Transition metal loading strongly influences distribution of ammonia oxidation products. The highest selectivity to N2 was measured for the catalysts with the lowest transition metal content.
Keywords: Hydrotalcite derived mixed metal oxides; Transition metals; Selective oxidation of NH; 3
Quantification of the in situ DRIFT spectra of Pt/K/γ-A12O3 NO x adsorber catalysts by Todd J. Toops; D. Barton Smith; William P. Partridge (pp. 245-254).
A method to quantify DRIFT spectral features associated with the in situ adsorption of gases on a NO x adsorber catalyst, Pt/K/Al2O3, is described. To implement this method, the multicomponent catalyst is analysed with DRIFT and chemisorption to determine that under operating conditions the surface comprised a Pt phase, a pure γ-Al2O3 phase with associated hydroxyl groups at the surface, and an alkalized-Al2O3 phase where the surface –OH groups are replaced by –OK groups. Both DRIFTS and chemisorption experiments show that 93–97% of the potassium exists in this form. The phases have a fractional surface area of 1.1% for the 1.7nm-sized Pt, 34% for pure Al2O3 and 65% for the alkalized-Al2O3. NO2 and CO2 chemisorption at 250°C is implemented to determine the saturation uptake value, which is observed with DRIFTS at 250°C. Pt/Al2O3 adsorbs 0.087μmol CO2/m2and 2.0μmol NO2/m2, and Pt/K/Al2O3 adsorbs 2.0μmol CO2/m2and 6.4μmol NO2/m2. This method can be implemented to quantitatively monitor the formation of carboxylates and nitrates on Pt/K/Al2O3 during both lean and rich periods of the NO x adsorber catalyst cycle.
Keywords: NO; x; adsorber catalysts; Lean NO; x; traps; Quantified DRIFTS; Chemisorption; Potassium; Pt; γ-Al; 2; O; 3; Pt/K/Al; 2; O; 3
Quantified NO x adsorption on Pt/K/gamma-Al2O3 and the effects of CO2 and H2O by Todd J. Toops; D. Barton Smith; William S. Epling; Jim E. Parks; William P. Partridge (pp. 255-264).
A multi-component NO x-trap catalyst consisting of Pt and K supported on γ-Al2O3 was studied at 250°C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H2O and CO2 on NO x storage. The Al2O3 support was shown to have NO x trapping capability with and without Pt present (at 250°C Pt/Al2O3 adsorbs 2.3μmols NO x/m2). NO x is primarily trapped on Al2O3 in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2μmols NO x/m2, and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al2O3 catalyst are coordinated on the Al2O3 support at saturation.When 5% CO2 was included in a feed stream with 300ppm NO and 12% O2, the amount of K-based nitrate storage decreased by 45% after 1h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H2O was included in a feed stream with 300ppm NO and 12% O2, the amount of K-based nitrate storage decreased by only 16% after 1h, but the Al2O3-based nitrates decreased by 92%. Interestingly, with both 5% CO2 and 5% H2O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al2O3 destabilized the K–CO2 bond; specifically, H2O mitigates the NO x storage capacity losses associated with carboxylate competition.
Keywords: Lean NO; x; traps; DRIFTS; Chemisorption; Potassium; Pt; γ-Al; 2; O; 3
Photooxidation reactions of polycyclic aromatic hydrocarbons over pure and Ag-loaded BiVO4 photocatalysts by Shigeru Kohtani; Misa Tomohiro; Kunihiro Tokumura; Ryoichi Nakagaki (pp. 265-272).
Photooxidative degradations of nine polycyclic aromatic hydrocarbons (PAHs) using pure and Ag-loaded BiVO4 photocatalysts have been examined in acetonitrile under visible light irradiation. Photoproducts have been identified by means of gas chromatography–mass spectrometry (GC–MS). Silver fine particles loaded on BiVO4 surface improved the reaction rates of all PAHs degradations. In particular, anthracene and benz[ a]anthracene (Bz[ a]A) are efficiently oxidized to anthraquinone and benz[ a]anthracene-7,12-dione, respectively. Photooxidation mechanism of anthracene and Bz[ a]A has been clarified by the GC–MS analyses by the use of H218O as a reactant. It has been proved that OH radicals are generated through the oxidation of water by valence band holes on the Ag-BiVO4 surface in acetonitrile, and greatly contribute to the degradations of these PAHs. The OH radical attack to PAHs probably determines the overall rates of oxidation of PAHs on Ag-BiVO4 photocatalyst.
Keywords: BiVO; 4; Ag-loaded BiVO; 4; Silver loading; Photooxidation; Polycyclic aromatic hydrocarbons; Visible light; OH radicals; Reaction mechanism
Oxygen storage capacity of La1− xA′xBO3 perovskites (with A′=Sr, Ce; B=Co, Mn)—relation with catalytic activity in the CH4 oxidation reaction by S. Royer; H. Alamdari; D. Duprez; S. Kaliaguine (pp. 273-288).
The aim of this work was to study the effect of cation-substitution on the reducibility of the perovskite, as well as the effect on the catalytic activity for the CH4 oxidation reaction. Six perovskites (LaCoO3, LaMnO3, La1− xSr xMnO3 ( x=0.2, 0.4), and La1− xCe xMnO3 ( x=0.05, 0.1)) were synthesized by reactive grinding. The reducibility of the perovskite was studied by means of the oxygen storage capacity (OSC) measurement. OSC was performed at different temperatures on LaCoO3 and LaMnO3, in order to elucidate the different mechanisms of reduction involved at each temperature. The substituted samples showed that reduction profile is modified at high-substitution degrees; however, no differences were observed on the OSC values (amount of most active oxygen, calculated after one pulse of CO) between the pure lanthanum sample and the substituted ones.Tested in the CH4 oxidation reaction, the LaCoO3 sample was found to present a little higher activity than LaMnO3, even if the cobalt-based sample presented a smaller specific surface area. Moreover, all the substituted samples presented very slightly higher activities than the pure LaMnO3 solid. Because of the supposed redox oxidation mechanism (Mars-Van-Krevelen), this agrees well with the OSC results obtained for the reducibility of the manganese on these samples, by which it was observed that substitution does not clearly affect the immediate reduction of the manganese.
Keywords: Perovskite; OSC measurement; Catalytic activity in methane oxidation; Sr- and Ce-substitution effect
Catalytic oxidation of naphthalene using a Pt/Al2O3 catalyst by Je-Lueng Shie; Ching-Yuan Chang; Jia-Hao Chen; Wen-Tien Tsai; Yi-Hung Chen; Chyow-San Chiou; Chiung-Fen Chang (pp. 289-297).
Polycylic aromatic hydrocarbons (PAHs) are listed as carcinogenic and mutagenic priority pollutants, belonging to the environmental endocrine disrupters. Most PAHs in the environment stem from the atmospheric deposition and diesel emission. Consequently, the elimination of PAHs in the off-gases is one of the priority and emerging challenges. Catalytic oxidation has been widely used in the destruction of organic compounds due to its high efficiency (or conversion of reactants), its economic benefits and good applicability.This study investigates the application of the catalytic oxidation using Pt/γ-Al2O3 catalysts to decompose PAHs and taking naphthalene (the simplest and least toxic PAH) as a target compound. It studies the relationships between conversion, operating parameters and relevant factors such as treatment temperatures, catalyst sizes and space velocities. Also, a related reaction kinetic expression is proposed to provide a simplified expression of the relevant kinetic parameters.The results indicate that the Pt/γ-Al2O3 catalyst used accelerates the reaction rate of the decomposition of naphthalene and decreases the reaction temperature. A high conversion (over 95%) can be achieved at a moderate reaction temperature of 480K and space velocity below 35,000h−1. Non-catalytic (thermal) oxidation achieves the same conversion at a temperature beyond 1000K. The results also indicate that Rideal–Eley mechanism and Arrhenius equation can be reasonably applied to describe the data by using the pseudo-first-order reaction kinetic equation with activation energy of 149.97kJ/mol and frequency factor equal to 3.26×1017s−1.
Keywords: Polycylic aromatic hydrocarbons; Catalytic oxidation; Pt/γ-Al; 2; O; 3; catalyst