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1.
Two compounds of formula BaO.Al 2O 3.2H 2O have been synthesised and studied by infra-red, X-ray and thermal analytical techniques. One form yielded another crystalline product, BaO.Al 2O 3.0.5H 2O, as an intermediate during dehydration. The results are interpreted in terms of the known crystal structure of one of the forms of the dihydrate. 相似文献
2.
The compound of approximate formula BaO.Al 2O 3.H 2O has been synthesised. Infra-red and X-ray studies suggest a structure based on a framework of AlO 4 tetrahedra. Thermal dehydration and differential thermal analysis curves are presented, and are explained in terms of the suggested structure. 相似文献
3.
α-BaO.Al 2O 3.4H 2O has been synthesised and studied by infrared, X-ray and thermal analytical techniques. The compound of approximate formula BaO.Al 2O 3.0.5H 2O, described in Part III, forms as a dehydration product, and appears to be identical with the compound β-BAH 2 described by other workers. A possible explanation for the discrepancy in water content is discussed. 相似文献
4.
The effects of thermal aging and H 2O treatment on the physicochemical properties of BaO/Al 2O 3 (the NO x storage component in the lean NO x trap systems) were investigated by means of X-ray diffraction (XRD), BET, TEM/EDX and NO 2 TPD. Thermal aging at 1000 °C for 10 h converted dispersed BaO/BaCO 3 on Al 2O 3 into low surface area crystalline BaAl 2O 4. TEM/EDX and XRD analysis showed that H 2O treatment at room temperature facilitated a dissolution/reprecipitation process, resulting in the formation of a highly
crystalline BaCO 3 phase segregated from the Al 2O 3 support. Crystalline BaCO 3 was formed from conversion of both BaAl 2O 4 and a dispersed BaO/BaCO 3 phase, initially present on the Al 2O 3 support material after calcinations at 1000 and 500 °C, respectively. Such a phase change proceeded rapidly for dispersed
BaO/BaCO 3/Al 2O 3 samples calcined at relatively low temperatures with large BaCO 3 crystallites observed in XRD within 10 min after contacting the sample with water. Significantly, we also find that the change
in barium phase occurs even at room temperature in an ambient atmosphere by contact of the sample with moisture in the air,
although the rate is relatively slow. These phenomena imply that special care to prevent the water contact must be taken during
catalyst synthesis/storage, and during realistic operation of Pt/BaO/Al 2O 3 NO x trap catalysts since both processes involve potential exposure of the material to CO 2 and liquid and/or vapor H 2O. Based on the results, a model that describes the behavior of Ba-containing species upon thermal aging and H 2O treatment is proposed. 相似文献
5.
Comparison of barium peroxide, Ba(OH) 2 and Ba(NO 3) 2 as the precursor of BaO for the preparation of NO x -storage BaO/Al 2O 3 material was carried out. The as prepared materials were calcined at 550 and 800 °C and characterized by N 2 physisorption, XRD, Raman and FT-IR spectroscopy. Measurements of the NO x storage performances of these BaO/Al 2O 3 materials by NO 2 adsorption and NO x -TPD experiments showed that the use of barium peroxide as the precursor of BaO inhibited the formation of BaAl 2O 4 and led to remarkable improvements in the thermal stability as well as NO x storage capacity of the final BaO/Al 2O 3 material calcined at 800 °C. 相似文献
6.
A new compound of formula 2BaO·Al 2O 3·2H 2O has been synthesised and studied by infra-red, X-ray and thermal analytical techniques. This hydrate yielded another crystalline product, 2BaO·Al 2O 3, during dehydration. 相似文献
7.
Transient experiments were performed to study sulfur deactivation and regeneration of Pt/BaO/Al 2O 3 and Pt/SrO/Al 2O 3 NO
x
storage catalysts. It was found that the strontium-based catalysts are more easily regenerated than the barium-based catalysts
and that a higher fraction of the NO
x
storage sites are regenerated when H 2 is used in combination with CO 2 compared to H 2 only. 相似文献
8.
The NO
x
adsorption mechanism on Pt/BaO/Al 2O 3 catalysts was investigated by performing NO
x
storage/reduction cycles, NO 2 adsorption and NO + O 2 adsorption on 2%Pt/(x)BaO/Al 2O 3 ( x = 2, 8, and 20 wt%) catalysts. NO
x
uptake profiles on 2%\Pt/20%BaO/Al 2O 3 at 523 K show complete uptake behavior for almost 5 min, and then the NO
x
level starts gradually increasing with time and it reaches 75% of the inlet NO
x
concentration after 30 min time-on-stream. Although this catalyst shows fairly high NO
x
conversion at 523 K, only ~2.4 wt% out of 20 wt% BaO is converted to Ba(NO 3) 2. Adsorption studies by using NO 2 and NO + O 2 suggest two different NO
x
adsorption mechanisms. The NO 2 uptake profile on 2%Pt/20%BaO/Al 2O 3 shows the absence of a complete NO
x
uptake period at the beginning of adsorption and the overall NO
x
uptake is controlled by the gas–solid equilibrium between NO 2 and BaO/Ba(NO 3) 2 phase. When we use NO + O 2, complete initial NO
x
uptake occurs and the time it takes to convert ~4% of BaO to Ba(NO 3) 2 is independent of the NO concentration. These NO
x
uptake characteristics suggest that the NO + O 2 reaction on the surface of Pt particles produces NO 2 that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NO
x
uptake, this spill-over process is very fast and so it is able to provide complete NO
x
storage. However, the NO
x
uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to Ba(NO 3) 2. The formation of Ba(NO 3) 2 around the Pt particles results in the development of a diffusion barrier for NO 2, and increases the probability of NO 2 desorption and consequently, the beginning of NO
x
slip. As NO
x
uptake by NO 2 spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO 2 uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase
NO 2 concentration. 相似文献
9.
Results of a study of the subsolidus structure of the BaO – Al 2O 3 – Fe 2O 3 system at 1300°C are presented. Based on thermodynamic and experimental data, the triangulation of the system, previously unreported, is carried out with regard for all phases stable at this temperature. Geometrical and topological characterization of the system is given. 相似文献
10.
Results of a thermodynamic and geometric-topological analysis of the subsolidus structure of the BaO – Al 2O 3 – Fe 2O 3 – SiO 2 system at temperatures above 1381 K are reported. Tetrahedratization of the system, with due regard for the existing high-temperature phases, is carried out. Data for solid-state reactions in the system obtained by thermodynamic and geometric-topological calculations are presented. Technological implications in the synthesis a new class of barium-containing cements with allowance for the specific subsolidus structure (associated with reversible solid-state reactions in ternary subsystems and the quaternary BaO – Al 2O 3 – Fe 2O 3 – SiO 2 system proper) are discussed. 相似文献
11.
The effect of Na addition on the performance of Rh/Al 2O 3 catalyst for NO reduction with CO in the presence of H 2O and O 2 was investigated. The reacted catalysts were analyzed by the FTIR technique to identify the products for further investigation on the possible catalytic reaction mechanisms and the reasons behind the H 2O poisoning. Experimental results show that the removal efficiency of NO by Rh/Al 2O 3 catalyst was 63% at 250 °C but that decreased as the H 2O content increased. Adding Na to modify the Rh/Al 2O 3 catalyst significantly enhanced the conversion of NO to 99% at 250–300 °C even as the H 2O content was 1.6 vol%. The FTIR analyses results reveal that the abundant H 2O in the flue gas can compete with NO to adsorb on the surfaces of Rh/Al 2O 3 and Rh-Na/Al 2O 3 catalysts and further enhance the formation of NO 3 that reacts with H. The effects of H 2O on Rh/Al 2O 3 and Rh-Na/Al 2O 3 catalysts can be eliminated by increasing the reaction temperature to higher than 300 °C. Rh-Na/Al 2O 3 is a feasible catalyst for NO reduction at such condition with relative high H 2O and O 2 contents. 相似文献
12.
In this investigation, a comparative study for a NO
X
storage catalytic system was performed focusing on the parameters that affect the reduction by using different reductants (H2, CO, C3H6 and C3H8) and different temperatures (350, 250 and 150 °C), for a Pt/BaO/Al2O3 catalyst. Transient experiments show that H2 and CO are highly efficient reductants compared to C3H6 which is somewhat less efficient. H2 shows a significant reduction effect at relatively low temperature (150 °C) but with a low storage capacity. We find that C3H8 does not show any NO
X
reduction ability for NO
X
stored in Pt/BaO/Al2O3 at any of the temperatures. The formation of ammonia and nitrous oxide is also discussed. 相似文献
13.
Results of a thermodynamic and geometric-topological analysis of the subsolidus structure of the BaO – Al 2O 3 – Fe 2O 3 – SiO 2 system at temperatures below 1381 K are reported. Tetrahedration of the system, with due regard for the existing high-temperature phases, is carried out and a topological graph for the relationship of elementary tetrahedra in the system is constructed. 相似文献
14.
The influence of PbO, B 2O 3, and Al 2O 3 additives on the glass formation and crystallization of glasses with a high total content of BaO and TiO 2 (65–75 wt % or 76–86 mol %) is investigated. It is shown that glasses of the compositions (wt %) 31–35 BaO, 12–17 PbO, 34–42 TiO 2, 10–13 Al 2O 3, and 2–3 B 2O 3 are promising materials for use in preparing glass-ceramic ferroelectrics based on the melting–molding–crystallization technology. These compounds are characterized by a relatively low melting temperature (1450°C), the absence of spontaneous crystallization during molding, and the possibility of controlling the phase composition of the material through the appropriate choice of the crystallization temperature. 相似文献
15.
Pt/Al 2O 3 and Pt/BaO/Al 2O 3 catalysts (1 wt% Pt, 10 wt%BaO) were sulfated under conditions simulating a real NSR catalyst operation. Comparative TPR
and XPS studies of sulfur removal from Pt/Al 2O 3 and Pt/BaO/Al 2O 3 catalysts indicate that the sulfur removal from Al 2O 3 surface precedes reductive decomposition of BaSO 4 (250–400 °C). Barium sulfate decomposition started with further increase in desulfation temperature at the point of surface
atomic ratio Ba:S = 1 (~450 o). Simultaneously, an intensive formation of sulfide species on the catalyst surface was observed. Thermodynamic analysis
of the desulfation process allows us to hypothesize that barium sulfide formation may hinder sulfur removal under reducing
conditions. 相似文献
16.
The reduction of lean NOx using ethanol in simulated diesel engine exhaust was carried out over Ag/Al 2O 3 catalysts in the presence of H 2O and SO 2. The Ag/Al 2O 3 catalysts are highly active for the reduction of lean NOx by ethanol but the reaction is accompanied by side reactions to
form CH 3CHO, CO along with small amounts of hydrocarbons (C 3H 6, C 2H 4, C 2H 2 and CH 4) and nitrogen compounds such as NH 3 and N 2O. The presence of H 2O enhances the NOx reduction while SO 2 suppresses the reduction. The presence of SO 2 along with H 2O suppresses the formation of acetaldehyde and NH 3. By infrared spectroscopy, it was revealed that the reactivity of NCO species formed in the course of the reaction was greatly
enhanced in the presence of H 2O. The NCO species readily reacts with NO in the presence of O 2 and H 2O at room temperature, being converted to N 2 and CO 2 (CO). Addition of SO 2 suppresses the formation of NCO species and lowers the reactivity of the NCO species. However, the reduction of NOx is still
kept at high conversion levels in the presence of H 2O and SO 2 over the present catalysts. About 80% of NOx in the simulated diesel engine exhaust was removed at 743 K.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
SO x uptake, thermal regeneration and the reduction of SO x via H 2(g) over ceria-promoted NSR catalysts were investigated. Sulfur poisoning and desulfation pathways of the complex BaO/Pt/CeO 2/Al 2O 3 NSR system was investigated using a systematic approach where the functional sub-components such as Al 2O 3, CeO 2/Al 2O 3, BaO/Al 2O 3, BaO/CeO 2/Al 2O 3, and BaO/Pt/Al 2O 3 were studied in a comparative fashion. Incorporation of ceria significantly increases the S-uptake of Al 2O 3 and BaO/Al 2O 3 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/Al 2O 3 and BaO/Pt/CeO 2/Al 2O 3 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/CeO 2/Al 2O 3 surface. Thus, under these conditions, BaO/Pt/CeO 2/Al 2O 3 surface stores more sulfur than that of BaO/Pt/Al 2O 3. BaO/Pt/CeO 2/Al 2O 3 reveals a significantly improved thermal regeneration behavior in vacuum with respect to the conventional BaO/Pt/Al 2O 3 catalyst. Ceria promotion remarkably enhances the SO x reduction with H 2(g). 相似文献
18.
Compared to the Ag/Al 2O 3 catalyst, a two‐stage catalyst composed of an Ag/Al 2O 3 layer followed by a Sn/Al 2O 3 layer shows higher low‐temperature activity and a wider temperature window. Its activity below 350 °C is enhanced in the presence of SO 2. Even in the presence of H 2O and SO 2, the performance of the same catalytic system is still satisfactory. 相似文献
19.
The effect of V 2O 5 nucleant on crystallization of stoichiometric cordierite glass ceramics in the presence of various amounts of BaO and Al 2O 3 additives were investigated by DTA, XRD and SEM. It was shown that 3 wt.% V 2O 5 and 1.5 wt.% BaO were the optimum amounts of the additives effective in inducing both surface and bulk crystallization in the above glass ceramics.This resulted in ~90 wt.% cordierite after a 3 h heat treatment at 1020 °C. The specimens possessing 4–5 wt.% Al 2O 3 in excess of the stoichiometric cordierite composition, developed mullite along with cordierite in the temperature range of 1045–1055 °C, whereas in the specimen containing 6 wt.% excess Al 2O 3, mullite was detected as the sole crystallization product. 相似文献
20.
The results of theoretical calculations and experimental studies of the ternary system BaO – Al 2O 3 – SiO 2 in the subsolidus range at temperatures of 1200 – 1400°C are considered. Complete splitting of this system into elementary polytypes is performed, a topological graph of the relationship between these polytypes is shown, and geometrical characteristics of the binary and ternary compounds comprising this system are supplied. 相似文献
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