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Applied Catalysis B, Environmental (v.56, #4)
NO decomposition over a Pd/MgO catalyst prepared from [Pd(acac)2] by Xianqin Wang; James J. Spivey; H. Henry Lamb (pp. 261-268).
A Pd/MgO catalyst was prepared by adsorption of palladium bis-acetylacetonate [Pd(acac)2] onto highly dehydroxylated MgO from toluene solution and subsequent reduction in flowing H2 at 300°C. The resultant catalyst was characterized by Pd K-edge X-ray absorption fine structure (XAFS) spectroscopy, temperature-programmed desorption (TPD), and Fourier transform infrared (FTIR) spectroscopy of adsorbed NO. The adsorbed [Pd(acac)2] species decomposes on heating in H2 to form 20–25Å supported Pd particles; however, organic residues from the acetylacetonate ligands remain on the catalyst surface. The FTIR spectrum of NO adsorbed on the reduced Pd/MgO catalyst at 25°C contains one principal band at 1722cm−1 due to atop Pd nitrosyl species. In situ XAFS of the Pd/MgO catalyst indicates that neither Pd oxidation nor particle sintering occurs during heating in flowing 1% NO/He to 300°C. NO decomposition over the Pd/MgO catalyst was investigated using temperature-programmed reaction spectroscopy (TPRS) and steady-state activity measurements. During the initial TPRS cycle in flowing 1% NO/He, nearly complete NO consumption occurs at ∼270°C due to oxidation of organic residues. O2 evolution commences at approximately 350°C, and steady-state catalytic decomposition of NO to N2 and O2 occurs at 600°C. Transient NO consumption during rapid cooling in 1% NO/He (after steady-state catalysis) is attributed to NO x adsorption on the Pd/MgO catalyst.
Keywords: NO decomposition; Temperature-programmed reaction; Temperature-programmed desorption; X-ray absorption fine structure spectroscopy; Fourier transform infrared spectroscopy
NO reduction by CH4 over Pd/Co-sulfated zirconia catalysts by Luis Fernando Córdoba; Wolfgang M.H. Sachtler; Consuelo Montes de Correa (pp. 269-277).
A series of sulfated zirconia supported Pd/Co catalysts was synthesized by the sol–gel method and examined for NO x reduction by methane. The NO conversion increased up to a Co/S ratio of 0.43, and then decreased at a higher Co loading (Co/S=0.95). Sulfate content was also essential for obtaining high selectivity to molecular nitrogen. A catalyst loaded with 0.06wt.% Pd, 2.1wt.% Co and 2.1wt.% S (Pd/Co-SZ-2) exhibited remarkable performance under lean conditions and displayed stability in a long-term durability test using a synthetic reaction mixture containing 10% water vapor. This catalyst exhibited the highest sulfur retention most probably as cobalt sulfide. Besides, the catalytic oxidation of NO to NO y groups was confirmed by FT-IR, in agreement with the general mechanism for the SCR of NO by hydrocarbons. In the absence of oxygen in the feed stream, the catalyst was highly active for NO reduction with methane. IR stretching bands assigned to N2O and adsorbed nitro groups were identified upon adsorbing NO on Pd/Co-SZ-2. This indicates that under rich conditions disproportionation of NO to N2O and NO2 occurs and confirms that the formation of NO2 species is an essential step for NO reduction by CH4.
Keywords: Nitrogen oxides; Methane; SCR; Sulfated zirconia; Sol–gel Co-SZ; Pd/Co-SZ
The role of lanthana loading on the catalytic properties of Pd/Al2O3-La2O3 in the NO reduction with H2 by Arturo Barrera; Margarita Viniegra; Sergio Fuentes; Gabriela Díaz (pp. 279-288).
The role of La2O3 loading in Pd/Al2O3-La2O3 prepared by sol–gel on the catalytic properties in the NO reduction with H2 was studied. The catalysts were characterized by N2 physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO.The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La2O3 inclusion. The selectivity depends on the NO/H2 molar ratio (GHSV=72,000h−1) and the extent of interaction between Pd and La2O3. At NO/H2=0.5, the catalysts show high N2 selectivity (60–75%) at temperatures lower than 250°C. For NO/H2=1, the N2 selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H2O vapor. The high N2 selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdO x phase interacting with La2O3. The formation of this phase is favored by the spreading of PdO promoted by La2O3. DTA shows that the phase transformation takes place at temperatures of 280–350°C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al2O3 the O2 uptake is consistent with the oxidation of PdO to PdO2, and when La2O3 is present the O2 uptake exceeds that amount (∼1.5 times). La2O3 in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250°C) favoring the formation of N2.
Keywords: Pd catalysts; Al; 2; O; 3; -La; 2; O; 3; binary oxide; Sol–gel method; Reducibility; NO desorption properties; NO reduction with H; 2; Selectivity to N; 2
Role of iron surface oxidation layers in decomposition of azo-dye water pollutants in weak acidic solutions by Jerzy A. Mielczarski; Gonzalo Montes Atenas; Ela Mielczarski (pp. 289-303).
While decomposition of water pollutants in the presence of metallic iron can be strongly influenced by the nature and structure of the iron surface layer, the composition and structure of the layer produced and transformed in the decomposition process, have been meagerly investigated. The studies presented here establish strong relationships between the composition and structure of the iron oxidized surface layer and the kinetics and reaction pathways of orange II decomposition. The most striking observation is a dramatic difference between dye decomposition at pH 3 and 4. Orange decomposition at pH 2 and 3 is a very fast process with pseudo-first-order kinetics, with a surface normalized rate constant kSA=0.18L/m2min at pH 3 and 30°C. Whereas at pH 4 and 5 the rate is lower with pseudo-zero-order kinetics, with normalized rate constant kSA=1.4×10−5mol/m2min at pH 5 and 30°C. At pH 3 the iron surface is covered by a polymeric Fe(OH)2 mixed with FeO very thin layer whose thickness remains almost constant with reaction time. There is a slow formation of an additional surface product with akaganeite-like structure. At pH 3 almost all oxidized iron is detected in solution, whereas at pH 5 almost total oxidized iron is cumulated on iron surface in the form of a lepidocrocite, γ-FeOOH, layer. The thickness of the layer increases continuously with time. The quantitative evaluation of the produced surface lepidocrocite and its surface distribution were performed by means of infrared reflection spectroscopy and spectral simulation methods. At higher temperature 40–50°C, other surface products such as goethite, α-FeOOH, and feroxyhite, β-FeOOH, are also observed. Decomposition of orange is a multi-step process, at pH 3 the orange molecule is at first adsorbed on the very thin iron oxidized layers through SO3 group and then undergoes reduction. Discoloration of orange II in aerobic solution takes place by reduction of theNN bond at the iron surface. The major intermediate is 1-amino-2-naphtol, which undergoes further decomposition without forming any aromatic species. The previously suggested sulfanilic acid as intermediate was not detected in solution. At pH 3 orange reduction and reduction of intermediates are governed by the combination of an electron transfer reaction, with the thin oxide surface layer as a mediator, and the catalytic hydrogenation reaction. At pH 4 and 5 continuous growing of lepidocrocite surface layer demonstrates the importance of the layer as a mediator in the electron transfer reaction. The layer shows a good conductivity, which results from adsorption and absorption of iron ions in the surface structure. It is observed that the decomposition reaction becomes significant at open circuit potential (OCP) below −120mV (SHE). At pH 3 this condition is fulfilled almost immediately after introduction of iron to aqueous solution, whereas at pH 4 and 5 the OCP of iron decreases very slowly. Iron surface layer composition and structure can be modified by an addition of Fe2+ to solution, which increases the dye decomposition rate. The performed observations make the treatment of waste water in the presence of metallic iron a promising environmental solution.
Keywords: Water pollutant; Orange II decomposition; Decomposition pathways; Iron surface compositions; pH dependence
Photocatalytic degradation of phenol on MWNT and titania composite catalysts prepared by a modified sol–gel method by Wendong Wang; Philippe Serp; Philippe Kalck; Joaquim Luís Faria (pp. 305-312).
Multi-walled carbon nanotubes (MWNT) and nanocrystalline titania composite catalysts were prepared by a modified sol–gel method. Thermogravimetric analysis, N2 adsorption–desorption isotherm measurements, powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy and UV–vis spectra were carried out to characterize the composite catalysts with different MWNT contents. The results suggest that the presence of MWNT embedding in the composite catalysts matrix prevents TiO2 particle agglomeration. Additionally, a correlation exists between the MWNT content and the changes in the UV–vis absorption properties. The photocatalytic degradation of phenol was chosen as a model reaction to evaluate the photocatalytic activities of the composite catalysts. An optimum of the synergetic effect was found for a weight ratio MWNT/TiO2 equal to 20%. The effects induced by MWNT on the composite catalysts may be explained in terms of a strong interphase interaction between MWNT and TiO2 in the composite catalysts.
Keywords: Photocatalysis; Phenol degradation; Titanium dioxide; MWNT; Composite catalyst
Catalytic combustion of hexane over transition metal modified zeolites NaX and CaA by Eva Díaz; Salvador Ordóñez; Aurelio Vega; José Coca (pp. 313-322).
Hexane deep oxidation was studied over NaX and CaA zeolites modified by ion exchange with transition metals (Mn2+, Co2+, Fe3+), the percentage of ion exchanged, determined by ICP-MS, varying between 39 and 98%. Parent and exchanged zeolites were characterized by X-ray diffraction (XRD), N2 physisorption, temperature-programmed reduction (TPR), oxygen and ammonia temperature-programmed desorption (TPD) and inverse gas chromatography (IGC). Catalytic activities were evaluated through the recording of light-off curves in a pulsed microreactor, catalytic activity being correlated with physicochemical properties of the solids (crystallinity, surface acidity, adsorption properties and morphological parameters). As general trend, CaA zeolites are more active than NaX zeolites. Mn-exchanged CaA zeolite was the most active catalyst for hexane oxidation, whereas the addition of Fe to the zeolites leads to strong chemical and morphological changes in the parent zeolite.
Keywords: Hexane; Deep oxidation; Adsorption; Transition metal exchanged zeolites; X and A zeolites