The oxidation resistance mechanism of SiC-B4C-xAl2O3 ceramics at 1400 ℃ in air atmosphere

The oxidation property of SiC-B₄C-xAl₂O₃ (x ranges from 0 wt% to 30 wt%) ceramics was studied in air at 1400 ℃. Results show that the porous oxide layer becomes dense and smooth with addition of Al₂O₃. When the content of Al₂O₃ is proper, the DOP (degree of polymerization) of borosilicate network can be improved with increase of Al₂O₃ content, inhibiting the migration of atoms and molecular groups. With that, the crystallization of SiO₂ and volatilization of B₂O₃ can be restrained. When the content of Al₂O₃ is excessive, the DOP of borosilicate network will be decreased, deteriorating the oxide layer morphology. It is believed that the damage of borosilicate network by excess Al₂O₃ should be responsible for this phenomenon. In this research, the SiC-B₄C ceramic with optimal oxidation resistant can be obtained when the content of Al₂O₃ is 15 wt%.     

A comparative study of the oxidation of CO and CH4 over Au/MOx/Al2O3 catalysts

The activity of Au/Al₂O₃ and Au/MO_x/Al₂O₃ (M=Cr, Mn, Fe, Co, Ni, Cu, and Zn) in low temperature CO oxidation and the oxidation of CH₄ was studied. Generally, addition of MO_x to Au/Al₂O₃ stabilizes small Au particles initially present on the support in heat treatments up to 700°C. All multi-component catalysts show a remarkable enhancement in low temperature CO oxidation compared to the mono-component catalysts. The observed activities are directly related to the average Au particle size, whereas the identity of MO_x is less important. The CH₄ oxidation activity of Au/Al₂O₃ is improved upon addition of MnO_x, FeO_x, CoO_x, and to a lesser extent NiO_x. Measured activities in CH₄ oxidation over Au/MO_x/Al₂O₃ decrease in the following order: CuO_x>MnO_x>CrO_x>FeO_x>CoO_x>NiO_x>ZnO_x. The high activity observed for CuO_x and CrO_x containing catalysts is assigned to the intrinsically high CH₄ oxidation capability of these oxides themselves. For Au/MnO_x/Al₂O₃ a lower apparent activation energy was found than for Au/Al₂O₃ and MnO_x/Al₂O₃, which might point to a promoting effect of MnO_x on Au in the oxidation of CH₄. The results presented support a similar model for both CO and CH₄ oxidation. In this model the reaction takes place at the Au/MO_x perimeter, which is defined as the boundary between Au, MO_x and the gas phase. The reductant, CO or CH₄, is adsorbed on Au, and MO_x is the supplier of O.     

Enhanced Sulfur Tolerance of Ceria-Promoted NO x Storage Reduction (NSR) Catalysts: Sulfur Uptake, Thermal Regeneration and Reduction with H2(g)

SO _x uptake, thermal regeneration and the reduction of SO _x via H₂(g) over ceria-promoted NSR catalysts were investigated. Sulfur poisoning and desulfation pathways of the complex BaO/Pt/CeO₂/Al₂O₃ NSR system was investigated using a systematic approach where the functional sub-components such as Al₂O₃, CeO₂/Al₂O₃, BaO/Al₂O₃, BaO/CeO₂/Al₂O₃, and BaO/Pt/Al₂O₃ were studied in a comparative fashion. Incorporation of ceria significantly increases the S-uptake of Al₂O₃ and BaO/Al₂O₃ under both moderate and extreme S-poisoning conditions. Under moderate S-poisoning conditions, Pt sites seem to be the critical species for SO _x oxidation and SO _x storage, where BaO/Pt/Al₂O₃ and BaO/Pt/CeO₂/Al₂O₃ catalysts reveal a comparable extent of sulfation. After extreme S-poisoning due to the deactivation of most of the Pt sites, ceria domains are the main SO _x storage sites on the BaO/Pt/CeO₂/Al₂O₃ surface. Thus, under these conditions, BaO/Pt/CeO₂/Al₂O₃ surface stores more sulfur than that of BaO/Pt/Al₂O₃. BaO/Pt/CeO₂/Al₂O₃ reveals a significantly improved thermal regeneration behavior in vacuum with respect to the conventional BaO/Pt/Al₂O₃ catalyst. Ceria promotion remarkably enhances the SO _x reduction with H₂(g).     

Kinetics and selectivity of methyl-ethyl-ketone combustion in air over alumina-supported PdOx–MnOx catalysts

A study is presented of the kinetics and oxidation selectivity of methyl-ethyl-ketone (MEK) in air over bimetallic PdO_x(0–1 wt% Pd)–MnO_x(18 wt% Mn)/Al₂O₃ and monometallic PdO_x(1 wt% Pd)/Al₂O₃ and MnO_x(18 wt% Mn)/Al₂O₃ catalysts. Reaction rate data were obtained at temperatures in the 443–523 K range and for MEK partial pressures in the reactor feed of between 6.5 and 126.6 Pa. Products of both MEK combustion and partial oxidation reactions were found. Monometallic Pd/Al₂O₃ was the most selective catalyst for complete oxidation whereas the partial oxidation of MEK in the presence of manganese oxides was significant. The maximum yield for the partial oxidation products (acetaldehyde, methyl-vinyl-ketone, and diacetyl) was always below 10%. Kinetic studies showed that the rates of CO₂ formation over PdO_x/Al₂O₃ were well-fitted by the surface redox Mars–van Krevelen (MvK) kinetic expression and also by a Langmuir–Hinshelwood (LH) model derived after considering the surface reaction between adsorbed MEK and oxygen as the rate-determining step. In the case of the Mn-containing catalysts the MvK model provides the best fit. Irrespective of the model, the kinetic parameters for the bimetallic Pd–Mn catalysts were between the values obtained for the monometallic samples, suggesting an additive rather than a cooperative effect between palladium and manganese species for MEK combustion.     

A New Catalyst for Selective Oxidation of CO in H2: Part 1, Activation by Depositing a Large Amount of FeO x on Pt/Al2O3 and Pt/CeO2 Catalysts

1% Pt/Al₂O₃ and 1% Pt/CeO₂ are markedly activated by the deposition of a large quantity of FeO _x , about 100 wt% in Fe with respect to the supports. In contrast, the activity of a Ru/Al₂O₃ catalyst was completely suppressed by the deposition of FeO _x , but a Ru-Pt/Al₂O₃ was markedly activated by the FeO _x . The activation depends on the sequence of the deposition, that is, no pronounced activation was observed on the Pt supported on a FeO _x /Al₂O₃ as well as on the Pt codeposited with a small amount of Fe on Al₂O₃, that is, the activity was almost equal to that of the Pt/Al₂O₃. The XPS analysis of the Pt/CeO₂ and FeO _x /Pt/CeO₂ proved that the Pt is effectively covered with the FeO _x . Selectivity for the oxidation of CO in H₂ was also improved on the FeO _x /Pt/Al₂O₃ and FeO _x /Pt/CeO₂ catalysts and it is rather independent of the conversion. In conformity with the feature of the FeO _x /Pt/Al₂O₃ and FeO _x /Pt/CeO₂ catalysts, we deduced that the deposited FeO _x is activated by the Pt and the FeO _x is responsible for the selective oxidation of CO.     

Enhancement of oxidation resistance at 1000–1400 °C of low carbon Al2O3–C refractories with pre-synthesized SiCnw/Al2O3

The oxidation resistance of low carbon Al₂O₃–C refractories with the addition of SiC_nw/Al₂O₃ composite powders and the enhancement mechanisms were investigated. The oxidation resistance was evaluated by oxidation index (O.I.) and oxidation rate constant (k). The enhancement mechanisms of SiC_nw/Al₂O₃ on oxidation resistance were analyzed based on the phases and microstructures. The results showed that the SiC_nw/Al₂O₃ can improve the oxidation resistance of Al₂O₃–C refractories, the O.I. and k of A6 (6 wt% SiC_nw/Al₂O₃ addition) were 26.0% and 34.5% lower than those of reference sample A0, respectively. The oxidation resistance of refractories was improved in a range of 1000–1400 °C due to the introduction of SiC_nw/Al₂O₃. The enhancement mechanisms can be explained that SiC_nw is more susceptible to be oxidized due to its high specific surface area, which expanded the action temperature range of other antioxidants and itself. The mullite and dense protective layer generated during oxidation is also beneficial to impede the diffusion of O₂.     

Correlations among sintering temperature,shrinkage, and open porosity of 3.5 YSZ/Al2O3 composites

Slip-cast ceramic samples of the system (100−x) (ZrO₂–3.5 mol% Y₂O₃)–xAl₂O₃ (abridged as (100−x) 3.5 YSZ/xAl₂O₃ composite, where x is expressed in wt%) were examined using dilatometry, isothermal sintering and electron microscopy methods. The shrinkage in the range 1100–1300 °C was found to be higher for the (100−x) 3.5 YSZ/xAl₂O₃ samples with prevailing fraction of PSZ than for the composites with a corundum matrix. When the weight fraction of corundum was increased, the relative shrinkage of the (100−x) 3.5 YSZ/xAl₂O₃ samples decreased and the open porosity of the ceramic materials grew. The effect of <gamma>-Al₂O₃ impurity on the sintering process and linear dimensions of ceramics is shown. Heat treatment of (50–40) 3.5 YSZ/(50–60) Al₂O₃ composites at 1300 °C are proposed as the optimum conditions for porous diaphragm formation.     

Combination of Ag/Al2O3 and Fe-BEA for High-Activity Catalyst System for H2-Assisted NH3-SCR of NO x for Light-Duty Diesel Car Applications

Low-temperature active Ag/Al₂O₃ and high-temperature active Fe-BEA zeolite were combined and tested for H₂-assisted NH₃-selective catalytic reduction (SCR) of NO _x . The catalysts were either washcoated onto separate monoliths that were placed up- or downstream of each other (dual-brick layout) or washcoated on top of each other in a sandwiched layout (dual-layer). Our results showed that it is highly preferred to have Ag/Al₂O₃ as the upstream or outer layer catalyst. Fe-BEA showed a high NH₃ oxidation giving an NH₃ deficit over the Ag/Al₂O₃. Ag/Al₂O₃ formed NO₂ which enhanced the activity over Fe-BEA through the “fast”-SCR reaction when Fe-BEA was placed downstream or as inner layer. When no H₂, which is needed for the SCR reaction over Ag/Al₂O₃, was added, the dual-layer layout was preferred. The shorter diffusion distance between the layers is a probable explanation.     

Potential of UV–Raman Spectroscopy for Characterization of Sub-monolayer MoOx Model Catalysts at Ambient Pressure

UV–Raman spectroscopy is demonstrated to be a valuable tool to study molybdenumoxide model catalysts at ambient pressure while visible light Raman yields no information. UV–Raman offers the possibility to characterize MoO _x /Al ₂O₃/Si and MoO _x /SiO₂/Si model systems with sub-monolayer loading of MoO _x even though the surface area of a Si-wafer is much lower than that of a bulk support. Anchoring of MoO _x onto Al₂O₃/Si and SiO₂/Si results in similar molybdenum oxide compounds as for bulk catalysts.     

Reaction Pathways in the Reduction of NO x Species by CO over Pt–Ba/Al2O3: Lean NO x Trap Catalytic Systems

The reduction by CO of NO _x species stored over Pt–Ba/Al₂O₃ Lean NO _x Trap systems is analysed in this work. The reaction mechanisms and pathways leading to N₂ formation both under dry and wet conditions are investigated by complementary transient dynamic experiments and FTIR analyses.     

H2-Induced NO x Adsorption/Desorption over Ag/Al2O3: Transient Experiments and TPD Study

NO adsorption/desorption over 1 wt% Ag/Al₂O₃ was studied by a combination of isothermal transient adsorption/desorption and NO _x temperature-programmed desorption (NO _x -TPD) methods. NO _x -TPD profiles obtained for Ag/Al₂O₃ were identified by comparison with decomposition profiles of “model” AgNO₃/Al₂O₃ and Al(NO₃)₃/Al₂O₃ prepared by impregnation of Al₂O₃ with individual AgNO₃ and Al(NO₃)₃ compounds. The data obtained indicate that H₂-induced NO adsorption leads to the formation of surface Ag and Al-nitrates. Their accumulation on the catalyst surface is accompanied by an intensive NO₂ evolution, which proceeds primarily via reaction of surface nitrates with NO. Thus, NO₂ formation appears to result from an intrinsic stage of the H₂-induced NO _x adsorption process, rather than from the direct oxidation of NO by gaseous oxygen catalyzed by Ag.     

Thermally stable composite system Al2O3-Ce0.75Zr0.25O2 for automotive three-way catalysts

Present-day three-way catalysts operate in contact with exhaust gases whose temperature is as high as >1000°C, so the problem of developing thermally stable catalytic compositions is still topical. A series of Al₂O₃-Ce_0.75Zr_0.25O₂ composites containing 0, 10, 25, and 50 wt % Al₂O₃ has been synthesized by direct precipitation. The as-prepared composites and those calcined in air at 1000 and 1100°C have been characterized by BET, X-ray diffraction, transmission electron microscopy, and temperature-programmed reduction methods. The composites aged at 1050°C in a 2% O₂ + 10% H₂O + 88% N₂ atmosphere have been used to prepare monolith catalysts, and the oxygen storage capacity (OSC) of the latter has been measured using a gas analysis setup. As the proportion of Al₂O₃ in the composite is raised, the mixing uniformity and degree of dispersion of Ce _x Zr_1?x O_2?δ particles increase, their chemical composition becomes homogeneous, and the amount of cerium involved in oxidation and reduction increases. The composite containing 50 wt % Al₂O₃ is a mixture of Ce _x Zr_1?x O_2?δ and Al₂O₃ crystallites, whose size is practically unaffected by calcination. The (Pt/Al₂O₃ + Al₂O₃-Ce_0.75Zr_0.25O₂) based on this composite has the highest OSC and is the most active. For this reason, full-scale testing of this catalyst is recommended.     

Cation Vacant Fe3?x?y V x �� y O4 Spinel-Type Catalysts for the Oxidation of Methanol to Formaldehyde

The potential of Fe?CV-oxide catalysts for use in methanol oxidation is explored. Our results show that although FeVO₄ is active and selective for formaldehyde (FA) formation, it is not completely stable towards volatilization under reaction conditions. Attempts to stabilize Fe?CV-oxide were made using titania, alumina and silica supports. However, we observe that although some stabilization is achieved using titania and alumina, the supported catalysts are sensitive to volatilization considering the relatively low content of active oxide. Compared with supported V-oxide, the results show that iron causes stabilization of vanadium decreasing its volatility. Considering the observation that the neat FeVO₄ restructures to form a spinel-type phase under influence of the catalysis, we prepared a series of cation vacant spinel-type Fe_3?x?y V _x ?? _y O₄ catalysts with various V/Fe ratio and consequent number of cation vacancies ??. Opposed to the activity, which is rather constant irrespectively of the vanadium content, the selectivity to FA passes through a maximum of about 90% for Fe/V = 14. A spinel-type phase with the composition Fe_2.62V_0.19??_0.20O₄ was prepared and subsequently preoxidized to different degree. It is observed that the spinel-type structure is stable and that the oxidation of vanadium and iron is balanced by an increasing number of cation vacancies. Moreover, irrespectively of the original degree of preoxidation, it is found that in methanol oxidation a steady state is reached where all samples are equally active and selective and have the same composition both in the bulk and at the surface. The results clearly demonstrate that the spinel-type catalysts are phase-stable, nonvolatile and flexible in that the cations can change oxidation state retaining the same basic structure type and Fe/V ratio.     

Oxide/oxide composites in the system Cr2O3-Y2O3-Al2O3

Ceramic compacts in the systems Al₂O₃–Y₂O₃, Cr₂O₃–Y₂O₃ and Y₃(Cr_yAl_1-y)₅O₁₂ (Cr-doped YAG) were prepared by solid state reaction in calcined co-precipitated powder mixtures of appropriate compositions. Various solid-solution phases were formed, e.g. Y₃(Al_1-xCr_x)₅O₁₂, YAl_yCr_1-yO₃ and Al_2-xCr_xO₃. Composite materials in the pseudo-binary or ternary systems Al₂O₃–Y₃Al₅O₁₂, Cr₂O₃–Y₂O₃ and Y₃(Al_1–xCr_x)₅O₁₂–YAl_yCr_1–yO₃–(Al_zCr_1−z)₂O₃ were obtained by hot-pressing appropriate powder precursors at 1600–1650°C for 1 h. The microstructure of the prepared materials was studied in a scanning electron microscope with element analysis facilities. X-ray diffraction was used to reveal the phases present and their lattice parameters. The chemical compatibility of these phases was investigated. The results are discussed with a special emphasis on the solubility of Cr in the YAG structure, and on the compatibility relationship between Cr-doped YAG and its neighbouring phases. A gel-coating process for preparing Al₂O₃–YAG composites with tailored microstructures is also described.     

Selective oxidation of ethane over hydrothermally synthesized Mo–V–Al–Ti oxide catalyst

Mo–V-based oxide catalysts are known highly active for various hydrocarbon selective oxidations. Particularly those which are monophasic giving XRD diffraction at d=4 Å are extremely active for alkane oxidations. We succeeded to synthesize this unique monophasic material by hydrothermal method and obtained Mo₆V₂Al₁O_x mixed oxide catalysts which showed activities for gas-phase ethane oxidation to ethene and acetic acid. The addition of titanium to the Mo₆V₂Al₁O_x oxide catalyst was found to result in a marked increase of the activity for the ethane selective oxidation, which was due to the morphological change of the catalyst particles and the increase of surface area by the addition of titanium. During the heat-treatment above 550°C under a nitrogen stream, the structural phase of the Mo₆V₂Al₁Ti_0.5O_x catalyst giving the XRD diffraction at d=4 Å transferred to (Mo_0.93V_0.07)₅O₁₄-like phase with drastic decreases of the oxidation activity and the surface area. On the basis of kinetic data and the fact that the lower reaction temperature and the existence of water vapor in the feed facilitated the formation of acetic acid, it is concluded that the breaking of C–H bond of ethane is the rate determining of the oxidation and acetic acid do not form through ethene.     

The diffuse interface phase-field model for in situ interlayer phase formations in low-temperature reaction bonding of alumina laminates—A theoretical and experimental comparison

A diffuse interface phase-field model is developed to study alumina seamless laminate composites formation via reaction interlayer diffusion bonding. The model was designed to indicate the important role of interface on phase transition and interfacial chemical reactions. The model has indicated the interfacial reactions, phase transitions, and phase evolution along the alumina/reaction layer interface. The theoretical results were assessed, validated, and confirmed by experimental data at similar bonding conditions via targeted diffusion bonding in aluminum-alkaline earth hydride and alanate interlayer systems Me (Me: Mg–Sr). The alanates and hydrides dissociations left pockets of intermetallic phases at interlayer as bonding constituents. The solid-state bonds have formed by diffusion bonding at laminate interfaces and partial oxidations. The elemental analysis showed the alkaline earth rich zones at interlayer and near interfaces. The crystalline composition of the interlayer materials was combinations of polycrystalline mixed oxide layers, formed due to different oxidation kinetics and diffusion rates. Interlayer oxidation products were combination of complex oxides with Sr_uMg_xAl_yO_z (u: 0.8–1, x: 0.7–1, y: 2–10, z: 4–17) stoichiometry.     

The effect of different types of additives on the catalytic activity of Au/Al2O3 in propene total oxidation: transition metal oxides and ceria

The total oxidation of C₃H₆ was investigated over Au/Al₂O₃, and multicomponent Au/MO_x/Al₂O₃ (M: Ce, Mn, Co, Fe) catalysts prepared by deposition-precipitation with urea. The catalysts have been characterized by means of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), total surface area (BET), and diffuse reflectance ultra-violet–visible spectroscopy DR/UV–Vis. Based on this characterization, it was concluded that gold is present as Au⁰. The most active catalyst is Au/CeO_x/Al₂O₃. Ceria and the transition metal oxides act as cocatalysts. It can supply oxygen via a Mars and van Krevelen mechanism.     

The effect of Al2O3 on the high‐temperature oxidation resistance of Si‐B‐C ceramic under air atmosphere

Si‐B‐C ceramics were prepared through reaction sintering, and the influence of Al₂O₃ addition on the high‐temperature (1100‐1300°C) oxidation behavior of the material under air atmosphere was studied. The erosion behavior and mechanism are determined from the measurement of weigh changes, microstructure observations, and characterization of the generated oxides on postexposure specimens. Results show that Al₂O₃ is enriched in the oxidized layer, inhibiting the volatilization of B₂O₃ and impeding the crystallization ability of oxide (cristobalite). Narrower erosion layer and less weigh change are observed with Al₂O₃. Low‐frequency Raman results reveals that with the increase in Al₂O₃, the bending vibrations of the BO₄ units and B‐O‐B stretching of the metaborate ring relative intensity are enhanced. Furthermore, high‐frequency Raman results shows that the relative proportion of high‐dimensional vibration modes Q³ and Q⁴ which result in a higher viscosity of melt and a greater resistance of oxygen diffusion are positively correlated with Al₂O₃.     

Novel Sn–Ce/Al2O3 Catalyst for the Selective Catalytic Reduction of NOx Under Lean Conditions

Effect of additives, Ce and Mn, on the catalytic performance of Sn/Al₂O₃ catalyst prepared by sol–gel method for the selective reduction of NO_x with propene under lean conditions was studied. Sn–Ce/Al₂O₃ catalysts exhibited higher activity than Sn/Al₂O₃ catalyst and the optimum Ce loading is 0.5–1%. The promoting effect of Ce is to enhance the oxidation of NO to NO₂ and facilitate the activation of propene, both of which are important steps for the NO_x reduction. The presence of oxygen contributes to the oxidation of NO and shows a promoting effect.     

Effect of TiO2 on phase evolution and microstructure of MgAl2O4 spinel in different atmospheres

The effect of TiO₂ on the formation and microstructure of magnesium aluminate spinel (MgAl₂O₄) at 1600 °C in air and reducing conditions were investigated. Under reducing conditions, stoichiometric MgAl₂O₄ spinel shifted toward alumina-rich types owing to volatilization of MgO, resulting in an increase in the porosity of fired samples. Addition of graphite to mixtures of MgO and Al₂O₃ intensified the reducing conditions and accelerated the formation of non-stoichiometric MgAl₂O₄. For TiO₂-containing samples on addition of MgAl₂O₄, magnesium aluminum titanium oxide (Mg_xAl_2(1−x)Ti_(1+x)O₅, x = 0.2 or 0.3) was detected as a minor phase. Under reducing conditions, XRD peak shifts were smaller for TiO₂-containing samples than for samples without TiO₂ owing to the formation of a solid solution of TiO₂ in MgAl₂O₄ and establishment of alumina-rich spinel, which have opposite effects on increasing the lattice parameter. In bauxite-containing samples, MgAl₂O₄ spinel, corundum, magnesium orthotitanate spinel (Mg₂TiO₄) and amorphous phases were identified. Mg₂TiO₄ spinel formed a complete solid solution with MgAl₂O₄ spinel but Mg₂TiO₄ remained as a distinct phase owing to the heterogeneous microstructure of bauxite-containing samples. Also dense microstructure established in air fired TiO₂ containing samples. The results are discussed with emphasis on the application and design of alumina-magnesia-carbon refractory materials, which are used in the steel industry.