Applied Catalysis B, Environmental (v.33, #1)
Effects of various alkali–acid additives on the activity of a manganese oxide in the catalytic combustion of ketones by L.M. Gandía; A. Gil; S.A. Korili (1-8).
The catalytic combustion of acetone and methyl-ethyl-ketone (MEK) has been studied over a manganese oxide, Mn2O3. The reactant conversion has been followed as a function of the reaction temperature and it has been observed that lower temperatures are required for the combustion of acetone than for MEK. The performance of Mn2O3 in the combustion reactions when alkali and acid ions are added to the oxide has also been investigated. A significant improvement of the catalyst performance is observed when Cs+ and Na+ are used as additives. The data of the ignition curves have been fitted to a simplified model where both ketone combustion reactions are assumed to have power law rate equations, which are first-order with respect to the corresponding organic molecule and do not depend on the oxygen partial pressure. Differences in the apparent activation energy values estimated with this model for acetone and MEK combustion are significant only when acid ions additives are present.
Keywords: Manganese oxide; Acetone catalytic combustion; Methyl-ethyl-ketone catalytic combustion; Alkali–acid additives;
Adsorption complexes of O2 on Fe/MFI and their role in the catalytic reduction of NO x by Zhi-Xian Gao; Qi Sun; Wolfgang M.H. Sachtler (9-23).
Exposure to oxygen of Fe/MFI catalysts, prepared by sublimation, results in an adsorbed oxygen species capable of reacting with nitrogen oxide or carbon monoxide at room temperature. The IR bands in the perturbed T-O-T vibration region indicate oxidation of Fe2+ at room temperature. The FTIR data also show that more nitro groups and nitrate ions are formed from NO+O2, if Fe/MFI is exposed to oxygen first, than if nitrogen oxide is dosed first. Formation of these NO y groups is faster on pre-reduced than on pre-oxidized catalysts, which suggests that diatomic oxygen is adsorbed on sites which, in the pre-oxidized catalysts, are occupied by oxide ions. Over pre-reduced catalysts, carbon monoxide oxidation with 18 O 2 yields significant amounts of C 18 O 16 O and a small amount of C 18 O 2 , even at room temperature.It is uncertain whether the adsorbed diatomic oxygen is present as a superoxide ion, as previously detected by ESR on Fe/MFI, Co/MFI and V/MFI at 77 K, or as a peroxo groups bridging over two iron ions, as identified in DFT analysis. Formation of nitrate ions is visualized by interaction of nitrogen oxide with a superoxide ion, possibly via a peroxynitrite ion.
Keywords: Superoxide ion; NO x reduction; Sublimation method; Bridging peroxo group; Fe/MFI catalyst;
Lean NO x reduction with dodecane over cerium and palladium loaded mordenite by Luis Fernando Córdoba; Maria Flytzani-Stephanopoulos; Consuelo Montes de Correa (25-33).
The selective catalytic reduction (SCR) of NO x by dodecane in excess oxygen-containing gas mixtures was studied on Ce and/or Pd-loaded HMOR. Under dry conditions the best fresh bimetallic catalyst, 6.2 wt.% Ce–0.08 wt.% Pd-HMOR, displayed a maximum of about 70% NO x conversion to N2 at 350°C and GHSV=30,000 h−1, while either Ce-HMOR or Pd-HMOR exhibited low activity for the SCR reaction. The presence of both Ce and Pd in the zeolite was crucial for high deNO x activity. The dodecane concentration is very important for the reaction. The catalyst is not active in the absence of the reducing agent and inhibition of NO x reduction is observed at dodecane concentrations higher than 440–500 ppm. The presence of 40 ppm SO2 in the gas feed suppresses the reaction. However, the coexistence of 15% H2O and 40 ppm SO2 has no appreciable effect on the catalyst activity. Enhanced activity and a broader temperature window was observed in the NO2+dodecane+O2 reaction in the presence of 15% H2O and 100 ppm SO2. In this reaction mixture, the catalyst was capable of retaining a stable NO2 conversion to N2 for a period of ten days. Characterization by XPS and UV–VIS–diffuse reflectance spectra (UV–VIS–DRS) of selected fresh and aged catalyst samples indicates that Pd exists mainly as Pd2+ cations in Ce–Pd-HMOR. Cerium is mostly present as CeO2 on the surface of the zeolite particles. However, part of the cerium in 6Ce–Pd-HMOR, exists as stable Ce3+ species.
Keywords: Cerium; Palladium; Mordenite; Nitrogen oxides; Dodecane; Lean SCR;
New spinel materials for catalytic NO–CO reaction: nonstoichiometric nickel–copper manganites by C Drouet; P Alphonse; A Rousset (35-43).
Nonstoichiometric nickel–copper spinel manganites were found to be highly active for the reduction of nitric oxide by carbon monoxide at low temperature (300°C). The activity increases with the level of nonstoichiometry of the oxides and almost linearly with their specific surface area. Moreover, this activity was found to depend strongly on the copper content, nickel manganites being almost inactive. The influence of the surface state of the oxides was also investigated. Cuprous cations are thought to play the determining role in the catalytic cycle.
Keywords: Manganite (nickel–copper); Spinel; deNO x ; Nitric oxide; Carbon monoxide;
Role of bulk and surface structures of La1−x Sr x NiO3 perovskite-type oxides in methane combustion by R.M. Garcı́a de la Cruz; H. Falcón; M.A. Peña; J.L.G. Fierro (45-55).
La1−x Sr x NiO3 (x=0.00–0.20) mixed oxides have been prepared and tested in the combustion of methane. X-ray diffraction (XRD) profiles revealed the presence of a single perovskite structure for substitutions 0≤x≤0.05, whereas a mixture of the major perovskite phase and minor NiO, SrO and SrCO3 phases were observed for substitutions x≥0.10. Temperature-programmed reduction (TPR) profiles showed: (i) a first reduction step at 520–650, due to reduction of Ni3+ to Ni2+, in which the perovskite structure is preserved although distorted by the oxygen vacancies; (ii) an intermediate reduction step at 589–650 K associated to reduction of the segregated NiO phase; (iii) a final step at 720–800 K assigned to the complete reduction of the Ni2+-containing phases into La2O3 and Nio. The partial substitution of Sr by La alters the oxide stoichiometry, yielding a mixture of NiII/NiIII oxidation states and oxygen vacancies, together with surface enrichment of Sr-containing phases. A close relationship has been found between substitution degree (x) and the oxygen non-stoichiometry. The highest intrinsic activity for the CH4 combustion in sample x=0.10 is associated not only with the largest proportion of oxygen non-stoichiometry but also with the lowest Sr segregation on the surface.
Keywords: Methane combustion; La1−x Sr x NiO3 perovskite-type oxides; Catalyst characterisation;
Photocatalytic mineralisation of mercaptans as environmental pollutants in aquatic system using TiO2 suspension by Mohammad Hossein Habibi; Shahram Tangestaninejad; Bahram Yadollahi (57-63).
The photocatalytic oxidation of aqueous solutions of 2-mercaptobenzothiazole (MBT), 2-mercaptobenzimidazole (MBI), 2-mercaptobenzoxazole (MBO), 2-mercaptopyridine (MP), para-tolylmercaptan (PTM) was investigated over a heterogeneous catalyst of TiO2, in a photocatalytic reactor. The effect of some physical and chemical parameters such as amount of catalyst, flow of oxygen, pH and time of irradiation were studied in order to optimise the process. These mercaptans are almost completely mineralised to carbon dioxide, ammonia and sulphate ion by photocatalytic method. The results obtained from photocatalytic oxidation of MBT have led us to propose a compound with the sum formula of benzothiazol-2-sulfonate (BTSO3) to be a main intermediate.
Keywords: Environmental pollutants; Mercaptans; Mineralisation; 2-Mercaptobenzothiazole (MBT); 2-Mercaptobenzimidazole (MBI); 2-Mercaptobenzoxazole (MBO); 2-Mercaptopyridine (MP); para-Tolylmercaptan (PTM); TiO2 photocatalyst; Odour control;
An investigation on the utilization of perovskite-type oxides La1−x Sr x MO3 (M = Co0.77Bi0.20Pd0.03) as three-way catalysts by H He; H.X Dai; C.T Au (65-80).
The perovskite-type oxides La1−x Sr x MO3 (M=Co0.77Bi0.20Pd0.03) have been investigated for three-way catalytic performance and characterized by means of temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The catalysts exhibited good activity in CO elimination: La0.8Sr0.2MO3 showed ca. 100% CO conversion at 160°C, 60 000 h−1, and λ=1.00. Under similar reaction conditions, the activity for C3H6 elimination decreased in the order of La0.2Sr0.8MO3>La0.8Sr0.2MO3>La0.4Sr0.6MO3>La0.6Sr0.4MO3>LaMO3, while the activity for NO elimination decreased in the order of La0.8Sr0.2MO3>La0.2Sr0.8MO3>La0.4Sr0.6MO3>La0.6Sr0.4MO3>LaMO3. With x<0.6, La1−x Sr x MO3 were single-phase and hexagonal in structure; at x=0.6 and 0.8, they were cubic and there was a trace amount of the La2O3 phase. The results of TPD, TPR, and XPS studies revealed the coexistence of over-stoichiometric oxygen vacancies in La0.8Sr0.2MO3, a criterion for good three-way catalytic performance.
Keywords: Perovskite-type oxides; La1−x Sr x MO3 (M=Co0.77Bi0.20Pd0.03); Three-way catalysts; Automotive exhaust purification; Oxidative nonstoichiometry;
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