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1.
An aqueous (NH 4) 2CO 3 coprecipitation method, based on that of Groppi et al. [Appl. Catal. A 104 (1993) 101–108] was used to synthesize Sr 1−xLa xMnAl 11O 19− hexaaluminates. These materials were first synthesized by alkoxide hydrolysis. This synthesis route requires special handling of the starting materials and is not likely to be commercially practical. The materials prepared by (NH 4) 2CO 3 coprecipitation have similar surface areas as those prepared by the alkoxide hydrolysis method. Their CH 4 oxidation activity, measured as the temperature needed for 10% conversion of methane, is higher than those prepared by alkoxide hydrolysis. The La-substantiated material, LaMnAl 11O 19−, shows high surface area with 19.3 m 2/g after calcination at 1400°C for 2 h. It is active for CH 4 oxidation with T10% at 450°C using 1% CH 4 in air and 70 000 cm 3/h g space velocity. The stability and activity of LaMnAl 11O 19− prepared by (NH 4) 2CO 3 coprecipitation method is a simple and important step forward for the application of CH 4 catalytic combustion for gas turbines. 相似文献
2.
The catalytic behaviour of SiO 2 supported MoO 2 and V 2O 5 catalysts in the partial oxidation of methane to formaldehyde with O 2 (MPO) in the range 400–800°C has been investigated by temperature programmed reaction (TPR) tests. Both the sequence of the onset temperature of product formation and the product distribution patterns signal that MPO on silica based oxide catalysts occurs mainly via a consecutive reaction path: CH 4 → HCHO → CO → CO 2. At T >/ 700°C a parallel surface assisted gas-phase reaction pathway leads to the formation of minor amounts of C 2 products both on SiO 2 and MoO 3/SiO 2 catalysts. The redox properties of MoO 3/SiO 2 and V 2O 5SiO 2 catalysts have been systematically evaluated by H 2 and CH 4 temperature programmed reduction (H 2-TPR, CH 4-TPR) measurements. H 2-TPR results do not account for the reactivity scale of oxide catalysts in the MPO. CH 4-TPR measurements indicate that the enhancement in the specific activity of the silica is controlled by the capability of MoO 3 and V 2O 5 promoters in providing ‘active’ lattice oxygen species. 相似文献
3.
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
4.
The reduction of NO 2 to N 2 at temperatures as low as 150°C without the use of an externally supplied reductant can be achieved with a microporous solid, (NH 4) 3PW 12O 40. Temperature-programmed experiments with NO, NO 2 and ammonium salts labelled with 15N show that NH 3 is not released from the solid until a temperature of 400°C or greater is attained. These observations, together with the high selectivities to N 15N, show that NO x is predominantly interacting with the bound NH 3 rather than that in the gas phase. After depletion of the NH 3 bound as NH +4 the parent acid remains, on which additional NO x is sorbed irreversibly. The sorbed NO x can be removed from the solid by water treatment at elevated temperatures and the ammonium salt can be regenerated by a precipitation process or by direct injection of NH 3 into the catalyst bed when the process is off-line. 相似文献
5.
The effects of cobalt and manganese oxides-doping on surface and catalytic properties of Cr 2O 3/MgO system have been investigated. The dopant concentration was changed between 1 and 5 mol% cobalt and manganese oxides. Pure and variously doped solids were subjected to heat treatment at 400 and 700 °C. The techniques employed were X-ray diffraction (XRD), nitrogen adsorption at –196 °C, catalytic conversion of iso-propanol at 200–400 °C using flow technique and catalytic decomposition of H 2O 2 at 20–40 °C. The results revealed that the doping process of the system investigated followed by calcinations at 400 or 700 °C, enhanced the solid–solid interactions between catalyst constituents yielding (-MgCrO 4, β-MgCrO 4) and MgCr 2O 4, respectively. Furthermore, manganese and cobalt oxide-doping for Cr 2O 3/MgO system increased its catalytic activity much towards H 2O 2-decomposition. The increase was, however, more pronounced in the case of manganese-doping. Opposite results have been observed in the case of iso-propanol conversion, which proceeds via dehydrogenation and dehydration reaction. The SBET of the investigated system was found to decrease by increasing the dopant concentration. The doping process did not modify the activation energy of the catalyzed reaction, but rather changed the concentration of the catalytically active constituents without changing their energetic nature. 相似文献
6.
A novel regenerable Fe/activated coke (AC) desulfurizer prepared by impregnation of Fe(NO 3) 3 on an activated coke was investigated. Experiment results showed that at 200 °C the SO 2 adsorption capacity of the Fe/AC was higher than that of AC or Fe 2O 3. Temperature-programmed desorption (TPD) revealed that H 2SO 4 and Fe 2(SO 4) 3 were generated on the desulfurizer upon adsorption of SO 2. Effect of desulfurization temperature was also investigated which revealed that with increasing temperature from 150 to 250 °C, the SO 2 removal ability gradually increases. The used Fe/AC can be regenerated by NH 3 at 350 °C to directly form solid ammonium-sulfate salts. 相似文献
7.
This paper presents an investigation into the complex interactions between catalytic combustion and CH 4 steam reforming in a co-flow heat exchanger where the surface combustion drives the endothermic steam reforming on opposite sides of separating plates in alternating channel flows. To this end, a simplified transient model was established to assess the stability of a system combining H 2 or CH 4 combustion over a supported Pd catalyst and CH 4 steam reforming over a supported Rh catalyst. The model uses previously reported detailed surface chemistry mechanisms, and results compared favorably with experiments using a flat-plate reactor with simultaneous H 2 combustion over a γ-Al 2O 3-supported Pd catalyst and CH 4 steam reforming over a γ-Al 2O 3-supported Rh catalyst. Results indicate that stable reactor operation is achievable at relatively low inlet temperatures (400 °C) with H 2 combustion. Model results for a reactor with CH 4 combustion indicated that stable reactor operation with reforming fuel conversion to H 2 requires higher inlet temperatures. The results indicate that slow transient decay of conversion, on the order of minutes, can arise due to loss of combustion activity from high-temperature reduction of the Pd catalyst near the reactor entrance. However, model results also show that under preferred conditions, the endothermic reforming can be sustained with adequate conversion to maintain combustion catalyst temperatures within the range where activity is high. A parametric study of combustion inlet stoichiometry, temperature, and velocity reveals that higher combustion fuel/air ratios are preferred with lower inlet temperatures (≤500 °C) while lower fuel/air ratios are necessary at higher inlet temperatures (600 °C). 相似文献
8.
Crystalline galliosilicates with the beta structure have been synthesized from alkali-free hydrogels with the composition Ga 2O 3: xSiO 2:12(TEA) 2O:1285H 2O, where x=40, 60, 80 and 100. The addition of an alkali (Ga 2O 3/Na 2O=1.0) to the hydrogel in the form of sodium gallate decreased crystallization times at 135°C to 10 days from 14 days. Crystallization reactions are not stoichiometric and the crystals' silica-to-gallia ratio (SGR) is always less than that in the parent hydrogel. Thermal analysis has shown that TEA ions first decompose and are then thermally desorbed, probably in the form of Hofmann decomposition products. Residual hydrocarbons are burned at T>400°C. The calcined crystals have surface area in the 500–625 m 2 g −1 range. FTIR experiments with chemisorbed pyridine have shown that the isomorphous substitution of Al(IV) with Ga(IV) atoms decreases the acid-site strength and changes the relative B/L acid-site ratio of the crystals. IR spectra at a desorption temperature of 500°C revealed that all the pyridine desorbed from B sites but not from L sites in Ga-beta, whereas the Al-beta analog retained pyridine on both L and B sites. Microcalorimetry experiments with ammonia at 150°C have revealed the existence of different acid-site (B+L) strengths and site populations. The total number of sites available to NH 3 chemisorption and the number of strong acid sites show the same dependence on the SGR value of the crystals. 29Si MAS-NMR spectra contain a resonance at −111 ppm attributed to T(4Si,0Ga) groups and a second resonance at −102 ppm attributed to T(3Si,1Ga) groups. 71Ga NMR spectra confirm that Ga(IV) is the dominant species and that Ga(VI) formation depends, in part, on the thermal pretreatment applied to the crystals. 1H NMR results have revealed that after calcination in air at 500°C, there are residual hydrocarbon compounds in the beta microporous structure. This can be avoided if the organic template is decomposed in nitrogen at 500°C for 1 h and the decomposition products are removed oxidatively by a second calcination step in air at 550°C for 2 h. 相似文献
9.
Ammonium aluminum sulphate was introduced to modify alumina bubbles by the wet chemical method. Distribution of ammonium aluminum sulphate, in situ decomposition characteristics, microstructure and mechanical properties of the modified alumina bubbles were investigated by SEM, XRD, BET and by a purposely designed experimental device for the measurement of compressive resistance. Experimental results showed that an ammonium aluminum sulphate thin film was formed on the surface of the alumina bubbles, at the same time, ammonium aluminum sulphate also infiltrated into the flaws and cavities of the alumina bubbles. After heat-treated at 900 °C, the ammonium aluminum sulphate on the alumina bubbles could be in situ decomposed to γ-Al 2O 3 with high activity. The compressive resistance of the modified alumina bubbles was enhanced from 15.6N to 38.7N after heat-treatment at 1700 °C for 2 h. 相似文献
10.
The nickel oxide—chromia gel powder solid-state reaction was followed at 400° – 1200 °C. The reaction fits kinetically the Jander and/or Ginstling-Brounshtein equation. Catalytic activity towards H 2O 2 decomposition was correlated with pertaining formation structural changes including tetragonal deformation of NiCr 2O 4. 相似文献
11.
Fe 2O 3-TiO 2 thin films were deposited on silica glass slides using three sol-gel solutions containing Ti-butoxide and different iron oxide precursors. The thermal evolution of the coatings was followed by DTA-TGA and XRD from 200 to 1000°C. All the iron-containing samples were amorphous up to 700°C, when a phase separation between iron and titanium oxides was evidenced by the presence of crystalline Fe 2O 3. Above that temperature a titanate compound (pseudobrookite) formed according to the equilibrium phase diagram. The use of different precursors did not affect the thermal evolution and all the different samples exhibited the same trend. 相似文献
12.
The catalytic performance of some metal oxides in the selective oxidation of H 2S in the stream containing water vapor and ammonia was investigated in this study. Among the catalysts tested, V 2O 5/SiO 2 and Fe 2O 3/SiO 2 catalyst showed good conversion of H 2S with very low selectivity to undesired SO 2. Hydrogen sulfide could be recovered as harmless solid products (elemental sulfur and various ammonium salts), and distribution of solid products was varied with types of catalyst and compositions of reactant. XRD and FT-IR analysis revealed that the salt was mixture of ammonium–sulfur–oxygen compounds. It was noteworthy that V 2O 5/SiO 2 catalyst produced elemental sulfur and ammonium thiosulfate, and that elemental sulfur was principal product on Fe 2O 3/SiO 2 catalyst. Small amount of ammonium sulfate was obtained with the Fe 2O 3/SiO 2 catalyst. In order to elucidate the reaction path, the effects of O 2/H 2S ratio and concentration of NH 3 and H 2O are also studied with the V 2O 5/SiO 2 catalyst. 相似文献
13.
借助ReaxFF-MD方法,对化学链燃烧过程Al 2O 3负载Fe 2O 3载氧体(Fe 2O 3/Al 2O 3)表面CH 4反应过程模拟,探究Al 2O 3惰性载体对Fe 2O 3-CH 4体系燃烧过程的调控机制。研究发现添加Al 2O 3惰性载体改变了化学链燃烧过程中Fe 2O 3载氧体反应性和Fe 2O 3/Al 2O 3-CH 4反应体系的热力学和动力学行为。主要是促进了Fe 2O 3载氧体表面CH 4氧化,并对CH 4反应过程、中间体、产物及其反应速率和放热量等均具有显著促进和调控作用。原因在于Al 2O 3惰性载体对Fe 2O 3活性相中晶格氧的活化作用促进了晶格氧的迁移-扩散-释放。添加惰性载体增强了Fe 2O 3载氧体在化学链燃烧过程晶格氧释放速率和释放量,有利于CH 4氧化燃烧向合成气的高效、清洁转化,强化了化学链燃烧过程,满足当前能源高效转化和碳减排目标。 相似文献
14.
借助ReaxFF-MD方法,对化学链燃烧过程Al 2O 3负载Fe 2O 3载氧体(Fe 2O 3/Al 2O 3)表面CH 4反应过程模拟,探究Al 2O 3惰性载体对Fe 2O 3-CH 4体系燃烧过程的调控机制。研究发现添加Al 2O 3惰性载体改变了化学链燃烧过程中Fe 2O 3载氧体反应性和Fe 2O 3/Al 2O 3-CH 4反应体系的热力学和动力学行为。主要是促进了Fe 2O 3载氧体表面CH 4氧化,并对CH 4反应过程、中间体、产物及其反应速率和放热量等均具有显著促进和调控作用。原因在于Al 2O 3惰性载体对Fe 2O 3活性相中晶格氧的活化作用促进了晶格氧的迁移-扩散-释放。添加惰性载体增强了Fe 2O 3载氧体在化学链燃烧过程晶格氧释放速率和释放量,有利于CH 4氧化燃烧向合成气的高效、清洁转化,强化了化学链燃烧过程,满足当前能源高效转化和碳减排目标。 相似文献
15.
蛋白土具有良好的应用前景,但是其自然白度较低制约了其开发应用。蛋白土硫酸铵煅烧法可去除其中的显色金属氧化物,提高蛋白土的白度,同时提取其中Al 2O 3。本文采用热重-差示扫描量热(TG-DSC)、同步热分析与红外质谱(TG-FTIR-MS)联用系统,结合X射线衍射对煅烧过程的固相及气相产物进行了表征分析,明析了其化学过程。结果表明,蛋白土中的Al 2O 3和Fe 2O 3在200~350℃时反应生成(NH 4) 3(Al,Fe)(SO 4) 3,同时逸出NH 3和H 2O;350~450℃时,进一步反应转化为NH 4(Al,Fe)(SO 4) 2,同时逸出NH 3、H 2O、SO 2和O 2;450~550℃时,NH 4(Al,Fe)(SO 4) 2分解生成(Al,Fe) 2(SO 4) 3,同时逸出NH 3、H 2O、SO 2和O 2;550~750℃时(Al,Fe) 2(SO 4) 3分解生成Al 2O 3和Fe 2O 3,同时逸出SO 2和O 2。采用Kissinger微分法与Ozawa积分法分别计算出4个阶段表观活化能后取二者平均值,分别为101.74kJ/mol、104.52kJ/mol、201.40kJ/mol、232.51kJ/mol,并获得对应4个热化学反应阶段的频率因子、反应级数和动力学方程。 相似文献
16.
The catalytic oxidation of unsymmetrical dimethylhydrazine (UDMH) by air has been studied in a vibro-fluidized catalyst bed laboratory kinetic setup over catalysts Cu xMg 1−xCr 2O 4/γ-Al 2O 3, 32.9%Ir/γ-Al 2O 3 and β-Si 3N 4 in a temperature range 150–400 °C. The catalyst Cu xMg 1−xCr 2O 4/γ-Al 2O 3 was found to be optimal regarding high yields of CO 2 and low yields of NO x. A probable mechanism of UDMH heterogeneous catalytic oxidation is proposed. Catalyst Cu xMg 1−xCr 2O 4/γ-Al 2O 3 has been further used in the pilot plant specially designed for the destruction of UDMH. Results of testing the main fluidized bed catalytic reactor for UDMH oxidation and the reactor for selective catalytic reduction of NO x with NH 3 are presented. These results prove that the developed UDMH destruction technology is highly efficient and environmentally safe. 相似文献
17.
The synthesis of a novel 3D aluminophosphate is described. The thermal properties of the material were investigated, and the existence of three high-temperature variants was revealed. The crystal structures of the as-synthesized material (UiO-26-as) and the material existing around 250°C (UiO-26-250) were solved from powder X-ray diffraction data. UiO-26-as with the composition [Al 4O(PO 4) 4(H 2O)] 2−[NH 3(CH 2) 3NH 3] 2+ crystallizes in the monoclinic space group P2 1/c (no. 14) with a=19.1912(5), b=9.3470(2), c=9.6375(2) Å and β=92.709(2)°. It exhibits a 3D open framework consisting of connections by PO 4 tetrahedra with AlO 4 tetrahedra, AlO 5 trigonal bipyramids and AlO 5(H 2O) octahedra forming two types of layers stacked along [1 0 0] and connected by Al–O–P bondings. The structure possesses a 1D 10-ring channel system running along [0 0 1], in which doubly protonated 1,3-diaminopropane molecules are located. UiO-26-250 with the composition [Al 4O(PO 4) 4] 2−[NH 3(CH 2) 3NH 3] 2+ crystallizes in the monoclinic space group P2 1/c with a=19.2491(4), b=9.27497(20), c=9.70189(20) Å and β=93.7929(17)°. The transformation to UiO-26-250 involves removal of the water molecule which originally is coordinated to aluminum. The rest of the structure remains virtually unchanged. The crystal structures of the two other variants existing around 400 (UiO-26-400) and 600°C (UiO-26-600) remain unknown. 相似文献
18.
Thermal reaction of niobium alkoxide in organic media at 300°C yielded amorphous niobia, which maintained a surface area above 130 m 2/g after calcination at 500°C. Niobium double oxides (LiNbO 3, Zr 6Nb 2O 17, CaNb 2O 6, CrNbO 4, FeNbO 4, ZnNb 2O 6 and R 3NbO 7 (R; rare earth)) were directly obtained by the reaction of niobium alkoxide with the corresponding metal alkoxide, acetate or acetylacetonate in 1,4-butanediol at 300°C. In some cases, amorphous product was obtained; however, double oxides (SrNb 2O 6, Co 4Nb 2O 9, etc.) crystallized from the products at low temperatures. 相似文献
19.
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
20.
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
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