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1.
In this work, a kinetic model is constructed to simulate sulfur deactivation of the NO x storage performance of BaO/Al 2O 3 and Pt/BaO/Al 2O 3 catalysts. The model is based on a previous model for NO x storage under sulfur-free conditions. In the present model the storage of NO x is allowed on two storage sites, one for complete NO x uptake and one for a slower NO x sorption. The adsorption of SO x is allowed on both of these NO x storage sites and on one additional site which represent bulk storage. The present model is built-up of six sub-models: (i) NO x storage under sulfur-free conditions; (ii) SO 2 storage on NO x storage sites; (iii) SO 2 oxidation; (iv) SO 3 storage on bulk sites; (v) SO 2 interaction with platinum in the presence of H 2; (vi) oxidation of accumulated sulfur compounds on platinum by NO 2. Data from flow reactor experiments are used in the implementation of the model. The model is tested for simulation of experiments for NO x storage before exposure to sulfur and after pre-treatments either with SO 2 + O 2 or SO 2 + H 2. The simulations show that the model is able to describe the main features observed experimentally. 相似文献
2.
Polyacrylamide/palygorskite(PAM/PA) as prepared by solution polymerization of acrylamide (AM) on the γ-methacryloxypropyl trimethoxy-silane (KH-570) modified palygorskite (PA). The influence of monomer concentration, polymerization time, initiator concentration and polymerization temperature on mercury adsorption of PAM/PA was investigated. The maximum adsorption capacity was 135.5 mg/g at optimum polymerization conditions. PAM/PA was characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). 相似文献
3.
The standard Gibbs free energies of formation of CuAlO 2 and CuAl 2O 4 were determined in the range 700° to 1100°C, using emf measurements on the galvanic cells (1) Pt,CuO +] Cu 2O/CaO-ZrO 2/O 2,Pt; (2) Pt,Cu +] CuAlO 2+] Al 2O 3/CaO-ZrO 2/ Cu +] Cu 2O,Pt; and (3) Pt,CuAl 2O 4+] CuAlO 2+]Al 2O 3/CaO-ZrO 2/O 2,Pt. The results are compared with published information on the stability of these compounds. The entropy of transformation of CuO from tenorite to the rock-salt structure is evaluated from the present results and from earlier studies on the entropy of formation of spinels from oxides of the rock-salt and corundum structures. The temperatures corresponding to 3-phase equilibria in the system Cu 2O-CuO-Al 2O 3 at specified O 2 pressures calculated from the present results are discussed in reference to available phase diagrams. 相似文献
4.
Transmission electron microscopy was used to study the ordered domain structures in undoped and La/Na-doped Pb(Mg 1/3Nb 2/3)O 3 (PMN), where the compositions of the doped samples were specifically chosen so as to elucidate the ordering mechanism. The results showed that the Mg 2+ ions and Nb 5+ ions are short-range ordered on the B-site sublattice in undoped PMN, with a domain size of 2 to 5 nm. This short-range ordering gives rise to B-site composition fluctuations occurring on a nanometer scale, and it is this compositional inhomogeneity which is believed to be responsible for the diffuse phase transition behavior. Donor doping with La 2O 3 can compensate for the local charge imbalance resulting from the short-range order and thus enhances the degree of ordering. Acceptor doping with Na 2O, however, increases the charge effect, and hence ordering is suppressed. The effect of Na doping and La doping on the dielectric properties of PMN is also discussed. 相似文献
5.
The influence of crystal volume fraction on fracture toughness ( K I C ) and indentation strength was analyzed for Li 2O-Al 2O 3-CaO-SiO 2 (LACS) and LACS glass-ceramics containing 0.58 mmol% AgNO 3 (LACS-0.58Ag) or 0.78 mmol% AgNO 3 (LACS-0.78Ag). The mean flexure strength, indentation strength, and K I C values of the LACS-0.78Ag groups increased with volume fraction of crystallinity. To achieve the greatest strength and K I C in LACS-Ag specimens, a high volume fraction of crystallinity (95%) had to be produced. However, the relationship between volume fraction of crystal phase and translucency had to be analyzed to determine the influence of crystallization on the potential esthetic results that are essential for dental applications. Addition of AgNO 3 to LACS glass produced a change from surface crystallization to bulk crystallization. 相似文献
6.
Single crystals of phenacite (Be 2SiO 4), bromellite (BeO), and tridymite (SiO 2) were grown from an Li 2MoO 4-MoO 3 flux. Phenacite, with rhombohedral symmetry, grew in three distinct shapes with aspect ratios (length/width) as follows: needles (>3), rods (>1.1 to 1.5), and rhombohedral-faced crystals (=1). The latter grew as single crystals; the others were twinned on the . For most experiments the temperature was held constant at 1165°C and the Li 2MoO 4/MoO 3 ratio at 1/16. The growth mechanism for crystallization was the evaporation of MoO 3. The system produced one to three phases, depending on the BeO/SiO 2 ratio. Bromellite grew until a BeO/SiO 2 ratio of 0.8 was attained. It grew as a hemipyramidal crystal having a short prism with a curved top or as a hexagonal plate. The pyramid- and prism-shaped crystals were twinned, although a few hexagonal plates were single. Tridymite grew in small hexagonal plates when the BeO/SiO 2 ratio was less than 1.5. The effect of temperature, nucleation, and flux composition on crystal shape, twinning, and occurrence is discussed. 相似文献
7.
Vanadium oxides supported on γ-Al 2O 3, SiO 2, TiO 2, and ZrO 2 were studied on their molecular structures and reactive performances for soot combustion. To investigate the effect of different alkali metals on the structures and reactivities of supported-vanadium oxide catalysts, they were doped into the V 4/TiO 2 catalyst which had the best intrinsic activity for soot combustion in the selected supported vanadium oxide catalysts. The experimental results demonstrated that the catalytic properties of these catalysts depended on the vanadium loading amount, support nature, and the presence or the absence of alkali metals. The spectroscopic analysis (FT-IR and UV–vis) and H 2-TPR results revealed that the higher activity of alkali-promoted vanadium oxide catalysts could be related to the ability of alkali metal promoting the redox cycle of the active vanadyl species. TG results showed that adding alkali to V m/TiO 2 catalyst was beneficial to lowering their melting points. Low melting points could ensure the good surface atom migration ability, which would improve the contact between the catalyst and soot. Due to the alkali metal components promoting the redox ability and the mobility of the catalysts, alkali-modified vanadium oxide catalysts could remarkably improve their catalytic activities for soot combustion. The catalytic activity order for soot combustion followed Li > Na > K > Rb > Cs in the catalyst system of alkali-V 4/TiO 2, and the reason why it followed this sequence was discussed. 相似文献
8.
Up to 50 vol% of TiB 2, TiC 0.5N 0.5, TiN, or TiC was added to Y 2O 3-stabilized tetragonal ZrO 2 polycrystals (Y-TZP) and hot pressed under vacuum. The influence of the type of secondary phase on the microstructure and mechanical properties was studied, as a function of the hot-pressing temperature. The influence of the secondary-phase content on the mechanical properties was studied by varying the TiB 2 content up to 50 vol%. Fully dense Y-TZP-based composites with very high toughness (up to 10 MPa·m 1/2), excellent bending strength (up to 1237 MPa), and increased hardness, with respect to ZrO 2 (Vickers hardness up to 1450 kg/mm 2), were obtained. 相似文献
9.
Bi 3.7Nd 0.3Ti 3O 12 (BNT0.3) films were fabricated on indium tin oxide/glass substrates using a metal organic decomposition method at temperatures ranging from 500° to 650°C. A predominantly (100)-oriented BNT0.3 film can be obtained even at 550°C. The growth mode of the predominantly (100)-oriented BNT0.3 films fabricated by the sequential layer annealing method was discussed based on the structure evolution with the annealing temperature and the film thickness. The largest values of the remanent polarization and piezoresponse are observed in the BNT0.3 film annealed at 650°C, which can be ascribed to the grain growth and the release of the in-plane residue tension stress. 相似文献
10.
A multi-component NO x-trap catalyst consisting of Pt and K supported on γ-Al 2O 3 was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H 2O and CO 2 on NO x storage. The Al 2O 3 support was shown to have NO x trapping capability with and without Pt present (at 250 °C Pt/Al 2O 3 adsorbs 2.3 μmols NO x/m 2). NO x is primarily trapped on Al 2O 3 in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO x/m 2, and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al 2O 3 catalyst are coordinated on the Al 2O 3 support at saturation. When 5% CO2 was included in a feed stream with 300 ppm NO and 12% O2, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H2O was included in a feed stream with 300 ppm NO and 12% O2, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al2O3-based nitrates decreased by 92%. Interestingly, with both 5% CO2 and 5% H2O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al2O3 destabilized the K–CO2 bond; specifically, H2O mitigates the NOx storage capacity losses associated with carboxylate competition. 相似文献
11.
The eutectic composition between Y 4Al 2O 9 and Y 2O 3 was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo- and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y 2O 3 and 29.5 mol% Al 2O 3). Based on the current results and other published data, the pseudobinary system between Al 2O 3 and Y 2O 3 is revised. 相似文献
12.
Transparent and highly oriented Ba 2NaNb 5O 15 (BNN) thin films have been prepared by using metal alkoxides. A homogeneous precursor solution was prepared by the controlled reaction of NaOC 2H 5, Nb(OC 2H 5) 5, and barium metal. The BNN precursor included a molecular-level mixture of NaNb(OC 2H 5) 6 and Ba[Nb(OC 2H 5) 6] 2 in ethanol. The alkoxy-derived powder crystallized to a low-temperature phase, and then transformed to orthorhombic BNN (tungsten bronze) at 600°C. BNN precursor films on substrates crystallized to orthorhombic BNN at 800°C via the low-temperature phase. Highly (002) oriented BNN films of tungsten bronze structure were successfully prepared on MgO (100) substrates at 700°C by using BNN underlayer. 相似文献
13.
Composite densification was studied by performing slip casting and sintering experiments on an Al 2O 3 matrix and Si 3N 4 whisker system. Even though all the slip-cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore-size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker-associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si 3N 4 affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the composites. 相似文献
14.
Zirconia doped with 3.2–4.2 mol% (6–8 wt%) yttria (3–4YSZ) is currently the material of choice for thermal barrier coating topcoats. The present study examines the ZrO 2-Y 2O 3-Ta 2O 5/Nb 2O 5 systems for potential alternative chemistries that would overcome the limitations of the 3–4YSZ. A rationale for choosing specific compositions based on the effect of defect chemistry on the thermal conductivity and phase stability in zirconia-based systems is presented. The results show that it is possible to produce stable (for up to 200 h at 1000°–1500°C), single (tetragonal) or dual (tetragonal + cubic) phase chemistries that have thermal conductivity that is as low (1.8–2.8W/m K) as the 3–4YSZ, a wide range of elastic moduli (150–232 GPa), and a similar mean coefficient of thermal expansion at 1000°C. The chemistries can be plasma sprayed without change in composition or deleterious effects to phase stability. Preliminary burner rig testing results on one of the compositions are also presented. 相似文献
15.
Single crystals of Al 2O 3 (sapphire) were grown from PbO-PbF 2 and MoO 3-PbF 2 fluxes; they varied from flat plates (PbF 2-rich melts) to equidimensional crystals (PbO- or MoO 3-rich melts). The primary growth planes are basal {0001}, first-order rhombohedral {1011}, and second-order rhombohedral {0112}. The habit change is interpreted on the basis of F - contamination and Pb 2+ surface adsorption. Possible ion species in the melts and their relative importance on crystal growth from these systems are discussed. 相似文献
16.
Concurrent thermogravimetry (TG) and evolved-gas analysis (EGA) were done for YBa 2Cu 3O 7-z and LaBa 2Cu 3-O 7-z superconductors. The sample weights were monitored by thermobalance and the evolved O 2 and CO 2 species were monitored by quadruple mass spectrometer (QMS). No diffraction peak for the impurity phase containing a carbonate group was observed in the X-ray diffraction patterns for these samples, but the release of CO 2 was detected by EGA. CO 2 gas began to evolve from YBa 2Cu 3O 7-z at 543°C and from LaBa 2Cu 3O 7-z at 692°C. Preparation of high-quality YBa 2Cu 3O 7-z and LaBa 2Cu 3O 7-z superconductors is discussed on the basis of results of these thermal analyses. 相似文献
17.
通过溶胶凝胶法,经多次涂覆在SBA-15上负载掺杂镍的纳米TiO 2,得到SBA-15/NiO/TiO 2复合物;再以TiO 2中还原态金属镍为催化剂,通过化学气相沉积法(CVD),在SBA-15/NiO/TiO 2表面原位生长碳纳米管,制得SBA-15/Ni/TiO 2/CNTs复合材料。通过XRD、SEM、TEM、UV-Vis和Raman等方法考察了SBA-15/Ni/TiO 2/CNTs复合材料的结构和性能,并通过降解亚甲基蓝溶液评价其光催化活性。结果表明,SBA-15/Ni/TiO 2/CNTs复合光催化剂的催化活性较SBA-15/NiO/TiO 2显著提高。 二次涂覆掺杂镍的二氧化钛制得的复合光催化剂的催化活性高于一次涂覆。 相似文献
18.
The dispersion of aqueous γ-Y 2Si 2O 7 suspensions, which contain only one component but have a complex ion environment, was studied by the introduction of two different polymer dispersants, polyethylenimine (PEI) and polyacrylic acid (PAA). The suspension without any dispersant remains stable in the pH range of 9–11.5 because of electrostatic repulsion, while it is flocculated upon stirring due to the readsorption of hydrolyzed ions on the colloid surface. However, suspensions with 1 dwb% PEI exhibit greater stability in the pH range of 4–11.5. The addition of PEI shifts the isoelectric point (IEP) of the suspensions from pH 5.8 to 10.8. Near the IEP (pH IEP=10.8), the stability of the suspensions with PEI is dominated by the steric effect. When the pH is decreased to acid direction, the stabilization mechanism is changed from steric hindrance to an electrosteric effect little by little. PAA also has the effect of reducing the hydrolysis speed via a "buffer effect" in the basic pH range, but the lack of adsorption between the highly ionized anionic polymer molecules and the negative colloid particle surfaces shows no positive effect on hydrolysis of colloids and on the stabilization of Y 2Si 2O 7 suspensions. 相似文献
19.
Characterization by XPS–UPS and XRD of commercial bulk WO 2 enabled us to identify the presence of four to five layers of WO 3 on the sample surface with an equal amount of W 5+, possibly W 20O 58 in the interface. The presence of these WO 3 and W 20O 58 on the WO 2 surface were not detected by XRD. Exposure of commercial bulk WO 2 to hydrogen at temperatures higher than 673 K results in the reduction of surface WO 3 to WO 2 and the formation of the bifunctional WO 2(H x) ac phase on its surface. A complete conversion of surface WO 3 to WO 2(H x) ac has been obtained following the exposure of the sample to hydrogen for at least 6 h at 773 K. A conversion of 52% of n-heptane at 573 K reaction temperature and a selectivity of 90% in isomerization products, mainly 2,3-MH and multibranched molecules were obtained. The isomerization products distribution is in agreement with the statistical and thermodynamic equilibrium of the methyl-shift mechanism. The stability of the active WO 2(H x) ac phase has been tested under prolonged exposure to hydrogen and the reaction mixture. Similar results were obtained in the case of bulk WO 3 and WO 3/TiO 2 systems. Dehydration and dehydrogenation of 2-propanol were studied on these systems at 393 K reaction temperature. 相似文献
20.
The cation solubility limits of the n = 2 and n = 3 superconducting phases in the Bi 2(Sr x Ca 1 − x ) n +1Cu n O y system were established along tie lines with compatible phases via electron probe microanalysis on bi- (or poly-) phasic samples prepared at 860°C. Pb additions (15 mol% of the Bi content) were used to facilitate formation of the n = 3 phase. In each case football-shaped volumes in composition space were established as the solubility limits which bordered on the nominal compositions 2212 or 2223 (Bi:Sr:Ca:Cu) with the long axis parallel to the Sr-Ca side of the quaternary (i.e., Sr-to-Ca intersolubility) but also extending toward Bi and Cu. This means that, for the most part, the superconducting phases are alkaline-earth deficient relative to the ideal 2212 and 2223 compositions. The Pb content in the 2223 phase is typically 10% of the Bi content. T c variations could be correlated with variations in Sr or (Sr + Bi) content and the length of the c -axis parameter. 相似文献
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