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1.
Dispersing La 2O 3 on δ- or γ-Al 2O 3 significantly enhances the rate of NO reduction by CH 4 in 1% O 2, compared to unsupported La 2O 3. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La 2O 3 precursor used, the pretreatment, and the La 2O 3 loading. The most active family of catalysts consisted of La 2O 3 on γ-Al 2O 3 prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m 2) occurred between the addition of one and two theoretical monolayers of La 2O 3 on the γ-Al 2O 3 surface. The best catalyst, 40% La 2O 3/γ-Al 2O 3, had a turnover frequency at 700°C of 0.05 s −1, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La 2O 3/Al 2O 3 catalysts exhibited stable activity under high conversion conditions as well as high CH 4 selectivity (CH 4 + NO vs. CH 4 + O 2). The addition of Sr to a 20% La 2O 3/γ-Al 2O 3 sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO =4 to the latter Sr-promoted La 2O 3/Al 2O 3 catalyst decreased activity although sulfate increased the activity of Sr-promoted La 2O 3. Dispersing La 2O 3 on SiO 2 produced catalysts with extremely low specific activities, and rates were even lower than with pure La 2O 3. This is presumably due to water sensitivity and silicate formation. The La 2O 3/Al 2O 3 catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications. 相似文献
2.
The role of La 2O 3 loading in Pd/Al 2O 3-La 2O 3 prepared by sol–gel on the catalytic properties in the NO reduction with H 2 was studied. The catalysts were characterized by N 2 physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO. The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La2O3 inclusion. The selectivity depends on the NO/H2 molar ratio (GHSV = 72,000 h−1) and the extent of interaction between Pd and La2O3. At NO/H2 = 0.5, the catalysts show high N2 selectivity (60–75%) at temperatures lower than 250 °C. For NO/H2 = 1, the N2 selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H2O vapor. The high N2 selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdOx phase interacting with La2O3. The formation of this phase is favored by the spreading of PdO promoted by La2O3. DTA shows that the phase transformation takes place at temperatures of 280–350 °C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al2O3 the O2 uptake is consistent with the oxidation of PdO to PdO2, and when La2O3 is present the O2 uptake exceeds that amount (1.5 times). La2O3 in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250 °C) favoring the formation of N2. 相似文献
3.
Well crystallised aluminium borate Al 18B 4O 33 has been synthesised from alumina and boric acid with a BET area of 18 m 2/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al 18B 4O 33 was prepared by conventional impregnation of Pd(NO 3) 2 aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al 18B 4O 33 in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al 2O 3. Pd/Al 18B 4O 33 exhibited a much lower activity than Pd/Al 2O 3 when treated in hydrogen at 500 °C or aged in O 2/H 2O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al 18B 4O 33 which became as active as Pd/Al 2O 3. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al 2O 3 and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al 18B 4O 33. The existence of two distinct types of PdO species formed on Al 18B 4O 33 and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O 2. 相似文献
4.
In this study, in order to develop low-temperature sintering ceramics for a multilayer piezoelectric transformer application, we explored CuO and Bi 2O 3 as sintering aids at low temperature (900 °C) sintering condition for Sb, Li and Mn-substituted 0.8Pb(Zr 0.48Ti 0.52)O 3–0.16Pb(Zn 1/3Nb 2/3)O 3–0.04Pb(Ni 1/3Nb 2/3)O 3 ceramics. These substituted ceramics have excellent piezoelectric and dielectric properties such as d33 347 pC/N, kp 0.57 and Qm 1469 when sintered at 1200 °C. The addition of CuO decreased the sintering temperature through the formation of a liquid phase. However, the piezoelectric properties of the CuO-added ceramics sintered below 900 °C were lower than the desired values. The additional Bi 2O 3 resulted in a significant improvement in the piezoelectric properties. The composition Sb, Li and Mn-substituted 0.8Pb(Zr 0.48Ti 0.52)O 3–0.16Pb(Zn 1/3Nb 2/3)O 3–0.04Pb(Ni 1/3Nb 2/3)O 3 + 0.5 wt% CuO + 0.5 wt% Bi 2O 3 showed the value of kp = 0.56, Qm = 1042 (planar mode), d33 = 350 pC/N, when it was sintered at 900 °C for 2 h. These values indicated that the newly developed composition might be suitable for multilayer piezoelectric transformer application. 相似文献
5.
Partial oxidation of methane to synthesis gas was carried out using supported iridium–nickel bimetallic catalysts, in order to reduce loading levels of iridium and nickel, and to avoid carbon deposition on nickel-based catalysts by adding iridium. The performance of supported iridium–nickel bimetallic catalysts in synthesis gas formation depended strongly upon the support materials. La 2O 3 gave the best performance among the support materials tested. Ir(0.25 wt%)–Ni(0.5 wt%)/La 2O 3 afforded 36% conversion of methane (CH 4/O 2=5) to give CO and H 2 with the selectivities of above 90% at 800°C, and those at 600°C were 25.3% conversion of methane and CO and H 2 selectivities of about 80%, respectively. Reduced monometallic Ir(0.25 wt%)/La 2O 3 and Ni(0.5 wt%)/La 2O 3 catalysts did not produce synthesis gas at 600°C. A higher conversion of methane was obtained by synergistic effects. The product concentrations of CO, H 2, and CO 2, and CH 4 conversion were maintained in high values, even increasing the space velocity of feed gas over Ir–Ni/La 2O 3 catalyst, indicating that rapid reaction takes place. As a by-product, a small amount of carbon deposition was observed, but carbon formation decreased with increasing the space velocity. On the other hand, with reduced monometallic Ni(10 wt%)/La 2O 3 catalyst, yield of synthesis gas and carbon decreased with increasing the space velocity. 相似文献
6.
Dependence of the electrical and optical properties of In 2O 3–10 wt% ZnO (IZO) thin films deposited on glass substrates by RF magnetron sputtering on the annealing atmosphere was investigated. The electrical resistivities of indium zinc oxide (IZO) thin films deposited on glass substrate can be effectively decreased by annealing in an N 2 + 10% H 2 atmosphere. Higher temperature (200 °C) annealing is more effective in decreasing the electrical resistivity than lower temperature (100 °C) annealing. The lowest resistivity of 6.2 × 10 −4 Ω cm was obtained by annealing at 200 °C in an N 2 + 10% H 2 atmosphere. In contrast, the resistivity was increased by annealing in an oxygen atmosphere. The transmittance of IZO films is improved by annealing regardless of the annealing temperature. 相似文献
7.
A series of CuO–ZnO/Al 2O 3 solids were prepared by wet impregnation using Al(OH) 3 solid and zinc and copper nitrate solutions. The amounts of copper and zinc oxides were varied between 10.3 and 16.0 wt% CuO and between 0.83 and 7.71 wt% ZnO. The prepared solids were subjected to thermal treatment at 400–1000°C. The solid–solid interactions between the different constituents of the prepared solids were studied using XRD analysis of different calcined solids. The surface characteristics of various calcined adsorbents were investigated using nitrogen adsorption at −196°C and their catalytic activities were determined using CO-oxidation by O 2 at temperatures ranged between 125°C and 200°C. The results showed that CuO interacts with Al2O3 to produce copper aluminate at ≥600°C and the completion of this reaction requires heating at 1000°C. ZnO hinders the formation of CuAl2O4 at 600°C while stimulates its production at 800°C. The treatment of CuO/Al2O3 solids with different amounts of ZnO increases their specific surface area and total pore volume and hinders their sintering (the activation energy of sintering increases from 30 to 58 kJ mol−1 in presence of 7.71 wt% ZnO). This treatment resulted in a progressive decrease in the catalytic activities of the investigated solids but increased their catalytic durability. Zinc and copper oxides present did not modify the mechanism of the catalyzed reaction but changed the concentration of catalytically active constituents (surface CuO crystallites) without changing their energetic nature. 相似文献
8.
Aluminium titanate films thicker than 0.5 μm have been synthesized by sol–gel methods. The films have been deposited via repetitive dip-coating on silicon wafers and their thermal stability has been tested as a function of the annealing time and temperature. The sol–gel approach has allowed the formation of the aluminium titanate phase at temperatures (700 °C) much lower than those necessary for solid-state reactions (1450 °C). Magnesium oxide has been used to improve the thermal stability of the films at high temperatures. The behavior of samples prepared with two different Mg content, i.e. Mg 0.2Al 1.6Ti 0.8O 5 and Mg 0.6Al 0.8Ti 1.6O 5, has been studied. The films have proven to be stable at 1150 °C, for up to 90 h. X-ray photoelectron spectroscopy has shown that after firing at 500 °C the surface chemical composition of the films is in accordance with the nominal one, whilst at higher annealing temperatures some differences, attributed to diffusion effects, have been observed. 相似文献
9.
Pure and lanthanum doped Bi 4Ti 3O 12 thin films were deposited on Pt/Ti/SiO 2/Si substrate using a polymeric precursor solution. Annealing in static air and oxygen atmosphere was performed at 700 °C for 2 h. The obtained films were characterized by X-ray diffraction and atomic force microscopy. The dielectric constant and dissipation factor were measured in the frequency region from 1 kHz to 1 MHz. Electrical characterization of the films pointed to ferroelectricity via hysteresis loop. Films annealed in static air possess a dielectric constant higher than films annealed in oxygen atmosphere due to differences in the grain size, crystallinity and structural defects. A regularly shaped hystereses loop is observed after annealing in static air. The obtained results suggest that the annealing in oxygen atmosphere can increase the trapped charge and the relaxation phenomenon. 相似文献
10.
Two new phases with ordered anion vacancies, La 8Mn 8O 23 and La 4Mn 4O 11, both having the general formula A nB nO 3n·1, form during the reduction of perovskites of the type LaMnO 3.07. The solid LaMnO 3.07, obtained in air at 1100°C and subsequently reduced in flowing carbon monoxide at 350°C, gives the stoichiometric perovskite LaMnO 3.00, whose reduction starts at 420°C and is completed at 450°C with formation of the phase La 8Mn 8O 23. The second reduction stage starts at approximately 500°C and is completed at 520°C with formation of La 4Mn 4O 11. The solids in the range of composition LaMnO 3.00-La 8Mn 8O 23 and La 8Mn 8O 23-La 4Mn 4O 11 would seem by X-ray examination to be biphasic but the behaviour during reduction is typical of a monophasic substance, the oxygen pressure of which at constant temperature progressively decreases as the composition tends to the limiting values La 8Mn 8O 23 and La 4Mn 4O 11. The crystal lattice of La 8Mn 8O 23 can be considered as resulting from a sequence of seven octahedral sheets, MnO 6, alternated with tetrahedral sheets, MnO 4, and the crystal lattice of La 4Mn 4O 11 from a sequence of three MnO 6 sheets alternating with MnO 4 sheets. 相似文献
11.
The formation of cubic solid solutions in the system La 2O 3–ZrO 2 by mechanochemical activation of a mixture of the oxides (molar ratio ZrO 2 82%–La 2O 3 18%) is studied. After 6 h of activation at room temperature, a poorly crystalline cubic solid solution is formed, with ultimate crystallite sizes in the nanometer range. The mixtures activated during 1–3 h form the solid solution on subsequent heating at 1000 °C, while the non-activated mixture does not react, even after thermal treatment at 1200 °C. The solid solution obtained at room temperature undergoes partial structural ordering at temperatures between 800 and 1000 °C. Long time heating at temperatures of 1000 °C and above results in the formation of La 2Zr 2O 7 and rejection of the excess ZrO 2. Mechanochemical activation offers interesting possibilities for the synthesis of these materials at temperatures lower than those used in conventional processing, and for the control of their physicochemical and microstructural properties. 相似文献
12.
Conductive perovskite lanthanum nickelate LaNiO 3 (LNO) thin films were fabricated on SiO 2/Si substrates through metal-organic chemical liquid deposition method. The effect of annealing temperature on the orientation and sheet resistance of the LNO films were investigated. XRD patterns showed that the LNO films deposited on SiO 2/Si substrates exhibited preferred-(1 1 0) orientation. The lowest sheet resistance of the LNO thin films, 250 Ω/□ was obtained after being annealed at 650 °C for 1 h. Subsequently, Pb 0.97La 0.02(Zr 0.85Sn 0.13Ti 0.02)O 3 (PLZST) antiferroelectric thin films were prepared on the LaNiO 3 buffered SiO 2/Si substrates via sol–gel process. And the crystallinity, microstructure and electric properties of the PLZST thin films were studied in details. 相似文献
13.
Regeneration of S-poisoned Pd/Al 2O 3 catalysts for the abatement of methane emissions from natural gas vehicles was addressed in this work. Investigations were devoted to determine the temperature threshold allowing for catalyst reactivation under different CH4 containing atmospheres. Under lean combustion conditions in the presence of excess O2, partial regeneration took place only above 750 °C after decomposition of stable sulphate species adsorbed on the support. Short CH4-reducing, O2-free pulses led to partial catalyst reactivation already at 550 °C and to practically complete regeneration at 600 °C. Also in this case reactivation was associated with SO2 release due to the decomposition of stable support sulphates likely promoted by CH4 activation onto the reduced metallic Pd surface. Rich combustion pulses with CH4/O2 = 2 were equally effective to CH4-reducing pulses in catalyst regeneration. These results suggest that a regeneration strategy based on periodical natural gas pulses fed to the catalyst by a by-pass line might be efficient in limiting the effects of S-poisoning of palladium catalysts for the abatement of CH4 emissions from natural gas engine. 相似文献
14.
The processing conditions, reaction mechanism, fine structure of the powders, microstructure, and dielectric properties of SrBiNb 2O 9 (SBN) were systematically studied. A relative density of >80% was obtained using a two-step sintering process at moderate pressure. XRD showed that a single phase with the layered perovskite structure of SrBi 2Nb 2O 9 (SBN) was formed after calcining at 600 °C. No intermediate phase was found during heat treatment at and above 600 °C. The crystallite size ( D) and the effective strain ( η) were found to be 38.8 nm and 0.01475, respectively, while the particle size obtained from TEM was laid between 25 and 36 nm. SEM revealed that the average grain size after sintering at 900 °C for 4 h was 0.67 μm. Dielectric constant and corresponding tangent loss were measured in the frequency range from 1 kHz to 1 MHz from which the Curie temperature ( Tc) was found to be at 450 °C. 相似文献
15.
La 2NiO 4 tubular membranes of relative density over 92% were used to separate oxygen from air and facilitate the partial oxidation of methane to H 2 and CO at 900 °C. When methane was fed into a tube of inner surface area 5.11 cm 2 at a rate of 10.5 ml/min, methane throughput conversion was 89%, CO selectivity 96%, H 2/CO ratio 1.5, and the equivalent oxygen flux was 6.8 ml/min. The surface of the La 2NiO 4 membrane exposed to CH 4 decomposed into La 2O 3 and Ni, while the surface in contact with air remained almost unchanged. It is suggested that the conversion of methane in the membrane reactor involves the reforming of methane by the H 2O and CO 2 catalyzed by nickel. 相似文献
16.
Since the electromechanical devices move towards enhanced power density, high mechanical quality factor ( Qm) and electromechanical coupling factor ( kp) are commonly needed for the high powered piezoelectric transformer with Qm≥2000 and kp=0.60. Although Pb(Mn 1/3Nb 2/3)O 3–PbZrO 3–PbTiO 3 (PMnN–PZ–PT) ceramic system has potential for piezoelectric transformer application, further improvements of Qm and kp are needed. Addition of 2CaO–Fe 2O 3 has been proved to have many beneficial effects on Pb(Zr,Ti)O 3 ceramics. Therefore, 2CaO–Fe 2O 3 is used as additive in order to improve the piezoelectric properties in this study. The piezoelectric properties, density and microstructures of 0.07Pb(Mn 1/3Nb 2/3)O 3–0.468PbZrO 3–0.462PbTiO 3 (PMnN–PZ–PT) piezoelectric ceramics with 2CaO–Fe 2O 3 additive sintered at 1100 and 1250 °C have been studied. When sintering temperature is 1250 °C, Qm has the maximum 2150 with 0.3 wt.% 2CaO–Fe 2O 3 addition. The kp more than 0.6 is observed for samples sintered at 1100 °C. The addition of 2CaO–Fe 2O 3 can significantly enhance the densification of PMnN–PZ–PT ceramics when the sintering temperature is 1250 °C. The grain growth occurred with the amount of 2CaO–Fe 2O 3 at both sintering temperatures. 相似文献
17.
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
18.
Nanoparticles of Ce xZr 1−xO 2 ( x = 0.75, 0.62) were prepared by the oxidation-coprecipitation method using H 2O 2 as an oxidant, and characterized by N 2 adsorption, XRD and H 2-TPR. Ce xZr 1−xO 2 prepared had single fluorite cubic structure, good thermal stability and reduction property. With the increasing of Ce/Zr ratio, the surface area of Ce xZr 1−xO 2 increased, but thermal stability of Ce xZr 1−xO 2 decreased. The surface area of Ce 0.62Zr 0.38O 2 was 41.2 m 2/g after calcination in air at 900 °C for 6 h. TPR results showed the formation of solid solution promoted the reduction of CeO 2, and the reduction properties of Ce xZr 1−xO 2 were enhanced by the cycle of TPR-reoxidation. The Pd-only three-way catalysts (TWC) were prepared by the impregnation method, in which Ce 0.75Zr 0.25O 2 was used as the active washcoat and Pd loading was 0.7 g/L. In the test of Air/Fuel, the conversion of C 3H 8 was close to 100% and NO was completely converted at λ < 1.025. The high conversion of C 3H 8 was induced by the steam reform and dissociation adsorption reaction of C 3H 8. Pd-only catalyst using Ce 0.75Zr 0.25O 2 as active washcoat showed high light off activity, the reaction temperatures ( T50) of 50% conversion of CO, C 3H 8 and NO were 180, 200 and 205 °C, respectively. However, the conversions of C 3H 8 and NO showed oscillation with continuously increasing the reaction temperature. The presence of La 2O 3 in washcoat decreased the light off activity and suppressed the oscillation of C 3H 8 and NO conversion. After being aged at 900 °C for 4 h, the operation windows of catalysts shifted slightly to rich burn. The presence of La 2O 3 in active washcoat can enhance the thermal stability of catalyst significantly. 相似文献
19.
The electrical and electromechanical properties of Pb(Mg 1/3Nb 2/3)O 3–Pb(Ni 1/3Nb 2/3)O 3–Pb(Zr,Ti)O 3 (PMN–PNN–PZT, PMN/PNN/PZT = 20/10/70) on Pt/Ti/SiO 2/Si substrates by chemical solution deposition was investigated. The PMN–PNN–PZT films annealed at 650 °C exhibited slim polarization hysteresis curves and a high dielectric constant of 2100 at room temperature. A broad dielectric maximum at approximately 140–170 °C was observed. The field-induced displacement was measured by scanning probe microscopy, the bipolar displacement was not hysteretic, and the effective piezoelectric coefficient ( d33) was 66 × 10 −12 m/V. The effective d33 decreased with temperature, but the value at 100 °C remained 45 × 10 −12 m/V. 相似文献
20.
Monodispersed nano-Au/γ-Al 2O 3 catalysts for low-temperature oxidation of CO have been prepared via a modified colloidal deposition route, which involves the deposition of dodecanethiolate self-assembled monolayer (SAM)-protected gold nanoparticles (C 12 nano-Au) in hexane on γ-Al 2O 3 at room temperature. The diameter of the gold nanoparticles deposited on the support is 2.5 ± 0.8 nm after thermal treatment, and their valence states comprise both the metallic and oxidized states. It is found that the thermal treatment temperature affects significantly the catalytic activity of the catalysts in the processing steps. The catalyst treated at 190 °C exhibits considerably higher activity as compared to catalysts treated at 165 and 250 °C. A 2.0-wt.% nano-Au/γ-Al 2O 3 catalyst treated at 190 °C for 15 h maintains the catalytic activity at nearly 100% CO oxidation for at least 800 h at 15 °C, at least 600 h at 0 °C, and even longer than 450 h at −5 °C. Evidently, the catalysts obtained using this preparation route show high catalytic activity, particularly at low temperatures, and a good long-term stability. 相似文献
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