A Sol–Gel Derived CuOx/Al2O3–ZrO2 Catalyst for the Selective Reduction of NO by Propane in the Presence of Excess Oxygen

Copper catalysts supported on alumina-doped zirconia were prepared by sol–gel processing followed by supercritical drying or aging in the mother solution at 100°C. After drying and calcination, the catalyst supports were impregnated with a copper(II) nitrate aqueous solution by the incipient wetness method to achieve a Cu loading of about 2%. The samples showed 90% NO conversion at 350–400°C. The catalytic performance of these systems appears to be determined by the degree of clustering of copper cations as probed by FTIR spectroscopy of adsorbed CO.     

Enhanced Activity of Spinel-type Ga2O3–Al2O3 Mixed Oxide for the Dehydrogenation of Propane in the Presence of CO2

Catalytic performance of a series of Ga₂O₃–Al₂O₃ mixed oxides prepared by alcoholic-coprecipitation method for the dehydrogenation of propane in the presence of CO₂ was investigated. It is shown that the combination of Ga and Al oxides greatly improved the performance of the Ga₂O₃-based materials for catalytic dehydrogenation of propane, with the highest performance attainable at a Ga₂O₃–Al₂O₃ catalyst with a 20 mol% aluminum content. While the same tendency was observed for the specific activity normalized by BET surface area, significantly enhanced stability was achieved for Ga₂O₃–Al₂O₃ with higher aluminum content. X-ray diffraction (XRD) revealed that a homogeneous spinel-type Ga₂O₃–Al₂O₃ solid solution is uniformly formed by substitution of Ga³⁺ for Al³⁺ in the Al₂O₃ lattice. The enhanced activity of Ga₂O₃–Al₂O₃ mixed oxides was accounted for by the abundance of surface weak acid sites due to the synergetic interaction between Ga₂O₃ and Al₂O₃ in the solid solution systems.     

Rh–Fe/Ca–Al2O3: A Unique Catalyst for CO-Free Hydrogen Production in Low Temperature Ethanol Steam Reforming

Low temperature ethanol steam reforming (ESR) was studied over a series of 1 wt% Rh–x % Fe catalysts with various Fe loading (x = 0–10 wt%) and on different supports (Ca–Al₂O₃, SiO₂ and ZrO₂). The results show that close interaction between Rh and Fe is required to reduce the CO selectivity to almost negligible values. In addition, Rh–Fe supported on Ca–Al₂O₃ exhibits the best performance in terms of CO selectivity and hydrogen yield as compared to other supports. Characterization by XPS and XANES indicates the presence of Fe_xO_y species upon reduction, resulting in the formation of coordinatively unsaturated ferrous (CUF) active sites along the Rh–Fe_xO_y interface. These CUF sites promote water–gas shift reaction during low temperature ESR. Temperature programmed oxidation and Raman spectroscopy of spent catalysts also indicate that the addition of iron oxide reduces coke deposition and forms more reactive coke. Hence, the catalyst lifespan is significantly extended.     

Effect of Si3N4 mesophase on the formation of Al2OC-AlNss in resin-bonded Al–Al2O3 composites

In this study, we developed a novel method for synthesising Al₂OC-AlNss using a solid nitrogen source: a Si₃N₄ mesophase. The two-step sintered Al–Al₂O₃ and Si₃N₄–Al–Al₂O₃ samples were prepared under an atmosphere of nitrogen to investigate the effect of Si₃N₄ on the formation of Al₂OC-AlNss in resin-bonded Al–Al₂O₃ composites. The samples were investigated via XRD and SEM. The results indicated that the synthesis of Al₂OC-AlNss with different morphologies was achieved via the Si₃N₄ mesophase, and its morphology was influenced by the source of AlN. Both Al₂OC-AlNss and Al₄O₄C were formed in the two-step sintered Al–Al₂O₃ sample, whereas only Al₂OC-AlNss was formed in the two-step sintered Si₃N₄–Al–Al₂O₃ sample. Induced by the AlN formed by the nitridation of Al, needle-like Al₂OC-AlNss was generated. Compared to that formed by the nitridation of Al, more AlN nuclei were provided by the reaction between Si₃N₄ and Al. Subsequently, columnar and granular Al₂OC-AlNss were formed. Furthermore, fibre-like Al₂OC-AlNss was also generated via the VS and VLS mechanism. The reaction model was established in this study.     

Stability in the system CaO–Al2O3–H2O

The solubility of AH₃, CAH₁₀, C₂AH_7.5, and C₃AH₆ was determined experimentally at 7 to 40 °C and up to 570 days. During the reaction of CA, at 20 °C and above initially C₂AH_7.5 formed which was unstable in the long-term. The solubility products calculated indicate that the solubilities of CAH₁₀, C₂AH_7.5 and C₄AH₁₉ increase with temperature while the solubility of C₃AH₆ decreases. Thus at temperatures above 20 °C, C₃AH₆ is stable, while at lower temperature also CAH₁₀ and C₄AH₁₉ are stable, depending on the C/A ratio.At early hydration times, CAH₁₀ can be stable initially at 30 °C and above, as the formation of amorphous AH₃ stabilises CAH₁₀ with respect to C₃AH₆ + 2AH₃. With time, as the solubility AH₃ decreases due to the formation of microcrystalline AH₃, CAH₁₀ becomes unstable at 20 °C and above.     

Synergetic effect of combined use of Cu–ZnO–Al2O3 and Pt–Al2O3 for the steam reforming of methanol

Commercial Cu–ZnO–Al₂O₃ catalysts are used widely for steam reforming of methanol. However, the reforming reactions should be modified to avoid fuel cell catalyst poisoning originated from carbon monoxide. The modification was implemented by mixing the Cu–ZnO–Al₂O₃ catalyst with Pt–Al₂O₃ catalyst. The Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture created a synergetic effect because the methanol decomposition and the water–gas shift reactions occurred simultaneously over nearby Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalysts in the mixture. A methanol conversion of 96.4% was obtained and carbon monoxide was not detected from the reforming reaction when the Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture was used.     

Autothermal Reforming of Methane Over CeO2–ZrO2–La2O3 Supported Rh Catalyst

Autothermal reforming of methane was studied over La-doped ceria–zirconia-supported Rh catalysts. The CH₄ conversion increased from 49 to 60% on increasing the content of La³⁺ from 5 to 15%, while further increase in the La³⁺ content led to a slight decrease on both CH₄ conversion and H₂ selectivity. H₂-TPR and UV–vis DRS spectrum showed that the interaction between Rh and the support was enhanced by increasing the content of La. We speculated that a so-called “Rh–La interfacial species” was formed on the surface of the support, which played an important role in catalytic activity. The balance between exposed Rh and the “Rh–La interfacial species” was necessary to improve the catalytic activity. Upon increasing the Rh loading on 15% La-doped ceria–zirconia support, the balance was built, i.e., CH₄ conversion increased from ~60 to 69% by increasing Rh loading from 0.1 to 0.5 wt% and only 2% conversion was elevated by doubling the Rh loading from 0.5 to 1.0 wt%.     

Flame-spraying synthesis of aluminate glasses in the Al2O3–La2O3 system

The paper deals with the synthesis and characterisation of binary aluminate glasses in the La₂O₃–Al₂O₃ system with Al₂O₃ contents changing between 74.6 and 86.9 mol% (48–65 wt.%), and of ternary glasses with 75.7 mol% Al₂O₃ doped with 1 mol% of Nd₂O₃ or Er₂O₃. Six binary and two ternary compositions were prepared. Flame synthesis facilitated the preparation of X-ray amorphous microspheres in the systems with 58 wt.% Al₂O₃, and with eutectic composition in the pseudobinary LaAlO₃–LaAl₁₁O₁₈ system doped with Er. Other systems contained low fractions of crystalline LaAlO₃ perovskite, regardless of the composition. The diameter of prepared microspheres ranged between 2 and 10 μm. They were transparent for visible light, as well as in the IR wavenumber range from 1300 to 4000 cm^?1.     

Effect of Soot During Operation of a Pt–K/Al2O3 LNT Catalyst

The effect of soot on the NO _x storage-reduction performances of a Pt–K/Al₂O₃ catalyst is analysed in this work by performing lean-rich cycles at constant temperature. The interaction between soot and the stored NO _x has been also investigated by temperature programmed methods. It has been found that the presence of soot reduces the NO _x storage capacity of the catalyst; besides the presence of soot favours the decomposition of the stored nitrates. A direct reaction between the stored nitrates and soot is suggested, that has been explained on the basis of the surface mobility of the adsorbed nitrates.     

Stable garnets in the Er2O3–Sc2O3–Al2O3 oxide system for optical ceramics application

Erbium-scandium-aluminum garnets (ErSAG) with the maximum possible concentrations of scandium cations in the dodecahedral and octahedral positions of the crystal lattice have been synthesized. It has been established that in ErSAG at a temperature of 1600 °C the maximum possible concentration of scandium in the dodecahedral and octahedral position reaches 64.7 ± 2.0 at. % and 92.0 ± 2.0 at. %, respectively. The maximum possible concentrations of scandium cations in the dodecahedral position and the set of range of possible ErSAG compositions decrease at increasing of calcination temperature. It was shown that ceramics technique allow to produce the optical ceramics with compositions {Er_1.5Sc_1.5}[Sc_0.2Al_1.8](Al₃)O₁₂ (SAG: Er 50 at%) and {Y_1.57Er_0.03Sc_1.4}[Sc_0.2Al_1.8](Al₃)O₁₂ (YSAG: Er 1 at%), which can be considered as analogs of yttrium-aluminum garnet single crystals (YAG: Er 50 at% and YAG: Er 1 at%).     

Cyclic Lean Reduction of NO by CO in Excess H2O on Pt–Rh/Ba/Al2O3: Elucidating Mechanistic Features and Catalyst Performance

This study provides insight into the mechanistic and performance features of the cyclic reduction of NO_x by CO in the presence and absence of excess water on a Pt–Rh/Ba/Al₂O₃ NO_x storage and reduction catalyst. At low temperatures (150–200 °C), CO is ineffective in reducing NO_x due to self-inhibition while at temperatures exceeding 200 °C, CO effectively reduces NO_x to main product N₂ (selectivity >70 %) and byproduct N₂O. The addition of H₂O at these temperatures has a significant promoting effect on NO_x conversion while leading to a slight drop in the CO conversion, indicating a more efficient and selective lean reduction process. The appearance of NH₃ as a product is attributed either to isocyanate (NCO) hydrolysis and/or reduction of NO_x by H₂ formed by the water gas shift chemistry. After the switch from the rich to lean phase, second maxima are observed in the N₂O and CO₂ concentrations versus time, in addition to the maxima observed during the rich phase. These and other product evolution trends provide evidence for the involvement of NCOs as important intermediates, formed during the CO reduction of NO on the precious metal components, followed by their spillover to the storage component. The reversible storage of the NCOs on the Al₂O₃ and BaO and their reactivity appears to be an important pathway during cyclic operation on Pt–Rh/Ba/Al₂O₃ catalyst. In the absence of water the NCOs are not completely reacted away during the rich phase, which leads to their reaction with NO and O₂ upon switching to the subsequent lean phase, as evidenced by the evolution of N₂, N₂O and CO₂. In contrast, negligible product evolution is observed during the lean phase in the presence of water. This is consistent with a rapid hydrolysis of NCOs to NH₃, which results in a deeper regeneration of the catalyst due in part to the reaction of the NH₃ with stored NO_x. The data reveal more efficient utilization of CO for reducing NO_x in the presence of water which further underscores the NCO mechanism. Phenomenological pathways based on the data are proposed that describes the cyclic reduction of NO_x by CO under dry and wet conditions.     

Y2O3–Al2O3–SiO2-based glass-ceramic fillers for the laser-supported joining of SiC

Four glass-ceramic filler compositions within the Y₂O₃–Al₂O₃–SiO₂ system were tested for their suitability in laser-supported joining of SiC materials. The compositions differed in terms of their primary crystallization behavior. Joint zone microstructures were investigated and joint tightness was determined using helium leak rate measurements after joining and subsequent annealing at 900 °C and 1050 °C.Yttria- and silica-rich compositions showed a partial crystallization of yttrium silicates during the short laser processing. Subsequent heat treatment effected further crystallization toward equilibrium conditions. Despite the strong change in the degree of crystallization no reduction of the tightness was observed for the best compositions; after 500 h annealing at 1050 °C tightness values of less than 10⁻⁸ mbar l s⁻¹ were measured. These results demonstrate the potential of the investigated filler system for high temperature stable hermetic sealing. At the same time the creation of homogeneously structured joints from glass-ceramic fillers using a laser-supported technology needs the understanding of the crystallization kinetics.     

Effect of MgO/Al2O3 ratio on the crystallization behaviour of Li2O–MgO–Al2O3–SiO2 glass-ceramic and its wettability on Si3N4 ceramic

The composition governs the crystallization ability, the type and content of crystal phases of glass-ceramics. Glass-ceramic joining materials have generated more research interest in recent years. Here, we prepared a novel Li₂O–MgO–Al₂O₃–SiO₂ glass-ceramic for the application of joining Si₃N₄ ceramics. We investigated the influence of the MgO/Al₂O₃ composition ratio on microstructure and crystallization behaviour. The crystallization kinetics demonstrated that the glasses had excellent crystallization ability and high crystallinity. β-LiAlSi₂O₆ and Mg₂SiO₄ were precipitated from the glass-ceramics, and the increase of MgO concentration was conducive to the precipitation of Mg₂SiO₄. Among the glass-ceramic samples, the thermal expansion coefficient of LMAS2 glass-ceramic was 3.1 × 10^?6/°C, which was very close to that of Si₃N₄ ceramics. The wetting test showed that the final contact angle of the glass droplet on the Si₃N₄ ceramic surface was 32° and the interface was well bonded.     

Performance of V2O5/NPTiO2–Al2O3-nanoparticle- and V2O5/NTiO2–Al2O3-nanotube model catalysts in the SCR–NO with NH3

In this study, the NO reduction by NH₃ over V₂O₅/NPTiO₂–Al₂O₃ (nanoparticles) and V₂O₅/NTiO₂–Al₂O₃ (nanotubes) catalysts synthesized by the sol–gel method with 10 and 5 wt.% of Al₂O₃ and V₂O₅, respectively, is reported. The V₂O₅/NPTiO₂–Al₂O₃ and V₂O₅/NTiO₂–Al₂O₃ catalysts showed remarkable conversion, high acidity, structure stability, N₂ selectivity and a high resistance to deactivation in the presence of 10 vol.% of H₂O within the 200 to 500 °C temperature interval. The nanostructured catalysts developed in this work are an excellent alternative to improve the SCR–NH₃ process, both expanding its operation window and preventing deactivation by H₂O at high temperatures.     

Al2O3-based binders for corrosion resistance optimization of Al2O3–MgAl2O4 and Al2O3–MgO refractory castables

This work addresses the main aspects related to the use of alternative binders [hydratable (HA) or colloidal alumina (ColAlu)] in castables containing different spinel sources (pre-formed or in situ generated), in order to point out: (i) the features that control the corrosion behavior of these materials, and (ii) the key factors to better select a refractory composition. Thermodynamic calculations, corrosion cup-test and SEM analyses were carried out in order to evaluate the slag attack of the designed refractory compositions. According to the attained results, the alumina-based binders (HA or ColAlu) induced a more effective sintering process due to their high specific surface area, improving the physical properties and the binding level of the generated microstructure. The spinel grain size also played an important role in the corrosion behavior of these refractories, as the finer the particles, the greater their dissolution was into the molten liquid, leading to further precipitation of spinel in the solid–liquid interface as a continuous and thick layer. Among the evaluated compositions and considering the presence of silica fume, the most suitable formulation with optimized corrosion resistance was the one with in situ spinel generation and HA as a binder.     

Highly Active CNT-Promoted Cu–ZnO–Al2O3 Catalyst for Methanol Synthesis from H2/CO/CO2

With types of in-house-synthesized multi-walled carbon nanotubes (CNTs) and the nitrates of the corresponding metallic components, highly active CNT-promoted Cu–ZnO–Al₂O₃ catalysts, symbolized as Cu _i Zn _j Al _k -x%CNTs, were prepared by the co-precipitation method. Their catalytic performance for methanol synthesis from H₂/CO/CO₂ was studied and compared with the corresponding CNT-free co-precipitated catalyst, Cu _i Zn _j Al _k . It was shown experimentally that appropriate incorporation of a minor amount of the CNTs into the Cu _i Zn _j Al _k could significantly increase the catalyst activity for methanol synthesis. Under the reaction conditions of 493 K, 5.0 MPa, H_2/CO/CO₂/N₂ = 62/30/5/3 (v/v), GHSV = 8000 h^-1, the observed CO conversion and methanol formation rate over a co-precipitated catalyst of Cu₆Zn₃Al₁-12.5%CNTs reached 36.8% and 0.291 mol CH₃OH s^-1 (m²-surf. Cu)^-1, which was about 44 and 25% higher than those (25.5% and 0.233 mol CH₃OH s^-1 (m²-surf. Cu)^-1) over the corresponding CNT-free co-precipitated catalyst, Cu₆Zn₃Al₁. Addition of a minor amount (10–15 wt%) of the CNTs to the Cu₆Zn₃Al₁ catalyst was found to considerably increase specific surface area, especially Cu surface area of the catalyst. H₂-TPD measurements revealed that the CNTs and the pre-reduced CNT-promoted catalyst systems could reversibly adsorb and store a considerably greater amount of hydrogen under atmospheric pressure at temperatures ranging from room temperature to 573 K. This unique feature would be beneficial for generating microenvironments with higher stationary-state concentration of active hydrogen adspecies on the surface of the functioning catalyst, especially at the interphasial active sites since the highly conductive CNTs might promote hydrogen spillover from the Cu sites to the Cu/Zn interphasial active sites, and thus be favorable for increasing the rate of the CO hydrogenation reactions. Alternatively, the operation temperature for methanol synthesis over the CNT-promoted catalysts can be 15–20 degrees lower than that over the corresponding CNT-free contrast system. This would contribute considerably to an increase in equilibrium CO conversion and CH₃OH yield. The results of the present work indicated that the CNTs could serve as an excellent promoter.     

Mechanism of the Reduction by Ammonia of Nitrates Stored onto a Pt–Ba/Al2O3 LNT Catalyst

The mechanism involved in the formation of N₂ and of N₂O during the reduction of nitrates stored onto a Pt–Ba/Al₂O₃ LNT catalyst is investigated using labeled NO and unlabeled ammonia, in the presence and in the absence of NO in the gas phase. The reduction of the stored NO _x species (labeled nitrates) with NH₃ leads to the selective formation of N₂. Based on the isotopic distribution, it appears that N₂ formation occurs primarily through the statistical coupling of N-atoms formed by dissociation of NO and NH₃ at metal Pt sites. When the reduction of the stored nitrates is carried out in the presence of NO in the gas phase, NO is preferentially reduced. This implies that the rate determining step of the reduction of nitrates by ammonia is likely associated with the release of stored NO _x . Negligible amounts of nitrous oxide have been observed during the NH₃-TPSR with adsorbed nitrates, whereas relevant quantities of N₂O have been detected at low temperatures (below 180 °C) in the runs performed in the presence of NO in the gas phase. The data converge to indicate that N₂O formation involves the presence of gaseous NO and this suggests that the formation of nitrous oxide occurs either through the coupling of two adsorbed NO molecules or the recombination of an adsorbed NO molecule with an adsorbed NH _x species.