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1.
NOx adsorption/desorption capacities of barium aluminates and BaSnO3 were measured under representative exhaust gas mixture at temperatures below 550°C and compared to those of bulk BaO. The capacities are high and the test of sorption–desorption is reproducible on barium aluminate and BaSnO3, while this is not the case on BaO. The difference is due to the electronic environment of barium oxide. If BaO is not engaged in a chemical bond, progressive formation of high stability carbonates is observed. This is not the case with barium aluminate and BaSnO3, where carbonation does not take place because the competition between nitrate and carbonate formation is in favour of the nitrate due to its chemical nature. An N-bounded nitrate, with IR frequencies at 1360 and 1415 cm−1, is formed on barium aluminate and BaSnO3 and not on bulk BaO. 相似文献
2.
S. Hodjati P. Bernhardt C. Petit V. Pitchon A. Kiennemann 《Applied catalysis. B, Environmental》1998,19(3-4):209-219
NOx adsorption/desorption capacities of barium aluminates were measured under representative exhaust gas mixture at temperatures below 550°C. The solid doped with Pt or not, exhibits good NO2 sorption capacities with a reversible adsorption to desorption process. With bulk BaO, desorption was observed at high temperature. The different behaviour between the two catalysts is explained by the fact that strongly bonded carbonates are formed on bulk BaO while they do not exist on barium aluminate, therefore allowing the formation of nitrates which can be decomposed by a thermal process. SO2 poisoning was also studied. 相似文献
3.
Joël Despres Manfred Koebel Oliver Krcher Martin Elsener Alexander Wokaun 《Applied catalysis. B, Environmental》2003,43(4):389-395
The influence of NO on the adsorption and desorption of NO2 on BaO/TiO2 has been studied under lean conditions. The adsorption of NO2 involves the disproportionation of NO2 into an adsorbed nitrate species and NO released to the gas phase with a 3:1 ratio,Three different nitrate species form on the catalyst: surface nitrates on the TiO2 support, surface nitrates on BaO, and bulk barium nitrate. The stability of the three species in different gas feeds was investigated by temperature-programmed desorption (TPD).
BaO+3NO2→NO+Ba(NO3)2.
The reverse reaction of the NO2 disproportionation has also been observed. If NO is added to the feed, nitrates previously formed on the sorbent will decompose into NO2. Therefore, the above chemical equation should be considered as an equilibrium reaction. Applying this finding to the NOx storage and reduction catalyst means that NO probably reacts with the previously formed nitrates yielding NO2 as an intermediate product. This NO2 is subsequently reduced by the reducing agents (hydrocarbons and CO) present during the regeneration period. 相似文献
4.
Sanaa Elbouazzaoui Xavier Courtois Patrice Marecot Daniel Duprez 《Topics in Catalysis》2004,(1):493-496
The deactivation of a Pt/Ba/Al2O3 NO
x
-trap model catalyst submitted to SO2 treatment and/or thermal ageing at 800 °C was studied by H2 temperature programmed reduction (TPR), X-ray diffraction (XRD) and NO
x
storage capacity measurements.The X-ray diffractogram of the fresh sample exhibits peaks characteristic for barium carbonate. Thermal ageing leads to the decomposition of barium carbonate and to the formation of BaAl2O4. The TPR profile of the sulphated sample shows the presence of (i) surface aluminium sulphates, (ii) surface barium sulphates, (iii) bulk barium sulphates. The exposure to SO2 after ageing leads to a small decrease of the surface barium-based sulphates, expected mainly as aluminate barium sulphates. This evolution can be attributed to a sintering of the storage material. TPR experiments also show that thermal treatment at 800 °C after the exposure to SO2 involves the decomposition of aluminium surface sulphates to give mainly bulk barium sulphates, also pointed out by XRD. Thus, the thermal treatment at 800 °C leads to a stabilization of the sulphates.These results are in accordance with the NO
x
storage capacity measurements. On non-sulphated catalysts, the treatment at 800 °C induces to a decrease of the NO
x
storage capacity, showing that barium aluminate presents a lower NO
x
storage capacity than barium carbonate. Sulphation strongly decreases the NO
x
storage capacity of catalysts, whatever the initial thermal treatment, showing that barium sulphates inhibit the NO2 adsorption. Moreover, the platinum activity for the NO to NO2 oxidation is lowered by thermal treatments. 相似文献
5.
6.
Nature of Cation Vacancies Formed to Compensate Donors during Oxidation of Barium Titanate 总被引:2,自引:0,他引:2
Oxidative cooling is a critical step in the processing of barrier layer electroceramics based on BaTiO3 . While it has been proposed that barium vacancies are formed at the grain boundaries to compensate donors,1, 2 no direct evidence for this mechanism exists. On the other hand, literature data can be found to support the compensation of donors in the bulk by either barium or titanium vacancies. As a result the defect(s) formed at electrically active titanate grain boundaries during oxidation has remained uncertain. We explore this phenomenon by observing changes in the surface composition of donor-doped BaTiO3 when cation vacancies are introduced during oxidation, using SAES (scanning Auger electron spectroscopy). Direct experimental support for the formation and in-diffusion of barium vacancies during oxidative cooling is obtained in a composition containing 0.7% Nb. It is suggested that barium vacancy compensation constitutes a metastable defect equilibrium in BaTiO3 . In a sample of lower concentration (0.3% Nb), results are inconclusive, perhaps because of slower oxidation limited by surface reaction kinetics. 相似文献
7.
Do Heui Kim Ja Hun Kwak Jnos Szanyi Sarah D. Burton Charles H.F. Peden 《Applied catalysis. B, Environmental》2007,72(3-4):233-239
Phase changes in high temperature treated (>900 °C) 8 or 20 wt% BaO supported on γ-Al2O3 model lean NOx trap (LNT) catalysts, induced by NO2 and/or H2O adsorption, were investigated with powder X-ray diffraction (XRD), solid state 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and NO2 temperature programmed desorption (TPD) experiments. After calcination in dry air at 1000 °C, the XRD and solid state 27Al MAS NMR results confirm that stable surface BaO and bulk BaAl2O4 phases are formed for 8 and 20 wt% BaO/Al2O3, respectively. Following NO2 adsorption over these thermally treated samples, some evidence for nanosized Ba(NO3)2 particles are observed in the XRD results, although this may represent a minority phase. However, when water was added to the thermally aged samples after NO2 exposure, the formation of bulk crystalline Ba(NO3)2 particles was observed in both samples. Solid state 27Al MAS NMR is shown to be a good technique for identifying the various Al species present in the materials during the processes studied here. NO2 TPD results demonstrate a significant loss of uptake for the 20 wt% model catalysts upon thermal treatment. However, the described phase transformations upon subsequent water treatment gave rise to the partial recovery of NOx uptake, demonstrating that such a water treatment of thermally aged catalysts can provide a potential method to regenerate LNT materials. 相似文献
8.
Pradeep P. Phule Srini Raghavan Subhash H. Risbud 《Journal of the American Ceramic Society》1987,70(5):108-C
A discussion of the relative merits and problems of using Ba(OH)2 , BaO, and Ba as starting materials for the synthesis of barium titanate by the alkoxide method is presented. The Ba(OH)2 process is promising because it does not involve the synthesis of barium alkoxide, whereas the BaO (93% pure) process suffers from complex chemical reactions; the impurities in BaO cause unknown effects on the composition and properties of the final ceramic. Use of high-purity Ba metal (99.99%) for the synthesis of barium alkoxide is more desirable since it results in high-purity, ultrafine barium titanate powders. 相似文献
9.
10.
Takaaki Nagaoka Yuji Iwamoto Koichi Kikuta Shin-ichi Hirano 《Journal of the American Ceramic Society》2000,83(7):1613-1616
Two types of barium aluminate binders were prepared by heat treatment of barium aluminate precursors, synthesized by using solution processes, at low temperatures ( T < 500°C). One was barium monoaluminate (BaAl2 O4 ), and the other was barium aluminate binder (BAH binder) composed of amorphous phase, barium aluminate monohydrate (BaAl2 O4 ·H2 O), and BaAl2 O4 . The setting time of the BAH binder was controlled by adjusting the heat-treatment temperature of the BAH binder precursor. The addition of the synthesized BaAl2 O4 powders to Al2 O3 powders improved the bending strength of Al2 O3 matrix green bodies. The synthesized BaAl2 O4 powders led to the in situ forming of barium hexaaluminate (BaO· x Al2 O3 , x = 6.9: BA6) platelets in the matrix by reacting with Al2 O3 during sintering. The formed BA6 platelets inhibited the grain growth of the matrix Al2 O3 grains. 相似文献
11.
L. F. Liotta A. Macaluso G. E. Arena M. Livi G. Centi G. Deganello 《Catalysis Today》2002,75(1-4):439-449
The behaviour of a Pt(1 wt.%) supported on CeO2–ZrO2(20 wt.%)/Al2O3(64 wt.%)–BaO(16 wt.%) as a novel NOx storage–reduction catalyst is studied by reactivity tests and DRIFT experiments and compared with that of Pt(1%)–BaO(15 wt.%) on alumina. The former catalyst, designed as a hydrothermally stable sample, is composed of an alumina modified with Ba ions and an overlayer of ceria-zirconia. The results pointed out that during the calcination barium ions migrates over the surface of the catalyst which thus show a good NOx storage–reduction behaviour comparable with that of Pt–BaO on alumina, although Ba ions result much better dispersed. 相似文献
12.
The effect of Nb2O5 as an additive to MgO catalyst for vapor phase hydrogen transfer reaction between methacrolein and ethanol to form methallyl alcohol and acetaldehyde has been studied. Nb2O5 itself was not effective for this reaction, but when Nb2O5 was added to MgO, the catalytic performance was enhanced. This result suggests that the preferable active sites on catalyst are increased by combination of Nb2O5 and MgO and so the catalytic performance is improved. On the other hand, when BaO or alkali metal oxide was added to Nb2O5, the catalytic performance became higher than that of Nb2O5 alone. This result suggests that the preferable active site is formed newly by combination of Nb2O5 and BaO or alkali metal oxide. 相似文献
13.
F. Laurent 《Chemical engineering science》2003,58(9):1793-1803
Modelling of the phenomena involved during the adsorption of NOx on NOx trap catalysts was developed. The aim of the model is the prediction of the quantity of stocked barium nitrate as well as the emissions of NO and NO2, as a function of time and temperature. The mechanism of the process is sounded on the adsorption of gas species (NO, NO2, O2) on platinum sites, equilibrium reaction between adsorbed species followed by the formation of Ba(NO3)2. This formation of barium nitrate is limited by the thermal decomposition reaction which liberates NO in the gas phase. The kinetic constant of decomposition of barium nitrate was determined by temperature programmed thermogravimetry on pure Ba(NO3)2, using the method of Freeman and Carroll. Other kinetic constants bound to the mechanism were estimated by fitting the results of the model to experimental results.The mechanism was validated for various values of the molar fraction of O2, the molar fraction of NO and various values of the NO/NO2 ratio in the gas entering the reactor. It was also tested with different catalyst compositions (variation of the platinum and BaO concentrations). The importance of oxygen in the process was clearly demonstrated as well as the promoting role of NO2. 相似文献
14.
Chung-Kook Lee Yolande Berta Robert F. Speyer 《Journal of the American Ceramic Society》1996,79(1):183-192
A glass crystallization method was utilized to synthesize nanosized BaO-6Fe2 O3 platelets from a 0.412BaO-0.258B2 O3 -0.330Fe2 O3 batch composition. Quenched ribbons were inhomogeneous, showing microclustering and ∼1 μm hematite crystals. Na2 O substitutions for BaO greatly enhanced the glass-forming tendency of quenched ribbons, though quenched-in ∼0.5 μm barium ferrite crystals were infrequently present. The improved homogeneity with Na2 O substitution was attributed to lower vapor pressure of BaO during batch melting, which increased its retention in the as-quenched ribbons. Quantities of BaO equal to or in excess of Fe2 O3 allowed iron ions to adopt stable network positions in the glass melt. With Na2 O substitution, devitrification of dispersed ∼40 nm barium ferrite particles from phase-separated regions occurred after secondary heat treatment. 5 mol% Na2 O batch substitution showed the lowest crystallinity in the as-quenched ribbons, and the highest crystallinity after secondary heat treatment. After optimum devitrification, the maximum values of saturation magnetization and coercivity were 21.22 emu/g and 2.82 kOe, respectively. 相似文献
15.
ROBERT H. THOMAS 《Journal of the American Ceramic Society》1950,33(2):35-44
Phase-equilibrium relations on the liquidus surface in the system Ba0-A12 O3 -SiO2 have been investigated by the quenching method. The compositions investigated within the ternary area were those containing less than 30%, A12 O3 and more than 20% SiO2 by weight. Petrographic and X-ray techniques were employed in the determination of the crystalline phases.
The crystal phases that separate from melts within the area investigated are barium orthosilicate (2BaO. SiO2 ,), barium metasilicate (BaO 2SO2 ,), solid solutions, sanbornite (BaO 2SiO2 ), tridymite and cristobalite (SO2 ), mullite (3A12 O3 2SiO2 ), and celsian (BaO A12 O3. 2SiO2 ). Diagrams show the isotherms and indices of refraction of the glasses.
Five quintuple points and eleven boundary curves have been determined within = .5yo compositional variations. The liquidus-surface temperatures have been obtained within limits of ± 125°C. 相似文献
The crystal phases that separate from melts within the area investigated are barium orthosilicate (2BaO. SiO
Five quintuple points and eleven boundary curves have been determined within = .5yo compositional variations. The liquidus-surface temperatures have been obtained within limits of ± 125°C. 相似文献
16.
Comparison of barium peroxide, Ba(OH)2 and Ba(NO3)2 as the precursor of BaO for the preparation of NO x -storage BaO/Al2O3 material was carried out. The as prepared materials were calcined at 550 and 800 °C and characterized by N2 physisorption, XRD, Raman and FT-IR spectroscopy. Measurements of the NO x storage performances of these BaO/Al2O3 materials by NO2 adsorption and NO x -TPD experiments showed that the use of barium peroxide as the precursor of BaO inhibited the formation of BaAl2O4 and led to remarkable improvements in the thermal stability as well as NO x storage capacity of the final BaO/Al2O3 material calcined at 800 °C. 相似文献
17.
The solubility and mode of incorporation for BaO in BaTiO3 were studied by X-ray powder diffraction, scanning and transmission electron microscopy, electron probe microanalysis, and equilibrium electrical conductivity measurements. The presence of barium orthotitanate, Ba2 TiO4 , as a second phase for samples containing >0.1 mol% excess BaO was confirmed by direct microscopic examination. There was no evidence to support the incorporation of excess BaO into BaTiO3 by a Ruddlesden-Popper type of superlattice ordering mechanism. Measurement of the equilibrium electrical conductivity showed no detectable shift in the conductivity profile resulting from excess BaO, thus setting an upper limit of 100 ppm for the solubility of BaO in BaTiO3 . 相似文献
18.
Makoto Kambara Takateru Umeda Minoru Tagami Xin Yao Eugene A. Goodilin Yuh Shiohara 《Journal of the American Ceramic Society》1998,81(8):2116-2124
Quasi-ternary phase diagrams of the NdO1.5 -BaO-CuO x system near the CuO x corner have been constructed near the peritectic temperature in air of the Nd1+ x Ba2− x Cu3 O6+delta (Nd123) solid-solution phase. Liquidus curves were determined by measuring the temperature dependences of the neodymium, barium, and copper solubilities in Nd-Ba-Cu-O solutions with different BaO:CuO ratios. Solidus line compositions and equilibrium tie lines were determined by analyzing the compositions of the Nd123 solid solution equilibrated with the melt by quenching samples held isothermally. Based on the tie-line features in the Nd123-and- liquid two-phase field, the Nd123 solid solution with the smaller substitution content was observed to be equilibrated with the solution melt with a higher BaO:CuO ratio, even in an air atmosphere. Nd123 crystal with a substitution content of ∼0.02 could be formed from the solution with the BaO:CuO ratio of greaterthan equal to0.75, which resulted in higher critical superconducting transition temperatures. 相似文献
19.
20.
J. E. SHELBY 《Journal of the American Ceramic Society》1984,67(8):557-560
The density, thermal expansion behavior, and transformation-range viscosity of barium germanate glasses containing 0 to 9 mol% BaO were measured. The addition of BaO to Ge02 resulted in phase separation at all BaO concentrations. The results of the property measurements indicate that glasses containing up to 3 mol% BaO consist of a matrix of essentially pure GeO2 with barium-rich droplets. Glasses containing 6 mol% BaO or more consist of two interconnected vitreous phases. The morphology of glasses containing between approximately 3.5 and 5.5 mol% BaO can be altered from the droplet/matrix form to that of two continuous phases by the appropriate heat treatment. 相似文献