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Applied Catalysis B, Environmental (v.93, #1-2)
Comparative study of La–Sr–Fe–O perovskite-type oxides prepared by ceramic and surfactant methods over the CH4 and H2 lean-deNO x by V.N. Stathopoulos; V.C. Belessi; T.V. Bakas; S.G. Neophytides; C.N. Costa; P.J. Pomonis; A.M. Efstathiou (pp. 1-11).
Mixed oxides of the general formula La0.8Sr0.2FeO3− x were prepared by ceramic and surfactant methods and tested towards the NO/H2/O2 and NO/CH4/O2 lean-deNO x reactions in the 200–450°C range. The materials were characterized for their bulk and surface composition by XRD, Mössbauer and XPS techniques. The main crystal phases detected were perovskites LaFeO3 and SrFeO3− x, and α-Fe2O3. The Mössbauer technique allowed the precise determination of the composition of Fe-containing phases in the solids prepared by both the ceramic and surfactant methods. Significant differences in the composition of the phases and the surface chemical states of Fe, Sr and La were revealed among the two solid compositions. For the CH4 lean-deNO x reaction at 350°C, the La0.8Sr0.2FeO3− x solid composition (LSF-2) prepared by the surfactant method exhibited an increase by 50% in the rate of NO conversion and by 30% in the rate of N2 formation (per gram basis) as compared to the solid prepared by the ceramic method (LSF-1). An opposite relative catalytic activity behavior was observed for the two solids when tested towards the H2 lean-deNO x reaction. At 375°C (maximum activity observed) the solid prepared by the ceramic method (LSF-1) exhibited an increase by 83% in the rate of N2 formation as compared to the solid prepared by the surfactant method (LSF-2). It is suggested that the mobility of surface lattice oxygen, the concentration of oxygen vacant sites, and the mixed valences of metal cations present in the La0.8Sr0.2FeO3− x solid all influenced by the preparation method used are the main parameters that largely influenced the catalytic behavior observed over the LSF-1 and LSF-2 solids.
Keywords: Perovskites; NO reduction by H; 2; NO reduction by CH; 4; Lean-deNO; x
NO x storage and reduction properties of Pt/Ce xZr1− xO2 mixed oxides: Sulfur resistance and regeneration, and ammonia formation by N. Le Phuc; E.C. Corbos; X. Courtois; F. Can; P. Marecot; D. Duprez (pp. 12-21).
The influence of the ceria-zirconia mixed oxide composition in Pt/Ce xZr1− xO2 catalysts was studied toward NO x storage capacity (NSC), including sulfur poisoning and sulfur regeneration, and NO x reduction efficiency in lean–rich cycling conditions. The results are compared with a Pt/Ba/Al model catalyst. The samples were characterized by N2 adsorption, XRD and H2-TPR. The behaviors of the ceria-zirconia supported catalysts are quite similar whatever their composition. They are sensitive to a reducing pretreatment which leads to an increase of (i) the cerium reducibility/oxygen mobility, (ii) the NO oxidation rate and (iii) the NO x storage capacity at 300 and 400°C. The sulfating treatment leads to a dramatic decrease of the NO x storage capacity for all catalysts, the decrease being more pronounced for the Zr-rich samples. H2-TPR experiments show that the sulfates amount and their stability tend to increase with the Zr content, but these sulfates are significantly less stable compared with Pt/Ba/Al. The sulfur elimination rates in rich mixture at 550°C are higher than 90% with the ceria-zirconia supported catalysts versus 56% with Pt/Ba/Al.The ceria-zirconia supported catalysts are able to convert NO x in lean–rich cycling condition. Compared with Pt/Ba/Al, the NO x conversions are a little lowered but the ammonia selectivity is significantly decreased with the Ce–Zr mixed oxides, with a beneficial influence of the cerium content.
Keywords: NO; x; Storage; Reduction; Ceria; Zirconia; Ammonia formation
Inverse hysteresis during the NO oxidation on Pt under lean conditions by W. Hauptmann; M. Votsmeier; J. Gieshoff; A. Drochner; H. Vogel (pp. 22-29).
It is shown that a platinum catalyst operated under a transient temperature profile exhibits a hysteresis behavior. The catalytic activity during heat-up phases exceeds the activity of the cool down phases. This effect is the opposite of the well-known hysteresis that can be caused by the existence of multiple steady states or the thermal inertia of the catalyst. It is therefore referred to as ‘inverse hysteresis’.If the catalyst is operated at one temperature for a longer time, a steady state activity is observed that is in-between the higher activity of the transient heat-up and the lower activity of the transient cool down.The most likely explanation for the observed hysteresis behavior is a reversible oxidation of the platinum surface by NO2 and a reduction of the platinum surface by NO. A micro-kinetic surface mechanism including the reversible surface oxidation has been developed and implemented in a monolith reactor model. The observed hysteresis effects can be quantitatively reproduced by the numerical model.If the feed contains CO/NO/O2 an inverse hysteresis is observed for NO and CO oxidation. This means that in the presence of NO the hysteresis behavior of the CO oxidation switches from the typical standard hysteresis to an inverse hysteresis.
Keywords: NO oxidation; Deactivation; Diesel exhaust; Platinum; Monolithic catalyst; Inverse hysteresis; Micro-kinetic model
Manganese oxide catalysts synthesized by exotemplating for the total oxidation of ethanol by S.S.T. Bastos; J.J.M. Órfão; M.M.A. Freitas; M.F.R. Pereira; J.L. Figueiredo (pp. 30-37).
Manganese oxides were synthesized by exotemplating using an activated carbon and a carbon xerogel as templates. The template effect and the impregnation method were studied. Up to a seven fold increase in the surface area was obtained for the materials prepared by exotemplating when compared to the standard calcination method. The catalytic performance of these manganese oxide materials was assessed in the total oxidation of ethanol. The catalytic activity was found to be mainly dependent on the manganese oxidation state and to a lower extent on the surface area. The best catalysts were able to oxidize ethanol into 100% of CO2 at 240°C. Experiments without oxygen in the feed suggest that the lattice oxygen can react with ethanol and is involved in the ethanol oxidation mechanism. There is a strong correlation between the catalytic activity and the lattice oxygen donating ability, which depends on the oxidation state of manganese and the surface area.
Keywords: Manganese oxide; Exotemplating; Catalytic oxidation; Ethanol; VOC
Structural properties, redox behaviour and methane combustion activity of differently supported flame-made Pd catalysts by Niels van Vegten; Marek Maciejewski; Frank Krumeich; Alfons Baiker (pp. 38-49).
Catalysts made up of Pd on different supports (Al2O3, SiO2, ZrO2, TiO2, MgAl2O4, and CeO2) have been synthesized by single-step flame-spray pyrolysis. The as-prepared, non-porous, nano-sized particles showed high surface area and compared to a conventional alumina supported palladium catalyst a relatively high content of PdO. Another characteristic feature of the flame-made materials is that part of the Pd-constituent was stabilized in the supporting oxide matrix, and thus inaccessible, as pertinent dissolution tests indicated. Characterization of the oxidation state of the Pd-component in different atmospheres and under reaction conditions revealed a strong correlation between the amount of PdO and methane combustion activity, although under reaction conditions, metallic Pd was still present. The origin of the observed hysteresis behaviour in methane conversion in heating–cooling cycles was investigated by in situ thermogravimetry combined with mass spectrometry, and confirmed in fixed-bed reactor tests. A strong correlation between the light-off temperature during methane oxidation and the shift of the characteristic PdO Raman signal at ca. 650cm−1 was observed. Samples with an increasing light-off temperature show a Raman shift shifting to that of bulk PdO, indicating an increase in light-off temperature with increasing particle size. Another property identified to be crucial for high activity is the thermal stability of the support, which was strongly correlated to the extent of PdO regeneration during cooling. Amongst the supports tested, the flame-made Al2O3 supported catalyst proved to possess the highest thermal stability and activity in methane combustion.
Keywords: Methane combustion; Flame-spray pyrolysis; In situ; TG–MS; Al; 2; O; 3; CeO; 2; MgAl; 2; O; 4; SiO; 2; TiO; 2; ZrO; 2; Oxidation state of Pd
Palladium–tin catalysts on conducting polymers for nitrate removal by Ibrahim Dodouche; Danns Pereira Barbosa; Maria do Carmo Rangel; Florence Epron (pp. 50-55).
Palladium–tin catalysts were prepared by successive impregnation or co-impregnation onto polyaniline and polypyrrole. The catalytic tests showed that this type of catalyst is active for nitrate reduction. The use of polymer support improves the selectivity of the catalyst toward nitrogen formation compared to a classical support, and avoids the apparition of intermediate nitrite. These better performances of the catalysts supported on electroactive polymers were explained by the ion-exchange properties of the conducting polymers. The most selective catalyst was Pd–Sn/PPy catalyst prepared by successive impregnation. The lower selectivity obtained with the Pani-supported catalyst was explained by the capability of this polymer to directly reduce nitrate into ammonium ions.
Keywords: Electroactive polymers; Palladium–tin; Catalytic properties; Nitrate reduction
Synthesis of pillared clays with Al–Fe and Al–Fe–Ce starting from concentrated suspensions of clay using microwaves or ultrasound, and their catalytic activity in the phenol oxidation reaction by A. Olaya; G. Blanco; S. Bernal; S. Moreno; R. Molina (pp. 56-65).
A new methodology for the synthesis of pillared clays with Al–Fe and Al–Fe–Ce was developed, in which the clay is added directly to the pillaring solution, and microwaves or ultrasound are used in the synthesis of the pillaring solution as well as in the intercalation process. That methodology allows the volumes of water and the processing time to decrease significantly compared to the conventional method of synthesis of pillared clays. The analysis of XRD, XRF and adsorption–desorption isotherms show that the synthesized solids present more intense and homogeneous signals of pillaring and better texture properties than the clay pillared using the conventional method of synthesis. The XPS analysis shows that the Fe and Ce species are located preferably in the inner parts of the pillared clays. The catalytic activity of the pillared clays was evaluated in the phenol oxidation reaction in a diluted aqueous medium where the synthesized materials showed catalytic properties comparable to those of the materials synthesized through the conventional method reported in the literature.
Keywords: Pillared clays; Microwaves; Ultrasound; Keggin; Phenol oxidation
Spatially resolved in situ FTIR analysis of CO adsorption and reaction on Pt/SiO2 in a silicon microreactor by Chung Kwang Christopher Tan; W. Nicholas Delgass; Chelsey D. Baertsch (pp. 66-74).
The design, fabrication and testing of a microreactor-FTIR imaging system is shown and used for the first time to demonstrate the ability to obtain in situ transmission FTIR analysis of working catalysts with both spatial and temporal resolution. MEMS (MicroElectroMechanical Systems) and microfabrication technologies were used to design and fabricate a microreactor with geometric and optical properties ideal for coupling with a high-throughput transmission FPA-FTIR system. CO adsorption and oxidation on Pt/SiO2 were used as a model catalyst system. Propagation of adsorbed species down the length of the microreactor was observed and fractional coverages were quantified during pulsed chemisorption experiments. The amount of adsorbed CO was also differentiated at different positions in the microreactor as a function of time during oxidation of the stored surface species.
Keywords: Focal plane array; MEMS; Microreactor; In situ; spectroscopy; Spatial resolution; CO oxidation
Development of conducting polypyrrole as corrosion-resistant catalyst support for polymer electrolyte membrane fuel cell (PEMFC) application by Sheng-Yang Huang; Prabhu Ganesan; Branko N. Popov (pp. 75-81).
Three polypyrrole (PPy) samples were synthesized via in situ chemical oxidative polymerization to study their applications as an alternate cathode catalyst support material for polymer electrolyte membrane fuel cells (PEMFCs). The electrochemical stabilities of the PPy samples and commercial carbon supports (Vulcan XC-72) were examined by cyclic voltammetry (CV). In contrast to the carbon supports, only a small anodic current was observed for the PPy samples up to 1.8V, indicating their resistance towards oxidation under high positive potentials. Among the PPy samples synthesized in this investigation, PPy-3 showed the highest electrical conductivity (1.67Scm−1) and BET surface area of 69.6m2g−1, and therefore was selected as a catalyst support for further studies. The wet reduction method was chosen to prepare supported platinum catalyst (50wt% Pt/PPy) by using sodium formate as reducing reagent. Transmission electron microscopy (TEM) image showed finely dispersed platinum crystallites with a diameter of dPt=3.6nm on the polypyrrole support. The potential cycling experiment revealed that Pt/PPy is electrochemically more stable than the Pt/C electrocatalyst. The oxygen reduction activity obtained from Tafel plots indicated twofold higher activity for Pt/PPy catalyst than that of Pt black catalyst at 0.9V. Comparison of ORR activity and fuel cell polarization curves demonstrated good oxygen reduction reaction kinetics and comparable fuel cell performance with that of commercial E-TEK Pt/C catalyst for the Pt/PPy catalyst.
Keywords: Conducting polymer; Polypyrrole; Cathode catalyst support; Corrosion-resistance; Oxygen reduction reaction; Proton exchange membrane fuel cell
Role of surface and lattice copper species in copper-containing (Mg/Sr)TiO3 perovskite catalysts for soot combustion by F.E. López-Suárez; S. Parres-Esclapez; A. Bueno-López; M.J. Illán-Gómez; B. Ura; J. Trawczynski (pp. 82-89).
Copper-containing (Mg/Sr)TiO3 perovskites have been prepared and characterized by ICP-OES, N2 adsorption at −196°C, XRD, XPS and H2-TPR. The catalytic activity for soot combustion has been studied in a fixed-bed reactor under a NO+O2 mixture, and the interaction between the reactive gas mixture and selected catalysts has been studied by in situ DRIFT experiments.The role of the catalysts is to oxidize NO to NO2, and a relationship between NO oxidation capacity and soot combustion activity has been obtained. SrTi0.89Cu0.11O3 is the most active catalyst among those studied because it combines a high NO x chemisorption capacity with a high oxidation rate of the NO x species chemisorbed. Surface CuO is the active component of the samples Cu/SrTiO3 and Cu/MgTiO3, where the support promotes the activity of CuO, SrTiO3 being preferable to MgTiO3. On the contrary, active sites are not necessarily linked to the copper cations in the catalysts SrTi0.89Cu0.11O3 and MgTi0.9Cu0.1O3. Lattice copper promotes significantly the catalytic activity of the mixed oxide SrTi0.89Cu0.11O3, but a much lower promoting effect is observed for MgTi0.9Cu0.1O3.
Keywords: Diesel soot; Soot combustion; Copper catalyst; Perovskite; NO; x; In situ; DRIFTS
Fabrication and characterization of brookite-rich, visible light-active TiO2 films for water splitting by Hui Pan; Xiaofeng Qiu; Ilia N. Ivanov; Harry M. Meyer; Wei Wang; Wenguang Zhu; M. Parans Paranthaman; Zhenyu Zhang; Gyula Eres; Baohua Gu (pp. 90-95).
We report that mild oxidation of Ti foils in air results in brookite-rich titanium oxide (TiO2) films with similar spectral response to that of dye-sensitized TiO2. X-ray powder diffraction and Raman spectroscopy show that the onset of brookite formation occurs at 500°C, and the material is characterized by a strong absorption band in the visible spectral range. The first-principle calculations show that enhanced visible light absorption correlates with the presence of Ti interstitials. Photocurrent density measurements of water splitting reveal that the brookite-rich TiO2 exhibits the highest photocatalytic performance among the different forms of TiO2 produced by oxidation of Ti foils. With increasing oxidation temperature transformation to the rutile phase accompanied by declining visible range photoactivity is observed.
Keywords: Brookite; TiO; 2; Water splitting; Catalysis; Bandgap; Photoenergy
New insights on solar photocatalytic degradation of phenol over Fe-TiO2 catalysts: Photo-complex mechanism of iron lixiviates by Cristina Adán; Arturo Martínez-Arias; Sixto Malato; Ana Bahamonde (pp. 96-105).
The influence of iron content and calcination temperature applied during catalyst preparation on the solar light assisted photo-degradation of phenol over iron-doped titania catalysts have been studied. Pure titania catalysts were also analysed for comparative purpose. An improvement in phenol and TOC removal was detected upon increasing calcination temperature. Although subtle differences in evolution and distribution of short-organic acids were observed upon increasing the calcination temperature, phenol photo-degradation apparently proceeded through the same oxidation pathway over all analysed catalysts. A photo-complex mechanism of iron lixiviates by which the reduced Fe(II) ions in solution become re-adsorbed on the catalyst surface and subsequently re-oxidized by photogenerated holes, leaving the catalyst ready for a new photo-degradation cycle, has been postulated to explain the obtained results. Iron is proposed to be extracted from the catalysts surface through formation of a [Fe-carboxylic acid]2+ complex within a process in which refractory carboxylic acids simultaneously undergo further oxidation. XPS results and the absence of differences in the iron content determined by chemical analysis when comparing fresh and used catalysts confirm the re-adsorption phenomena of iron lixiviates on the catalyst surface.
Keywords: Titania; Iron; Solar; Photocatalysts; Phenol; Photo-complex; Lixiviates
Destruction of PAH and dioxin precursors using selective oxidation over zeolite catalysts. Influence of the presence of ammonia in the flue gas by S.C. Marie-Rose; T. Belin; J. Mijoin; E. Fiani; M. Taralunga; X. Chaucherie; F. Nicol; P. Magnoux (pp. 106-111).
This work reports the influence of ammonia during 1-methylnaphthalene (1-MN) and 1-methylnaphthalene/1,2-dichlorobenzene mixture oxidation over USHY zeolite and 0.8%PtUSHY catalyst. The USHY zeolite was able to transform 1-MN into carbon dioxide from 400°C whatever the reaction conditions. However the presence of ammonia improves the transformation rate of 1-MN: 1-MN was totally transformed from 300°C when the reaction was carried out in the presence of ammonia. Whatever the reaction temperature, only carbon dioxide was formed from 1-MN and no aromatic by-products were observed. Over the PtUSHY catalyst, the same phenomenon was observed in the 200–300°C temperature range. Indeed, the 1-MN was totally transformed from 200°C when the reaction was carried out with ammonia against 250°C when 1-MN was alone. It was also shown that the presence of NH3 favoured the 1-MN transformation into adsorbed oxygenated polyaromatic compounds over the USHY zeolite. In the case of PtUSHY catalyst, the presence of NH3 improves the transformation rate of 1-MN into carbon dioxide by formation of NO2, which can participate in the oxidation reaction of 1-MN.The oxidation reaction of 1-methylnaphthalene/1,2-dichlorobenzene (1-MN/1,2PhCl2) mixture was also studied in the absence and presence of NH3 over PtUSHY catalyst. The presence of 1,2PhCl2 and 1,2PhCl2/NH3 had no influence over 1-MN transformation in the temperature range studied (250–400°C). Indeed, 1-MN was completely converted into CO2 from 250°C. 1,2PhCl2 appears as the most difficult compound to destroy. However, the presence of NH3 leads to a promoting effect over 1,2PhCl2 transformation rate. 1,2PhCl2 was totally transformed from 350°C in the presence of NH3 against 400°C in the absence of NH3. As for 1-MN this promoting effect can be explained by the NH3 oxidation over Pt atoms leading to NO2 formation. Some experiments carried out with NO confirm that NO2 formation (formed by the oxidation of NH3) participate in the oxidation reaction. However, for an initial concentration of NH3 up to 100ppm, an inhibitory effect over the 1,2PhCl2 transformation rate was observed.
Keywords: POPs; Faujasite; USHY; Platinum; NO; NO; 2
Photocatalytic inactivation of bacteria in water using suspended and immobilized silver-TiO2 by Rafael van Grieken; Javier Marugán; Carlos Sordo; Patricia Martínez; Cristina Pablos (pp. 112-118).
Incorporation of silver to titanium dioxide is of great interest for photocatalytic disinfection applications since in addition to the enhancement of the electron–hole separation and interfacial charge transfer and the increase in the visible light response, silver compounds present a strong bactericidal effect. Ag/TiO2 materials used in suspension and immobilized in two different configurations (catalytic wall and fixed-bed reactors) have been prepared, characterized and tested using Escherichia coli as model microorganism. Although the incorporation of silver to powdered Degussa P25 TiO2 increases the activity, the thermal treatment required for the stabilization of the supported metal particles reduces the global efficiency. The comparison with experiments of dye photodegradation indicates that the activity of Ag/TiO2 is mainly due to the bactericidal role of silver and not to the enhancement of the photocatalytic mechanism. The best tested system has been proved to be the Ag/TiO2 catalytic wall reactor with a 0.6wt.% of Ag loading, showing a high activity both in relative (per gram of TiO2) and absolute terms, an optimal use of the radiation source, and a good stability of the film with negligible silver lixiviation, allowing the continuous treatment of water.
Keywords: Photocatalysis; Disinfection; Silver; E. coli; Immobilized TiO; 2
Free-standing TiO2 nanotube array films sensitized with CdS as highly active solar light-driven photocatalysts by Chin Jung Lin; Yi Hsien Yu; Ya Hsuan Liou (pp. 119-125).
Self-organized, large-scale, free-standing TiO2 nanotube (TiNT)-array films were fabricated via anodization of titanium (Ti) foil in fluorine-containing ethylene glycol, followed by a facile detachment method to flake the TiNT arrays off the Ti substrate. After annealing, the resulting film consisted of well-ordered, vertically oriented, nanocrystalline TiNT arrays of ∼130nm pore diameter, ∼25nm wall thickness, and ∼46μm length, corresponding to a high aspect ratio (the length/diameter) of ∼250. The surface modification of the TiNT-array film using a simple solution-based method was carried out to fabricate a CdS nanoparticle-sensitized TiNT heterostructure-array (CdS/TiNT) film. Except for the significant shift in spectral photoresponse to a lower energy, the induced electron–hole pairs were extracted efficiently by their type-II band alignments in the CdS/TiNT arrays. This was confirmed by the incident photon-to-electron efficiency measurements and the kinetics of photocurrent decay in response to on-off irradiation. As a result of the strong absorption within the solar spectrum and the effective suppression of electron–hole pair recombination, a significant increase of ∼10 times in the apparent first-order rate constant ( k obs) of methylene blue (MB) photo-degradation under simulated sunlight (AM 1.5G) illumination was obtained as compared with the commercial Degussa P25 film. This free-standing film provides the advantage of the use of back-side illumination where light directly strikes the CdS/TiNT arrays, largely reducing incident light loss through the solution. In this case, the k obs value was further increased by ∼30% compared with that in front-side illumination. The ease of free-standing TiO2 nanotube array film fabrication and CdS sensitization in this process will facilitate the development of environmental contaminants’ solar light-driven decomposition.
Keywords: Titanium dioxide; Nanotube; Photocatalyst; Visible light
Hydrogen production and performance of nickel based catalysts synthesized using supercritical fluids for the gasification of biomass by André D. Taylor; Gregory J. DiLeo; Kai Sun (pp. 126-133).
A method for synthesizing carbon nanotube/fiber and aluminosilicate supported Ni catalysts using supercritical methanol is suggested. This one-pot synthesis method allows the metal precursor (nickel acetylacetonate) to reduce directly onto the surface of the carbon structure without the pretreatment of surface functional groups. The synthesized catalysts are confirmed to be active in the gasification of biomass in supercritical water—specifically for the production of hydrogen-rich gases. Although the synthesized catalysts can produce similar hydrogen yields (∼8mmol H2/g biomass) compared to two commercially available aluminosilicate supported Ni catalysts, the gas product ratio for producing hydrogen over methane for the synthesized catalysts was 3.5mol H2/mol CH4 compared to 1.3mol H2/mol CH4 for the commercial catalysts under the same conditions.
Keywords: Supercritical fluids; Nickel; Gasification; Biomass; Nanomaterials; Catalysts; Carbon nanotubes; Carbon fibers; Supercritical water gasification; Hydrogen generation; Renewable energy
Sulfated tin oxide as solid superacid catalyst for transesterification of waste cooking oil: An optimization study by Man Kee Lam; Keat Teong Lee; Abdul Rahman Mohamed (pp. 134-139).
Biodiesel is a renewable, biodegradable and non-toxic fuel which can be easily produced through transesterification reaction. However, current commercial usage of refined vegetable oils for biodiesel production is impractical and uneconomical due to high feedstock cost and priority as food resources. Low-grade oil, typically waste cooking oil can be a better alternative; however, the high free fatty acids (FFA) content in waste cooking oil does not allow efficient production of biodiesel via current commercial homogeneous transesterification process. Therefore in the present study, superacid sulfated tin oxide catalyst, SO42−/SnO2 has been prepared using impregnation method for biodiesel production via heterogeneous tranesterification process. The bi-metallic effect of the catalyst was also studied, in which SnO2 was mixed with SiO2 and Al2O3, respectively, at different weight ratios in order to enhance the catalytic activity of SnO2. The effect of different reaction parameters such as calcination temperature and period of the catalyst, reaction temperature, catalyst loading, methanol-to-oil ratio and reaction time were studied to optimize the reaction conditions. It was found that SO42−/SnO2-SiO2 with weight ratio 3 exhibited an exceptional high activity with an optimum yield of 92.3% at reaction temperature 150°C, catalyst loading 3wt%, methanol-to-oil ratio 15 and reaction time 3h. The physical and chemical properties of the catalysts were also characterized using XRD analysis and FT-IR imaging.
Keywords: Solid acid catalyst; Sulfated tin oxide; Biodiesel; Waste cooking oil
Ionic liquid templated TiO2 nanoparticles as a support in gold environmental catalysis by Rafael S. Avellaneda; Svetlana Ivanova; Oihane Sanz; Francisca Romero-Sarria; Miguel Angel Centeno; Jose Antonio Odriozola (pp. 140-148).
This work presents the synthesis of a nanostructured titania support and its subsequent utilization for the gold particles deposition and application in the reaction of the CO oxidation. A functionalized ionic liquid has been used as a templating agent for the titanium oxide synthesis resulting in a high specific surface nanostructured titania anatase. The as prepared support was then used for gold nanoparticles deposition without ionic liquid removal in order to study the possible role of the latter in the stabilization of the gold particles. The presence of ionic liquid in the catalysts results in an unusual catalytic behaviour—strong dependence on the presence of CO and changed kinetics and rate of oxidation.
Keywords: TiO; 2; nanoparticles; Ionic liquid; Gold catalysts; CO oxidation
Hydrothermal N-doped TiO2: Explaining photocatalytic properties by electronic and magnetic identification of N active sites by Massimiliano D’Arienzo; Roberto Scotti; Laura Wahba; Chiara Battocchio; Edoardo Bemporad; Angeloclaudio Nale; Franca Morazzoni (pp. 149-155).
N-doped TiO2 nanocrystals with high photoactivity in the visible range, were successfully synthesized by hydrothermal method, followed by thermal annealing at different temperatures (350–600°C), in order to allow differential nitrogen diffusion into the TiO2 lattice. Optical and magnetic properties, studied by diffuse reflectance spectroscopy, electron paramagnetic resonance and X-ray photoelectron spectroscopy analysis, revealed that TiO2 was effectively doped. The thermal treatment induces insertion of nitrogen into TiO2 lattice in the form of nitride anion N−, detected as N by EPR, whose ionic character varies with the temperature of annealing. The amount of N increases till 450°C, then it decreases. Similar trend was observed for the photomineralization of phenol under visible light irradiation ( λ>385nm): the photoactivity of N-doped samples becomes maximum for N–TiO2 annealed at 450°C. The overall results suggest that the efficacy of the catalyst depends on the ability of N− centers to trap photogenerated holes. This effect lowers the rate of electron–hole recombination and allows the N (N−+h+) center acts as strong oxidizing agent.
Keywords: Nitrogen-doped titanium dioxide; Photocatalytic activity under visible light; Optical absorption; EPR spectroscopy
Development of non-precious metal oxygen-reduction catalysts for PEM fuel cells based on N-doped ordered porous carbon by Gang Liu; Xuguang Li; Prabhu Ganesan; Branko N. Popov (pp. 156-165).
N-doped ordered porous carbon (CN x) was synthesized via a nano-casting process using polyacrylonitrile (PAN) as the carbon and nitrogen precursor and mesoporous silica as a hard template. Nitrogen adsorption/desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the synthesized CN x and the derived non-precious metal oxygen-reduction catalysts. The CN x exhibited a highly ordered porosity and high graphitization with a surface area of 1132m2g−1 and a nitrogen content of 6.88at.%. The non-precious metal oxygen-reduction catalysts were prepared by pyrolyzing iron acetate-impregnated CN x in argon, followed by post-treatments. Optimizations of the iron loading and the pyrolyzing temperature were also explored. The catalytic activities of the CN x products for the oxygen reduction reaction (ORR) were examined by rotating disc electrode (RDE) measurements and single-cell tests. The onset potential for oxygen reduction in 0.5M H2SO4 of the best catalyst was as high as 0.88V vs. normal hydrogen electrode (NHE). The current density obtained in an H2/O2 proton exchange membrane fuel cell (PEMFC) was as high as 0.6Acm−2 at 0.5V with a cathode catalyst loading of 2mgcm−2.
Keywords: N-doped carbon; Ordered porosity; Oxygen reduction reaction; Non-precious metal catalysts; PEM fuel cells
The role of monomeric iron during the selective catalytic reduction of NO x by NH3 over Fe-BEA zeolite catalysts by Martin Høj; Matthias Josef Beier; Jan-Dierk Grunwaldt; Søren Dahl (pp. 166-176).
A series of highly active Fe-BEA iron zeolite catalysts for selective catalytic reduction (SCR) of NO x with ammonia have been prepared by incipient wetness impregnation and aqueous ion exchange with Fe(NO3)3 solutions. While the catalysts were stable during short term hydrothermal ageing, extended periods of ageing did reduce activity. The catalysts were investigated by diffuse reflectance UV–vis spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, ammonia temperature programmed desorption (NH3-TPD), X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) both ex-situ and in-situ with online gas analysis to measure catalytic activity.UV–vis, EPR and EXAFS showed that low iron loadings (≤1.2wt.% Fe) resulted in mostly iron monomers, especially for the ion exchanged samples. In contrast a mixture of monomers, oligomers and hematite particles was formed at intermediate to high loadings (2.5–5.1wt.% Fe). Linear combination analysis of UV–vis spectra, with real spectra and one Gaussian as basis, was used for quantifying the three iron species. A good correlation between iron monomer content and NO conversion was observed. In-situ XANES and EXAFS showed that heating in oxygen atmosphere removes water from the iron coordination sphere and the resultant vacant positions are occupied by NO, NH3 or H2O upon exposing the catalyst to a gas mixture of NO, NH3 and O2 in He. Heating in this gas mixture gave a distinct correlation between the catalytic performance and the oxidation state of iron, which is more pronounced in the catalysts where mostly iron monomers are present. Iron was more partially reduced when the SCR activity increased. These results support a mechanism where the oxidation of iron or NO is the rate-determining step.
Keywords: SCR; Iron BEA zeolite; UV–vis; In-situ EXAFS; Structure-performance relationship
The influence of chemical and thermal aging on the catalytic activity of a monolithic diesel oxidation catalyst by Alexander Winkler; Davide Ferri; Myriam Aguirre (pp. 177-184).
Chemical and thermal aging effects and their influence on the catalytic activity were analyzed for a commercial engine aged diesel oxidation catalyst (DOC). Samples were taken from the inlet (DOCin), the middle (DOCmid) and the outlet (DOCout) area of the ceramic monolith and were compared with a new (DOCnew) and thermal aged (DOC800°C) diesel oxidation catalyst. In all aged DOCs the platinum particles were sintered with particle diameters typically ranging between 10 and 20nm. DOCmid, DOCout and DOC800°C exhibited platinum particle morphology with edges and facets. Round platinum particles without discernable edges and facets were detected in the case of DOCin. DOCin showed two types of continuous, μm-thick contaminant deposits (up to 5 and 20μm) with differing elemental compositions covering the washcoat. Catalytic activity towards the abatement of CO, propene and NO did not show any substantial difference between the different engine aged DOC samples. Thermal aging seems to be the primary cause for the observed loss in catalytic activity compared to DOCnew. However, NO2 production above 275°C was increasing along the monolith in the order of DOCinmidout revealing a possible influence of the type of aging on activity towards NO oxidation.
Keywords: Diesel oxidation catalyst; Aging; Activity; NO oxidation; Sintering; Poisoning; Particle morphology; CO adsorption; TEM; DRIFTS
On the mechanism of the SCR reaction on Fe/HBEA zeolite by D. Klukowski; P. Balle; B. Geiger; S. Wagloehner; S. Kureti; B. Kimmerle; A. Baiker; J.-D. Grunwaldt (pp. 185-193).
The mechanism of the so-called standard SCR reaction on an efficient HBEA zeolite catalyst modified with 0.25wt.% Fe (0.25Fe/HBEA) was elucidated. The studies were systematically performed by using DRIFTS, TPD, XANES and kinetic studies. SCR kinetics were examined between 175 and 275°C employing a gradient-free loop reactor. DRIFTS and NO x-TPD showed formation of Fe3+-NO species as well as their reaction with NH3. But, only minor adsorption and conversion of NO x on the zeolite substrate occurred. On the contrary, DRIFTS and NH3-TPD showed mainly adsorption of NH3 on the zeolite, whereas XANES and kinetic examinations indicated NH3 adsorption on the Fe3+ sites as well. Moreover, the XANES studies evidenced deNO x on the iron component which is considered to be the major pathway in standard SCR. From our experimental investigations we suggest a dual site mechanism implying the adsorption and reaction of NO and NH3 on neighbouring Fe3+ sites. The uptake of NH3 results in partial reduction of Fe3+ sites which are finally recycled by O2. However, some contribution from a single site mechanism cannot be ruled out completely.Numerical modelling of the NH3-TPD pattern provides kinetic parameters of the adsorption and desorption of NH3 on 0.25Fe/HBEA differentiating the specific adsorption sites of the substrate. As a result, the uptake and release of NH3 on the zeolite is found to be much faster than that of SCR. In connection with a previous SSITKA study, this comparison suggests that NH3 undergoes several adsorption/desorption cycles on the substrate before adsorbing and reacting on the Fe3+ sites.
Keywords: SCR; Mechanism; Fe; BEA zeolite; DRIFTS; XANES; TPD; Kinetics; Numerical modelling
Effect of manganese substitution on the structure and activity of iron titanate catalyst for the selective catalytic reduction of NO with NH3 by Fudong Liu; Hong He; Yun Ding; Changbin Zhang (pp. 194-204).
Selective catalytic reduction (SCR) of NO with NH3 over manganese substituted iron titanate catalysts was fully studied. The low temperature SCR activity was greatly enhanced when partial Fe was substituted by Mn, although the N2 selectivity showed some decrease to a certain extent. The Mn substitution amounts showed obvious influence on the catalyst structure, redox behavior and NH3/NO x adsorption ability of the catalysts. Among Fe aMn1− aTiO x ( a=1, 0.75, 0.5, 0.2, 0) serial catalysts, Fe0.5Mn0.5TiO x with the molar ratio of Fe:Mn=1:1 showed the highest SCR activity, because the interaction of iron, manganese and titanium species in this catalyst led to the largest surface area and the highest porosity, the severest structural distortion and most appropriate structural disorder, the enhanced oxidative ability of manganese species, the highest mobility of lattice oxygen, the proper ratio of Brønsted acid sites and Lewis acid sites together with the enhanced NO x adsorption capacity.
Keywords: Selective catalytic reduction; Iron titanate catalyst; Mn substitution amounts; Low temperature activity; Redox behavior