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1.
The system Al 2O 3–ZrO 2 was studied by differential thermal analysis in inert atmosphere and in vacuum. The eutectic was located at 1866°C and 40% mass of ZrO 2. Zirconia solid solution at the eutectic temperature is up to 1.1±0.3% mass of Al 2O 3. Enthalpy of melting of this eutectic is 1080±90 J/g. Pure ZrO 2 transforms from monoclinic to tetragonal at 1162±7°C, but the saturated solid solution of ZrO 2, with 0.7±0.2% mass Al 2O 3 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 ZrO 2 from monoclinic to tetragonal phase is 42±5 J/g (5.2±0.6 J/mol) but only 30±5 J/g for a ZrO 2 saturated solid solution. 相似文献
2.
Mixed oxides of alumina and zirconia having a relative composition of 50, 80 and 100% Zr 2O were synthesized by means of sol–gel methods. The catalysts were sulfated with H 2SO 4 1N, and were loaded with 0.3% Pt metal using the incipient wetness technique. The characterization of the physicochemical properties was carried out using XRD, N 2-adsorption at 78 K, and SEM. The catalytic properties of the Al 2O 3–ZrO 2 series were studied by means of dehydration of 2-propanol at 180°C and isomerization of n-hexane at 250°C, 1 atm. The sulfated solids presented a high surface acidity and a limited crystallinity, together with high activity for alcohol dehydration (i.e. 2-propanol). On the other hand, the Al 2O 3–ZrO 2 solid solutions (i.e. those having a 20–80% composition) turned out to be the most active ones for the isomerization of n-hexane. 相似文献
4.
Effect of additives, In 2O 3, SnO 2, CoO, CuO and Ag, on the catalytic performance of Ga 2O 3–Al 2O 3 prepared by sol–gel method for the selective reduction of NO with propene in the presence of oxygen was studied. As for the reaction in the absence of H 2O, CoO, CuO and Ag showed good additive effect. When H 2O was added to the reaction gas, the activity of CoO-, CuO- and Ag-doped Ga 2O 3–Al 2O 3 was depressed considerably, while an intensifying effect of H 2O was observed for In 2O 3- and SnO 2-doped Ga 2O 3–Al 2O 3. Of several metal oxide additives, In 2O 3-doped Ga 2O 3–Al 2O 3 showed the highest activity for NO reduction by propene in the presence of H 2O. Kinetic studies on NO reduction over In 2O 3–Ga 2O 3–Al 2O 3 revealed that the rate-determining step in the absence of H 2O is the reaction of NO 2 formed on Ga 2O 3–Al 2O 3 with C 3H 6-derived species, whereas that in the presence of H 2O is the formation of C 3H 6-derived species. We presumed the reason for the promotional effect of H 2O as follows: the rate for the formation of C 3H 6-derived species in the presence of H 2O is sufficiently fast compared with that for the reaction of NO 2 with C 3H 6-derived species in the absence of H 2O. Although the retarding effect of SO 2 on the activity was observed for all of the catalysts, SnO 2–Ga 2O 3–Al 2O 3 showed still relatively high activity in the lower temperature region. 相似文献
5.
In the present study, both t-phase zirconia and m-phase zirconia particles are incorporated into an alumina matrix. Dense Al 2O 3/(t-ZrO 2+m-ZrO 2) composites were prepared by sintering pressurelessly at 1600 °C. The microstructure of the composites are characterized, the elastic modulus, strength and toughness determined. Because the ZrO 2 inclusions are close to each other in the Al 2O 3 matrix, the yttrium ion originally in t-ZrO 2 particles can diffuse to nearby m-ZrO 2 particles during sintering, and the m-phase zirconia is thus stabilized after sintering. The strength of the Al 2O 3/(t-ZrO 2+m-ZrO 2) composites after surface grinding can reach values as high as 940 MPa, which is roughly three times that of Al 2O 3 alone. The strengthening effect is contributed by microstructural refinement together with the surface compressive stresses induced by grinding. The toughness of alumina is also enhanced by adding both t-phase and m-phase zirconia, which can reach values as high as two times that of Al 2O 3 alone. The toughening effect is attributed mainly to the zirconia t–m phase transformation. 相似文献
6.
Ni–Mo/Al 2O 3–MCM-41 supported catalysts have been investigated for modification of MCM-41 by using sol–gel alumina incorporation method. Different catalysts were synthesized with variation of Si/Al molar ratios of 10, 50, 100 and 200. High specific surface area ordered meso-porous solid (MCM-41) was synthesized by using organic template method. In order to modify the low acidity of silica solid, the surface of MCM-41 was modified by incorporation of alumina. The surface acidity of solids modified significantly with variation of alumina content in the supports. The sol–gel method of alumina incorporation was used, which does not modify extensively the pore characteristics of MCM-41 material during the preparation of Al 2O 3–MCM-41. The X-ray diffraction intensities indicated that alumina as well as MCM-41 were present in the synthesized supports. Additionally, the hydrothermal stability of the Al 2O 3–MCM-41 materials was maintained up to 873 K using sever conditions like 100% water vapor stream. The catalytic activity of the catalysts was tested in the hydrodesulfurization (HDS) of dibenzothiophene (DBT). Selectivity was oriented mainly to the production of biphenyl (BP) and for high Si/Al ratios toward cyclohexylbenzene (CHB) and showed a higher conversion and better selectivity to hydrogenation (cyclohexylbenzene). 相似文献
7.
Stable suspensions based on methylethylketone (MEK), n-butylamine and nitrocellulose were developed for the electrophoretic deposition (EPD) of Al 2O 3 and ZrO 2 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. 相似文献
8.
The subsolidus region of the BaO–La 2O 3–V 2O 5 phase diagram has been redetermined. Previously reported binary phases of LaVO 4, La 8V 2O 17, La 3VO 7, BaLa 2O 4, Ba 3V 2O 8, Ba 2V 2O 7, Ba 3V 4O 13 and BaV 2O 6 have all been confirmed. The ternary phases Ba 2LaV 3O 11, Ba 3LaV 3O 12 and Ba 3La 40V 12O 93 have also been confirmed. The new phase “BaLa 10V 4O 26” has been synthesised for the first time and is reported here along with the amended phase diagram. The previously omitted phase of La 1.42V 0.58O 3.58 has also been included and reasons for this are cited. The diagram is shown for two temperatures (1000 and 600 °C) due to the low melting point of V 2O 5-rich compounds. 相似文献
9.
CeO 2–ZrO 2–La 2O 3 (CZL) mixed oxides were prepared by citric acid sol–gel method. The as-received gel was calcined at 500, 700, 900 and 1050 °C to obtain the so-called C5, C7, C9 and CK, respectively. The C5, C7 and C9 powders were impregnated with H 2PtCl 6 and then calcined at 500 °C to prepare P5C5, P5C7 and P5C9, respectively. The impregnated CK powders were calcined at 500, 700 and 900 °C to prepare P5CK, P7CK and P9CK, respectively. The XRD and XPS analyses show that the surface distribution of Pt is evidently influenced by the structural and textural properties of the support. The CO adsorption followed by FTIR reveals that the dispersion and the chemisorption sites of Pt are reduced as the calcination temperature of CZL support increases. The chemisorption ability of the CK samples is even completely deactivated. The encapsulation mechanism, which has been applied to explain the so-called strong metal–support interaction (SMSI) after reductive treatment, is introduced here to demonstrate the abnormal observations though the samples were prepared in oxidative atmosphere. The HRTEM results also confirm this explanation. The effects of oxygen vacancies, the chemisorption sites on the Pt surface and Pt/Ce interfacial sites on the three-way catalytic activities are discussed. 相似文献
10.
Ceramics in the system BaO-Li 2O–Nd 2O 3–TiO 2 (BNT–LNT) were prepared by the mixed oxide route. Powders were mixed, milled, calcined and sintered at 1475°C for 4 h. Fired densities decreased steadily along the series from BNT to LNT. The microstructures of samples rich in BNT were dominated by small needle-like grains; the LNT samples comprised larger (6 μm) cubic grains. X-ray diffraction showed that there was a transition from orthorhombic BNT to cubic LNT; small amounts of LNT could be accommodated in BNT, but between 10–20% LNT there was the development of the second phase. Small additions of LNT led to a small increase in relative permittivity, but decreased the dielectric Q-value (from the maximum of 1819 at 4 GHz). As BNT and LNT exhibit negative and positive temperature dependencies of permittivity respectively, the addition of 10–20% LNT to BNT should yield samples with zero temperature dependence of r Impedance spectroscopy showed that data could only be acquired at elevated temperatures for BNT rich samples (above 500°C), but at modest temperatures (less than 100°C) for the more conductive LNT. 相似文献
11.
This work aims at exploring the thermal ageing mechanism of Pt on ceria-based mixed oxides and the corresponding effect on the oxygen storage capacity (OSC) performance of the support material. Pt was supported on low-surface-area CeO 2–ZrO 2–La 2O 3 mixed oxides (CK) by impregnation method and subsequently calcined in static air at 500, 700 and 900 °C, respectively. The evolutions of textural, microstructural and redox properties of catalysts after the thermal treatments were identified by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and high-resolution transmission electron microscope (HRTEM). The results reveal that, besides the sintering of Pt, encapsulation of metal by the mixed oxides occurs at the calcination temperature of 700 °C and above. The burial of Pt crystallites by support particles is proposed as a potential mechanism for the encapsulation. Further, the HRTEM images show that the distortion of the mixed oxides lattice and other crystal defects are distributed at the metal/oxides interface, probably indicating the interdiffusion/interaction between the metal and mixed oxide. In this way, encapsulation of Pt is capable to promote the formation of Ce 3+ or oxygen vacancy on the surface and in the bulk of support. The OSC results show that the reducibility and oxygen release behavior of catalysts are related to both the metal dispersion and metal/oxides interface, and the latter seems to be more crucial for those supported on low-surface-area mixed oxides. Judging by the dynamic oxygen storage capacity (DOSC), oxygen storage capacity complete (OSCC) and oxygen releasing rate, the catalyst calcined at 700 °C shows the best OSC performance. This evident promotion of OSC performance is believed to benefit from the partial encapsulation of Pt species, which leads to the increment of Ce 3+ or oxygen vacancies both on the surface and in the bulk of oxides despite a loss of chemisorption sites on the surface of metal particles. 相似文献
12.
The catalytic performance of mono- and bimetallic Pd (0.6, 1.0 wt.%)–Pt (0.3 wt.%) catalysts supported on ZrO 2 (70, 85 wt.%)–Al 2O 3 (15, 0 wt.%)–WO x (15 wt.%) prepared by sol–gel was studied in the hydroisomerization of n-hexane. The catalysts were characterized by N 2 physisorption, XRD, TPR, XPS, Raman, NMR, and FT-IR of adsorbed pyridine. The preparation of ZrW and ZrAlW mixed oxides by sol–gel favored the high dispersion of WO x and the stabilization of zirconia in the tetragonal phase. The Al incorporation avoided the formation of monoclinic-WO 3 bulk phase. The catalysts increased their SBET for about 15% promoted by Al 2O 3 addition. Various oxidation states of WO x species coexist on the surface of the catalysts after calcination. The structure of the highly dispersed surface WO x species is constituted mainly of isolated monotungstate and two-dimensional mono-oxotungstate species in tetrahedral coordination. The activity of Pd/ZrW catalysts in the hydroisomerization of n-hexane is promoted both with the addition of Al to the ZrW mixed oxide and the addition of Pt to Pd/ZrAlW catalysts. The improvement in the activity of Pd/ZrAlW catalysts is ascribed to a moderated acid strength and acidity, which can be correlated to the coexistence of W 6+ and reduced-state WO x species (either W 4+ or W 0). The addition of Pt to the Pd/ZrAlW catalyst does not modify significantly its acidic character. Selectivity results showed that the catalyst produced 2MP, 3MP and the high octane 2,3-dimethylbutane (2,3-DMB) and 2,2-dimethylbutane (2,2-DMB) isomers. 相似文献
13.
Homogeneous-eutectic microstructure of Y 3Al 5O 12–Al 2O 3 system without coarse primary crystals was formed at an off-eutectic composition. This method utilizes a low migration rate in an amorphous phase. A mixture of Y 2O 3 and Al 2O 3 having the off-eutectic composition was melted and quenched rapidly to form an amorphous phase. A heat-treatment of the amorphous phase at 1000 °C and 1300 °C for 30 min formed Y 3Al 5O 12 and Al 2O 3 phases. SEM observation of this material, which was formed from the amorphous phase at 1300 °C for 30 min, showed homogeneous eutectic-like microstructure. The formation of the primary crystals (coarse Al 2O 3), which are always observed in the off-eutectic compositions by ordinary method, was completely suppressed. 相似文献
14.
Al 2O 3–MoSi 2 composites with MoSi 2 volume fractions between 16 and 40% were fabricated from commercial ceramic Al 2O 3 and intermetallic MoSi 2 powders by granulation, cold isostatic pressing and vacuum-sintering. The addition of MoSi 2 had only a slight influence on the densification of the composites, with sintered densities of 98% for samples with 16 vol.% MoSi 2 and 94% for samples with 40 vol.% MoSi 2. Composites with MoSi 2 contents of 20 vol.% and higher were electroconductive due to the formation of a three-dimensional percolating network of the conductive MoSi 2 phase. 相似文献
15.
Nanosized particles dispersed uniformly on Al 2O 3 particles were prepared from the decomposition of precursor Cr(CO) 6 by metal organic chemical vapor deposition (MOCVD) in a fluidized chamber. These nanosized particles consisted of Cr 2O 3, CrC 1−x, and C. A solid solution of Al 2O 3–Cr 2O 3 and an Al 2O 3–Cr 2O 3/Cr 3C 2 nanocomposite were formed when these fluidized powders were pre-sintered at 1000 and 1150 °C before hot-pressing at 1400 °C, respectively. In addition, an Al 2O 3–Cr 2O 3/Cr-carbide (Cr 3C 2 and Cr 7C 3) nanocomposite was formed when the particles were directly hot pressed at 1400 °C. The interface between Cr 3C 2 and Al 2O 3 is non-coherent, while the interface between Cr 7C 3 and Al 2O 3 is semi-coherent. 相似文献
16.
The effect of the TiO 2–Al 2O 3 mixed oxide support composition on the hydrodesulfurization (HDS) of gasoil and the simultaneous HDS and hydrodenitrogenation (HDN) of gasoil+pyridine was studied over two series of CoMo and NiMo catalysts. The intrinsic activities for gasoil HDS and pyridine HDN were significantly increased by increasing the amount of TiO 2 into the support, and particularly over rich- and pure-TiO 2-based catalysts. It is suggested that the increase in activity be due to an improvement in reducing and sulfiding of molybdena over TiO 2. The inhibiting effect of pyridine on gasoil HDS was found to be similar for all the catalysts, i.e., was independent of the support composition. The ranking of the catalysts for the gasoil HDS test differed from that obtained for the thiophene test at different hydrogen pressures. In the case of gasoil HDS, the activity increases with TiO 2 content and large differences are observed between the catalysts supported on pure Al 2O 3 and pure TiO 2. In contrast, in the case of the thiophene test, the pure Al 2O 3-based catalyst appeared relatively more active than the catalysts supported on mixed oxides. Also, in the thiophene test the difference in intrinsic activity between the pure Al 2O 3-based catalyst appeared relatively more active than the catalysts supported on mixed oxides. Also in the thiophene test, the difference in intrinsic activity between the pure Al 2O 3- and pure TiO 2-based catalysts is relatively small and dependent on the H 2 pressure used. Such differences in activity trend among the gasoil and the thiophene tests are due to a different sensitivity of the catalysts (by different support or promoter) to the experimental conditions used. The results of the effect of the H 2 partial pressure on the thiophene HDS, and on the effect of H 2S concentration on gasoil HDS demonstrate the importance of these parameters, in addition to the nature of the reactant, to perform an adequate catalyst ranking. 相似文献
17.
Surface-phase ZrO 2 on SiO 2 (SZrOs) and surface-phase La 2O 3 on Al 2O 3 (SLaOs) were prepared with various loadings of ZrO 2 and La 2O 3, characterized and used as supports for preparing Pt/SZrOs and Pt/SLaOs catalysts. CH 4/CO 2 reforming over the Pt/SZrOs and Pt/SLaOs catalysts was examined and compared with Pt/Al 2O 3 and Pt/SiO 2 catalysts. CO 2 or CH 4 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. 相似文献
18.
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. 相似文献
19.
Catalytic activities of Al 2O 3–TiO 2 supporting CoMo and NiMo sulfides (CoMoS and NiMoS) catalysts were examined in the transalkylation of isopropylbenzene and hydrogenation of naphthalene as well as the hydrodesulfurization (HDS) of model sulfur compounds, conventional gas oil (GO), and light cycle oil (LCO). Al 2O 3–TiO 2 supporting catalysts exhibited higher activities for these reactions except for the HDS of the gas oil than a reference Al 2O 3 supporting catalyst, indicating the correlation of these activities. Generally, more content of TiO 2 promoted the activities. Inferior activity of the catalyst for HDS of the gas oil is ascribed to its inferior activity for HDS of dibenzothiophene (DBT) in gas oil as well as in model solvent decane, while the refractory 4,6-dimethyldibenzothiophene (4,6-DMDBT) in gas oil as well as in decane was more desulfurized on the catalyst. Characteristic features of Al 2O 3–TiO 2 catalyst are discussed based on the paper results. 相似文献
20.
A deoxidizing catalyst was prepared in this paper. Several characterization techniques (XRD, SEM–EDS, TEM, TPD and TPR) were used to study its structure and properties. A normal pressure micro-reactor was built to study its deoxidizing performance. Results show that when inlet O 2 concentration was 0.1%, space velocity was 3000–12 000 h −1 and operation temperature was above 80 °C, the outlet residual O 2 can be as low as 1.0 × 10 −6 (v/v). 300 h continuous operation shows that its deoxidizing activity was stable. Through comparison of the deoxidizing activities for fresh and deactivated catalyst and by simulating the water vapor contents in system, the mechanism of deactivation and reactivation was studied. 相似文献
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