Applied Catalysis B, Environmental (v.67, #1-2)


Effect of synthesis parameters on catalytic properties of CuO-CeO2 by George Avgouropoulos; Theophilos Ioannides (1-11).
A modified citrate method incorporating hydrothermal treatment of the precursors has been employed for the synthesis of CuO-CeO2 catalysts with varying CuO content. Catalyst characterization was done by N2 physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and H2-TPR. The activity and selectivity of the catalysts for the selective oxidation of CO in excess H2 was examined employing a simulated reformate gas mixture. Hydrothermal treatment of the citrate precursors was found to induce more efficient mixing of Cu2+ and Ce4+ ions in the precursor compound. CuO-CeO2 catalysts containing highly dispersed, XRD-invisible CuO species were synthesized with this method at even higher CuO contents than those reported previously. Interaction between CuO and CeO2 leads to structural and thermal stabilization of the catalysts, while surface segregation of Cu2+ ions takes place in all cases. CuO-CeO2 catalysts are significantly more active, more selective and less inhibited by CO2 than pure CuO. The activation temperature and the CuO content of the catalysts seem to play an important role by influencing, on one hand, the degree of interaction between CuO and CeO2 and, on the other hand, the phenomena of sintering and surface area loss.
Keywords: Selective CO oxidation; Hydrogen; Copper oxide; Cerium oxide; Citrate; Hydrothermal;

Catalytic wet air oxidation of phenol over active carbon catalyst by A. Eftaxias; J. Font; A. Fortuny; A. Fabregat; F. Stüber (12-23).
The catalytic wet air oxidation of phenol has been studied in an integral trickle bed reactor using active carbon as catalyst. Temperature, oxygen partial pressure and liquid space time have been varied from 120 to 160 °C, 1–2 bar and 0.05–0.55 kgcat  kgL −1  h, respectively. The active carbon catalyst employed is stable and performs phenol and COD conversions over 99% and 85% with a low residual content of bio-toxic aromatic compounds. The distribution of the main intermediates has been quantitatively assessed by means of HPLC analysis to develop a global kinetic model for the phenol oxidation over active carbon. Several rate models and reaction pathways have been tested in the kinetic modelling. The involved model parameters have been optimised with the stochastical simulated annealing algorithm leading to a very satisfactory fit of the whole experimental data. On cost of a higher, but still reasonable, computational time demand, this easy to implement algorithm proves to be a robust alternative to the classical gradient-based Levenberg–Marquardt algorithm for multi-parameter identification.
Keywords: Phenol; Active carbon; Catalytic wet air oxidation; Trickle bed reactor; Reaction pathway; Kinetics; Simulated annealing;

Gas and liquid phase reactions on MCM-41/SZ catalysts by E. Ghedini; M. Signoretto; F. Pinna; G. Cerrato; C. Morterra (24-33).
A series of sulphated zirconia (SZ) catalysts supported on MCM-41 were prepared by the liquid-crystal template method (LCT), using cetyltrimethylammonium bromide (CTABr) as template. The effect of metal oxides promotion (Al2O3, Fe2O3 and Ga2O3) on structural and textural property was studied. These materials were characterized by ion chromatography (for the determination of sulphates content), nitrogen-adsorption measurements, X-ray diffraction, FT-IR spectroscopy, TG–DSC analysis and TPR–MS measurements. The catalytic performance of the catalysts in both gas and liquid phase reactions was investigated. A correlation between morphological and chemical physical features of the systems and their catalytic activity is here proposed.
Keywords: Sulphated zirconia catalysts; Liquid-crystal template method; SZ/MCM-41; Acylation; Isomerization;

Electrochemical methods have proved to be particularly effective both for water detoxification (abatement of heavy metals and organic impurities) and sterilization, from natural resources to utilization points, combining these features with low costs and easier handling of equipments. The efficiency of electrochemical methods strongly depends on electrode nature, anodes being of particular importance because they must stand much more severe polarization conditions and have to exhibit good catalytic activity for complex reactions like oxygen and chlorine evolution. The present work is devoted to the preparation and characterization of mixed-oxide electrode coatings based on IrO2, RuO2 and SnO2, with the scope of finding optimal compositions guaranteeing longer service-life under the critical conditions where oxygen evolution is concomitant with chloride oxidation.
Keywords: Electrocatalysis; Oxide electrodes; Chlorine evolution reaction; Oxygen evolution reaction; Water sterilization;

Cu-doped TiO2 systems with improved photocatalytic activity by G. Colón; M. Maicu; M.C. Hidalgo; J.A. Navío (41-51).
Photocatalytic oxidation of phenol was performed over copper doped TiO2 prepared by a sol–gel method. Different preparation methods were followed and a comparison with undoped system was also made. Wide structural and surface characterization of catalysts was carried out in order to establish a correlation between the effect of sulphuric and nitric acids present in the initial solution and the Cu–TiO2 photocatalytic properties. The presence of sulphuric acid clearly stabilizes Cu–TiO2 and TiO2 structure and surface against sintering, maintaining anatase phase and relatively high surface area values with respect non sulphated Cu–TiO2 or TiO2. Best photocatalytic behaviour is found for sulphated TiO2, as previously reported. In addition, incorporation of copper ions into the structure seems to enhance the photoactivity of the system for acidified systems, being the sulphated one the most favourable photocatalyst. The optimum metal loading is found for 0.5 M% of copper ion. A possible explanation of this photocatalytic improvement might be related to the stabilization of Cu2O species in doped TiO2 prepared in the presence of sulphuric acid. This stabilization could be related to the presence of oxygen vacancies generated in the preparation procedure using sulphuric acid.
Keywords: Cu-doped TiO2; Photocatalysis; Sulphated TiO2; Anatase stabilization; Oxygen vacancies;

Abatement of CO from relatively simple and complex mixtures by Alexander J. Dyakonov; Christine A. Little (52-59).
Oxidation of carbon monoxide on CeO2/C and Pd/CeO2/C catalysts was investigated at temperatures ranging from 50 to 700 °C until the catalysts disintegrated. The CeO2/C catalyst, which was stored in air for an extended period of time, was found to be more active than the same catalyst which was annealed in inert atmosphere, which suggested that the catalyst could accumulate active surface oxygen over time. The effects of surface oxidation were further evident after preparing more active Pd catalysts on both oxidized and annealed forms of CeO2/C, with the latter catalyst showing a higher activity. Nitric oxide was found to inhibit CO oxidation on ceria and promote it on supported Pd catalysts. ESR spectra showed the presence of active oxygen, possibly in the forms of superoxide and peroxide on the surface of carbon and ceria. The reduced Pd clusters could generate superoxide radicals whereas they were stabilized by partially reduced Ce3+ and carbon. Adsorbed water most likely protected ceria sites from a complete oxidation into Ce4+. Long-term storage of a sealed container of Pd/CeO2/C at room temperature aged the catalyst, which possibly involved a Pd-assisted slow partial reduction of Ce4+ by carbon and an accumulation of Ce3+⋯O2 complexes.
Keywords: Abatement; CO; NO; Oxidation; Pd/CeO2/C catalyst; Carbon; Smoke; ESR;

Conventional Cu-ZSM-5 has for many years been recognized as a unique catalyst for direct NO decomposition. Zeolite-based catalysts have a crystallographically well-defined microporous structure. In such microporous catalysts, the creation and accessibility of the active sites is often influenced by the geometry of the pore system. To improve these catalysts, secondary mesoporous systems can benefitially be introduced into the structure of conventional zeolites. Here, this approach was used with Cu-containing ZSM-5 and ZSM-11 type mesoporous zeolite catalysts, which were used as catalysts for direct NO decomposition. It was discovered that introducing mesoporosity into the conventional materials leads to a significant improvement of the catalytic activity. Additionally, mesoporous Cu-ZSM-11 catalyst was found to be about twice as active as mesoporous Cu-ZSM-5. This difference is attributed to the fact that ZSM-11 has only straight microporous channels, while ZSM-5 has both straight and sinusoidal channels. Apparently, there is a preferential formation of active sites and/or improved accessibility in the straight channels compared to the sinusoidal channels, which make the ZSM-11 material a better catalysts than ZSM-5. All samples were characterized by XRPD, SEM, TPD-NH3, in situ EPR and N2 physisorption measurements. Thus, this confirms our previous observation that the special pore structure of Cu-ZSM-5 is not a decisive factor for the catalytic activity in NO decomposition.
Keywords: NO decomposition; Cu-ZSM-5; Cu-ZSM-11; Zeolite; Mesopores; Carbon; Zeolite single crystals;

Hydrodechlorination of 4-chlorophenol in aqueous phase using Pd/AC catalysts prepared with modified active carbon supports by L. Calvo; M.A. Gilarranz; J.A. Casas; A.F. Mohedano; J.J. Rodríguez (68-76).
The hydrodechlorination of 4-chlorophenol with hydrogen in aqueous phase has been studied using different home-made Pd/active carbon catalysts. The active carbons employed were subjected to oxidation with nitric acid, hydrogen peroxide and ammonium persulfate and to thermal treatment under nitrogen atmosphere at temperatures ranging from 200 to 900 °C in order to modify their surface chemistry. High conversion values for 4-chlorophenol, well above 95%, were obtained working under mild conditions of temperature (50–75 °C) and pressure (2.4 bar). Modifications on the surface composition of the active carbon support upon oxidation and thermal treatment proved to affect to both conversion of 4-chlorophenol and selectivity towards cyclohexanol, the less toxic product in the reaction pathway. Increasing thermal treatment temperature showed to be detrimental in both respects whereas oxidation with nitric acid favored a higher selectivity to cyclohexanol. Those effects can be attributed more specifically to the relative presence of some oxygen groups on the surface of the support. Thus, the highest 4-chlorophenol conversions were obtained with the catalysts whose supports yielded higher amounts of CO2 upon temperature programmed desorption analysis. In particular, carboxylic acid and lactone groups where found to be relevant for a high conversion of 4-chlorophenol and their presence was also important to improve the selectivity to cyclohexanol. The presence of such groups can be related with a more homogeneous distribution of palladium on the catalysts surface.
Keywords: Surface oxygen groups; Active carbon; Palladium catalysts; Hydrodechlorination; 4-Chlorophenol;

Adsorbents for removal of hydrogen sulfide from moist air were prepared from sewage sludge, waste oil sludge and their 50:50 mixture by pyrolysis at 650 and 950 °C for either half an hour or an hour. The resulting materials were characterized using sorption of nitrogen, thermal analysis, pH measurements and X-ray diffraction. Their catalytic performance was evaluated in hydrogen sulfide reactive adsorption via a dynamic test. The adsorbents have high capacity for hydrogen sulfide removal and high selectivity for its conversion to elemental sulfur. That sulfur is stored in the pore system, mainly mesopores. In some cases, after the oxidation, a secondary pore system is formed within the deposited sulfur. Either the capacity or selectivity depends on the pyrolysis temperature, time, and the specific chemistry of the sludge precursors. They govern the development of porosity and formation of new mineral-like catalytic components through solid-state reaction.
Keywords: Sewage sludge; Industrial sludge; Pyrolysis; Adsorbents; Hydrogen sulfide; Catalytic activity;

Combining photo-Fenton process with aerobic sequencing batch reactor for commercial hetero-bireactive dye removal by Julia García-Montaño; Francesc Torrades; José A. García-Hortal; Xavier Domènech; José Peral (86-92).
A synthetic textile effluent containing an hetero-bireactive dye (Cibacron Red FN-R, 250 mg l−1) was treated by coupling photo-Fenton process with an aerobic sequencing batch reactor (SBR). The study is focused on the selection of suitable mild photo-Fenton conditions (Fe(II)/H2O2 reagents and irradiation time) that performs as a pre-treatment of a secondary biological treatment. Decolourisation (Abs542.5), biodegradability enhancement (determined by BOD5/COD index and respirometric methods) and dye degradation intermediates toxicity (Biotox® technique) were the parameters employed to verify the chemical stage effectiveness. From different assessed photo-Fenton conditions, a Fenton reagent concentration of 20 mg l−1 Fe(II) and 250 mg l−1 H2O2, applied for an irradiation time of 90 min, was found to be an effective process to bio-compatibilize the studied wastewater. The SBR, working under steady conditions in a 24-h-cycle mode, made available an 80% DOC removal in the combined oxidation system.
Keywords: Reactive azodye; Photo-Fenton's oxidation; Biodegradability enhancement; Aerobic sequencing batch reactor; Coupled oxidation system;

Electro-Fenton and photoelectro-Fenton degradation of indigo carmine in acidic aqueous medium by Cristina Flox; Salah Ammar; Conchita Arias; Enric Brillas; Aída Viridiana Vargas-Zavala; Ridha Abdelhedi (93-104).
Acidic aqueous solutions containing concentrations of the dye indigo carmine up to 0.9 g l−1 have been degraded by environmentally friendly electrochemical methods such as electro-Fenton and photoelectro-Fenton using an undivided cell with a Pt or boron-doped diamond (BDD) anode and an O2-diffusion cathode at 35.0 °C. The great oxidation ability of these systems is due to the large production of hydroxyl radical (•OH) at the anode surface from water oxidation and in the medium mainly from Fenton's reaction between catalytic Fe2+ and cathodically generated H2O2. Complete mineralization is feasible using electro-Fenton with a BDD anode and 1.0 mM Fe2+ and when 1.0 mM Fe2+ and 0.25 mM Cu2+ are combined as catalysts in photoelectro-Fenton with a Pt anode. The first method yields similar degradation rate in the pH range 2.0–4.0, whereas the second one is more potent up to 0.44 g l−1 of the dye at pH 3.0. Mineralization is enhanced with increasing current density and initial dye concentration. The indigo carmine decay always follows a pseudo zero-order reaction. Isatin 5-sulfonic acid, indigo and isatin are detected as aromatic products, which are degraded to oxalic and oxamic acids. The electro-Fenton process with BDD yields the destruction of Fe3+-oxalate and Fe3+-oxamate complexes by •OH at the anode surface. In the presence of Fe2+ and Cu2+ the photoelectro-Fenton process with Pt involves the photolysis of Fe3+-oxalate complexes under the action of UVA light, while competitive Cu2+-oxalate and Cu2+-oxamate complexes are mineralized with •OH produced by Fenton's reaction. The nitrogen of the dye is mainly converted into NH4 +.
Keywords: Indigo carmine; Boron-doped diamond anode; Electro-Fenton; Photoelectro-Fenton; Catalysis;

The influence of the ceria catalysts preparation method on their structure and activity in water gas shift reaction has been studied. Ceria supports were prepared by the precipitation method from aqueous and organic solutions (e.g. alcohols) and the impregnation method used for active metal loading. Ceria catalysts were also prepared, by the coprecipitation method for total concentration of nitrate salts ranging from 0.006 to 0.18 mol/L. All catalysts were tested under conditions of the water gas shift (WGS) reaction. The catalysts were characterized by BET, XRD and SEM analyses and their activities were tested in a flow reactor in the temperature range of 150–350 °C with gas hourly space velocity (GHSV) in the range of 750–16,000 h−1. The catalysts prepared by the coprecipitation method present the highest activity.
Keywords: Water gas shift; Ceria-based catalysts; Catalysts activity;

The objective of this work was to analyze the activity of petroleum coke and of activated carbons prepared from this material in the ozonation of aromatic contaminants of low biodegradability. 1,3,6-Naphthalenetrisulphonic acid (NTS) was selected as model compound for the study. The presence of the original coke during 1,3,6-naphthalenetrisulphonic acid ozonation produced a slight increase in the NTS degradation rate, indicating the generation of highly oxidant species in the medium. The coke was chemically activated with KOH. The activation process considerably develops the micro-, meso- and macroporosity of the raw material. The surface chemical nature of the original coke was also modified by the activation process, increasing its surface basicity. The coke activation increased its activity in the NTS ozonation process due to: (i) development of the porosity of the coke, allowing greater accessibility of the ozone to its surface active sites and mineral matter and (ii) an increase in the surface basicity of the original material. The presence of either the original or activated coke reduces the concentration of total dissolved organic carbon (DOC) during NTS ozonation. This reduction is due to: (i) mineralization of organic carbon to CO2 because of the generation of •OH radicals enhanced by the presence of the coke in the system (catalytic contribution), and (ii) adsorption of NTS oxidation by-products on the coke samples (adsorptive contribution).
Keywords: Ozone; Petroleum coke; 1,3,6-Naphthalenetrisulphonic acid (NTS);

A new recirculating enzymatic membrane reactor for ester synthesis in ionic liquid/supercritical carbon dioxide biphasic systems by Francisco J. Hernández; Antonia P. de los Ríos; Demetrio Gómez; Manuel Rubio; Gloria Víllora (121-126).
Dynamic membranes with immobilized Candida antarctica lipase B (CALB) were successfully applied for butyl propionate synthesis in a recirculating bioreactor in supercritical carbon dioxide medium and in room temperature ionic liquid/supercritical carbon dioxide biphasic systems at 50 °C and 80 bar.In room temperature ionic liquid/supercritical carbon dioxide biphasic systems, the immobilized enzyme coated with three different ionic liquids based on dialkylimidazolium cations, showed an increase in the selectivity of the process (up to 99.5%) compared with supercritical carbon dioxide assayed in the absence of room temperature ionic liquid. For the RTILs assayed, it was observed that the increase in activity was in agreement with an increase in the partition coefficients of the substrates between RTIL and hexane.Additionally, a model based on the Ping Pong Bi–Bi mechanism with competitive alcohol inhibition is proposed to describe the kinetics of the transesterification reaction in supercritical carbon dioxide.
Keywords: Ionic liquids; Supercritical fluids; Lipase; Membrane reactor;

The interaction of NO x (N2O, NO and NO2) with γ-Al2O3 taken as a mineral dust model at 225–360 K is studied by mass spectrometry at low pressures corresponding to typical concentrations of these pollutants in the atmosphere. It is found that, at low coverage (<0.01 monolayer), adsorbed NO x molecules are strongly bonded to surface centers and cannot be removed by evacuation at room temperature. According to temperature-programmed desorption data, desorption energies of N2O and NO fall in the intervals of 120–140 and 90–135 kJ mol−1, respectively. N2O adsorption is accompanied by partial dissociation into N2 and O2, whereas NO2 adsorption leads to complete dissociation to NO and O2. Adsorption of N2O and NO2 is accompanied by chemiluminescence with a maximum emission at 420 nm. NO adsorption does not result in appreciable dissociation. Upon UV-irradiation of Al2O3 in a N2O flow, partial N2O decomposition into N2 and O2 and parallel N2O desorption from the surface are observed, while in the case of a NO flow, only photoinduced NO desorption is detected. It is assumed that the dissociative adsorption of N2O and NO2, chemiluminescence and photoinduced N2O dissociation occur via the capture by the adsorbed molecules of electrons from surface F- and F+-centers of Al2O3 to give rise to Oads 2− and/or Oads species. The photodesorption efficiency of N2O and NO correlates with absorption spectrum of Al2O3 in the UV range and may be associated with light-induced decomposition of complexes formed by these molecules with surface centers. Thermodesorption of preadsorbed N2O and NO from Al2O3 occurs predominantly without decomposition of these molecules, whereas the major desorption product of NO2 is NO. The desorption-rate maxima and the product distribution depend on the initial surface coverage indicating inhomogeneity of the adsorption centers.
Keywords: Air pollution; Mineral dust; Adsorption; Alumina; NO x ; UV-irradiation; Mass spectrometry; Temperature-programmed desorption;

Simulation of heterogeneously MgO-catalyzed transesterification for fine-chemical and biodiesel industrial production by Tanguy F. Dossin; Marie-Françoise Reyniers; Rob J. Berger; Guy B. Marin (136-148).
A heterogeneous magnesium oxide catalyst is a good alternative for homogeneous catalysts for the transesterification of alkyl esters for the production of fine-chemicals as well as for the production of biodiesel. The transesterification of ethyl acetate with methanol was used as a model reaction to simulate fine-chemical production in a batch slurry reactor at industrial conditions. The transesterification of triolein with methanol to methyl oleate was chosen to simulate continuous production of biodiesel from rapeseed oil. A kinetic model based on a three-step ‘Eley–Rideal’ type of mechanism in the liquid phase was used in both process simulations. The transesterification reaction occurs between methanol adsorbed on a magnesium oxide free basic site and ethyl acetate or the glyceride from the liquid phase. Methanol adsorption is assumed to be rate-determining in both processes. Activity coefficients were required to account for the significant non-ideality of the reaction mixture in the simulations of both processes. The simulations indicate that a production of 500 tonnes methyl acetate per year can be reached at ambient temperature in a batch reactor of 10 m3 containing 5 kg of MgO catalyst, and that a continuous production of 100,000 tonnes of biodiesel per year can be achieved at 323 K in a continuous stirred reactor of 25 m3 containing 5700 kg of MgO catalyst. Although various assumptions and simplifications were made in these explorative simulations the assumptions concerning the reaction kinetics used, the results indicate that for both processes a heterogeneous magnesium oxide catalyst shows promising potential as a viable industrial scale alternative.
Keywords: Transesterification; Solid base catalyst; Magnesium oxide; Reactor modeling; Industrial simulation; Biodiesel; Rapeseed oil; Kinetics;