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Applied Catalysis B, Environmental (v.57, #3)
Fe–Zn manganite spinels and their carbonate precursors: preparation, characterization and catalytic activity by G. Fierro; M. Lo Jacono; R. Dragone; G. Ferraris; G.B. Andreozzi; G. Graziani (pp. 153-165).
Iron–zinc manganites at different Fe content [ x=Fe/(Fe+Zn)=0, 0.01, 0.05, 0.10] were prepared by thermal decomposition of carbonate precursors obtained by coprecipitation at constant pH. Precursors were characterized by diffuse reflectance spectroscopy (DRS), thermogravimetric analysis (TGA-DTA) and magnetic susceptibility measurements. All samples are made by a single rhodochrosite-like phase, Fe xZn yMn(1− x− y)CO3, in which besides Zn2+ and Mn2+, only 5% of the total iron was Fe2+, the remaining 95% surprisingly being Fe3+ ions. The carbonate monophasic precursors were decomposed at 723 and 973K in air and solid solutions of iron in the zinc manganite spinel-like phase were obtained at both temperatures. Zinc manganite, ZnMn2O4, is a ‘normal’ spinel in which the Zn2+ ions occupy the tetrahedral sites and Mn3+ ions the octahedral sites of the crystal lattice. When iron is loaded, it is present as Fe3+ ions in the octahedral sites of the spinel together with Mn3+ ions. The catalyst reducibility in H2 was studied by temperature-programmed reduction (TPR). The iron-containing catalysts resulted to be slightly more resistant towards reduction than pure ZnMn2O4, suggesting that Fe3+ ions play a stabilizing effect on the spinel structure. Results of a Mössbauer investigation on the iron chemical state and its coordination symmetry are extensively reported. Some preliminary results of the catalytic behaviour for the reduction of NO by hydrocarbons are presented.
Keywords: Iron–zinc manganites; Carbonate precursors; Mössbauer spectroscopy; Temperature-programmed reduction; Diffuse reflectance spectroscopy; Magnetic susceptibility measurements; X-Ray diffraction; Reduction of NO by propane and propene
Redistribution of cobalt species in Co-ZSM5 during operation under wet conditions in the reduction of NO x by propane by Angel Martínez-Hernández; Gustavo A. Fuentes (pp. 167-174).
The presence of H2O introduced significant changes in the catalytic behavior of Co-ZSM5 during NO x reduction with propane at 500°C. Analysis of fresh and spent catalysts using ultraviolet–visible spectroscopy and temperature-programmed reduction in H2 were consistent with a deactivation mechanism involving nucleation growth of CoO x species. At short times on stream (TOS≤12h), the changes in activity (including an increase in activity) were caused by migration of Co species toward Co-free binding sites in the zeolite. At TOS of the order of tens of hours, CoO x moieties nucleated and then grew to form small crystallites of Co3O4 inside the zeolite channels. This process resulted in irreversible deactivation of the catalysts. The presence of a residual catalytic activity was ascribed to the existence of a hard-to-reduce Co2+ fraction. Structural characterization with X-ray diffraction,29Si and27Al magic angle spinning–nuclear magnetic resonance (MAS–NMR) showed that dealumination was not significant in our Co-ZSM5 samples.
Keywords: Co-ZSM5; NO; x; reduction; Deactivation; Propane; Hydrothermal treatment; Migration
Novel Pd/Sn xZr1− xO2 catalysts for methane total oxidation at low temperature and their18O-isotope exchange behavior by W. Lin; L. Lin; Y.X. Zhu; Y.C. Xie; K. Scheurell; E. Kemnitz (pp. 175-181).
Sn xZr1− xO2 solid solution-supported palladium catalysts were prepared and evaluated for the catalytic combustion of methane at low temperature. Catalyst Pd/Sn0.4Zr0.6O2 with the highest activity ( T90% = 378°C, GHSV = 48,000h−1) and excellent stability is one of the most active catalysts so far reported. The18O-isotope exchange measurements show that both partial and complete heterogeneous exchanges, as well as oxygen release, were observed for all catalysts. Compared with other catalysts, Pd/Sn0.4Zr0.6O2 has much higher activity for oxygen-exchange reaction and higher temperature for bulk palladium oxide decomposition. The results of TPR and18O-isotope exchange experiments demonstrate that the excellent activity of Pd/Sn0.4Zr0.6O2 is due to its high oxygen mobility and moderate reducibility. XPS results indicate that the dispersion of Pd has little influence on the catalytic activity.
Keywords: Methane total oxidation; Palladium; Sn–Zr solid solution; Reducibility; Oxygen mobility
An environmentally benign route to γ-butyrolactone through the coupling of hydrogenation and dehydrogenation by Yu-Lei Zhu; Jun Yang; Gen-Quan Dong; Hong-Yan Zheng; Hao-Hong Zhang; Hong-Wei Xiang; Yong-Wang Li (pp. 183-190).
The separated dehydrogenation cyclization of 1,4-butanediol, the hydrogenation of maleic anhydride, and the coupled process were carried out in a fixed-bed reactor over a Cu–Zn–Al catalyst, under different conditions of reaction temperatures and liquid hourly space velocity (LHSV). Compared to conventional processes, the coupled process has several advantages, e.g., improved γ-butyrolactone yield, good energy efficiency, optimal hydrogen utilization, and environmentally benign process. In addition, the coupled process can dramatically inhibit the formation of de-carbonization compounds, namely n-propanol and CO, and facilitate the decrease of γ-butyrolactone production cost in an industrial plant. The coupled operation leads to other advantages, such as easy temperature control in a tubular fixed-bed, due to its moderate heat release compared to the single maleic anhydride hydrogenation reaction, which can avoid the formation of apparent hotspots or coldspots in the practical process. Otherwise, the active hydrogen species released from the dehydrogenation cyclization of 1,4-butanediol is pretty suitable for the hydrogenation of maleic anhydride, and improves the selectivity of γ-butyrolactone. The coupled operation shows the improved technology and presents the goal of green chemistry, namely atom economic way, in the view-points of material, energy utilization and environment.
Keywords: Maleic anhydride hydrogenation; 1,4-Butanediol dehydrogenation; γ-Butyrolactone; Coupling reaction; Environmentally benign process
Influence of feed composition on the activity of Mn and PdMn/Al2O3 catalysts for combustion of formaldehyde/methanol by V.A. de la Peña O'Shea; M.C. Álvarez-Galván; J.L.G. Fierro; P.L. Arias (pp. 191-199).
The activity of supported manganese and palladium–manganese oxides over γ-alumina for the removal of a mixture of formaldehyde/methanol was evaluated using two different feed-streams. One consisted of air feed and the other that surrogates a gas exhaust type feed, with low proportion in oxygen, high concentration in water and a small amount of carbon monoxide. The catalysts were characterised by X-ray diffraction (XRD), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). These techniques pointed to a mixture of MnIV/MnIII oxide phases in both monometallic and bimetallic catalysts together with a PdO phase in the bimetallic systems. The TPR and XPS data of used catalysts depict a change in the active phase for the samples used in a feed-stream poor in oxygen. A decrease in the activity is observed when a feed that surrogates a gas exhaust stream is used. These facts are explained in terms of the mechanism by which VOCs are oxidized by lattice oxygen from Pd and/or Mn oxide phases and the slower re-oxidation of these phases by the oxygen-poor feed-stream. Besides, the high concentration of water in the feed favours deactivation of catalyst due to the competition of VOCs and water molecules for the active site.
Keywords: VOCs; Catalytic combustion; Manganese catalysts; Palladium; Formaldehyde
Decomposition and combined reforming of methanol to hydrogen: a FTIR and QMS study on Cu and Au catalysts supported on ZnO and TiO2 by Maela Manzoli; Anna Chiorino; Flora Boccuzzi (pp. 201-209).
Methanol decomposition and methanol combined reforming to hydrogen have been studied by in situ FTIR spectroscopy and quadrupole mass spectrometry (QMS) on Cu and Au catalysts supported on ZnO and TiO2 at increasing temperatures. Methoxy species on ZnO and TiO2 supported catalysts are produced by the reaction of methanol with OH surface groups already at room temperature. The evolution of the adsorbed species with the increase of the temperature is different on the four catalysts. The reaction mixture, the nature of the metal and the preparation method are the controlling parameters. Although both ZnO supported catalysts have been prepared by the same coprecipitation method, formate species are produced on Cu/ZnO, while they are completely lacking on the Au/ZnO sample during the same thermal treatment in pure methanol. On the contrary, the surface species on the TiO2 supported catalysts that have been both prepared by deposition–precipitation evolve quite similarly with the temperature. The different behaviour of the two coprecipitated ZnO based catalysts is mainly related to the formation of a solid solution precursor phase in the copper catalyst, not produced in Au/ZnO as a consequence of gold size. Therefore, gold is not able to activate the support towards the oxidative dehydrogenation of methanol to formaldehyde and it does not affect the defect equilibria of ZnO.In the methanol combined reforming reaction, the activity towards H2 production is beneficially influenced on the copper based catalysts and it is negatively affected by the presence of TiO2 as support.
Keywords: In situ FTIR spectroscopy; QMS; Copper catalysts; Gold catalysts; Methanol decomposition; Methanol combined reforming; Hydrogen production
Steam reforming of dimethyl ether over composite catalysts of γ-Al2O3 and Cu-based spinel by Yohei Tanaka; Ryuji Kikuchi; Tatsuya Takeguchi; Koichi Eguchi (pp. 211-222).
Cu-based spinel-type oxides were investigated for steam reforming of dimethyl ether (DME). Addition of γ-Al2O3 to Cu catalysts improved DME conversion since hydrolysis of DME was promoted over acid-sites on γ-Al2O3. Higher catalytic activity was shown over the composite of γ-Al2O3 and Cu–Mn, or Cu–Fe, or Cu–Cr oxide than that of γ-Al2O3 and Cu/ZnO/Al2O3. Cu–Fe and Cu–Mn catalysts demonstrated high activity for methanol steam reforming, which was ascribed to high performance for DME steam reforming. XPS measurements revealed that mono- and zero-valent copper species co-existed on Cu–Mn, Cu–Fe, and Cu/ZnO/Al2O3 catalysts subjected to in situ H2 reduction following methanol steam reforming.In the system of γ-Al2O3 and Cu–Mn–Fe oxide, Mn/Fe ratio was optimized. Mn/Fe ratio exhibited influence on CO2 and CO selectivity. Mn-rich composite catalysts produced more CO than Fe-rich catalysts. The optimized atomic ratio of Cu/Mn/Fe was 2/1/3. DME conversion was not influenced by steam/DME ratio in the feed gas, while CO formation was suppressed with a rise in steam/DME ratio. Complete DME conversion was attained below 350°C over the optimized composite catalyst. The optimal Al2O3/Cu–Mn–Fe weight ratio was between 1/2 and 1. High catalytic activity and thermal stability of the optimal composite was found by the time-on-steam test at as high as 400°C for 100h.
Keywords: Steam reforming; Dimethyl ether; Cu–Mn–Fe spinel-type oxide; γ-Al; 2; O; 3; Methanol steam reforming; XPS; Auger electron spectroscopy
Photocatalytic degradation of phenol in aqueous nitrogen-doped TiO2 suspensions with various light sources by Zhengpeng Wang; Weimin Cai; Xiaoting Hong; Xiaolian Zhao; Fang Xu; Chuenguang Cai (pp. 223-231).
In this study, the nitrogen-doped TiO2 polycrystalline powders were synthesized by calcination of the hydrolysis product of tetra-butyl titanate with ammonia. According to the XRD, SEM, XPS, UV–vis absorption and BET analysis, we found that the N-doped TiO2 has mesoporous structures and the nitrogen doping causes the absorption edge of TiO2 to shift to a lower energy region. It is considered that nitrogen atoms in doped TiO2 polycrystalline powder are responsible for the significant enhancement in the doped TiO2 photoactivity under visible light irradiation. We explored the photocatalytic activity of N-doped TiO2 under various irradiation conditions. We found the photocatalytic activity of the N-doped TiO2 with anatase phases was higher than that of the commercial TiO2 photocatalyst Degussa P25 for phenol decomposition under visible light irradiation, while Degussa P25 showed higher photocatalytic activity under sunlight irradiation. The FT-IR spectra showed that products of phenol decomposition adsorbed on the catalyst surface after photodegradation of phenol.
Keywords: Photocatalyst; Nitrogen-doped TiO; 2; Phenol; Visible light; Sunlight
Erratum to ‘Study on the mechanism of the reaction of NO2 with aluminium oxide’ [Appl. Catal. B: Environ. 51 (2004) 43–50]
by N. Apostolescu; T. Schröder; S. Kureti (pp. 233-233).