Showing papers in "Applied Catalysis B-environmental in 2006"

Study of the electronic structure and photocatalytic activity of the BiOCl photocatalyst

Abstract: A novel bismuth oxychloride (BiOCl) was synthesized by a hydrolysis method. The powder sample was characterized by X-ray diffraction, field emission scanning electron microscope and UV–vis spectrophotometer. The layered compound BiOCl was the first oxyhalide to be used as a photocatalyst. The prepared material has an optical indirect band-gap of 3.46 eV. For its photocatalytic reactivity, the methyl orange (MO) dye degradation was chosen to be investigated. In a comparison of a three-cycle measurement of BiOCl with that of TiO 2 (P25, Degussa), it was found that BiOCl had better performance than P25 at every recycle. The calculated electronic structure of BiOCl confirms that it has an indirect band-gap, and the Cl 3p and Bi 6p states dominate the highest occupied molecular orbitals (HOMO) and the lowest unoccupied orbitals (LUMO), respectively. The open crystal structure and indirect optical transitions of BiOCl play important roles in its excellent photocatalytic activity.

Size dependency of nanocrystalline TiO2 on its optical property and photocatalytic reactivity exemplified by 2-chlorophenol

Abstract: Anatase TiO2 nanocrystallines (17–29 nm) were successfully synthesized by the metal–organic chemical vapor deposition method (MOCVD). Moderate manipulation of system parameters of MOCVD can control the particle size. The electro-optical and photocatalytic properties of the synthesized TiO2 nanoparticles were studied along with several commercially available ultra-fine TiO2 particles (e.g., 3.8–5.7 nm). The band gap of the TiO2 crystallines was determined using the transformed diffuse reflectance technique according to the Kubelka–Munk theory. Results showed that the band gap of TiO2 monotonically decreased from 3.239 to 3.173 eV when the particle size decreased from 29 to 17 nm and then increased from 3.173 to 3.289 eV as the particle size decreased from 17 to 3.8 nm. The results of band gap change as a function of particle size agreed well with what was predicted by the Brus’ equation, i.e., the effective mass model (EMM). However, results of the photocatalytic oxidation of 2-chlorophenol (2-CP), showed that the smaller the particle size, the faster the degradation rate. This is attributed in part to the combined effect of band gap change relative to the spectrum of the light source and the specific surface area (or particle size) of the photocatalysts. The change of band gap due to particle size represents only a small optical absorption window with respect to the total spectrum of the light source, i.e., from 380 to 400 nm versus >280 nm. Consequently, the gain in optical property of the larger particles was severely compromised by their decrease in specific surface area. Our results clearly indicated the importance of specific surface area in controlling the photocatalytic reactivity of photocatalysts. Results also showed that the secondary particle size grew with time due mainly to particle aggregation. The photocatalytic rate constants decreased exponentially with increase in primary particle size. Primary particle size alone is able to predict the photocatalytic rate as it is closely related to the electro-optical properties of photocatalysts.

MnOx-CeO2 mixed oxide catalysts for complete oxidation of formaldehyde: Effect of preparation method and calcination temperature

Abstract: MnO x –CeO 2 mixed oxides prepared by sol–gel method, coprecipitation method and modified coprecipitation method were investigated for the complete oxidation of formaldehyde. Structure analysis by H 2 -TPR and XPS revealed that there were more Mn 4+ species and richer lattice oxygen on the surface of the catalyst prepared by the modified coprecipitation method than those of the catalysts prepared by sol–gel and coprecipitation methods, resulting in much higher catalytic activity toward complete oxidation of formaldehyde. The effect of calcination temperature on the structural features and catalytic behavior of the MnO x –CeO 2 mixed oxides prepared by the modified coprecipitation was further examined, and the catalyst calcined at 773 K showed 100% formaldehyde conversion at a temperature as low as 373 K. For the samples calcined below 773 K, no any diffraction peak corresponding to manganese oxides could be detected by XRD measurement due to the formation of MnO x –CeO 2 solid solution. While the diffraction peaks corresponding to MnO 2 phase in the samples calcined above 773 K were clearly observed, indicating the occurrence of phase segregation between MnO 2 and CeO 2 . Accordingly, it was supposed that the strong interaction between MnO x and CeO 2 , which depends on the preparation route and the calcination temperature, played a crucial role in determining the catalytic activity toward the complete oxidation of formaldehyde.

Catalytic performance and mechanism of a Pt/TiO2 catalyst for the oxidation of formaldehyde at room temperature

Abstract: The performance of TiO2 supported noble metal (Pt, Rh, Pd and Au) catalysts was examined and compared for the catalytic oxidation of formaldehyde (HCHO). Among them, the Pt/TiO2 was the most active catalyst. The effects of Pt loading and gas hourly space velocity (GHSV) on Pt/TiO2 activity for HCHO oxidation were investigated at a room temperature (20 degrees C). The optimal Pt loading is 1 wt.%. At this loading, HCHO can be completely oxidized to CO2 and H2O over the Pt/TiO2 in a GHSV of 50,000 h(-1) at 20 degrees C. The 1% Pt/TiO2 was characterized using BET, XRD, high resolution (HR) TEM and temperature programmed reduction (TPR) methods. The XRD patterns and HR TEM image show that Pt particles on TiO2 are well dispersed into a size smaller than 1 nm, an important feature for the high activity of the 1% Pt/TiO2. The mechanism of HCHO oxidation was studied with respect to the behavior of adsorbed species on Pt/TiO2 surface at room temperature using in situ DRIFTS. The results indicate that surface formate and CO species are the main reaction intermediates during the HCHO oxidation. The formate species could decompose into adsorbed CO species on the catalyst surface without the presence Of O-2, and the CO was then oxidized to CO, with the presence of O-2. Based on these results, a simplified mechanism for the catalytic oxidation of HCHO over 1% Pt/TiO2 was proposed. (c) 2006 Elsevier B.V. All rights reserved.

Cu-doped TiO2 systems with improved photocatalytic activity

Abstract: Photocatalytic oxidation of phenol was performed over copper doped TiO 2 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–TiO 2 photocatalytic properties. The presence of sulphuric acid clearly stabilizes Cu–TiO 2 and TiO 2 structure and surface against sintering, maintaining anatase phase and relatively high surface area values with respect non sulphated Cu–TiO 2 or TiO 2 . Best photocatalytic behaviour is found for sulphated TiO 2 , 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 Cu 2 O species in doped TiO 2 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.

Ag-ZnO catalysts for UV-photodegradation of methylene blue

Abstract: High surface area Ag-ZnO catalysts have been made by flame spray pyrolysis (FSP) and characterized by X-ray diffraction (XRD), nitrogen adsorption, UV–vis spectroscopy and electron microscopy (SEM and transmission electron microscopy (TEM)) combined with energy dispersive X-ray spectroscopy (EDXS) for elemental mapping. Silver metal clusters deposited directly on ZnO nanocrystals were obtained from this process. The Ag loading (1–5 at.%) controlled the Ag cluster size from 5 to 25 nm but did not influence the ZnO crystal size. Photodegradation of 10 ppm methylene blue (MB) solution was used to evaluate the performance of these FSP-made Ag-ZnO and was compared to wet-made Ag-ZnO and reference titania photocatalysts. The rate of photodegradation was optimal for Ag loading around 3 at.%. The best photocatalytic performance was exhibited by flame-made Ag-ZnO produced at the longest high-temperature residence times having high crystallinity as determined by XRD and UV–vis.

Sol–gel preparation of mesoporous photocatalytic TiO2 films and TiO2/Al2O3 composite membranes for environmental applications

Abstract: This study describes the application of novel chemistry methods for the fabrication of robust nanostructured titanium oxide (TiO2) photocatalysts. Such materials can be applied in the development of efficient photocatalytic systems for the treatment of water. Mesoporous photocatalytic TiO2 films and membranes were synthesized via a simple synthesis method that involves dip-coating of appropriate substrates into an organic/inorganic sol composed of isopropanol, acetic acid, titanium tetraisopropoxide, and polyoxyethylenesorbitan monooleate surfactant (Tween 80) followed by calcination of the coating at 500 8C. Controlled hydrolysis and condensation reactions were achieved through in-taking of water molecules released from the esterification reaction of acetic acid with isopropanol. The subsequent stable incorporation of Ti–O–Ti network onto self-assembled surfactants resulted in TiO2 photocatalysts with enhanced structural and catalytic properties. The properties included high surface area (147 m 2 /g) and porosity (46%), narrow pore size distribution ranging from 2 to 8 nm, homogeneity without cracks and pinholes, active anatase crystal phase, and small crystallite size (9 nm). These TiO2 photocatalysts were highly efficient for the destruction of methylene blue and creatinine in water. High water permeability and sharp polyethylene glycol retention of the prepared photocatalytic TiO2/Al2O3 composite membranes evidenced the good structural properties of TiO2 films. In addition, the multi-coating procedure made it possible to effectively control the physical properties of TiO2 layer such as the coating thickness, amount of TiO2, photocatalytic activity, water permeability and organic retention. # 2005 Elsevier B.V. All rights reserved.

Hydrothermal doping method for preparation of Cr3+-TiO2 photocatalysts with concentration gradient distribution of Cr3+

Abstract: Cr3+-doped anatase titanium dioxide photocatalysts were prepared by the combination of sol–gel process with hydrothermal treatment. The samples were characterized by UV–vis diffuse reflectance spectroscopy, X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) specific surface area (SBET), transmission electron microscopy (TEM), atomic absorption flame emission spectroscopy (AAS), electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS). It was confirmed that Cr substitutes Ti4+ in TiO2 lattice in trivalent ionic state, and the concentrations of dopants Cr3+ decrease from the exterior to the interior of doped TiO2. The photocatalytic activity of Cr-TiO2 was investigated for the photocatalytic degradation of XRG aqueous solution both under UV and visible light irradiation. Due to the excitation of 3d electron of Cr3+ to the conduction band of TiO2, Cr-TiO2 shows a good ability for absorbing the visible light to degrade XRG. Doping of chromium ions effectively improves the photocatalytic activity under both UV light irradiation and visible light irradiation with an optimal doping concentration of 0.15% and 0.2%, respectively. The special distribution of dopants Cr3+ seems having a good effect on enhancing the photocatalytic activity of Cr-TiO2.

Co3O4/CeO2 composite oxides for methane emissions abatement: Relationship between Co3O4–CeO2 interaction and catalytic activity

Abstract: Co 3 O 4 /CeO 2 composite oxides with different cobalt loading (5, 15, 30, 50, 70 wt.% as Co 3 O 4 ) were prepared by co-precipitation method and investigated for the oxidation of methane under stoichiometric conditions. Pure oxides, Co 3 O 4 and CeO 2 were used as reference. Characterization studies by X-ray diffraction (XRD), BET, temperature programmed reduction/oxidation (TPR/TPO) and X-ray photoelectron spectroscopy (XPS) were carried out. An improvement of the catalytic activity and thermal stability of the composite oxides was observed with respect to pure Co 3 O 4 in correspondence of Co 3 O 4 –CeO 2 containing 30% by weight of Co 3 O 4 . The combined effect of cobalt oxide and ceria, at this composition, strongly influences the morphological and redox properties of the composite oxides, by dispersing the Co 3 O 4 phase and promoting the efficiency of the Co 3+ –Co 2+ redox couple. The presence in the sample Co 3 O 4 (30 wt.%)–CeO 2 of a high relative amount of Ce 3+ /(Ce 4+ + Ce 3+ ) as detected by XPS confirms the enhanced oxygen mobility. The catalysts stability under reaction conditions was investigated by XRD and XPS analysis of the used samples, paying particular attention to the Co 3 O 4 phase decomposition. Methane oxidation tests were performed over fresh (as prepared) and thermal aged samples (after ageing at 750 °C for 7 h, in furnace). The resistance to water vapour poisoning was evaluated for pure Co 3 O 4 and Co 3 O 4 (30 wt.%)–CeO 2 , performing the tests in the presence of 5 vol.% H 2 O. A methane oxidation test upon hydrothermal ageing (flowing at 600 °C for 16 h a mixture 5 vol.% H 2 O + 5 vol.%O 2 in He) of the Co 3 O 4 (30 wt.%)–CeO 2 sample was also performed. All the results confirm the superiority of this composite oxide.

Recent progress in direct ethanol proton exchange membrane fuel cells (DE-PEMFCs)

Abstract: Direct ethanol fuel cells (DEFCs) belong to the family of proton exchange membrane fuel cells (PEMFCs), in which ethanol is directly used as the fuel. In the present work, the main aspects related to DEFCs such as electrocatalysts, membrane electrode assembly (MEA) preparation and their corresponding effects on the total cell performance are summarized and discussed. Furthermore, the issues about the disadvantages such as ethanol crossover and the electrolyte membrane's thermal and mechanical stability, as well as the challenges for DEFC's rapid development and commercialization are addressed.

The methanol oxidation reaction on platinum alloys with the first row transition metals The case of Pt-Co and -Ni alloy electrocatalysts for DMFCs: A short review

Abstract: In recent years there has been much activity in examining Pt alloys with first row transition metals as catalysts materials for DMFCs In this work, the electrochemical oxidation of methanol on Pt–Co and –Ni alloy electrocatalysts is reviewed The effect of the transition metal on the electrocatalytic activity of Pt–Co and –Ni for the methanol oxidation reaction (MOR) has been investigated both in half-cell and in direct methanol fuel cells Conflicting results regarding the effect of the presence of Co(Ni) on the MOR are examined and the primary importance of the amount of non-precious metal in the catalyst is remarked For low base metal contents, an enhancement of the onset potential for the MOR with increasing Co(Ni) amount in the catalyst is observed, whereas for high contents of the base metal, a drop of the MOR onset potential with increasing Co(Ni) is found As well as the base metal content, an important role on the MOR activity of these catalysts has to be ascribed to the degree of alloying

Photocatalytic properties of nanosized Bi2WO6 catalysts synthesized via a hydrothermal process

Abstract: Nanosized Bi 2 WO 6 was synthesized by a hydrothermal crystallization process. The as-prepared samples were characterized by X-ray diffraction, Brunauer–Emmet–Teller surface area and porosity measurements, transmission electron microscopy, Raman spectra, and diffuse reflectance spectroscopy. The photoactivities of the as-prepared samples for the rhodamine-B photodegradation were investigated systematically. As a result, the sample prepared at 180 °C exhibited the highest photochemical activity under visible-light irradiation. The further experiments revealed that the catalyst was active in a wide spectral range. Density functional theory calculations suggested that the visible-light response was due to the transition from the valence band formed by the hybrid orbitals of Bi 6s and O 2p to the conduction band of W 5d. The photoactivity of the catalyst in relationship with the hydrothermal temperature, the crystal and band structure were also discussed in detail.

Aqueous-phase reforming of ethylene glycol over supported Pt and Pd bimetallic catalysts

Abstract: More than 130 Pt and Pd bimetallic catalysts were screened for hydrogen production by aqueous-phase reforming (APR) of ethylene glycol solutions using a high-throughput reactor. Promising catalysts were characterized by CO chemisorption and tested further in a fixed bed reactor. Bimetallic PtNi, PtCo, PtFe and PdFe catalysts were significantly more active per gram of catalyst and had higher turnover frequencies for hydrogen production ( TO F H 2 ) than monometallic Pt and Pd catalysts. The PtNi/Al2O3 and PtCo/Al2O3 catalysts, with Pt to Co or Ni atomic ratios ranging from 1:1 to 1:9, had TO F H 2 values (based on CO chemisorption uptake) equal to 2.8–5.2 min−1 at 483 K for APR of ethylene glycol solutions, compared to 1.9 min−1 for Pt/Al2O3 under similar reaction conditions. A Pt1Fe9/Al2O3 catalyst showed TO F H 2 values of 0.3–4.3 min−1 at 453–483 K, about three times higher than Pt/Al2O3 under identical reaction conditions. A Pd1Fe9/Al2O3 catalyst had values of TO F H 2 equal to 1.4 and 4.3 min−1 at temperatures of 453 and 483 K, respectively, and these values are 39–46 times higher than Pd/Al2O3 at the same reaction conditions. Catalysts consisting of Pd supported on high surface area Fe2O3 (Nanocat) showed the highest turnover frequencies for H2 production among those catalysts tested, with values of TO F H 2 equal to 14.6, 39.1 and 60.1 min−1 at temperatures of 453, 483 and 498 K, respectively. These results suggest that the activity of Pt-based catalysts for APR can be increased by alloying Pt with a metal (Ni or Co) that decreases the strengths with which CO and hydrogen interact with the surface (because these species inhibit the reaction), thereby increasing the fraction of catalytic sites available for reaction with ethylene glycol. The activity of Pd-based catalysts for APR can be increased by adding a water-gas shift promoter (e.g. Fe2O3).

Single-reactor process for sequential aldol-condensation and hydrogenation of biomass-derived compounds in water

Abstract: A bifunctional Pd/MgO-ZrO 2 catalyst was developed for the single-reactor, aqueous phase aldol-condensation and hydrogenation of carbohydrate-derived compounds, furfural and 5-hydroxymethylfurfural (HMF), leading to large water-soluble intermediates that can be converted to liquid alkanes. The cross aldol-condensation of these compounds with acetone results in formation of water-insoluble monomer (C 8 –C 9 ) and dimer (C 13 –C 15 ) product species, which are subsequently hydrogenated in the same batch reactor to form water-soluble products with high overall carbon yields (>80%). After a cycle of aldol-condensation followed by hydrogenation, the Pd/MgO-ZrO 2 catalyst undergoes a loss in selectivity by 18% towards heavier product (dimer) during subsequent runs. However, the catalytic activity and dimer selectivity are completely recovered when the catalyst is recycled with an intermediate calcination step at 873 K. The optimum temperatures for aldol-condensation of furfural with acetone and for condensation of HMF with acetone are 353 and 326 K, respectively, representing a balance between dimer selectivity and overall carbon yield for the process. The product selectivity can be controlled by the molar ratio of reactants. When the molar ratio of furfural-acetone increases from 1:9 to 1:1, the selectivity for the formation of dimer species increases by 31% and this selectivity increases further by 12% when the ratio increases from 1:1 to 2:1. It is likely that this active, stable, and heterogeneous catalyst system can be applied to other base and/or metal catalyzed reactions in the aqueous phase.

Photocatalytic degradation study of diclofenac over aqueous TiO2 suspensions

Abstract: 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 1 h), 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 2 h 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.

Catalytic wet peroxide oxidation of phenol with a Fe/active carbon catalyst

Abstract: A Fe on activated carbon catalyst has been prepared and tested for phenol oxidation with H 2 O 2 in aqueous solution at low concentration (100 mg/L). Working at 50 °C, initial pH 3 and a dose of H 2 O 2 corresponding to the stoichiometric amount (500 mg/L) complete removal of phenol and a high TOC reduction (around 85%) has been reached. Oxidation of phenol gives rise to highly toxic aromatic intermediates which finally disappear completely evolving to short-chain organic acids. Some of these last showed to be fairly resistant to oxidation being responsible for the residual TOC. In long-term continuous experiments the catalyst undergoes a significant loss of activity in a relatively short term (20–25 h) due to Fe leaching, this being related with the amount of oxalic acid produced. Deactivation may also be caused by active sites blockage due to polymeric deposits on whose formation some evidences were found. Washing with 1N NaOH solution allows to recover the activity although complete restoration was not achieved.

Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5

Abstract: 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 NOx 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/NOx = 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/NOx = 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/NOx

Synthesis and characterisation of La1−xCaxFeO3 perovskite-type oxide catalysts for total oxidation of volatile organic compounds

Abstract: La1−xCaxFeO3 perovskite-type oxides with x = 0, 0.2 and 0.4 were prepared by the citrate method and characterised by means of X-ray diffraction (XRD), X-ray fluorescence (XRF), surface area measurement BET, X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FT-IR), laser Raman spectroscopy (LRS), oxygen temperature-programmed desorption (O2-TPD) and temperature-programmed reduction (TPR). The citrate method shows to be simple and appropriate to obtain single phases avoiding segregation and/or contamination. Moreover, controlling the calcination temperature, specific surface areas adequate for catalysts to be used in oxidation reactions are achieved. The structure refinement by using the Rietveld method indicates that the partial calcium substitution modifies the orthorhombic structure of the LaFeO3 perovskite towards a less distorted one. From XRF and XPS, a slight surface enrichment in lanthanum and calcium was detected. XRD, FT-IR and TPR results indicated that the electronic debalance caused by the partial substitution for La3+ by Ca2+ is compensated by an oxidation state increase of a part of Fe3+ to Fe4+. O2-TPD results revealed that at a substitution level higher than x = 0.2, oxygen vacancies are also formed to preserve the electroneutrality. Finally, an improvement of the catalytic activity in propane and ethanol combustion was observed on the substituted perovskites. Correlating this with the characterisation results, the active sites would be associated to the Fe4+ ions.

Improved Pd-on-Au bimetallic nanoparticle catalysts for aqueous-phase trichloroethene hydrodechlorination

Abstract: Groundwater remediation through the catalytic breakdown of the undesired contaminants is a more effective and desirable approach than the conventional physical displacement methods of air-stripping and carbon adsorption. Palladium-on-gold nanoparticles (Pd/Au NPs) have recently been shown to catalyze the hydrodechlorination of trichloroethene in water, at room temperature, and in the presence of hydrogen, with the most active Pd/Au material found to be >70 times more active than Pd supported on alumina on a per-Pd atom basis. The potential of this catalyst as a groundwater remediation technology could be improved by synthesizing Pd/Au NPs with smaller diameters and immobilizing them on a solid support. For this study, we synthesized Pd/Au NPs with a core diameter of 4 nm and with different Pd loadings and studied them in colloidal form for aqueous-phase trichloroethene hydrodechlorination. The most active catalysts were considerably more active (>1900 L/gPd/min) than Pd NPs (55 L/gPd/min) and conventionally synthesized Pd/Al2O3 (47 L/gPd/min). Accounting for a gas–liquid mass transfer effect and converting the Pd loading to Pd surface coverage using a magic cluster model for the Pd/Au NPs, the reaction rates in terms of initial turnover frequencies were >1.4, 4.35 � 10 � 2 , and 3.76 � 10 � 2 s � 1 , respectively. These materials exhibited volcano-like catalytic activity, in which hydrodechlorination rate was maximum near 70% Pd surface coverage. Au appeared to promote catalysis through geometric and electronic effects. Immobilization of the NPs on alumina, magnesia, and silica supports yielded active oxide-supported catalysts. # 2006 Elsevier B.V. All rights reserved.

Photocatalytic performance of Ru doped anatase mounted on silica for reduction of carbon dioxide

Abstract: Ru doped anatase supported on silica was prepared by solid-state dispersion method and examined for the photocatalytic reduction of carbon dioxide in aqueous medium at ambient conditions. To assist in interpreting the photocatalytic behaviour of Ru-TiO2/SiO2, reference systems consisting of Ru doped TiO2 and TiO2 supported on SiO2 were also analyzed and the conditions were optimized. Ru/TiO2 photocatalysts with metal loadings of 0.1, 0.3, 0.5 and 1.0 wt% were prepared by impregnation method and a series of TiO2/SiO2 catalysts with low TiO2 (1, 3, 5 and 10 wt%) contents were prepared by solid-state dispersion method. The photocatalysts were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), UV–vis DRS, FT-IR and Raman spectroscopy. The results showed that TiO2 particles with Ru ions have higher photocatalytic activity than undoped TiO2 and the optimum Ru loading was found to be 0.5 wt%. Nevertheless, the yield increased notably when TiO2 was supported on SiO2. This strong enhancement suggests that in 10 wt% TiO2/SiO2 the efficiency of charge separation is strongly influenced through the presence of Ti O Si bridging bonds. On the contrary, Ru-TiO2/SiO2 has no significant improvement in activity over TiO2/SiO2 except that it shows nearly quadruple times higher activity for the formation of methanol than Ru/TiO2. The difference in the photocatalytic activity is related to its physico-chemical properties.

Electro-Fenton and photoelectro-Fenton degradation of indigo carmine in acidic aqueous medium

Abstract: Acidic aqueous solutions containing concentrations of the dye indigo carmine up to 09 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 350 °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 10 mM Fe2+ and when 10 mM Fe2+ and 025 mM Cu2+ are combined as catalysts in photoelectro-Fenton with a Pt anode The first method yields similar degradation rate in the pH range 20–40, whereas the second one is more potent up to 044 g l−1 of the dye at pH 30 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+

Phenol degradation using hydroxyl radicals generated from zero-valent iron and hydrogen peroxide

Abstract: The degradation of the model pollutant phenol with hydroxyl radicals generated from zero-valent iron and hydrogen peroxide has been investigated by means of HPLC analysis of the intermediates. The optimum conditions for degradation utilise the continuous presence of iron metal, acidic pH and relatively concentrated hydrogen peroxide (9.5 M). When less stringent conditions were used, the products obtained from the decomposition are broadly the same but are formed over a much longer timescale. The intermediates in the oxidation were identified as catechol, hydroquinone, benzoquinone, maleic acid and a relatively stable product, tentatively identified as an organic complex of iron and oxidised catechol.

Effect of synthesis parameters on catalytic properties of CuO-CeO2

Abstract: 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.

Photocatalytic properties of nc-Au/ZnO nanorod composites

Abstract: Photodegradation of methyl orange (MO) under 365-nm irradiation has been performed to investigate the photocatalytic activities of the nc-Au/ZnO nanorod composites in comparison with those of the ZnO nanorods and the ZnO film. The ZnO nanorods and the ZnO film were deposited on the Si substrates using chemical vapor deposition. The apparent first-order rate constant of MO photodegradation using the ZnO nanorods is two orders magnitude larger than that using the ZnO film. Photosyntheses of Au nanoparticles on the ZnO nanorods were conducted in HAuCl 4 /ethanol solutions under 365-nm irradiation. The diameters and the densities of the Au nanoparticles formed on the surface of ZnO nanorods are tunable through varying the HAuCl 4 concentration and the irradiation period. The enhancement of the photocatalytic activity for degradation of MO is achieved by loading Au nanoparticles with sizes smaller than 15 nm on the ZnO nanorods and is more pronounced as the size of the Au nanoparticles is reduced to 5 nm. However, the photocatalytic activity of the nc-Au/ZnO nanorod composite is much lower than that of the Zn nanorods when the diameter of the nc-Au is enlarged to 30 nm. The photocatalytic activity of the nc-Au/ZnO nanorods for degradation of MO is dependent on the diameter and the density of the nc-Au.

Total oxidation of ethanol and propane over Mn-Cu mixed oxide catalysts

Abstract: Mn-Cu mixed oxides were prepared by co-precipitation varying the aging time for 4, 18 and 24 h. 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.

Photo-catalytic production of hydrogen form ethanol over M/TiO2 catalysts (M = Pd, Pt or Rh)

Abstract: 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 350 K. 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 10 nm. 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.

Selective catalytic reduction of NOx with NH3 over Cu-ZSM-5—The effect of changing the gas composition

Abstract: The selective catalytic reduction of nitrogen oxides (NOx) with ammonia over ZSM-5 catalysts was studied with and without water vapor. The activity of H-, Na- and Cu-ZSM-5 was compared and the result showed that the activity was greatly enhanced by the introduction of copper ions. A comparison between Cu-ZSM-5 of different silica to alumina ratios was also performed. The highest NO conversion was observed over the sample with the lowest silica to alumina ratio and the highest copper content. Further studies were performed with the Cu-ZSM-5-27 (silica/alumina = 27) sample to investigate the effect of changes in the feed gas. Oxygen improves the activity at temperatures below 250 °C, but at higher temperatures O2 decreases the activity. The presence of water enhances the NO reduction, especially at high temperature. It is important to use about equal amounts of nitrogen oxides and ammonia at 175 °C to avoid ammonia slip and a blocking effect, but also to have high enough concentration to reduce the NOx. At high temperature higher NH3 concentrations result in additional NOx reduction since more NH3 becomes available for the NO reduction. At these higher temperatures ammonia oxidation increases so that there is no ammonia slip. Exposing the catalyst to equimolecular amounts of NO and NO2 increases the conversion of NOx, but causes an increased formation of N2O.

Effects of Sn dopant on the photoinduced charge property and photocatalytic activity of TiO2 nanoparticles

Abstract: In this paper, Sn-doped TiO 2 nanoparticles were prepared by a sol–gel method, and also were characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared spectrum (IR), ultraviolet–visible diffuse reflection spectrum (UV–vis DRS), photoluminescence spectrum (PL), surface photovoltage spectroscopy (SPS) and electrical field induced surface photovoltage spectroscopy (EFISPS). The sample activity was evaluated by photocatalytic oxidation reactions of phenol solution. The effects of thermal treatment temperature and Sn amount on photoinduced charge property, mainly involving charge separation and bound excitons resulting from surface states, and photocatalytic activity of TiO 2 nanoparticles were principally investigated. The results show that an appropriate calcination temperature and Sn dopant amount can greatly enhance the SPS responses of TiO 2 nanoparticles related not only to the band–band transitions but also to the bound excitons, and obviously weaken the PL signal, while the photocatalytic activity remarkably raises. These demonstrate that the separation rate of photoinduced charges of TiO 2 nanoparticles can be effectively improved by doping Sn, which is responsible for the obvious increase in the photocatalytic activity. Moreover, the existence of bound excitons related to surface states also favor the photocatalytic activity. In addition, it can be found that the SPS responses related to the bound excitons could easily exhibit in the TiO 2 sample consisting of much of anatase and little of rutile, which is possibly ascribed to the heterojunction between the anatase and rutile phase.

Effects of chloride ions on the iron(III)-catalyzed decomposition of hydrogen peroxide and on the efficiency of the Fenton-like oxidation process

Abstract: The effects of chloride concentration on the rates of decomposition of H 2 O 2 by ferric ion and on the rate of oxidation of an organic solute in homogeneous aqueous solution have been investigated. Experiments were carried out in a batch reactor, in the dark, at pH ≤ 3, 25.0 ± 0.5 °C and at controlled ionic strength (≤1 M). The concentrations of chloride ranged from 0 to 1 M ([Fe(III)] 0 = 0.2 or 1 mM, [H 2 O 2 ] 0 = 1, 10 or 50 mM). The spectrophotometric study shows that chloride ions compete with hydrogen peroxide for the complexation of Fe(III) and that H 2 O 2 does not form complexes with iron(III)-chlorocomplexes. The kinetic study showed that the rates of decomposition of H 2 O 2 decreased in the presence of chloride. The measured rates were accurately predicted by a kinetic model which incorporates the formation of iron(II) and iron(III)-chlorocomplexes and reactions involving Cl 2 − radicals. At a fixed pH, the pseudo-first-order rate constants were found to decrease linearly with the molar fraction of Fe(III) complexed with chloride. The kinetic model was also able to predict the rate of oxidation of a probe compound (atrazine) by Fe(III)/H 2 O 2 in the presence of chloride. Computer simulations indicate that Cl 2 − which represents the predominant radical contributes to the oxidation of atrazine.

A novel oxidative desulfurization process to remove refractory sulfur compounds from diesel fuel

Abstract: Manganese and cobalt oxide catalysts supported on γ-Al 2 O 3 have been found to be effective in catalyzing air oxidation of the sulfur impurities in diesel to corresponding sulfones at a temperature range of 130–200 °C and atmospheric pressure. The sulfones were removed by extraction with polar solvent to reduce the sulfur level in diesel to as low as 40–60 ppm. Oxidation of model compounds showed that the most refractory sulfur compounds in hydrodesulfurization of diesel were more reactive in oxidation. The oxidative reactivity of model impurities in diesel follows the order: trialkyl-substituted dibenzothiophene > dialkyl-substituted dibenzothiophene > monoalkyl-substituted dibenzothiophene > dibenzothiophene.