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Applied Catalysis B, Environmental (v.59, #1-2)
The effects of the use of weak organic acids on the improvement of oxygen storage and release properties of aged commercial three-way catalysts by P.S. Lambrou; S.Y. Christou; A.P. Fotopoulos; F.K. Foti; T.N. Angelidis; A.M. Efstathiou (pp. 1-11).
The efficiency of weak organic acid solutions for improving the oxygen storage and release properties of aged commercial three-way catalysts (TWC) has been investigated. For the first time, three candidate leaching solutions, namely acetic acid (CH3COOH), oxalic acid ((COOH)2) and citric acid (C6H8O7) were tested for their efficiency in removing various contaminants, such as P, Fe, Zn, Cr, Pb, Ni and Cu that potentially reduce the oxygen storage capacity (OSC) of three-way catalysts. It was shown that over a severely aged (242,000km) commercial TWC, oxalic acid appears to be the most efficient in removing P and Cu, while citric acid in removing Zn, Fe, Cr, Pb and Ni. For all three weak organic acid solutions investigated, higher quantities of contaminants were removed with increasing aging of TWC. All regeneration procedures applied led to the partial recovery of the OSC of aged TWCs likely due to the removal of at least some of the contaminants mostly responsible for their negative effect on the processes of oxygen storage and release in CeO2 and Ce xZr1− xO2 “oxygen storage� materials present. It was found that oxalic acid leads to the largest improvement of OSC quantities, very likely because of its ability to remove P, the latter being known to cause severe reduction of OSC. Based on transient redox experiments in measuring the dynamic OSC of TWC (O2/He→He→CO/He), it was found that the peak maximum position of the transient response curve of CO2 obtained over the regenerated TWC samples shifts to higher reaction times compared to that of aged TWC. This behavior is very likely related to the increase of the rate constant ( k) of the oxygen back-spillover step, based on mathematical simulations of the same transient experiment performed on a model Pd/CeO2 catalyst.
Keywords: TWC regeneration; TWC deactivation; OSC; Transient kinetics
The effect of calcination temperature on the oxygen storage and release properties of CeO2 and Ce–Zr–O metal oxides modified by phosphorus incorporation by C. Larese; M. López Granados; R. Mariscal; J.L.G. Fierro; P.S. Lambrou; A.M. Efstathiou (pp. 13-25).
The present work reports on the effects of calcination temperature ( Tc) on the oxygen storage and release properties of CeO2 and Ce0.8Zr0.2O2 solids deliberately contaminated with CePO4. Pulsed-feed and stepped-feed dynamic OSC,18O2 temperature programmed isotopic exchange (TPIE), and18O2 transient isothermal isotopic exchange experiments were conducted in order to investigate the alteration of the OSC process by the applied Tc. The characterisation of the solids calcined at a low temperature (ca. 873K) by XRD, Raman and XPS confirms the presence of CePO4 crystallites covering to a large extent the surface of the solid metal oxides. However, calcination at a higher temperature (ca. 1273K) results in the sintering of the crystallites of the metal oxides and CePO4, the sintering of the latter being more intense. This sintering effect causes the rearrangement of the surface of CeO2 and Ce0.8Zr0.2O2 solids giving rise to the partial uncovering of it from CePO4. The OSC properties of all P-contaminated solids were found to be largely worse than the corresponding ones of P-free solids due to the presence of CePO4 in the surface/subsurface region of the former solids. However, the OSC properties of the phosphated Ce0.8Zr0.2O2 solid were partially recovered after calcination in air at 1273K. This effect was not observed in the case of CeO2 and it must be related to the surface/subsurface reorganization of the solid driven by calcination at 1273K, and also to the intrinsic OSC properties of Ce0.8Zr0.2O2 solid. Therefore, Ce xZr1− xO2 solid solutions appear to be excellent oxygen storage materials for commercial TWC applications and their OSC properties display an enhanced resistance against P-deterioration compared to those of pure CeO2.
Keywords: TWC deactivation; CePO; 4; Monazite; OSC; Oxygen isotopic exchange
Effect of SO2 on the activity of Ag/γ-Al2O3 catalysts for NO x reduction in lean conditions by Paul W. Park; Carrie L. Boyer (pp. 27-34).
The lean-NO x performance and the effect of SO2 on silver doped gamma alumina catalysts were investigated with simulated diesel exhaust in order to understand the active site and poisoning phenomena of the catalysts. The samples were prepared by impregnating γ-alumina with silver nitrate or silver sulfate using incipient wetness technique. Among the catalysts tested in the absence of SO2, 2wt.% Ag/Al2O3 was found to be the optimum silver loading for NO x reduction, while further increasing the Ag loading resulted in lower selectivity due to the fact that metallic Ag species in high loading samples favors propene oxidation. However, the common catalyst performance paradigm dramatically changed when SO2 was present in the feed stream. The NO x reduction over the 2wt.% Ag/Al2O3 initially was enhanced by SO2 exposure and then decreased until the N2 yield leveled off at 40%. Upon removal of SO2 from the feed stream, the catalyst was completely regenerated to the original N2 yield. The NO x reduction performance of a high silver loading catalyst (8wt.% Ag/Al2O3) increased significantly after SO2 was introduced in the gas stream from its initial performance of 1–28%.XPS analysis showed that the dispersion of silver species decreased by exposure to the feed gas containing SO2. Crystallite silver sulfate (Ag2SO4) was observed in the XRD pattern for the 8wt.% Ag/Al2O3. The catalyst performance and characterization results suggested that the silver sulfate species enhanced the NO x reduction capability via producing more intermediate species (NCO). Eventual decrease in the performance could be attributed to SO2 poisoning of the alumina sites, which are responsible for NO x reduction to N2. A sample containing selectively sulfated silver sites showed higher NO x reduction performance than a sample containing silver oxide. However, when alumina sites were sulfated it demonstrated lower NO x reduction than a pure alumina. With the results obtained from this study the order of apparent catalytic activity for silver species to enhance NO x reduction on alumina is proposed as the following: Ag2SO4>Ag2O≫Ag0.
Keywords: NO; x; Lean; Reduction; Silver; Gamma alumina; SO; 2; XPS; XRD; DRIFTS
Coke formation over zeolites andCeO2-zeolites and its influence on selective catalytic reduction ofNOx by K. Krishna; M. Makkee (pp. 35-44).
Selective catalytic reduction, various possible reasons of coke formation, and temperature programmed oxidation of coke deposits are studied over HFER, HZSM-5 and 15 wt%CeO2-Hzeolites. The materials are characterised by TGA,NH3-TPD and in-situ FTIR measurements. HFER based catalysts showed superiorNOx (NO+NO2) conversion in SCR with propene compared with HZSM-5 based catalysts. It is found thatNO2 (formed by the oxidation of NO) is not the only important intermediate in determining the extent ofNOx conversion. The topology and acidity of the zeolites play an important role in selective activation of propene and its reaction with NO2. Over HZSM-5 based catalysts the rate of deposition of carbonaceous compounds is higher than the rate of reaction of activated propene withNO2, leading to unselective reduction to NO. The nature and the amount of the carbonaceous products deposited over the zeolites are found to depend on the acidity, structure of the zeolite and reaction conditions (inert or oxidative atmosphere). Coke deposition rate is enhanced in the presence of oxygen and most of the coke is retained by the zeolite which is detrimental forNOx reduction. in-situ IR studies show that hydrocarbon deposits are more heterogeneous and carbon rich over HZSM-5 compared with HFER. TPO studies show that only a negligible fraction of hydrocarbon deposits are active inNOx conversion.
Keywords: SCR; NO; x; reduction; Coke formation; HFER; HZSM-5; FTIR; TGA; TPD; TPO
Water-gas shift: an examination of Pt promoted MgO and tetragonal and monoclinic ZrO2 by in situ drifts by Emilie Chenu; Gary Jacobs; Adam C. Crawford; Robert A. Keogh; Patricia M. Patterson; Dennis E. Sparks; Burtron H. Davis (pp. 45-56).
In situ DRIFTS measurements on unpromoted and Pt promoted MgO and ZrO2 (both tetragonal and monoclinic) indicate that at high H2O/CO ratios, where the reaction rate has been reported to be zero order in H2O and first order in CO, the mechanism involved in the catalysis of water-gas shift is likely a surface formate mechanism, in agreement with Shido and Iwasawa. Pt was found to catalyze the removal of surface carbonates and to facilitate the generation of active OH groups relative to the unpromoted catalyst. Comparison with Pt/ceria revealed that the OH groups involved in the catalysis of magnesia and zirconia may be those of the bridging variety which occur at defect sites. That is, water dissociated over vacancies to produce bridging OH groups, as observed by infrared spectroscopy. The existence of such an adsorbed species is implied in the zero reaction order for water, where kinetics suggests that the surface should be saturated by an adsorbed water species. The lower extent of vacancy formation for magnesia and zirconia-based materials in comparison with ceria could explain a lower surface population of active bridging OH groups. CO was used as a probe molecule of the reduced centers, as it reacts with bridging OH groups to generate surface formates, a proposed WGS intermediate, and the decomposition of which is proposed to be the rate-limiting step. The trends in formate intensity by CO adsorption and CO conversion in WGS catalytic testing both followed the order: Pt/ceria>Pt/m-zirconia>Pt/t-zirconia>Pt/magnesia. In all cases, a normal kinetic isotope effect was observed in switching from H2O to D2O, consistent with a link between the rate-limiting step and the decomposition of surface formates, as noted previously by Shido and Iwasawa for Rh/ceria, MgO, and ZnO.
Keywords: Magnesia; MgO; Zirconia; ZrO; 2; Tetragonal; Monoclinic; Platinum; Pt; Ceria; CeO; 2; Vacancies; Water-gas shift; LTS; Kinetic isotope effect; KIE; DRIFTS
Effect of the support on tin distribution in Pd–Sn/Al2O3 and Pd–Sn/SiO2 catalysts for application in water denitration by Anthony Garron; Károly Lázár; Florence Epron (pp. 57-69).
The reduction of nitrate to nitrogen is a key process for nitrate removal in water. Among others bimetallic, palladium tin catalysts are well suited for this purpose. In the present study Pd–Sn catalysts are used and characterized in the mentioned process. Alumina and silica supported catalysts were prepared by deposition of tin onto palladium particles by using controlled surface reaction. The accessibility of palladium and the amount of reducible tin are tested by O2/H2/O2 titration. The structural characterization is based on119Sn Mössbauer spectroscopy, TEM, XRD, and catalytic performances are tested in nitrate and nitrite reductions at room temperature. Remarkably, the properties of Pd–Sn/Al2O3 and Pd–Sn/SiO2 are different, although they were prepared according to the same procedure. Namely, the silica supported catalyst is more stable and selective, but is less active than the alumina supported one. This could be explained by the difference of (i) the interaction between the two metals and (ii) the affinity of tin with the support depending on the support used. The characterization results are interpreted with respect to the catalytic behavior of the materials.
Keywords: Pd–Sn catalysts; Catalytic reduction in water; Bimetallic catalyst; 119; Sn Mössbauer spectroscopy; Support effect
On the kinetic and structure sensitivity of lean reduction of NO with C3H6 over nanodispersed Pt crystals by Balint Ioan; Akane Miyazaki; Ken-ichi Aika (pp. 71-80).
Well-defined platinum nanocrystals (≈52% cubic) with an average diameter of 12nm were prepared by colloid method and then supported on alumina. The effect of amount of the exposed Pt (controlled either by metal loading or by the catalyst weight) on the catalytic performances for lean-burn deNO x reaction was taken firstly under investigation. An optimum peak NO x conversion of ≈56% was observed at 250°C. Typically, the propylene conversion level was in the 85–100% range at peak NO x conversions. The catalytic data free of transport effects have been identified and then used to calculate kinetic parameters for NO and C3H6 conversions (turnover frequencies and activation energies). The average TOF values for NO and C3H6 conversions, determined in the 225–275°C temperature domain, ranged between 0.04–0.27 and 0.05–0.50s−1, respectively. The average activation energies for NO and C3H6 conversions were 91 and 108kJmol−1, respectively. The structure sensitivity of NO/O2/C3H6 reaction as well as the morphological evolution of the well-structured platinum nanocrystals in reaction conditions have been clearly evidenced. The deNO x catalytic behavior was mainly related to the shape (facet effect) and to a lesser extent to the size (bulk effect) of the Pt nanoparticles. The N2/N2O ratio was higher (≈1:1) for the catalyst statistically rich in Pt nanoparticles with low index facets, relatively free of defects, compared to the polycrystalline one (≈1:2). High concentration of edges, corners, kinks, and surface defects of polycrystalline platinum particles was the main factor responsible for the overall catalytic activity for NO x conversion. Large (24nm) as well as small (2.4nm) polycrystalline Pt particles showed in great lines the same catalytic behavior for NO conversion. In light of experimental results, it is suggested that further improvement in the catalytic activity and selectivity for lean deNO x reaction (increase in the specific catalytic activity and N2/N2O ratio) can be foreseen through an optimum morphological (size and facet) control of the supported Pt particles.
Semiconductor-sensitized photodegradation of dichlorvos in water using TiO2 and ZnO as catalysts by E. Evgenidou; K. Fytianos; I. Poulios (pp. 81-89).
In the present study, the photocatalytic degradation of an organophosphorous insecticide, dichlorvos, using two different photocatalysts (TiO2 and ZnO) has been investigated. The blank experiments for either illuminated dichlorvos solution or the suspension containing TiO2 or ZnO and dichlorvos in the dark showed that both illumination and the catalyst were necessary for the destruction of the pesticide. Dichlorvos disappearance is achieved in ∼20min when treated with illuminated TiO2 and in ∼120min with illuminated ZnO. The degradation of the organic molecule follows first-order kinetics according to the Langmuir–Hinshelwood model. It was observed that the initial rate increases linearly with an increase in the amount of catalyst up to a level where it reaches a plateau. Temperature and pH also affect the rate of the reaction. The addition of an oxidant (H2O2 or K2S2O8) to the TiO2 suspensions leads to an increase in the rate of photooxidation. On the contrary, the addition of hydrogen peroxide in ZnO suspensions caused a decrease in the reaction rate. Moreover, illuminated TiO2 suspensions were proved to be quite effective in mineralizing dichlorvos. Measurements of chlorine and phosphate ions gave valuable information about how this process is achieved. On the other hand, ZnO suspensions could not lead in complete mineralization of the insecticide. Addition of the oxidants enhances mineralization for both photocatalytic systems. Finally, toxicity measurements showed that toxicity increases during treatment independently from the type of catalyst or the oxidant that has been used, indicating the inefficient detoxification capacity of the processes.
Keywords: Photocatalysis; Dichlorvos; TiO; 2; ZnO; Mineralization; Toxicity
Cerium incorporated ordered manganese oxide OMS-2 materials: Improved catalysts for wet oxidation of phenol compounds by M. Abecassis-Wolfovich; R. Jothiramalingam; M.V. Landau; M. Herskowitz; B. Viswanathan; T.K. Varadarajan (pp. 91-98).
Cryptomelane type manganese oxide OMS-2 material was synthesized by redox reaction between potassium permanganate and manganese sulphate in acidic medium under reflux or hydrothermal crystallization conditions. Ce was added by ion-exchange, impregnation or directly at crystallization stage. The chemical composition, structure, texture, morphology and thermal stability of the materials were measured by EDX spectroscopy, XRD, N2-adsorption, SEM and TGA. Results of the catalytic wet oxidation of phenol at 100°C indicate best performance of the well-crystallized octahedral molecular sieve with cryptomelane structure where all the accessible potassium ions were exchanged for cerium cations. OMS-2 materials containing pure CeO2 phase and excess of active oxygen species had lower stability, activity and capacity for oxidative reactive adsorption of phenol. Implementation of Ce-exchanged crystalline OMS-2 catalyst improves the wastewater treatment capacity by a factor of 1.5–2 and 3 compared with co-precipitated Mn–Ce-oxide and activated carbon, respectively.
Keywords: Mn–Ce-oxide catalyst; Cryptomelane; Phenol; Wet oxidation
Hydrogen peroxide-assisted photocatalytic oxidation of phenolic compounds by M.A. Barakat; J.M. Tseng; C.P. Huang (pp. 99-104).
The effect of hydrogen peroxide (H2O2) on photocatalytic oxidation of phenol and monochlorophenols (CP) in aqueous suspensions of commercial TiO2 rutile was investigated. Various concentrations of H2O2 were used without and with the presence of TiO2 under different atmospheres, e.g., N2 or O2. Sources of hydroxyl radicals for photocatalytic processes are suggested through the surface hydroxyl group reacting with hole, dissolved oxygen trapping an electron, and photolytic H2O2. The combination of TiO2 and H2O2 under UV illumination can greatly enhance the degradation rates of the phenol and chlorophenols. The photocatalytic oxidation with the H2O2/UV/TiO2 system was found to be much more effective than either UV/TiO2 or UV/H2O2 alone. The efficiency of the photocatalytic degradation of phenol was improved from 30 to 97% due to the presence of H2O2. As the H2O2 concentration increases, more hydroxyl radicals are produced, and the phenol oxidation rate increases. At high H2O2 concentration (≥10−2M), O2 or N2 atmospheres are not important factors for phenol oxidation in the H2O2/UV/TiO2 system.
Keywords: H; 2; O; 2; /UV/TiO; 2; system; Photocatalytic degradation; H; 2; O; 2; concentration
Selective catalytic reduction of NO with ammonia over V2O5 supported sulfated titanium-pillared clay catalysts: influence of V2O5 content by L. Khalfallah Boudali; A. Ghorbel; P. Grange; F. Figueras (pp. 105-111).
A series of V2O5 supported sulfated titanium-pillared clay catalysts with various amounts of vanadia were prepared and characterized by BET, DRX, XPS and NH3-TPD then studied for selective catalytic reduction (SCR) of NO by NH3 in the presence of oxygen. It was found that vanadia supported on sulfated Ti-pillared clay is more active at any temperature than sulfated Ti-pillared clay for the SCR of NO by NH3. The NO conversions over V2O5 supported sulfated Ti-pillared clay catalysts were found to increase with vanadia content, but the selectivity into nitrogen decreases upon sulfation, suggesting that there is an optimum of activity as a function of the number of acid sites.
Keywords: Sulfated Ti-pillared clay; V; 2; O; 5; SCR of NO by ammonia; acidity
Linear alkylbenzensulphonic acids (LAS) oxidation by H2O2 and O2: an investigation by gas- and liquid-chromatography coupled with mass spectrometry by Angela Cuzzola; Andrea Raffaelli; Piero Salvadori (pp. 113-120).
The degradation of linear alkylbenzensulphonic acids (LAS) by means of H2O2 and O2 as oxidant was investigated by mass spectrometry identification and characterization of partial oxidation products. Polar water-soluble compounds were analyzed by LC/MS with electrospray ionization (ESI), and the volatile products were trapped by means of headspace solid phase micro-extraction (HS-SPME) and investigated by GC/MS with EI and CI ionization. The shortening of LAS alkylic chain and the oxidation of the terminal part to carboxylic acids or the insertion of oxygen atoms to give hydroxylic derivatives, were observed. The reaction leads also to volatile aldehydes, with H2O2, and acids and lactones, with O2. On the basis of demolition product structures a relevant difference in selectivity between H2O2 and O2 was pointed out, obtaining important information on the reaction mechanism.
Keywords: Surfactants; Oxidation; Mass spectrometry; Reaction mechanism
Platinum and ruthenium catalysts on mesoporous titanium and zirconium oxides for the catalytic wet air oxidation of model compounds by Nina Perkas; Doan Pham Minh; Pierre Gallezot; Aharon Gedanken; Michèle Besson (pp. 121-130).
Pt and Ru catalysts on mesoporous (MSP) TiO2 and ZrO2 were developed using sonochemical irradiation. The catalysts were characterized by XRD, TEM, HR TEM, EDX, and BET methods. The high homogeneity of the active metal phase was confirmed by electron microscopy. The catalysts were examined for the removal of organic pollutants from model wastewaters using the wet air oxidation (WAO) process of acetic, succinic and p-coumaric acids in an autoclave reactor at 140 and 190°C and 50bar total air pressure. The high activity and stability of Pt supported on a TiO2 (MSP) catalyst in the removal of succinic and p-coumaric acids, and intermediates of their oxidation was demonstrated. The catalytic performances of Ru/TiO2 (MSP) were similar to those of the catalysts prepared by incipient-wetness impregnation of commercial TiO2 supports.
Keywords: Catalytic wet air oxidation; Heterogeneous catalysts; Mesoporous support; Nanoparticles; Sonochemical irradiation
A new photocatalytic membrane reactor (PMR) for removal of azo-dye Acid Red 18 from water by Sylwia Mozia; Maria Tomaszewska; Antoni W. Morawski (pp. 131-137).
A very promising method for solving problems concerning separation of photocatalyst as well as products and byproducts of photodecomposition from the reaction mixture is application of photocatalytic membrane reactors (PMRs). The PMRs described in literature combine photocatalysis with pressure-driven membrane techniques, such as nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF). A serious problem with application of these membrane techniques is membrane fouling, which is especially observable with MF and UF. The presented paper describes the results of the investigation on the possibility of coupling photocatalysis and membrane distillation (MD) for degradation of organic pollutants in aqueous solution. Acid Red 18 was applied as a model dye and titanium dioxide Aeroxide® P25 (Degussa, Germany) was used as a photocatalyst. In the first step of the research, the effect of the presence of TiO2 in a feed on the permeate flux was investigated. It was found that the addition of TiO2 P25 did not affect the distillate flux, regardless of the catalyst concentration applied. In order to select the best process conditions in terms of photocatalyst concentration and temperature of feed solution the batch tests were performed. On the basis of these experiments, the TiO2 concentration was set on a level of 0.3g/dm3 and the feed temperature in the reactor amounted to 333K. The hybrid process photocatalysis – MD – was conducted in a simultaneous mode. After 5h of the processes performance, the model dye was removed completely, whereas TOC concentration was lowered for ca. 80%.
Keywords: Photocatalytic membrane reactors; Photocatalyst separation; Titanium dioxide; Membrane distillation; Dye decomposition