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Applied Catalysis B, Environmental (v.75, #1-2)
Palladium-catalyzed aqueous hydrodehalogenation in column reactors: Modeling of deactivation kinetics with sulfide and comparison of regenerants by Naoko Munakata; Martin Reinhard (pp. 1-10).
Palladium (Pd) based catalysts are increasingly important in environmental applications; however, sulfide, a known poison, has been identified as a potential issue in laboratory and field studies. This paper develops a quantitative model for deactivation kinetics with aqueous sulfide; investigates the effects of pH on a catalyzed dehalogenation reaction and sulfide deactivation; and characterizes regeneration with acids, bases, and oxidizing agents. Results obtained with trichloroethylene show no inherent catalyst deactivation in deionized water. Deactivation increased with sulfide concentration and exposure time. Deactivation was slowly reversible by flushing the catalyst with deionized water at pH 10.4. Treatment with 20mM sodium hypochlorite quickly and completely regenerated the catalyst, and was significantly more effective than hydroxide, hydrochloric acid, hydrogen peroxide, and air-saturated water. The time required for regeneration increased with increasing sulfide concentrations and exposure times. These results have important implications for maintaining catalyst activity with Pd or bimetallic catalyst systems.
Keywords: Palladium (Pd); Sulfide deactivation kinetics; Sulfur diffusion; Regeneration; Groundwater remediation; Trichloroethene (TCE); Zero valent iron; Bimetallic catalysts
Catalytic removal of NO x and soot from diesel exhaust: Oxidation behaviour of carbon materials used as model soot by N. Nejar; M. Makkee; M.J. Illán-Gómez (pp. 11-16).
For NO x/soot laboratory experiments, different carbon materials have been used as model soot. In this work, the behaviour of two carbon materials (a charcoal and a synthetic soot from Degussa) is compared with that of a real diesel soot. The performance of the three carbon materials in synthetic air and in an O2+NO gas mixture was compared, both in the presence and in the absence of the KCu2/Al2O3 catalyst. The properties of the carbon materials, including microstructure, elemental analysis, ash content and ash composition determine its reactivity, which is higher in NO x atmosphere than in synthetic air. The use of the KCu2/Al2O3 catalyst increases the NO x reduction, the soot consumption rate and the selectivity towards N2 and CO2. Charcoal A3 and Printex-U can be accepted as model substances representing upper and lower limits of diesel soot reactivity.
Keywords: Soot; NO; x; TEM; DRIFTS; K; Cu
Solar photoelectro-Fenton degradation of cresols using a flow reactor with a boron-doped diamond anode by Cristina Flox; Pere-Lluís Cabot; Francesc Centellas; José Antonio Garrido; Rosa María Rodríguez; Conchita Arias; Enric Brillas (pp. 17-28).
The solar photoelectro-Fenton degradation of 2.5l of acidic solutions containing o-cresol, m-cresol and p-cresol up to ca. 1gl−1, 0.05M Na2SO4 and Fe2+ as catalyst has been studied using a flow plant with a one-compartment filter-press electrolytic reactor with a boron-doped diamond (BDD) anode and an O2-diffusion cathode, both of 20-cm2 area, coupled to a solar photoreactor. In this environmentally friendly indirect electrooxidation method pollutants are mainly oxidized by hydroxyl radical formed at the anode surface from water oxidation and in the medium from Fenton's reaction between Fe2+ and cathodically electrogenerated H2O2, giving rise to complexes of Fe(III) with final carboxylic acids that are rapidly photodecomposed by UVA light supplied by solar irradiation. Electrolyses performed in batch using 1.0mM Fe2+ at pH 3.0 and 50mAcm−2 yield complete mineralization of all cresols up to ca. 0.5gl−1. The effects of current density, solution pH and concentrations of Fe2+ and cresols on the degradation rate, efficiency and energy costs of the solar photoelectro-Fenton process have been examined. The decay kinetics for all cresols follows a pseudo-first-order reaction. Initial hydroxylation of o-cresol and m-cresol gives 2-methyl- p-benzoquinone via 2-methylhydroquinone, whereas dihydroxylation of p-cresol leads to 5-methyl-2-hydroxy- p-benzoquinone. These aromatic intermediates are rapidly converted into a mixture of carboxylic acids, being oxalic and acetic acids the most persistent final products. Overall mineralization is attained by the efficient photodecarboxylation of Fe(III)-oxalate complexes. Solar photoelectro-Fenton with a BDD anode appears to be a viable method to remove cresols in wastewaters at industrial scale.
Keywords: Cresols; Boron-doped diamond anode; Photoelectro-Fenton; Solar light; Mineralization
Decomposition of gas-phase toluene by the combination of ozone and photocatalytic oxidation process (TiO2/UV, TiO2/UV/O3, and UV/O3) by Kuo-Pin Yu; Grace W.M. Lee (pp. 29-38).
The gas-phase toluene removal efficiencies by photocatalytic oxidation (TiO2/UV), the combination of ozone and photocatalytic oxidation (TiO2/UV/O3), and the UV/O3 reaction were tested using a quartz tube photoreactor. The experiments were conducted under various ozone concentrations (3.3–15ppm), toluene concentrations (1–9ppm), relative humidity (5–80%), and gas flow rates (200–1200mL/min). The toluene oxidation rates (TORs) of TiO2/UV/O3, and UV/O3 reactions were proportional to the ozone concentrations. The TORs of TiO2/UV, TiO2/UV/O3, and UV/O3 reactions increased with toluene concentration. However, there were negative correlations between the toluene removal efficiencies of these three kinds of reactions and the toluene concentrations. The order of the TORs and the CO2 yield rates of these three reactions were TiO2/UV/O3>TiO2/UV>UV/O3. The kinetics of TiO2/UV, and TiO2/UV/O3 reactions fit the Langmuir–Hinshelwood rate form. The rate constants ( k) and Langmuir adsorption constants ( K) are as follows: TiO2/UV: k=0.0102ppmm/s, K=0.146ppm−1; TiO2/UV/O3: k=0.0268ppmm/s, K=0.0796ppm−1. The reciprocal of UV/O3 reaction rate showed a positive linear relationship with the reciprocals of humidity and of toluene concentration. Ozone, also an air pollutant, was removed in the TiO2/UV/O3, and UV/O3 reactions. The ozone removal efficiency of TiO2/UV/O3 reaction in the presence and absence of toluene ranged from 61.1 to 99.5% and 38.1 to 95.1%, respectively.
Keywords: Photocatalyst; Ozone; Toluene; Advance oxidation process; Langmuir–Hinshelwood kinetics
Estimating the temperatures of the precious metal sites on a lean NO x trap during oxidation reactions by Joseph R. Theis; Erdogan Gulari (pp. 39-51).
The temperatures of the platinum sites on a 0.64cm long monolithic platinum/potassium/alumina lean NO x trap have been estimated during CO oxidation from the conversion of a trace amount of hydrocarbon (HC) and a HC conversion versus temperature calibration curve. The base exhaust contained 5% O2, 10% H2O, 10% CO2, and 200ppm of the HC in N2. For CO levels of 2 to 5%, CO injection times of 5 to 180s, and base temperatures of 300 to 500°C, the inferred platinum site temperatures were in good agreement with tracer gases of propane, butane, hexane, and methane (when used), even though the levels of HC conversion were significantly different. At a base temperature of 300°C with 2% CO, the steady-state precious metal temperature was approximately 130°C higher than the temperature of the exhaust gas exiting the catalyst. The difference in PGM temperature and gas temperature decreased with increasing base temperature and increasing CO level, suggesting that radiation is an important mechanism of heat transfer within a catalyst during exothermic reactions.
Keywords: Catalyst; Precious metal; Temperature; CO oxidation; Radiation
Photocatalytic properties of phosphor-doped titania nanoparticles by L. Lin; W. Lin; J.L. Xie; Y.X. Zhu; B.Y. Zhao; Y.C. Xie (pp. 52-58).
Phosphor-doped titania nanoparticles in an anatase phase were prepared by a simple modified sol–gel method with hypophosphorous acid as a precursor. The resulting materials were characterized by differential thermal analysis-thermogravimetry (DTA-TG), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), nitrogen physical adsorption at 77K, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), FT-IR pyridine adsorption spectroscopy, and UV–vis spectroscopy. It was found that the phosphor-doped species could significantly increase the surface area of the materials, and consequently gave them a higher content of surface hydroxyl groups. Moreover, the phosphor-doping improved the thermal stability of titania and decreased the phase transformation of anatase to rutile to a certain extent. UV–vis spectra proved that the modification by phosphor shifted the absorption edge of titania to the visible region, making it an effective photocatalyst in visible light. This was demonstrated by the degradation of MB and 4CP under visible-light (>400nm) irradiation. The excellent photocatalytic activity of phosphor-doped titania compared with pure titania could be explained by its high surface area and small crystallite size.
Keywords: Phosphor-doped titania; Photocatalysis; Methylene blue; 4-Chlorophenol
Structured bimetallic Pd-Pt/γ-Al2O3 catalysts on FeCrAlloy fibers for total combustion of methane by Andrea Maione; Floriane André; Patricio Ruiz (pp. 59-70).
Bimetallic Pd-Pt/Al2O3 catalysts were deposited by dip coating on FeCrAlloy-type fibers. The influence of different gaseous pretreatments (under O2/H2 or H2 flows) on the physico-chemical properties and the catalytic activity in the total combustion of methane were investigated. Dispersion of Pd and Pt appeared to be a crucial parameter for the catalytic performances. All directly calcined catalysts displayed better catalytic behaviour than the pretreated ones. Long duration tests demonstrated that a bimetallic Pd-Pt catalyst has a better catalytic behaviour than a Pd-based one, which undergoes through sintering as evidenced by the formation of larger agglomerates. After the long duration test, palladium is in a fully oxidized state (PdO) (as well as platinum (PtO)). It is suggested that the better catalytic behaviour of the Pt-Pd catalyst could be due to an active cooperation between PtO, formed during the early stages of the catalytic reaction, and PdO. Two hypotheses can be put forward; in the first case, PtO could supply oxygen atoms to PdO, which actually would play the active role in the catalytic reaction. In the second case, CH4 would be adsorbed and dissociated on Pt atoms, and PdO would supply oxygen atoms leading to the formation of CO2.
Keywords: Methane combustion; Palladium; Platinum; FeCrAlloy fibers; Long duration tests
Treatment of olive oil mill wastewater by catalytic wet air oxidation by Doan Pham Minh; Pierre Gallezot; Michèle Besson (pp. 71-77).
Catalytic wet air oxidation of an aqueous solution of p-hydroxybenzoic acid chosen as a representative phenolic pollutant of olive mill wastewater was conducted over ruthenium catalysts supported on TiO2 or ZrO2 in a stirred batch autoclave and in a fixed bed reactor at 140°C and 50bar air. The objective was to examine the stability of these catalysts by conducting catalyst recycling experiments in batch reactor and long-term stability tests in trickle-bed reactor. A small deactivation was observed during the first recycling of ruthenium catalyst in batch experiments, which was totally reversible after catalyst reduction. A small deactivation was also observed at the start of the continuous runs. This moderate deactivation was attributed to an over-oxidation of the surface or subsurface of ruthenium particles. Then the catalysts proved to be very stable in ca. 50-day continuous runs. Noticeably, no leaching from the catalysts was observed at all under these operating conditions. Catalysts prepared on ZrO2 from ruthenium nitrosyl nitrate showed the highest performances.
Keywords: Wet air oxidation; p; -Hydroxybenzoic acid; Ruthenium catalysts; Stability; Batch reactor; Trickle-bed reactor
NO x storage and reduction over Pt based catalysts with hydrogen as the reducing agent by Anna Lindholm; Neal W. Currier; Erik Fridell; Aleksey Yezerets; Louise Olsson (pp. 78-87).
The NO x storage and reduction performance have been investigated by means of transient flow reactor experiments for three different types of model catalysts: Pt/Ba/Al, Pt/Al and Pt/Si. The objectives of the work were to study the behavior of the catalysts when using hydrogen as a reducing agent and to investigate the influence of H2O and CO2. It was found that the Pt/Ba/Al catalyst possesses the highest storage capacity of the examined catalysts. However, the Pt/Al catalyst stores a large amount of NO x and the optimum storage temperature occurs at 200°C. The performed experiments show that the storage capacity of the alumina based catalysts is higher in the absence of H2O and CO2. Carbon dioxide has a greater effect than H2O on the storage on barium and water has a greater effect than CO2 on the storage on alumina. The results show that N2O and NH3 can be formed when reducing NO x with H2 and the NO x reduction, NH3 formation and N2O formation depend on the H2 concentration. A delay in the ammonia signal was observed and this was explained by an SCR reaction between nitrites/nitrates adsorbed on the storage components and ammonia. Furthermore, CO2 was found to have a promoting effect on the NH3 formation at lower temperatures.
Keywords: NO; x; storage; NO; x; reduction; Barium; Platinum; NSR; LNT; H; 2; O; CO; 2; NH; 3; formation; N; 2; O formation
Catalysts based on tin and beta zeolite for the reduction of NO x under lean conditions in the presence of water by A.E. Palomares; J.G. Prato; F.E. Imbert; A. Corma (pp. 88-94).
The catalytic performance of catalysts based on beta zeolite and tin for the NO reduction under lean burn conditions is discussed. The tin has been incorporated in the catalyst by different ways: in the zeolite framework, ion exchanged and in a physical mixture with the zeolite. The best results are obtained with a physical mixture of tin dioxide and the acid zeolite. The activity of this system has been improved by adding a second metal such as manganese or copper. At high oxygen concentration and using a catalyst formed by a physical mixture of Mn2O3, SnO2 and H-BEA, a 60% NO conversion, in presence of 14% of oxygen, has been obtained. These results suggest that the physical mixtures of metals with zeolites could be an interesting and inexpensive way to prepare active catalysts for this reaction. The role of the zeolite and the metal centers is discussed along the text.
Keywords: NO; x; removal; SnO; 2; Lean burn; Acid sites; Physical mixture
Electrocatalytic materials for the electrochemical oxidation of synthetic dyes by Marco Panizza; Giacomo Cerisola (pp. 95-101).
The electrocatalytic properties of Ti–Ru–Sn ternary oxide, platinum, lead dioxide and boron-doped diamond anodes for the oxidation of methyl red have been compared by potentiodynamic measurements and bulk electrolysis. The results of the cyclic voltammetries have shown that in the potential region of supporting electrolyte stability polymeric materials, which result in electrode deactivation, are formed on the electrode surfaces. While Ti–Ru–Sn ternary oxide and platinum cannot restore their initial activity by polarization, lead dioxide and boron-doped diamond anodes can be reactivated by electrolysis in the potential region of electrolyte decomposition due to the electrogeneration of hydroxyl radicals.The bulk electrolysis showed that the complete COD and colour removal were only achieved using lead dioxide and boron-doped diamond while Ti–Ru–Sn ternary oxide and platinum only permitted a partial oxidation of methyl red.
Keywords: Electrocatalysis; Methyl red; Oxidation; Cyclic voltammetry; Bulk electrolysis
Beneficial and problematic interactions between NO x trapping materials and carbonaceous particulate matter by James A. Sullivan; Orla Keane; Andrew Cassidy (pp. 102-106).
The presence of a NO x trapping BaO component with a Pt active phase does not per-se promote carbon combustion. However when coupled with trap regeneration, promotion of soot combustion is seen. Operating NO x trapping materials promote combustion due to the periodic localised increases in [NO2](g) generated during the regeneration rather than due to a co-generated exotherm. However, it has been noted that the presence of soot prevents NO x traps from employing their full trapping capacity.
Keywords: Particulate combustion; NO; x; trap promotion; Transient studies
The influence of silver on the structural, redox and catalytic properties of the cryptomelane-type manganese oxides in the low-temperature CO oxidation reaction by Wojciech Gac (pp. 107-117).
Cryptomelane-type manganese oxides modified with silver were prepared by the redox precipitation technique. The catalysts were obtained by the direct introduction of silver to the synthesis mixture containing manganese and the impregnation technique. The catalysts showed high activity in the low-temperature CO oxidation reaction. Studies of the nitrogen adsorption/desorption, powder X-ray diffraction, Fourier transform infrared spectroscopy in the photoacoustic mode (FT-IR/PAS), and atomic force microscopy (AFM) evidenced strong influence of the preparation methods and the pre-treatment conditions on the structural and catalytic properties of the samples. It was found that the presence of silver in the synthesis mixture caused partial replacement of potassium ions in the tunnels, leading to the partial decomposition of the well ordered structure of the manganese oxides, increasing reducibility and activity of the catalyst. Structural changes were not observed in the samples prepared by the impregnation technique. Slight increase of activity of the impregnated samples was related to the increase of oxygen mobility.
Keywords: Manganese oxide; Cryptomelane; Silver; TPR; FT-IR/PAS; AFM low-temperature CO oxidation
New method of improving photocatalytic activity of commercial Degussa P25 for azo dyes decomposition by M. Janus; A.W. Morawski (pp. 118-123).
New method of improving photocatalytic activity of commercial TiO2 (Degussa P25, Germany) is presented. Modified P25 has better photocatalytic activity and “practical efficiency” than unmodified TiO2 Degussa P25. The modification of P25 was conducted under elevated pressure in organic solvent atmosphere. It leads to built in carbon into TiO2 structure without surface hydroxyl groups reduction. The photocatalytic activity of modified P25 was tested during three azo dyes (reactive red, reactive black, direct green – Boruta-Kolor Company, Poland) decomposition under UV light irradiation. The photocatalytic activity of modified P25 was two times higher than for unmodified P25. Moreover after the process the new photocatalyst had better sedimentation and filtration was easier.
Keywords: Carbon-modified TiO; 2; Photocatalysis; Dye decomposition; TiO; 2; –P25
On the promoting effect by quaternary ammonium salts in the multiphase hydrodechlorination with hydrogen gas on Raney nickel catalyst by Sergei Zinovyev; Pietro Tundo (pp. 124-128).
Hydrodechlorination of 1,3,5-trichlorobenzene with hydrogen gas on Raney nickel catalyst in the biphasic system composed of isooctane, aqueous KOH, and a quaternary ammonium compound as a promoter at 70°C and atmospheric pressure was studied. The promoter increased the rate of reaction above two orders of magnitude. Several quaternary chloride salts were studied, such as Aliquat 336, trioctylmethylammonium, tridodecylmethylammonium, cetyltrimethylammonium, tetrabutylammonium and tetraethylammonium. The relationship of the reaction rate with adsorption, micelle formation, and the structural properties of the promoter are discussed.
Keywords: Hydrodechlorination; Quaternary ammonium; Raney nickel; Multiphase; Promoter
Synthesis and characterization of metal nanoparticles-decorated PPY–CNT composite and their electrocatalytic oxidation of formic acid and formaldehyde for fuel cell applications by V. Selvaraj; M. Alagar; K. Sathish Kumar (pp. 129-138).
Polymer composite films encompassed of polypyrrole (PPY) and multi-walled carbon nanotubes (MWCNT) [PPY–CNT] were synthesized by in situ polymerization of pyrrole on carbon nanotubes in 0.1M HCl containing (NH4)S2O8 as oxidizing agent at 0–5°C. 2–4nm sized metal nanoparticles, viz. Pt and Pt–Pd were then deposited on the prepared polypyrrole/CNT composites by chemical reduction of the corresponding metal salts using HCHO as reducing agent at pH 11. The fabricated nanoparticles-decorated composite films were then used as a probe towards formic acid and formaldehyde oxidation. Furthermore, the addition of CNT to polypyrrole imparts a high activity, which might be due to the higher electrochemically accessible surface areas, electronic conductivity and easier charge-transfer at polymer/carbon nanotubes interfaces allowing a higher dispersion and utilization of deposited Pt and Pt–Pd nanoparticles. Results from formic acid and formaldehyde oxidation showed that Pt–Pd nanoparticles-modified Ppy–CNT exhibits better catalytic activity and stability in contrast to Pt supported on Ppy–CNT. Pt and Pt–Pd nanoparticles are found homogeneously dispersed in Ppy–MWCNT composites ascertained from various analytical data like scanning electron microscopy (SEM), transmission electron microscopy (TEM). Energy dispersive X-ray analysis (EDAX) is used to characterize the nature of metal present in the nanoparticles-modified electrodes.
Keywords: Polypyrrole; Pt nanoparticles; Pt–Pd nanoparticles; Multi-walled carbon nanotubes; Formic acid and formaldehyde oxidation; Fuel cells
Catalytic combustion of benzofuran and of a benzofuran/1,2-dichlorobenzene binary mixture over zeolite catalysts by M. Taralunga; B. Innocent; J. Mijoin; P. Magnoux (pp. 139-146).
The catalytic combustion of benzofuran and of benzofuran/1,2-dichlorobenzene binary mixture over zeolite catalysts was investigated. The benzofuran is much easier to oxidize compared to chlorinated aromatic compounds and PtNaY is the most adapted catalyst for this oxidation reaction. Unfortunately, in the presence of the benzofuran/1,2-dichlorobenzene mixture, PtNaY catalyst deactivates and becomes less adapted for oxidation reaction than a catalyst based on acid zeolite like HY (Si/Al=5). Thus, PtHY proved to be a promising catalyst for 1,2-dichlorobenzene mixture degradation. The presence of benzofuran in mixture clearly improves the conversion of chlorinated compound, decreases the production of chlorinated by-products and enhances the selectivity into CO2.The oxidation of benzofuran/1,2-dichlorobenzene mixture is also influenced by the platinum content deposited on zeolite, the activity of PtHY catalyst increasing linearly with the platinum level. Thus, at 350°C, the 1.2% PtHY catalyst is able to completely oxidize the 1,2-dichlorobenzene in mixture into CO2 with production of a low amount of PhClx by-products.Deactivation has been studied on 1.2% PtHYcatalyst, catalyst initially deactivates during the first 24h reaction. The main cause of this initial deactivation is certainly a Pt sintering leading to a collapse of the cristallinity of the zeolite. However, after 24h reaction PtHY catalyst was stable and allows to oxidize 1,2-dichlorobenzene with a limited formation of polychlorobenzene by-products.
Keywords: Catalytic combustion; Dioxin; Furan; Benzofuran; 1,2-Dichlorobenzene; Faujasite; Platinum