Fe2O3/Al2O3 catalysts for the N2O decomposition in the nitric acid industry

Fe₂O₃ catalysts supported on Al₂O₃ were used to remove nitrous oxide from the nitric acid plant simulated process stream (containing O₂, NO and H₂O). Catalysts were prepared by the coprecipitation method and were characterized for their physico-chemical properties by BET, XRD, AFM and TPR analysis. A strong influence of the post-preparation heating conditions on the structural and catalytic properties of the catalysts has been evidenced. Laboratory tests revealed 95% conversion of N₂O at temperature 750 °C and a slight decrease in activity in the presence of H₂O and NO. The catalysts were inert towards decomposition of NO. The pilot-plant reactor and real plant studies (up to 3300 h time-on-stream) confirmed high activity and very good mechanical stability of the catalysts as well as no decomposition of nitric oxide.     

NO decomposition and reduction by CH4 over Sr/La2O3

Both NO decomposition and NO reduction by CH₄ over 4%Sr/La₂O₃ in the absence and presence of O₂ were examined between 773 and 973 K, and N₂O decomposition was also studied. The presence of CH₄ greatly increased the conversion of NO to N₂ and this activity was further enhanced by co-fed O₂. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH₄ in the absence of O₂, and reduction with 1% O₂ in the feed were 8.3·10⁻⁴, 4.6·10⁻³, and 1.3·10⁻² μmol N₂/s m², respectively. This oxygen-enhanced activity for NO reduction is attributed to the formation of methyl (and/or methylene) species on the oxide surface. NO decomposition on this catalyst occurred with an activation energy of 28 kcal/mol and the reaction order at 923 K with respect to NO was 1.1. The rate of N₂ formation by decomposition was inhibited by O₂ in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH₄ continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O₂ with equal activation energies of 26 kcal/mol. The addition of O₂ increased the reaction order in CH₄ at 923 K from 0.19 to 0.87, while it decreased the reaction order in NO from 0.73 to 0.55. The reaction order in O₂ was 0.26 up to 0.5% O₂ during which time the CH₄ concentration was not decreased significantly. N₂O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10⁻⁴ μmol N₂/s m² at 623 K and 1220 ppm N₂O and an activation energy of 24 kcal/mol. The addition of CH₄ inhibits this decomposition reaction. Finally, the use of either CO or H₂ as the reductant (no O₂) produced specific activities at 773 K that were almost 5 times greater than that with CH₄ and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H₂ to reduce NO to N₂ over these REO catalysts.     

Catalytic decomposition of N2O and catalytic reduction of N2O and N2O + NO by NH3 in the presence of O2 over Fe-zeolite

The decomposition of N₂O, and the catalytic reduction by NH₃ of N₂O and N₂O + NO, have been studied on Fe-BEA, -ZSM-5 and -FER catalysts. These catalysts were prepared by classical ion exchange and characterized by TPR after various activation treatments. Fe-FER is the most active material in the catalytic decomposition because “oxo-species” reducible at low temperature, appearing upon interaction of Fe^II-zeolite with N₂O (-oxygen), are formed in largest amounts with this material. The decomposition of N₂O is promoted by addition of NH₃, and even more with NH₃ + NO in the case of Fe-FER and -BEA. It is proposed that the NO-promoted reduction of N₂O originated from the fast surface reaction between -oxygen O^* and NO^* to yield NO₂^*, which in turn reacts immediately with NH₃.     

ZrO2/SiO2- and La2O3/Al2O3-supported platinum catalysts for CH4/CO2 reforming

Surface-phase ZrO₂ on SiO₂ (SZrOs) and surface-phase La₂O₃ on Al₂O₃ (SLaOs) were prepared with various loadings of ZrO₂ and La₂O₃, characterized and used as supports for preparing Pt/SZrOs and Pt/SLaOs catalysts. CH₄/CO₂ reforming over the Pt/SZrOs and Pt/SLaOs catalysts was examined and compared with Pt/Al₂O₃ and Pt/SiO₂ catalysts. CO₂ or CH₄ pulse reaction/adsorption analysis was employed to elucidate the effects of these surface-phase oxides.

The zirconia can be homogeneously dispersed on SiO₂ to form a stable surface-phase oxide. The lanthana cannot be spread well on Al₂O₃, but it forms a stable amorphous oxide with Al₂O₃. The Pt/SZrOs and Pt/SLaOs catalysts showed higher steady activity than did Pt/SiO₂ and Pt/Al₂O₃ by a factor of three to four. The Pt/SZrOs and Pt/SLaOs catalysts were also much more stable than the Pt/SiO₂ and Pt/Al₂O₃ catalysts for long stream time and for reforming temperatures above 700 °C. These findings were attributed to the activation of CO₂ adsorbed on the basic sites of SZrOs and SLaOs.     

Mechanistic aspects of N2O and N2 formation in NO reduction by NH3 over Ag/Al2O3: The effect of O2 and H2

A mechanistic scheme of N₂O and N₂ formation in the selective catalytic reduction of NO with NH₃ over a Ag/Al₂O₃ catalyst in the presence and absence of H₂ and O₂ was developed by applying a combination of different techniques: transient experiments with isotopic tracers in the temporal analysis of products reactor, HRTEM, in situ UV/vis and in situ FTIR spectroscopy. Based on the results of transient isotopic analysis and in situ IR experiments, it is suggested that N₂ and N₂O are formed via direct or oxygen-induced decomposition of surface NH₂NO species. These intermediates originate from NO and surface NH₂ fragments. The latter NH₂ species are formed upon stripping of hydrogen from ammonia by adsorbed oxygen species, which are produced over reduced silver species from NO, N₂O and O₂. The latter is the dominant supplier of active oxygen species. Lattice oxygen in oxidized AgO_x particles is less active than adsorbed oxygen species particularly below 623 K. The previously reported significant diminishing of N₂O production in the presence of H₂ is ascribed to hydrogen-induced generation of metallic silver sites, which are responsible for N₂O decomposition.     

Co-Cu-Ce-Fe/γ-Al_2O_3催化剂催化分解N_2O的性能

以等体积浸渍法制备γ-Al_2O_3负载的Co、Cu、Ce和Fe氧化物催化剂,利用正交试验设计实验条件,采用XRD、BET和H_2-TPR等对催化剂进行表征,并考察活性组分对催化剂催化分解N_2O活性的影响。结果表明,催化剂具有尖晶石结构,其BET比表面积随着金属氧化物负载量增加而降低。催化剂中铜的氧化物可以降低还原峰温度,进而明显提高催化活性,Co和Fe的加入对活性有一定的提高,Ce对催化活性没有明显影响。     

Catalytic decomposition of N2O over CeO2 promoted Co3O4 spinel catalyst

A series of CeO₂ promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N₂O). Addition of CeO₂ to Co₃O₄ led to an improvement in the catalytic activity for N₂O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N₂O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O₂, H₂O or NO. Methods of XRD, FE-SEM, BET, XPS, H₂-TPR and O₂-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO₂ could increase the surface area of Co₃O₄, and then improve the reduction of Co³⁺ to Co²⁺ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N₂O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO₂, are responsible for the enhancement of catalytic activity of Co₃O₄.     

A zirconium-containing sandwich-type dimer based on trivacant α- and β-[GeW9O34] units, [Zr3O(OH)2(α-GeW9O34)(β-GeW9O34)]

A new zirconium-containing sandwich-type dimer based on trivacant α- and β-[GeW₉O₃₄]¹⁰⁻ units, [Zr₃O(OH)₂(α-GeW₉O₃₄)(β-GeW₉O₃₄)]¹²⁻, was synthesized and characterized by IR spectroscopy, UV electronic spectroscopy and single crystal X-ray diffraction. The central bent of the polyanion contains three oxygen atoms alternating with the three zirconium atoms. The polyanion represents a rare example of the sandwich POMs containing two different isomer subunits.     

Promoting effect of calcium on reduction of NO by C3H6 over V2O5/ZrO2 catalysts

The reduction of nitrogen monoxide by propene on V₂O₅/ZrO₂ doped with or without calcium has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Considerable promoting effect of calcium doping on the reduction of nitrogen monoxide by propene was observed on the V₂O₅/ZrO₂ catalysts. For the reaction of a mixture of NO+C₃H₆, carbonyl and carboxylate species were observed above 373 K, although nitrate species formed at room temperature on V₂O₅/ZrO₂ doped with calcium. No bands due to a compound including both carbon and nitrogen atoms were observed. Thus, the redox mechanism, i.e. propene reduces the catalyst and nitrogen monoxide oxidizes the catalyst, is confirmed on V₂O₅/ZrO₂ catalysts doped with or without calcium. The analysis of the V=O band in the region of 1100–900 cm⁻¹ indicates that this promotion is mainly due to new V=O species formed by the addition of calcium onto the catalyst. This species is easily reproduced in comparison with the other V=O species on the surface in the reoxidation process of the catalyst.     

Interfacial phenomena in Al2O3—liquid metal and Al2O3—liquid alloy systems

The sessile drop technique has been used to measure the contact angle of liquid metals and liquid alloys in contact with polycrystalline alumina. The experiments were carried out in argon atmosphere at various temperatures. The measured contact angles exhibit no wettability (θ > 90°). The linear temperature functions of the work of adhesion as well as of the interfacial energy were also determined in the investigated systems. The partial high values of the work of adhesion in several Al₂O₃-liquid metal systems can be attributed to a chemical bond establishment at the interface. The values of the interfacial energies at the melting point of the metals, for non-reactive Al₂O₃-liquid metal systems, vary in a restricted region (2·35–2·75) J m⁻²). An empirical relation is proposed for evaluation of the interfacial energy of the metals at their melting point. The agreement between experimental and calculated values is satisfactory.     

Phase diagram of the system: Al2O3–ZrO2

The system Al₂O₃–ZrO₂ was studied by differential thermal analysis in inert atmosphere and in vacuum. The eutectic was located at 1866°C and 40% mass of ZrO₂. Zirconia solid solution at the eutectic temperature is up to 1.1±0.3% mass of Al₂O₃. Enthalpy of melting of this eutectic is 1080±90 J/g. Pure ZrO₂ transforms from monoclinic to tetragonal at 1162±7°C, but the saturated solid solution of ZrO₂, with 0.7±0.2% mass Al₂O₃ at this temperature, transforms at 1085±5°C. Inverse transitions occur with hysteresis correspondingly at 1055±5 and 995±5°C. Enthalpy of transformation of pure ZrO₂ from monoclinic to tetragonal phase is 42±5 J/g (5.2±0.6 J/mol) but only 30±5 J/g for a ZrO₂ saturated solid solution.     

Catalytic oxidation of naphthalene using a Pt/Al2O3 catalyst

Polycylic aromatic hydrocarbons (PAHs) are listed as carcinogenic and mutagenic priority pollutants, belonging to the environmental endocrine disrupters. Most PAHs in the environment stem from the atmospheric deposition and diesel emission. Consequently, the elimination of PAHs in the off-gases is one of the priority and emerging challenges. Catalytic oxidation has been widely used in the destruction of organic compounds due to its high efficiency (or conversion of reactants), its economic benefits and good applicability.

This study investigates the application of the catalytic oxidation using Pt/γ-Al₂O₃ catalysts to decompose PAHs and taking naphthalene (the simplest and least toxic PAH) as a target compound. It studies the relationships between conversion, operating parameters and relevant factors such as treatment temperatures, catalyst sizes and space velocities. Also, a related reaction kinetic expression is proposed to provide a simplified expression of the relevant kinetic parameters.

The results indicate that the Pt/γ-Al₂O₃ catalyst used accelerates the reaction rate of the decomposition of naphthalene and decreases the reaction temperature. A high conversion (over 95%) can be achieved at a moderate reaction temperature of 480 K and space velocity below 35,000 h⁻¹. Non-catalytic (thermal) oxidation achieves the same conversion at a temperature beyond 1000 K. The results also indicate that Rideal–Eley mechanism and Arrhenius equation can be reasonably applied to describe the data by using the pseudo-first-order reaction kinetic equation with activation energy of 149.97 kJ/mol and frequency factor equal to 3.26 × 10¹⁷ s⁻¹.     

Water-induced bulk Ba(NO3)2 formation from NO2 exposed thermally aged BaO/Al2O3

Phase changes in high temperature treated (>900 °C) 8 or 20 wt% BaO supported on γ-Al₂O₃ model lean NO_x trap (LNT) catalysts, induced by NO₂ and/or H₂O adsorption, were investigated with powder X-ray diffraction (XRD), solid state ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and NO₂ temperature programmed desorption (TPD) experiments. After calcination in dry air at 1000 °C, the XRD and solid state ²⁷Al MAS NMR results confirm that stable surface BaO and bulk BaAl₂O₄ phases are formed for 8 and 20 wt% BaO/Al₂O₃, respectively. Following NO₂ adsorption over these thermally treated samples, some evidence for nanosized Ba(NO₃)₂ particles are observed in the XRD results, although this may represent a minority phase. However, when water was added to the thermally aged samples after NO₂ exposure, the formation of bulk crystalline Ba(NO₃)₂ particles was observed in both samples. Solid state ²⁷Al MAS NMR is shown to be a good technique for identifying the various Al species present in the materials during the processes studied here. NO₂ TPD results demonstrate a significant loss of uptake for the 20 wt% model catalysts upon thermal treatment. However, the described phase transformations upon subsequent water treatment gave rise to the partial recovery of NO_x uptake, demonstrating that such a water treatment of thermally aged catalysts can provide a potential method to regenerate LNT materials.     

Ketone-amine based suspensions for electrophoretic deposition of Al2O3 and ZrO2

Stable suspensions based on methylethylketone (MEK), n-butylamine and nitrocellulose were developed for the electrophoretic deposition (EPD) of Al₂O₃ and ZrO₂ powder. Deposits with a high green density, smooth surface and high deposition yield were obtained upon adding 10–15 vol.% n-butylamine in combination with 1 wt.% nitrocellulose. The influence of the reaction between MEK and n-butylamine, forming water and imines, on the electrophoretic deposition behaviour was investigated. Experimental results revealed that the zeta potential is not a straightforward indication of the stability of these suspensions, since the maximum absolute zeta potential did not correspond with a maximum suspension stability, due to the additional electrosteric stabilisation of the adsorbed charged nitrocellulose.     

Matrix–filler interactions in polysilazane-derived ceramics with Al2O3 and ZrO2 fillers

Interactions between a poly(vinyl)silazane and Al₂O₃ or Y₂O₃-stabilised ZrO₂ fillers were studied during the fabrication of polysilazane-derived bulk ceramics in order to investigate the influence of oxide fillers on resulting properties. Specimens were produced by coating of the filler powders with the polysilazane, warm-pressing of the resulting composite powders, and pyrolytic conversion in flowing N₂ at various temperatures between 1000 °C and 1400 °C. Significant differences in densification were observed, depending on the filler used. Reactions between the polysilazane-derived matrix and Al₂O₃ or ZrO₂ at temperatures ≥1300 °C resulted in the formation of Si₅AlON₇ or ZrSiO₄, respectively. Reactivity in the polysilazane-derived component was a result of SiO₂ contamination caused primarily by adsorbed species on the filler particle surface. Knowledge of polysilazane/filler interface processes is found to be decisive for the prediction of properties such as shrinkage and porosity, which heavily influence performance of a material.     

Ionic conductivity of PEO9: Cu(CF3SO3)2: Al2O3 nano-composite solid polymer electrolyte

Cu⁺⁺ ion containing solid polymer electrolytes exhibit interesting electrochemical properties. In particular, the polymer electrolyte PEO₉:Cu(CF₃SO₃)₂ made by complexing copper triflate (CuTf₂) with PEO appears to show scientifically intriguing transport properties. Although some copper ion transport in these systems has been seen from plating stripping processes, the detailed mechanism of ionic transport and the species involved are yet to be established. In order to obtain enhanced ionic conductivities and also to contribute towards understanding the ionic transport process in Cu⁺⁺ ion containing, PEO based composite polymer electrolytes, we have studied the system PEO₉: CuTf₂: Al₂O₃ incorporating 10 wt.% of alumina filler particles of grain size 10 μm, 37 nm, 10–20 nm and also particles of pore size 5.8 nm. Thermal and electrical measurements show that the system remains amorphous down to room temperature. The composite electrolyte is predominantly an ionic conductor with electronic conductivity less than 2%. The triflate (CF₃SO₃⁻) anions appear to be the dominant carriers. The presence of alumina grains has enhanced the conductivity significantly from room temperature up to 100 °C. The nano-porous grains with 5.8 nm pore size and 150 m²/g specific surface area exhibited the maximum conductivity enhancement. This enhancement has been attributed to Lewis acid–base type surface interactions of ionic species with O²⁻ and OH⁻ groups on the filler grain surface.     

Activity enhancement of SnO2-doped Ga2O3–Al2O3 catalysts by coexisting H2O for the selective reduction of NO with propene

Catalytic reduction of NO by propene in the presence of oxygen was studied over SnO₂-doped Ga₂O₃–Al₂O₃ prepared by sol–gel method. Although SnO₂-doped Ga₂O₃–Al₂O₃ gave lower NO conversion than Ga₂O₃–Al₂O₃ in the absence of H₂O, the activity was enhanced considerably by the presence of H₂O and much higher than that of Ga₂O₃–Al₂O₃. The presence of SnO₂ and Ga₂O₃–Al₂O₃ species having intimate Ga–O–Al bondings was found to be essential for the promotional effect of H₂O. The promotional effect of H₂O was interpreted by the following two reasons. The first one is the removal of carbonaceous materials deposited on the catalyst surface by H₂O. The other is the selective inhibition by H₂O of the reaction steps resulting in propene oxidation to CO_x without reducing NO.     

Characterizations and activities of the nano-sized Ni/Al2O3 and Ni/La–Al2O3 catalysts for NH3 decomposition

A series of nano-sized Ni/Al₂O₃ and Ni/La–Al₂O₃ catalysts that possess high activities for NH₃ decomposition have been successfully synthesized by a coprecipitation method. The catalytic performance was investigated under the atmospheric conditions and a significant enhancement in the activity after the introduction of La was observed. Aiming to study the influence of La promoter on the physicochemical properties, we characterized the catalysts by N₂ adsorption/desorption, XRD, H₂-TPR, chemisorption and TEM techniques. Physisorption results suggested a high specific surface area and XRD spectra showed that nickel particles are in a highly dispersed state. A combination of XRD, TEM and chemisorption showed that Ni⁰ particles with the average size lower than 5.0 nm are always obtained even though the Ni loading ranged widely from 4 to 63%. Compared with the Ni/Al₂O₃ catalysts, the Ni/La–Al₂O₃ ones with an appropriate amount of promoter enjoy a more open mesoporous structure and higher dispersion of Ni. Reduction kinetic studies of prepared catalysts were investigated by temperature-programmed reduction (TPR) method and the fact that La additive partially destroyed the metastable Ni–Al mixed oxide phase was detailed.     

Phase diagram of the Al2O3–ZrO2–Nd2O3 system

The phase diagram of the Al₂O₃–ZrO₂–Nd₂O₃ system was constructed in the temperature range 1250–2800 °C. The liquidus surface of the phase diagram reflects the preferentially eutectic interaction in the system. Two new ternary and one new binary eutectics were found. The minimum melting temperature is 1675 °C and it corresponds to the ternary eutectic Nd₂O₃·11Al₂O₃ + F-ZrO₂ + NdAlO₃. The solidus surface projection and the schematic of the alloy crystallization path confirm the preferentially congruent character of phase interaction in the ternary system. The polythermal sections present the complete phase diagram of the Al₂O₃–ZrO₂–Nd₂O₃ system. No ternary compounds or regions of remarkable solid solution were found in the components or binaries in this ternary system.     

Mn和Zr共掺杂钛酸锶钡/氧化镁复合陶瓷材料的微结构和介电性能

通过固相反应法制备Mn、Zr共掺杂钛酸锶钡/氧化镁陶瓷粉体,经干压成型后在空气气氛中于1450℃烧结4h,通过扫描电子显微镜和X射线衍射研究了ZrO2和MnO2共掺杂的Ba0.6Sr0.4TiO3/MgO复合陶瓷材料的微结构和介电性能。结果表明:ZrO2可以显著降低材料的介电常数和介电损耗,有效提高了陶瓷材料的温度稳定性;随ZrO2添加量的增加,体系的晶胞参数略有增加,MgO在钛酸锶钡中以独立相的形式存在;制备出的BST铁电陶瓷材料的25℃相对介电常数较低(εr<110),介质损耗小于1.0×10–3(在频率为10kHz时),温度系数小于6.012×10–3,可调性大于20%(8.0kV/mm),适用于制作移相器。