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Applied Catalysis B, Environmental (v.74, #3-4)
The effect of treatment of activated carbon by H2O2 or HNO3 on the decomposition of pentachlorobenzene by Masaki Takaoka; Hiromu Yokokawa; Nobuo Takeda (pp. 179-186).
We investigated the reactivity between modified activated carbon and pentachlorobenzene (P5CBz) to better understand how the nature of activated carbon affects the decomposition of chlorinated aromatics. To modify and control the overall characteristics, activated carbon was treated with either nitric acid or hydrogen peroxide. According to ESR, the number of free radicals on the activated carbon was found to increase in response to a treatment in 70% nitric acid at 70°C for 24h. Additionally, certain functional groups, such as carboxyl and hydroxyl groups, are believed to have formed on the surface of the activated carbon as indicated by the results of FT-IR and XPS. The catalytic decomposition experiments involving P5CBz were conducted using a pulse-type, packed-bed reactor. Using the activated carbon that was previously treated with nitric acid for 24h, P5CBz molecules were completely decomposed at a temperature of 350°C, irrespective of either an air or nitrogen atmosphere. The mechanism of decomposition of P5CBz depends on the kind of oxygen functional group and free radicals present.
Keywords: Activated carbon; Chemical treatment; Pentachlorobenzene; Oxygen functional groups
Photocatalytic degradation of two volatile fatty acids in monocomponent and multicomponent systems: Comparison between batch and annular photoreactors by Pierre-François Biard; Abdelkrim Bouzaza; Dominique Wolbert (pp. 187-196).
This study investigates the influence of the inlet concentrations on the degradation rates of a binary mixture of volatile fatty acids (propionic and butyric acids). TiO2-coated non-woven fibre textile was used as the photocatalyst in a batch reactor and in an annular reactor at laminar flow regime. The humidity level, temperature, UV intensity and flow rate were kept constant. First, the adsorption isotherms were determined and these showed that the propionic acid is less adsorbed than the butyric acid onto the catalyst. Conversely to the adsorption, the photodegradation of pure propionic acid is better than the degradation of pure butyric acid. Photodegradation is greatly inhibited when the two acids are mixed in comparison to the pure compounds. The propionic acid is more affected. This inhibition is attributed to the competitive adsorption of the two species on the active sites of the catalyst. A model which takes into account the competitive adsorption is proposed in order to determine the outlet concentrations of the compounds knowing the inlet concentrations in the annular reactor.
Keywords: Photocatalysis; Competitive adsorption; Mixture; Multicomponent system; Kinetic modelling
Preparation of supported Pt-M catalysts (M=Mo and W) from anion-exchanged hydrotalcites and their catalytic activity for low temperature NO-H2-O2 reaction by S. Hamada; S. Hibarino; K. Ikeue; M. Machida (pp. 197-202).
The NO-H2-O2 reaction was studied over supported bimetallic catalysts, Pt-Mo and Pt-W, which were prepared by coexchange of hydrotalcite-like Mg-Al double layered hydroxides by Pt(NO2)42−, MoO42−, and/or WO42− and subsequent heating at 600°C in H2. The Pt–Mo interaction could obviously be seen when the catalyst after reduction treatment was exposed to a mixture of NO and H2 in the absence of O2. The Pt-HT catalyst showed the almost complete NO conversion at 70°C, whereas the Pt-Mo-HT showed a negligible conversion. Upon exposure to O2, however, Pt-Mo-HT exhibited the NO conversion at the lowest temperature of ≥30°C, compared to ≥60°C required for Pt-HT. EXAFS/XANES, XPS and IR results suggested that the role of Mo is very sensitive to the oxidation state, i.e., oxidized Mo species residing in Pt particles are postulated to retard the oxidative adsorption of NO as NO3 and promote the catalytic conversion of NO to N2O at low temperatures.
Keywords: NO-H; 2; -O; 2; reaction; Platinum; Molybdenum; Hydrotalcite
Ozonation of phenolic wastewaters in the presence of a perovskite type catalyst by M. Carbajo; F.J. Beltrán; O. Gimeno; B. Acedo; F.J. Rivas (pp. 203-210).
The ozonation of different wastewaters has been carried out in the presence of perovskites. Wastewaster A (a mixture of syringic, pyruvic and gallic acids in water) was firstly ozonated for 30min in the absence of the catalyst. Thereafter, the solid was added to the solution and the effect of the main operating variables investigated. Catalyst activity and stability was confirmed by consecutive experiments with the same solid. Concentration of perovskites did show no appreciable influence for values above 0.1gL−1. Additionally, the higher the initial chemical oxygen demand, the higher the observed removal rate. Mineralization level was favoured by temperature while pH did exert a positive effect when raising this parameter to values as high as 11. No clear trend after free radical scavengers addition could be envisaged for the catalytic experiments. Wastewaters B (from wine distillery), C (from olive debittering) and D (from olive oil production) showed different behaviours depending on their nature. Thus, activity of perovskites in the ozonation process was only experienced for those effluents showing a certain refractory character towards single ozonation. No differences were observed between catalytic and non-catalytic runs for easily oxidised wastewaters.
Keywords: Perovskite; Ozone; Heterogeneous catalysis; Wastewater
Steam reforming of tar from a biomass gasification process over Ni/olivine catalyst using toluene as a model compound by D. Świerczyński; S. Libs; C. Courson; A. Kiennemann (pp. 211-222).
A Ni/olivine catalyst, previously developed for biomass gasification and tar removal during fluidized bed steam gasification of biomass, was tested in a fixed bed reactor in toluene steam reforming as a tar destruction model reaction. The influence of the catalyst preparation parameters (nickel precursor, calcination temperature and nickel content) and operating parameters (reaction temperature, steam to carbon S/C ratio and space-time) on activity and selectivity was examined showing a high toluene conversion and a low carbon formation compared to olivine alone. The steam reforming of toluene was found to be of zero order for water and first order for toluene. Activation energy required for Ni/olivine was determined to be about 196kJmol−1 in accordance with literature. Catalyst activity and stability and its resistance against carbon formation were discussed on the basis of X-ray diffraction (XRD), transmission electron microscopy (TEM) and temperature programmed oxidation (TPO) results. Characterization before test (XRD, temperature programmed reduction (TPR), Mössbauer spectroscopy) have shown the presence of NiO–MgO solid solution, formed on the surface of olivine support, which explains the efficiency of the catalyst calcined at 1100°C. After test, Ni–Fe alloys were observed (TEM, Mössbauer spectroscopy). It was suggested that magnesium oxide enhanced steam adsorption, facilitating the gasification of surface carbon and that Ni–Fe alloys prevented carbon deposition by dilution effect.
Keywords: Ni/olivine catalyst; Tar removal; Toluene steam reforming; Carbon formation
On the impact of the choice of model VOC in the evaluation of V-based catalysts for the total oxidation of dioxins: Furan vs. chlorobenzene by D.P. Debecker; F. Bertinchamps; N. Blangenois; P. Eloy; E.M. Gaigneaux (pp. 223-232).
V-based catalysts, widely developed for the catalytic abatement of dioxins, are usually studied and optimized by investigating the oxidation of model chlorinated aromatic compounds (e.g. chlorobenzene). Even though the oxygenated function included in the central aromatic ring of the molecular structure of a dioxin could influence major aspects of the catalytic process, it has never been taken into account in the reported works. In this study, furan is chosen as a model for the central oxygenated ring of a polychlorinated dibenzo furan (PCDF) and its oxidation is compared to the case of chlorobenzene. The strategy was to check systematically if the improvements of formulations enlightened from our previous investigation on chlorobenzene also remain beneficial with furan. It turned out that the use of a sulfate containing TiO2 as support for the active VO x phase as well as the doping of the formulation with Mo or W oxides had very different impacts in the two cases. Some improvement strategies prove to be inefficient or deleterious in the case of furan. Competition tests further suggest that the adsorption behavior of dioxin could be better imitated by furan than by chlorobenzene. These observations highlight, in the case for which working with the target pollutant is difficult (as with dioxins), that the choice of the model molecule is critical.
Keywords: Dioxin; Chlorobenzene; Furan; Catalytic oxidation; VOC combustion; VO; x; /TiO; 2
Photocatalytic degradation of sulfamethoxazole in aqueous suspension of TiO2 by M.N. Abellán; B. Bayarri; J. Giménez; J. Costa (pp. 233-241).
The presence of drugs in the aquatic media has emerged in the last decade as a new environmental risk. The aim of this study is the evaluation of photocatalysis as a suitable process to degrade an antibiotic, the sulfamethoxazole. In this way, sulfamethoxazole in aqueous solution was treated by using titania in suspension as catalyst, and UV light. Sulfamethoxazole degradation and TOC reduction were improved when titania concentration was increased, until an optimum located between 0.5–1.0g TiO2/L. Under the studied conditions, 82% of sulfamethoxazole degradation and 23% of TOC reduction was achieved when working with 0.5g TiO2/L. The initial pH also seemed to influence the process in some extent, although the antibiotic degradation was not affected by this variable, TOC reduction was dramatically decreased when the initial pH was 2, probably due to interferences caused by the sulfate anion. The LC/MS study has been also carried out, and a mechanism has been proposed, through the identification of five intermediates. Sulfate and ammonium ions were also monitored in the solution finding that, as long as the sulfamethoxazole is degraded, the total amount of releasable ions was not reached. The SUVA parameter along the reaction shows a decrease on the aromatic content, but there is still a notable presence of the aromatic compounds after 15h of reaction. Finally, the experimental data were fitted to different kinetic models. The best results were obtained for a model including the sulfamethoxazole and intermediates concentration.
Keywords: Sulfamethoxazole; Photocatalysis; UV radiation; Pollutants treatments; Radiation field
Stability of palladium-based catalysts during catalytic combustion of methane: The influence of water by Katarina Persson; Lisa D. Pfefferle; William Schwartz; Anders Ersson; Sven G. Järås (pp. 242-250).
The stability of methane conversion was studied over a Pd/Al2O3 catalyst and bimetallic Pd–Pt/Al2O3 catalysts. The activity of methane combustion over Pd/Al2O3 gradually decreased with time, whereas the methane conversion over bimetallic Pd–Pt catalysts was significantly more stable. The differences in combustion behavior were further investigated by activity tests where additional water vapor was periodically added to the feed stream. From these tests it was concluded that water speeds up the degradation process of the Pd/Al2O3 catalyst, whereas the catalyst containing Pt was not affected to the same extent. DRIFTS studies in a mixture of oxygen and methane revealed that both catalysts produce surface hydroxyls during combustion, although the steady state concentration on the pure Pd catalyst is higher for a fixed temperature and water partial pressure. The structure of the bimetallic catalyst grains with a PdO domain and a Pd–Pt alloy domain may be the reason for the higher stability, as the PdO domain appears to be more affected by the water generated in the combustion reaction than the alloy. Not all fuels that produce water during combustion will have stability issues. It appears that less strong binding in the fuel molecule will compensate for the degradation.
Keywords: Palladium; Platinum; Bimetal; Methane; Catalytic combustion; Stability; Water; DRIFTS
Thermal sintering studies of an autothermal reforming catalyst by Sauri Gudlavalleti; Tijmen Ros; Dick Lieftink (pp. 251-260).
This paper describes a novel approach to life studies on catalysts used in non-isothermal reactors, using a single long-term experiment. Temperature dependence of catalyst aging is determined by comparing the activity reduction of portions of the catalyst from different sections of the reactor, subjected to different temperatures. Time dependence is determined by fitting the drift in catalyst temperatures to a time-dependent reaction rate via a thermodynamic reactor model. Experimentally, a monolithic autothermal reforming catalyst was subjected to thermally accelerated aging under reforming conditions in an adiabatic laboratory mini-flow reactor for 1000h. Methane was used as the fuel. The axial temperature profile of the catalyst was monitored using thermocouples placed at various locations along the catalyst. A gradual change in temperature profile, with increasing temperatures due to decreasing steam-reforming activity, was observed. The aged monolith was cut up into short pieces centered on the thermocouple locations. The pieces, each aged at a different temperature due to its location, were tested individually for activity. The reduced activities were correlated with the aging temperature to obtain the temperature dependence of thermal sintering rates. A generalized power-law equation (GPLE) model for sintering was fit to the activity data. A plug flow reactor (PFR) model describing the reaction was built and the sintering kinetics were incorporated. The PFR model was used to predict changes in catalyst performance due to sintering under normal operating conditions. Thermal sintering deactivation for this catalyst was found to be within acceptable limits for commercial applications.
Keywords: Sintering; Power-law equation; Autothermal reforming; Chemical reactor model
Evaluation of catalytic properties of tungsten carbide for the anode of microbial fuel cells by Miriam Rosenbaum; Feng Zhao; Marion Quaas; Harm Wulff; Uwe Schröder; Fritz Scholz (pp. 261-269).
In this communication we discuss the properties of tungsten carbide, WC, as anodic electrocatalyst for microbial fuel cell application. The electrocatalytic activity of tungsten carbide is evaluated in the light of its preparation procedure, its structural properties as well as the pH and the composition of the anolyte solution and the catalyst load. The activity of the noble-metal-free electrocatalyst towards the oxidation of several common microbial fermentation products (hydrogen, formate, lactate, ethanol) is studied for microbial fuel cell conditions (e.g., pH 5, room temperature and ambient pressure). Current densities of up to 8.8mAcm−2 are achieved for hydrogen (hydrogen saturated electrolyte solution), and up to 2mAcm−2 for formate and lactate, respectively. No activity was observed for ethanol electrooxidation.The electrocatalytic activity and chemical stability of tungsten carbide is excellent in acidic to pH neutral potassium chloride electrolyte solutions, whereas higher phosphate concentrations at neutral pH support an oxidative degradation.
Keywords: Tungsten carbide; Microbial fuel cell; Biofuel cell; Biohydrogen; Formate oxidation; Lactate
Improved performance of non-thermal plasma reactor during decomposition of trichloroethylene: Optimization of the reactor geometry and introduction of catalytic electrode by M. Magureanu; N.B. Mandache; V.I. Parvulescu; Ch. Subrahmanyam; A. Renken; L. Kiwi-Minsker (pp. 270-277).
The decomposition of trichloroethylene (TCE) by non-thermal plasma was investigated in a dielectric barrier discharge (DBD) reactor with a copper rod inner electrode and compared with a plasma-catalytic reactor. The particularity of the plasma-catalytic reactor is the inner electrode made of sintered metal fibers (SMF) coated by transition metal oxides. In order to optimize the geometry of the plasma reactor, the efficiency of TCE removal was compared for different discharge gap lengths in the range of 1–5mm. Shorter gap lengths (1–3mm) appear to be more advantageous with respect to TCE conversion. In this case TCE conversion varies between 67% and 100% for input energy densities in the range of 80–480J/l, while for the 5mm discharge gap the conversion was lower (53–97%) for similar values of the input energy. As a result of TCE oxidation carbon monoxide and carbon dioxide were detected in the effluent gas. Their selectivity was rather low, in the range 14–24% for CO2 and 11–23% for CO, and was not influenced by the gap length. Several other chlorinated organic compounds were detected as reaction products.When using MnOx/SMF catalysts as the inner electrode of the DBD reactor, the TCE conversion was significantly enhanced, reaching ∼95% at 150J/l input energy. The selectivity to CO2 showed a major increase as compared to the case without catalysts, reaching 58% for input energies above 550J/l.
Keywords: Non-thermal plasma; Dielectric barrier discharge; Chlorinated volatile organic compounds; Trichloroethylene; Plasma-catalysis
Correlation between the surface properties and deNO x activity of ceria-zirconia catalysts by Małgorzata Adamowska; Sébastien Muller; Patrick Da Costa; Andrzej Krzton; Philippe Burg (pp. 278-289).
The catalytic activities of ceria-zirconia mixed oxides Ce xZr1− xO2 ( x=0.17, 0.42, 0.62 and 0.8) are determined by isothermal steady-state experiments using a representative mixture of exhaust gases of coal combustion. Results show that all supports are active in deNO x reaction in the presence of the previous hydrocarbons. However, their catalytic activity varies with the content of cerium and goes through a maximum for x=0.62, leading to 27% NO x consumption. This activity was correlated with physicochemical properties determined by a linear solvation energy relationship (LSER) approach. Moreover, a mechanism of HC assisted reduction of NO is proposed on ceria-zirconia supported catalysts. This mechanism is divided in three catalytic cycles involving: (i) the oxidation of NO into NO2, (ii) the reaction of NO2 and the hydrocarbons leading to RNO x species and C xH yO z, and finally (iii) the decomposition of NO assisted by these latter C xH yO z species.
Keywords: deNO; x; Coal; Ceria-zirconia; LSER; Mechanism
Structure evolution of nanocrystalline CeO2 and CeLnOx mixed oxides (Ln=Pr, Tb, Lu) in O2 and H2 atmosphere and their catalytic activity in soot combustion by Małgorzata A. Małecka; Leszek Kępiński; Włodzimierz Miśta (pp. 290-298).
This paper reports results of studies on structure and activity in soot combustion of nanocrystalline CeO2 and CeLnOx mixed oxides (Ln=Pr, Tb, Lu, Ce/Ln atomic ratios 5/1). Nano-sized (4–5nm) oxides with narrow size distribution were prepared by a microemulsion method W/O. Microstructure, morphology and reductivity of the oxides annealed up to 950°C in O2 and H2 were analyzed by HRTEM, XRD, FT-IR, Raman spectroscopy and H2-TPR. Obtained mixed oxides had fluorite structure of CeO2 and all exhibited improved resistance against crystal growth in O2, but only CeLuOx behaved better than CeO2 in hydrogen.The catalytic activity of CeO2, CeLnOx and physical mixtures of CeO2+Ln2O3 in a model soot oxidation by air was studied in “tight contact” mode by using thermogravimetry. Half oxidation temperature T1/2 for soot oxidation catalysed by nano-sized CeO2 and CeLnOx was similar and ca. 100°C lower than non-catalysed oxidation. However, the mixed oxides were much more active during successive catalytic cycles, due to better resistance to sintering. Physical mixtures of nanooxides (CeO2+Ln2O3) showed exceptionally high initial activity in soot oxidation (decrease in T1/2 by ca. 200°C) but degraded strongly in successive oxidation cycles. The high initial activity was due to the synergetic effect of nitrate groups present in highly disordered surface of nanocrystalline Ln2O3 and enhanced reductivity of nanocrystalline CeO2.
Keywords: Nanocrystalline CeO; 2; Mixed oxides; Lanthanide doped CeO; 2; Soot combustion; TEM; Microemulsion
Direct decomposition of NO into N2 and O2 on BaMnO3-based perovskite oxides by Hideharu Iwakuni; Yusuke Shinmyou; Hiroshi Yano; Hiroshige Matsumoto; Tatsumi Ishihara (pp. 299-306).
Effects of dopant in BaMnO3 perovskite oxide on the NO direct decomposition activity were investigated. NO direct decomposition activity was greatly elevated by doping La and Mg for Ba and Mn site in BaMnO3, respectively. The highest N2 yield was achieved on Ba0.8La0.2Mn0.8Mg0.2O3. The NO decomposition rate increased with increasing NO partial pressure with PNO1.19. Coexistence of oxygen lowered the N2 yield withPO2−0.18; however, N2 yield of 40% was sustained even under coexisting of 5% O2 at 1123K. Adsorption state of oxygen was also studied with temperature programmed desorption (TPD) method and the desorption temperature of oxygen was lowered by doping Mg for Mn site in BaMnO3.
Keywords: NO decomposition; Perovskite oxide; BaMnO; 3; Dopant effects
Characterization and photocatalytic activity of N-doped TiO2 prepared by thermal decomposition of Ti–melamine complex by M. Sathish; B. Viswanathan; R.P. Viswanath (pp. 307-312).
Nitrogen doped spherical TiO2 has been prepared by thermal decomposition of Ti–melamine complex in air atmosphere. A clear shift in the onset light absorption from UV region (<400) to visible region (>520nm) has been observed for the N-doped samples. It has been deduced from the optical absorption spectra that the higher calcination temperature results in the decrease in the amount of N-doping. The XRD results revealed the phase transition of TiO2 from anatase to rutile crystalline phase, starting at calcination temperature ≥600°C. The electron microscopic images reveal the formation of spherical and flakes of TiO2 nanocrystals (25nm). The chemical nature of N in the N-TiO2 has been evolved through X-ray photoelectron spectroscopy. The presence of different types of N species have been observed corresponding to different oxidation states and the presence of Ti–N and O–Ti–N have been confirmed from the observed binding energy values. Photocatalytic decomposition of methylene blue has been carried out both in the visible region and UV+visible region. In the visible region, N-TiO2 showed higher activity compared to the undoped commercial TiO2 (Degussa P25).
Keywords: N-doped TiO; 2; Anion doping; Photocatalysis; Visible light absorption
The selective reduction of NO x with propene on Pt-beta catalyst: A transient study by J.M. García Cortés; M.J. Illán Gómez; C. Salinas Martínez de Lecea (pp. 313-323).
The mechanism of the NO/C3H6/O2 reaction has been studied on a Pt-beta catalyst using transient analysis techniques. This work has been designed to provide answers to the volcano-type activity behaviour of the catalytic system, for that reason, steady state transient switch (C3H6/NO/O2→C3H6/Ar/O2, C3H6/Ar/O2→C3H6/NO/O2, C3H6/NO/O2→Ar/NO/O2, Ar/NO/O2→C3H6/NO/O2, C3H6/NO/O2→C3H6/NO/Ar and C3H6/NO/Ar→C3H6/NO/O2) and thermal programmed desorption (TPD) experiments were conducted below and above the temperature of the maximum activity ( Tmax). Below Tmax, at 200°C, a high proportion of adsorbed hydrocarbon exists on the catalyst surface. There exists a direct competition between NO and O2 for Pt free sites which is very much in favour of NO, and therefore, NO reduction selectively takes place over hydrocarbon combustion. NO and C3H6 are involved in the generation of partially oxidised hydrocarbon species. O2 is essential for the oxidation of these intermediates closing the catalytic cycle. NO2 is not observed in the gas phase. Above Tmax, at 230°C, C3H6ads coverage is negligible and the surface is mainly covered by Oads produced by the dissociative adsorption of O2. NO2 is observed in gas phase and carbon deposits are formed at the catalyst surface. From these results, the state of Pt-beta catalyst at Tmax is inferred. The reaction proceeds through the formation of partially oxidised active intermediates(CxHyOzNw) from C3H6ads and NOads. The combustion of the intermediates with O2(g) frees the Pt active sites so the reaction can continue. Temperature has a positive effect on the surface reaction producing active intermediates. On the contrary, formation of NOads and C3H6ads are not favoured by an increase in temperature. Temperature has also a positive effect on the dissociation of O2 to form Oads, consequently, the formation of NO2 is favoured by temperature through the oxygen dissociation. NO2 is very reactive and produces the propene combustion without NO reduction. These facts will determine the maximum concentration of active intermediates and consequently the maximum of activity.
Keywords: NO; x; -SCR mechanism; Propene; Pt-beta catalyst; Transient experiments
Platinum-based ternary catalysts for low temperature fuel cells by Ermete Antolini (pp. 324-336).
Pt-based ternary catalysts have been proposed as electrode materials for low temperature fuel cells. Pt–Ru-based ternary catalysts were tested as anode materials with improved CO tolerance or enhanced activity for methanol or ethanol oxidation. Ternary catalysts based on platinum alloyed with first row transition metals were tested as cathode materials with improved activity for the oxygen reduction. This paper presents an overview of the preparation methods and structural characteristics of these ternary catalysts.
Keywords: Ternary alloys; Fuel cells; Platinum; Catalysts; Nanomaterials
Platinum-based ternary catalysts for low temperature fuel cells by Ermete Antolini (pp. 337-350).
The development of high performance electrode materials is currently one of the main activities in the field of the low temperature fuel cells, fuelled with H2/CO or low molecular weight alcohols. A promising way to attain higher catalytic performance is to add a third element to the best binary catalysts actually used as anode and cathode materials. In Part I of this review an overview of the preparation and structural characteristics of Pt-based ternary catalysts was presented. This part of the review deals with the electrochemical properties of these catalysts regarding their CO tolerance and electrocatalytic activity for methanol and ethanol oxidation in the case of anode materials, and their activity for oxygen reduction and stability in fuel cell conditions when used as cathode materials.
Keywords: Ternary alloys; Fuel cells; Platinum; Catalysts; Electrochemical properties