Forgot your password?
New user?
New Window Opens on the Secret Life of Microbes: Scientists Develop First Microbial Profiles of Ecosystems
CAS announces the release of SciFinder Scholar for Mac OS X
Press Releases
Press Releases
| Check out our New Publishers' Select for Free Articles |
Applied Catalysis B, Environmental (v.67, #3-4)
Platinum catalysed decomposition of hydrogen peroxide in aqueous-phase pulsed corona electrical discharge by Selma Međedović; Bruce R. Locke (pp. 149-159).
Electrical discharges in water produced by a pulsed high voltage power supply generate chemically active species (OH, H2, O2, H2O2, HO2 and O) that are capable of degrading various hazardous chemicals. Previous experimental studies showed that platinum high voltage electrodes in a pulsed corona electrical discharge lead to significantly higher pollutant removal in comparison to that with other electrode materials. In the present work it was observed that when nickel–chromium was used as a high voltage electrode, the pulsed corona electrical discharge in water produces hydrogen peroxide at a constant rate regardless of the initial pH of the solution. Replacement of the nickel–chromium electrode with a platinum high voltage electrode leads to the decomposition of hydrogen peroxide where the rate of decomposition increases with increasing pH. An Eley-Rideal mechanism describing heterogeneous catalytic hydrogen peroxide decomposition is proposed. It is assumed that the decomposition occurs on the surface of the platinum particles ejected from the platinum high voltage electrode. Combination of the experimental measurements and a mathematical model describing the platinum catalysed hydrogen decomposition suggests that the pH dependent hydrogen peroxide decomposition is caused by the adsorption of molecular hydrogen produced by the discharge and hydroxyl ions on the platinum surface. The influence of gases bubbled into the reactor (argon, oxygen and hydrogen) on the hydrogen peroxide decomposition was also tested by both experiments and the model. Finally, the model was utilized to predict molecular hydrogen and oxygen concentrations at three pH values when either nickel–chromium or platinum high voltage electrodes are used.
Keywords: Hydrogen peroxide; Platinum; pH; Hydrogen; Pulsed corona discharge; Eley-Rideal
Manganese based materials for diesel exhaust SO2 traps by Kirill Tikhomirov; Oliver Kröcher; Martin Elsener; Markus Widmer; Alexander Wokaun (pp. 160-167).
The storage of SO2 in manganese based materials was investigated in flow reactor experiments. Manganese oxide precipitated with ammonia and hydrogen peroxide stored about 76wt.% of SO2 at a high diffusion rate into the bulk. Doping with potassium increases the SO2 storage rate substantially at 200°C, but has an only minor effect at 400°C. Kinetic studies showed that the storage of SO2 in pure and potassium doped manganese oxide is controlled by the kinetics of the sulfate formation reaction on the catalyst surface up to complete sulfation, whereas the storage on manganese cerium mixed oxide is limited by internal diffusion of the formed sulfate. The sulfate formation reaction was found to be first order with respect to both SO2 and manganese oxide. For the potassium doped catalyst sulfur was found to be bound on manganese sites being transferred to potassium afterwards.
Keywords: Diesel; SO; 2; Trap; Sulfur; Poisoning; Storage; Manganese; Potassium
Kinetics of methane combustion over CVD-made cobalt oxide catalysts by Naoufal Bahlawane (pp. 168-176).
The present investigation provides the required kinetic parameters to evaluate and to predict the rate of the catalytic combustion of methane over cobalt oxide. For this purpose, monolithic cordierites with low specific surface area were uniformly coated with cobalt oxide thin films of controlled thickness using the chemical vapor deposition (CVD) process. The obtained catalysts were tested in the catalytic combustion of methane in oxygen-deficient and -rich conditions. Catalysts with loadings above 0.46wt.% are active starting at a temperature of 250°C and completely convert methane to CO2 below 550°C where the conversion rate reaches 35μmol (CH4)/gcats. The involvement of the bulk-oxide-ions in the catalytic reaction was supported by the constant value of the normalized reaction rate to the weight of deposited cobalt oxide. The experimental data fit well to the Mars–Van Krevelen redox model and can be approximated with a power rate law in oxygen-rich mixtures. The resulting activation energies and frequency factors allow the identification of the rate-limiting step and accurately reproduce the effect of the temperature and partial pressure of the reactants on the specific reaction rate.
Keywords: Catalytic combustion of methane; Kinetics; Cobalt oxide; Chemical vapor deposition
Catalytic ozonation of phenolic compounds by M. Carbajo; F.J. Beltrán; F. Medina; O. Gimeno; F.J. Rivas (pp. 177-186).
The catalytic ozonation of gallic acid has been carried out in the presence of a perovskite type material (LaTi0.15Cu0.85O3). Some of the main operating parameters like temperature, catalyst and ozone doses exerted a positive influence in the ozonation rate. Contrarily, initial gallic acid concentration showed a negative effect in terms of conversion while the optimum working pH was found to be around 2 (3.5 if deactivation is avoided). Addition of tert-butyl alcohol (free radical scavenger) had no effect on the catalytic removal of gallic acid, although its presence decreased the mineralization degree achieved. The catalyst demonstrated a high stability in terms of the acid removal, however, a partial deactivation was experienced in terms of organic carbon elimination if compared the first and second reuses. The catalyst kept its activity after further reutilisations. Due to the moderate regime developed in the homogeneous phase, the process was better simulated by an empirical expression rather than using a detailed model.
Keywords: Perovskite; Ozone; Gallic acid; Heterogeneous catalysis; Kinetics
Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5 by Mukundan Devadas; Oliver Kröcher; Martin Elsener; Alexander Wokaun; Nicola Söger; Marcus Pfeifer; Yvonne Demel; Lothar Mussmann (pp. 187-196).
The influence of NO2 on the selective catalytic reduction (SCR) of NO with ammonia was studied over Fe-ZSM5 coated on cordierite monolith. NO2 in the feed drastically enhanced the NO x removal efficiency (DeNOx) up to 600°C, whereas the promoting effect was most pronounced at the low temperature end. The maximum activity was found for NO2/NO x=50%, which is explained by the stoichiometry of the actual SCR reaction over Fe-ZSM5, requiring a NH3:NO:NO2 ratio of 2:1:1. In this context, it is a special feature of Fe-ZSM5 to keep this activity level almost up to NO2/NO x=100%. The addition of NO2 to the feed gas was always accompanied by the production of N2O at lower and intermediate temperatures. The absence of N2O at the high temperature end is explained by the N2O decomposition and N2O-SCR reaction. Water and oxygen influence the SCR reaction indirectly. Oxygen enhances the oxidation of NO to NO2 and water suppresses the oxidation of NO to NO2, which is an essential preceding step of the actual SCR reaction for NO2/NO x<50%. DRIFT spectra of the catalyst under different pre-treatment and operating conditions suggest a common intermediate, from which the main product N2 is formed with NO and the side-product N2O by reaction with gas phase NO2.
Keywords: Fe-ZSM5; Ammonia SCR; Selective catalytic reduction; Coated monolith; NO; 2; N; 2; O formation; DRIFT spectroscopy
Photocatalytic degradation study of diclofenac over aqueous TiO2 suspensions by P. Calza; V.A. Sakkas; C. Medana; C. Baiocchi; A. Dimou; E. Pelizzetti; T. Albanis (pp. 197-205).
Diclofenac (2-[2′,6′-(dichlorophenyl)amino]phenylacetic acid) is a non-steroidal anti-inflammatory drug used to treat inflammatory and painful diseases of rheumatic and non-rheumatic origin. The present work deals with the photocatalytic transformation of diclofenac, under simulated solar irradiation using titania suspensions as catalyst, to assess the decomposition of the pharmaceutical compound, to identify intermediates, as well as to elucidate some mechanistic details of the degradation. The variation of TiO2 amount and initial diclofenac concentration on the reaction rate, were systematically investigated. The use of the response surface methodology allowed to fit the optimal values of the parameters leading to the degradation of the pollutant. Also, a single polynomial expression modeling the reaction was obtained. Photomineralization of the substrate in terms of chlorine ions release was rather a quick process (within 1h), while the amino moiety is mainly transformed into NH4+ and in a lesser extend into NO3− ions. Evolution of CO2 (loss of TOC) was found to occur within 2h of irradiation. LC/MS was brought to bear in assessing the temporal course of the photocatalyzed process. Based on our findings a tentative degradation pathway is proposed for the photocatalytic degradation of diclofenac based on the formation of hydroxy-derivatives before the complete mineralization of the starting molecule. In addition Microtox bioassay ( Vibrio fischeri) was employed in evaluating the ecotoxicity of solutions treated by photocatalysis. Results clearly demonstrate the efficiency of the photocatalytic process in the detoxification of the irradiated solution.
Keywords: Photocatalysis; TiO; 2; Experimental design; Mineralization; Toxicity; Diclofenac
Reaction pathway of the catalytic wet air oxidation of phenol with a Fe/activated carbon catalyst by A. Quintanilla; J.A. Casas; A.F. Mohedano; J.J. RodrÃguez (pp. 206-216).
Catalytic wet air oxidation (CWAO) of phenol with molecular oxygen using a home-made Fe/activated carbon catalyst at mild operating conditions (100–127°C; 8atm) has been studied in a trickle-bed reactor. Ring compounds (hydroquinone, p-benzoquinone and p-hydroxybenzoic acid) and short-chain organic acids (maleic, malonic, oxalic, acetic and formic) have been identified as intermediate oxidation products. CWAO experiments using each one of these intermediates as starting compound have been carried out (at 127°C and 8atm) in order to elucidate the reaction pathway. It was found that phenol is oxidized through two different ways. It can be either para-hydroxylated to hydroquinone, which is instantaneously oxidized to p-benzoquinone or para-carboxylated to p-hydroxybenzoic acid. p-Benzoquinone is majorly mineralized to CO2 and H2O through oxalic acid formation whereas p-hydroxybenzoic acid gives rise to short-chain acids. Only acetic acid showed to be refractory to CWAO under the operating conditions used in this work. The catalyst avoids the presence of ring-condensation products in the reactor effluent which were formed in absence of it. This is an additional important feature because of the ecotoxicity of such compounds.
Keywords: Phenol; Fe/AC catalyst; Oxidation pathway; Catalytic wet air oxidation
Photo-catalytic production of hydrogen form ethanol over M/TiO2 catalysts (M=Pd, Pt or Rh) by Y.Z. Yang; C.-H. Chang; H. Idriss (pp. 217-222).
The photo-catalytic production of hydrogen from liquid ethanol, a renewable bio-fuel, over Rh/TiO2, Pd/TiO2 and Pt/TiO2, anatase, has been studied. In the absence of the metal, TiO2 shows negligible production of molecular hydrogen. The addition of Pd or Pt dramatically increases the production of hydrogen and a quantum yield of about 10% is reached at 350K. On the contrary, the Rh doped TiO2 is far less active. The low activity of Rh compared to that of Pd and Pt is not due to poor dispersion or low available Rh sites on the surface, as analyzed by XPS and TEM. For all three catalysts, TEM shows most particles with a size less than 10nm. XPS results show that while the state of Pd and Pt particles in the as-prepared catalysts was mostly metallic that of the Rh was composed of non-negligible contribution of Rh cations. The extent of reaction of a series of alcohols was also studied, for comparison, on Pt/TiO2. It was found that the reaction is governed by the solvation of the alcohol. In that regard, the production of molecular hydrogen over Pt/TiO2 showed the following trend: methanol≈ethanol>propanol≈isopropanol> n-butanol.
Keywords: Photo-catalytic reactions of ethanol; Hydrogen-production; TiO; 2; Rh/TiO; 2; Pd/TiO; 2; Pt/TiO; 2; Polarizability; Polarity; Quantum yield
Treatability studies on domestic wastewater using UV/H2O2 process by Taner Yonar; Kadir Kestioglu; Nuri Azbar (pp. 223-228).
Advanced oxidation processes (AOPs) are emerging and promising technology both as an alternative treatment to conventional wastewater treatment methods and enhancement of current biological treatment methods especially dealing with highly toxic and low biodegradable wastes. In this paper, the results of domestic wastewater treatment using H2O2/UV process in both batch and continuous mode are presented. Over 95% reduction in COD was achieved in less than 60min of reaction time. Optimum conditions for pH and H2O2 dosage for this process was found to be 3 and 50mgL−1, respectively. A pretreatment in the form of removal of turbidity is recommended for the success of the process in the long run. Electric energy required is estimated to be 10kWhkg−1 COD on the average.
Keywords: Advanced oxidation; Domestic wastewater; Hydrogen peroxide; UV
Total oxidation of ethanol and propane over Mn-Cu mixed oxide catalysts by MarÃa Roxana Morales; Bibiana P. Barbero; Luis E. Cadús (pp. 229-236).
Mn-Cu mixed oxides were prepared by co-precipitation varying the aging time for 4, 18 and 24h. The catalytic performance in propane and ethanol total oxidation on these samples was better than on Mn2O3 and CuO pure oxides. The increase of the aging time enhanced the activity and the selectivity to CO2. The nature and disposition of the phases forming the catalytic system as well as the effect of the precipitated aging time was determined by means of specific surface area measurements, X-ray diffractometry (XRD), infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and temperature programmed desorption of oxygen (O2-TPD). The catalytic behaviour seems related to the existence of a Cu1.5Mn1.5O4 mixed phase and the easier reducibility of the catalysts.
Keywords: Manganese; Copper; Mixed oxide; Catalytic combustion; Propane; Ethanol
Gold supported on ceria and ceria–alumina promoted by molybdena for complete benzene oxidation by D. Andreeva; P. Petrova; J.W. Sobczak; L. Ilieva; M. Abrashev (pp. 237-245).
New gold–molybdena catalysts supported on ceria and ceria–alumina in reaction of complete benzene oxidation were studied. The catalysts were characterized by means of XRD, TPR, XPS and Raman spectroscopy. High and stable catalytic activity was established in the temperature region 200–240°C. The presence of gold causes a modification in ceria structure leading to an increase of Ce3+ and oxygen vacancies formation. The loading of Al3+ increases additionally the oxygen vacancies, while a tendency of decrease of Ce3+ amount was observed. The presence of alumina results also in a larger share of active oxygen species proved by analysis of O 1s XPS spectra. The differences in the activities within the starting temperature range (150–180°C) and in the region of 100% conversion (200–240°C) could be explained by supposing that in the LT region the electron transfer between nanosized gold and ceria particles via oxygen vacancies has a crucial role. In the HT region the oxygen mobility, provoked by the defective structure of ceria due to the presence of Al3+, becomes of prevailing importance. It was also concluded that alumina prevents the gold and ceria agglomeration, which is the main factor to avoid deactivation under extreme reaction conditions.
Keywords: Gold; Gold–molybdena; Ceria–alumina; Hydrocarbon oxidation; XRD; XPS; TPR; Raman spectroscopy
NO oxidation over supported Pt: Impact of precursor, support, loading, and processing conditions evaluated via high throughput experimentation by P.J. Schmitz; R.J. Kudla; A.R. Drews; A.E. Chen; C.K. Lowe-Ma; R.W. McCabe; W.F. Schneider; C.T. Goralski Jr. (pp. 246-256).
This work summarizes the development and validation of a high throughput approach for automated design, synthesis, screening, and statistical analysis of supported heterogeneous catalyst systems. The catalytic oxidation of NO over supported Pt is used as a tractable case study to demonstrate consistency with published results and to generate a self-consistent set of data to evaluate the influence of synthesis and processing variables on catalytic performance using a five-factor two-level full factorial design-of-experiment (DOE). A novel approach is demonstrated for data descriptor generation whereby a simple model is used to extract kinetic parameters that can be used independently to evaluate performance, or used to calculate secondary performance metrics. The kinetic data obtained revealed a compensation effect for this reaction over Al2O3 and SiO2 supported Pt and is the first reported observation of this phenomenon for the oxidation of NO to NO2. Relative performance rankings of Pt supported on Al2O3 and SiO2 show an enhanced intrinsic rate for all the Pt/SiO2 catalysts. Both the Pt/Al2O3 and Pt/SiO2 systems were found to exhibit structure sensitivity, with the change in rate with average particle size appearing stronger for the Pt/Al2O3 system. Statistical analysis of the performance data was used to identify the main effects impacting the NO oxidation rate at 200°C. The relative order of importance of the factors evaluated was found to be support>pretreatment>loading>calcination atmosphere>calcination temperature>precursor salt.
Keywords: NO oxidation; Platinum; Design of experiment; Combinatorial catalysis; High throughput experimentation
A quantitative evalution of the photocatalytic performance of TiO2 slurries by Claudio Minero /; Davide Vione (pp. 257-269).
This paper reports the results of a comparison between two TiO2 photocatalysts that differ for particle size and absorption/scattering optical properties. The catalyst with larger particles and lower surface area performed better in the degradation of phenol than the specimen with smaller particles and larger surface area. Following carefully designed experiments, it is possible to assess the relative role of light absorption/scattering properties and catalyst-related efficiency by means of a basic kinetic model for the rate of photocatalytic reactions. Explicit relationships are derived in the framework of the steady-state approximation for the quantum yield as a function of one a-dimensional number collecting surface kinetic constants for charge carrier reactions at the interface, absorbed light and surface substrate concentrations. The dimensionality change to volume-defined quantities allows derivation of the explicit dependence of the quantum yield on substrate concentration and partition constants, catalyst concentration, and the rate of volumetric light absorption. Following this approach, the rate expression for slurry systems, valid in the absence of back reactions, is directly derived. Some further simplification of the rate equation for the case of low quantum yield regime leads to analytical relationships able to account for the dependence of the rate on catalyst concentration and absorbed light in the case of stirred and unstirred conditions. The reported properly designed experiments allow the estimation of catalyst-specific micro-kinetic constants.
Keywords: Photocatalysis; Titanium dioxide; Photocatalytic rate; Quantum yield; Catalyst loading; Slurry
Influence of support acid pretreatment on the behaviour of CoO x/γ-alumina monolithic catalysts in the CH4-SCR reaction by J.C. MartÃn; P. Ã?vila; S. Suárez; M. Yates; A.B. MartÃn-Rojo; C. Barthelemy; J.A. MartÃn (pp. 270-278).
The effect of treatment with different mineral acids (H2SO4, H3PO4, HNO3 and HCl) on the activity of monolithic CoO x/γ-Al2O3 catalysts in the reduction of nitric oxide with methane in the presence of oxygen (CH4-SCR of NO x) was studied. Their behaviour in the methane oxidation reaction in both the presence and absence of NO x was determined in order to interpret the results in terms of intrinsic activity and competition between both processes. Depending on the nature of the acid used, significant differences were observed in the catalytic activities which were related to the textural states, surface acidities and the nature of the detected species. The best results were obtained after treatment with H2SO4, which increased the activity towards NO x elimination compared to the other catalysts. This behaviour was attributed not only to an increase in surface acidity but also to the stabilisation of the active Co2+ species, thus avoiding the formation of Co3O4 spinel that is responsible for the strongly adsorbed NO x species that lead to NO2 formation which increase the rate of the undesired methane oxidation reaction at high temperatures.
Keywords: Nitrogen oxides removal; DeNO; x; HC-SCR; CoO; x; /γ-Al; 2; O; 3; SCR monolithic catalysts
Photodegradation of polycyclic aromatic hydrocarbons in fossil fuels catalysed by supported TiO2 by Maria J. GarcÃa-MartÃnez; Ignacio Da Riva; Laureano Canoira; Juan F. Llamas; Ramon Alcántara; Jose Luis R. Gallego (pp. 279-289).
This paper describes the photodegradation behavior of polycyclic aromatic hydrocarbons present in different types of fossil fuels (commercial diesel, Arabian light crude, heavy fuel oil from the Prestige oil spill and coal from an abandoned coal dump) suspended in artificial seawater or ultrapure water, under irradiation in a stirred photochemical reactor for 14 days. The reactor was continuously fed with air from a compressor at a constant rate of 6NLh−1, and thin films of TiO2 (anatase) supported on pyrex glass raschig rings were used as catalyst. Dark control samples were carried out simultaneously for all the experiments, and both phases, aqueous and organic, were analyzed by gas chromatography–mass spectrometry in the experimental and dark control samples, allowing to calculate a photodegradation ratio. The polycyclic aromatic hydrocarbons reached a high degree of photodegradation in the water-soluble fraction of the samples, but the organic fractions remained almost unaffected in most of the experiments. Some photodegradation products have been also identified in the aqueous and organic fractions of the samples.
Keywords: Fossil fuels; Photodegradation; Polycyclic aromatic hydrocarbons; Seawater; TiO; 2; catalyst