共查询到20条相似文献,搜索用时 31 毫秒
1.
Zhendong Zhang Xiaoxia Yan Bozhi Tian Chengzhong Yu Bo Tu Guangshan Zhu Shilun Qiu Dongyuan Zhao 《Microporous and mesoporous materials》2006,90(1-3):23-31
Ordered mesoporous silicates with tailorable pore structures and small pore sizes have been synthesized by using polyoxyethylene alkyl amine surfactant PN-430 [CH3(CH2)17N(EO)x(EO)y, x + y = 5] as a structure-directing agent under acidic condition. Two-dimensional (2-D) hexagonal (p6mm) mesoporous silicates have been prepared via an evaporation-induced self-assembly (EISA) process. The N2 sorption isotherms show that the product has a small uniform pore size distribution of 1.8 nm by BJH model, a BET surface area of 730 m2/g and a pore volume of 0.36 cm3/g. 3-D cubic (Pm-3n) mesoporous silicate with small uniform pore size (1.76 nm) can also be prepared at high concentration of PN-430 by EISA method in tetrahydrofuran solvent. The solvothermal post-treatment by n-hexane at 70 °C for 3 d to the above material results in the phase transition of the mesostructure from Pm-3n to P63/mmc based on XRD and TEM analyses. In comparison, by using nonionic oligometric alkyl-ethylene oxide surfactant such as Brij 78 (C18H37EO20) or Triton X-100 (CH3C(CH3)2CH2C(CH3)2C6H4 EO10) as co-templates, high-quality hexagonal (p6mm) small pore mesoporous silicates have also been prepared in ethanol media. Our results show that the blend templates composed of PN-430 and a small amount of nonionic surfactant can increase the efficiency of organic and inorganic hybrid species assembly, improve the quality of the structural regularity, and decrease the pore size to about 1.65 nm. 相似文献
2.
M. Richter M. Langpape S. Kolf G. Grubert R. Eckelt J. Radnik M. Schneider M. -M. Pohl R. Fricke 《Applied catalysis. B, Environmental》2002,36(4):261-277
A quaternary catalyst library of 56 samples comprising all combinations of four elements, viz. Ag, Co, Cu, In, with six equally spaced atomic fraction increments from 0 to 1 was prepared by impregnation of a proprietary mesoporous alumina support. Catalytic properties of the library were tested in the selective catalytic reduction (SCR) of NOx by propane under lean conditions in the temperature range 400–500 °C. The catalytic data acquired by a parallel 64-channel microreactor system with automated time-of-flight mass spectrometric analysis have been evaluated regarding selectivity–compositional relationships, synergistic effects for NOx conversion, and efficiency of propane utilization. Full conversion of NOx is achieved over Ag–Co combinations at 450 °C with N2 selectivities of more than 90% and reductant utilization of 20% in a feed of 1500 ppm NO, 1500 ppm propane and 5 vol.% O2 (space velocity of 36,000 cm3 gcat−1 h−1). For the single-component catalysts Ag/Al2O3, Co/Al2O3, Cu/Al2O3, and In/Al2O3, the state of the elements on the mesoporous alumina was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Cobalt forms a spinel-like cobalt aluminate phase whereas copper and indium are present as oxides with small sizes not detectable by XRD. Silver occurs in both metallic state and as Ag2O, and forms Agn clusters of at least two different sizes, predominantly with diameters of about 30 nm. The conclusions are consistent with the reducibility of the single-component catalysts samples by H2. Surface area measurements and pore size distributions revealed reasonable modifications of the textural properties. The main pore size of the alumina support is decreased from 7 to ca. 5 nm after loading of the active components. 相似文献
3.
Jiaguo Yu Guohong Wang Bei Cheng Minghua Zhou 《Applied catalysis. B, Environmental》2007,69(3-4):171-180
Bimodal nanocrystalline mesoporous TiO2 powders with high photocatalytic activity were prepared by a hydrothermal method using tetrabutylorthotitanate (TiO(C4H9)4, TBOT) as precursor. The as-prepared TiO2 powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and N2 adsorption–desorption measurements. The photocatalytic activity of the as-prepared TiO2 powders was evaluated by the photocatalytic degradation of acetone (CH3COCH3) under UV-light irradiation at room temperature in air. The effects of hydrothermal temperature and time on the microstructures and photocatalytic activity of the TiO2 powders were investigated and discussed. It was found that hydrothermal treatment enhanced the phase transformation of the TiO2 powders from amorphous to anatase and crystallization of anatase. All TiO2 powders after hydrothermal treatment showed bimodal pore-size distributions in the mesoporous region: one was intra-aggregated pores with maximum pore diameters of ca. 4–8 nm and the other with inter-aggregated pores with maximum pore diameters of ca. 45–50 nm. With increasing hydrothermal temperature and time, the average crystallite size and average pore size increased, in contrast, the Brunauer-Emmett-Teller (BET) specific surface areas, pore volumes and porosity steadily decreased. An optimal hydrothermal condition (180 °C for 10 h) was determined. The photocatalytic activity of the prepared TiO2 powders under optimal hydrothermal conditions was more than three times higher than that of Degussa P25. 相似文献
4.
The sintering behavior and dielectric properties of the monoclinic zirconolite-like structure compound Bi2(Zn1/3Nb2/3)2O7 (BZN) and Bi2(Zn1/3Nb2/3−xVx)2O7 (BZNV, x = 0.001) sintered under air and N2 atmosphere were investigated. The pure phase were obtained between 810 and 990 °C both for BZN and BZNV ceramics. The substitution of V2O5 and N2 atmosphere accelerated the densification of ceramics slightly. The influences on microwave dielectric properties from different atmosphere were discussed in this work. The best microwave properties of BZN ceramics were obtained at 900 °C under N2 atmosphere with r = 76.1, Q = 850 and Qf = 3260 GHz while the best properties of BZNV ceramics were got at 930 °C under air atmosphere with r = 76.7, Q = 890 and Qf = 3580 GHz. The temperature coefficient of resonant frequency τf was not obviously influenced by the different atmospheres. For BZN ceramics the τf was −79.8 ppm/°C while τf is −87.5 ppm/°C for BZNV ceramics. 相似文献
5.
Three inorganic adsorbents were applied as templates to produce porous carbons from polystyrene-based organic polymers. As matrices, amorphous silica gel, mesoporous alumina and microporous zeolite 13X were used. Organic precursors were polystyrene sulfonic acid (co-maleic acid) sodium salt and polystyrene co-maleic acid isobutyl/methyl mixed ester. The impregnated templates were carbonized at 800 °C. After removal of inorganic matrices porous carbons were obtained. Materials were characterized by adsorption of nitrogen, thermal analysis, potentiometric titration and SEM. Owing to the template carbonization, highly mesoporous carbons were obtained (SBET up to 1500 m2/g, Vt up to 3 cm3/g) with majority of pores with sizes between 20–200 Å. Although the carbons were not replicas of their matrices, the carbonization within the confined space with utilization of self-released pore formers resulted in unique carbonaceous materials with very acidic surface. That acidity is linked to either exothermic effect of sodium reactivity with moist air or susceptibility for air oxidation of small graphene layers formed in the confined pore space. 相似文献
6.
Leonid V. Gorobinskii Gleb Yu. Yurkov Dmitry A. Baranov 《Microporous and mesoporous materials》2007,100(1-3):134-138
Cerium oxide nanoparticles modified montmorillonite was obtained by interaction of a clay with (NH4)2Ce(NO3)6. The mean size of cerium oxide nanoparticles in clay was at 3.5 nm. The product was an amorphous solid and showed high permanent porosity and stability at high temperatures. The amorphous structure of the sample was proven by X-ray diffraction and electronic diffraction. The porous structure was studied by means of chemisorption and it was shown that samples calcined at 550 °C had SBET = 239 m2/g; micropore volume = 0.1839 cm3/g; average pore diameter = 3.07 nm. 相似文献
7.
We have investigated the regeneration of a nitrated or sulphated model Pt/Ba-based NOx trap catalyst using different reductants. H2 was found to be more effective at regenerating the NOx storage activity especially at lower temperature, but more importantly over the entire temperature window after catalyst ageing. When the model NOx storage catalyst is sulphated in SO2 under lean conditions at 650 °C almost complete deactivation can be seen. Complete regeneration was not achieved, even under rich conditions at 800 °C in 10% H2/He. Barium sulphate formed after the high temperature ageing was partly converted to barium sulphide on reduction. However, if the H2 reduced sample was exposed to a rich condition in a gas mixture containing CO2 at 650 °C, the storage activity can be recovered. Under these rich conditions the S2− species becomes less stable than the CO32−, which is active for storing NOx. Samples which were lean aged in air containing 60 ppm SO2 at <600 °C, after regeneration at λ=0.95 at 650 °C, have a similar activity window to a fresh catalyst. It is, therefore, important that CO2 is present during the rich regenerations of the sulphated model samples (as of course it would be under real conditions), as suppression of carbonate formation can lead to sulphide formation which is inactive for NOx storage. 相似文献
8.
Preparation of three-dimensional ordered macroporous SiCN ceramic using sacrificing template method 总被引:4,自引:0,他引:4
Hao Wang Shuang-yang Zheng Xiao-dong Li Dong-pyo Kim 《Microporous and mesoporous materials》2005,80(1-3):357-362
Three-dimensional (3D) long range well ordered macroporous SiCN ceramics were prepared by infiltrating sacrificial colloidal silica templates with the low molecular weight preceramic polymer, polysilazane. This was followed by a thermal curing step, pyrolysis at 1250 °C in a N2 atmosphere, and finally the removal of the templates by etching with dilute HF. The produced macroporous SiCN ceramics showed high BET surface areas (pore volume) in the range 455 m2/g (0.31 cm3/g)–250 m2/g (0.16 cm3/g) with the pore sizes of 98–578 nm, which could be tailored by controlling the sizes of the sacrificial silica spheres in the range 112–650 nm. The sphere-inversed macropores were interconnected by 50 ± 30 nm windows and 3–5 nm mesopores embedded in the porous SiCN ceramic frameworks, which resulted in a trimodal pore size distribution. The surface of the achieved porous SiCN ceramic was then modified by Pt–Ru nanoparticle depositing under mild chemical conditions. 相似文献
9.
Paul Inge Dahl Reidar Haugsrud Hilde Lea Lein Tor Grande Truls Norby Mari-Ann Einarsrud 《Journal of the European Ceramic Society》2007,27(16):4461-4471
Powders of pure and 5% ytterbium substituted strontium cerate (SrCeO3/SrCe0.95Yb0.05O3−δ) were prepared by spray pyrolysis of nitrate salt solutions. The powders were single phase after calcination in nitrogen atmosphere at 1100 °C (SrCeO3) and 1200 °C (SrCe0.95Yb0.05O3−δ). Dense SrCeO3 and SrCe0.95Yb0.05O3−δ materials were obtained by sintering at 1350–1400 °C in air. Heat treatment at 850 and 1000 °C, respectively, was necessary prior to sintering to obtain high density. The dense materials had homogenous microstructures with grain size in the range 6–10 μm for SrCeO3 and 1–2 μm for SrCe0.95Yb0.05O3−δ. The electrical conductivity of SrCe0.95Yb0.05O3−δ was in good agreement with reported data, showing mixed ionic–electronic conduction. The ionic contribution was dominated by protons below 1000 °C and the proton conductivity reached a maximum of 0.005 S/cm above 900 °C. In oxidizing atmosphere the p-type electronic conduction was dominating above 700 °C, while the contribution from n-type electronic conduction only was significant above 1000 °C in reducing atmosphere. 相似文献
10.
Toshihide Horikawa Masahiro Katoh Tahei Tomida 《Microporous and mesoporous materials》2008,110(2-3):397-404
Nitrogen-doped mesoporous titania aerogel photocatalysts were prepared by supercritical drying technique with carbon dioxide (SCCO2) and calcination the urea impregnated TiO2 aerogel at 773 K under NH3 or N2 + NH3aq gaseous atmosphere. The pore properties were investigated from nitrogen adsorption measurement at 77 K. The prepared N-doped TiO2 aerogel had a high specific surface area (116 m2/g), a total pore volume (0.33 cm3/g) and a sharp pore radius distribution (rpeak = 4.2 nm). The doping of the nitrogen atom into the TiO2 lattice is expected from X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), and UV–Vis spectroscopy. Nitrogen states in the lattice and crystalline structure were measured using X-ray photoelectron spectroscopy and X-ray diffractometry. The N-doped TiO2 aerogel absorbed well into the visible region up to 600 nm. 相似文献
11.
Sandra Dir Riccardo Ceccato Stefano Gialanella Florence Babonneau 《Journal of the European Ceramic Society》1999,19(16):1090-2858
Polydimethylsiloxane–zirconia nanocomposites have been prepared by hydrolysis of diethoxydimethylsilane and zirconium n-propoxide in different molar ratios. Transparent, homogeneous and non-porous xerogels have been obtained up to 70 mol% ZrO2 content. The starting xerogels have been pyrolyzed under argon atmosphere up to 1400°C and the structural evolution of samples treated at different temperatures has been followed by X-ray diffraction, transmission electron microscopy, infrared and 29Si solid state nuclear magnetic resonance spectroscopies, thermal analyses and N2 sorption measurements. The polymer-to-ceramic conversion leads to the structural rearrangement of the siloxane component with the production at 600°C of high surface area materials with pore sizes below 3 nm. Samples are amorphous up to 800°C. At 1000°C, the structural evolution of the silicon moiety produces an amorphous oxycarbide phase whereas the primary crystallisation of tetragonal zirconia takes place, with crystallinity and crystallite sizes depending on the ZrO2 content. At 1400°C, the silicon oxycarbide phase generates a mixture of amorphous silica and crystalline silicon carbide polymorphs. In this matrix, tetragonal and monoclinic ZrO2 phases are present with ZrO2 average crystallite dimensions never exceeding 20 nm, for ZrO2 content ≤50 mol%. The tetragonal/monoclinic ratio as well as the crystallite sizes appear strictly related to the chemical composition. © 相似文献
12.
Shihong Xu Wenfeng Shangguan Jian Yuan Mingxia Chen Jianwei Shi 《Applied catalysis. B, Environmental》2007,71(3-4):177-184
A magnetically separable nitrogen-doped photocatalyst TiO2−xNx/SiO2/NiFe2O4 (TSN) with a typical ferromagnetic hysteresis was prepared by a simple process: the magnetic SiO2/NiFe2O4 (SN) dispersion prepared by a liquid catalytic phase transformation method and the visible-light-active photocatalyst TiO2−xNx were mixed, sonificated, dried, and calcined at 400 °C. The prepared photocatalyst is photoactive under visible light irradiation and easy to be separated from a slurry-type photoreactor under the application of an external magnetic field, being one of promising photocatalysts for wastewater treatment. Transmission electron microscope (TEM) and X-ray diffractometer (XRD) were used to characterize the structure of the TSN photocatalyst. The results indicate that the magnetic SiO2/NiFe2O4 (SN) nanoparticles adhere to the surface of TiO2−xNx congeries. The magnetic photocatalyst TSN shows high catalytic activity for the degradation of methyl orange in water under UV and visible light irradiation (λ > 400 nm). SiO2 coating round the surface of NiFe2O4 nanoparticles prevents effectively the injection of charges from TiO2 particles to NiFe2O4, which gives rise to the increase in photocatalytic activity. Moreover, the recycled TSN exhibits a good repeatability of the photocatalytic activity. 相似文献
13.
Enhanced high-potential and elevated-temperature cycling stability of LiMn2O4 cathode by TiO2 modification for Li-ion battery 总被引:1,自引:0,他引:1
The surface of spinel LiMn2O4 was modified with TiO2 by a simple sol–gel method to improve its electrochemical performance at elevated temperatures and higher working potentials. Compared with pristine LiMn2O4, surface-modification improved the cycling stability of the material. The capacity retention of TiO2-modified LiMn2O4 was more than 85% after 60 cycles at high potential cycles between 3.0 and 4.8 V at room temperature and near to 90% after 30 cycles at elevated temperature of 55 °C at 1C charge–discharge rate. SEM studies shows that the surface morphology of TiO2-modified LiMn2O4 was different from that of pristine LiMn2O4. Powder X-ray diffraction indicated that spinel was the only detected phase in TiO2-modified LiMn2O4. Introduction of Ti into LiMn2O4 changed the electronic structures of the particle surface. Therefore a surface solid compound of LiTixMn2−xO4 may be formed on LiMn2O4. The improved electrochemical performance of surface-modified LiMn2O4 was attributed to the improved stability of crystalline structure and the higher Li+ conductivity. 相似文献
14.
Hong Zheng Ian M. Reaney Duncan Muir Tim Price David M. Iddles 《Journal of the European Ceramic Society》2007,27(16):4479-4487
Ten weight percent BBZS (Bi2O3, B2O3, ZnO and SiO2) glass was added to x(Ba4Nd9.333Ti18O54) − (1 − x)(BaLa4Ti4O15) (BNLT, 0 ≤ x ≤ 1) composite dielectric ceramics to lower their sintering temperature whilst retaining microwave properties useful for low temperature co-fired ceramic and antenna core technology. With the addition of 10 wt% BBZS glass, dense BNLT composite ceramics were produced at temperatures between 950 and 1140 °C, depending on composition (x), an average reduction of sintering temperature by 350 °C. X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy studies revealed that there was limited inter-reaction between BLT/BNT and the BBZS glass. Microwave property measurement showed that the addition of BBZS glass to BNLT ceramics had a negligible effect on r and τf, although deterioration in the measured quality factor (Qf) was observed. The optimised composition (xBNT − (1 − x)BLT)/0.1BBZS (x = 0.75) had r 61, τf 38 ppm/°C and Qf 2305 GHz. 相似文献
15.
Krisztina Lszl Gbor Dobos Gyrgy Onyestyk Erik Geissler 《Microporous and mesoporous materials》2007,100(1-3):103-110
Carbon–silica molecular sieves were prepared by carbonization of Scotch fir (Pinus sylvestris) after impregnation with aqueous waterglass (NaxSiyOz, where x, y and z may take a range of values). Compared to Si-free samples, doping significantly modifies the structure that forms during the carbonization process. For carbonization temperatures between 600 °C and 1000 °C, doped samples shrink less than undoped samples, indicating increased mechanical strength. The specific surface area and pore volume develop in a combined self-activation and chemical vapour deposition (CVD) process. Nevertheless, the presence of the sodium silicates limits self-activation and thus reduces the porosity. Doping drastically reduces the specific surface area, measured both by gas adsorption and small angle X-ray scattering. The latter technique demonstrates that in both doped and undoped samples the specific surface area is isotropic. X-ray photoelectron spectroscopy (XPS) reveals that the spatial distribution of Na and Si atoms within the samples are not identical. The open honeycomb structure, conserved during the heat treatment from the original wood, provides easy access for gas adsorption and separation applications. The ratios of the microporous diffusion time constants of N2 and O2 from frequency response (FR) measurements gave separation factors 3.0, 4.3, 2.7 and 1.3 for samples prepared at 600 °C, 700 °C, 800 °C and 900 °C, respectively. 相似文献
16.
Mari Uenishi Masashi Taniguchi Hirohisa Tanaka Mareo Kimura Yasuo Nishihata Junichiro Mizuki Tetsuhiko Kobayashi 《Applied catalysis. B, Environmental》2005,57(4):267-273
In order to elucidate the superior start-up activity of LaFePdOx catalysts in practical automotive emission control, the redox property of Pd species in a Perovskite-type LaFe0.95Pd0.05O3 catalyst was studied at temperatures ranging from 100 to 400 °C using X-ray spectroscopic techniques. In a reductive atmosphere, and even at temperatures as low as 100 °C, Pd0 species is partially segregated out onto the catalyst surface from the B-site of the Perovskite-type matrix of LaFe0.95Pd0.05O3. Passing through successive oxidizing atmospheres, the segregated Pd0 species is re-oxidized into Pd2+ at 200–300 °C. The formation of a solid solution between the re-oxidized Pd species and the Perovskite-type matrix begins to be seen at around 400 °C and accelerates at higher temperatures. Thus a quasi-reversible redox reaction between the surface Pd0 and the cationic Pd in the LaFe0.95Pd0.05O3 matrix takes place. The start-up activity of LaFePdxOx catalysts can be attributed to Pd0 that segregates under the reductive atmosphere which is a natural part of the redox fluctuation in automotive exhaust gases at 100–200 °C. 相似文献
17.
A 5 wt% CoOx/TiO2 catalyst has been used to study the effect of calcination temperature on the activity of this catalyst for CO oxidation at 100 °C under a net oxidizing condition in a continuous flow type fixed-bed reactor system, and the catalyst samples have been characterized using TPD, XPS and XRD measurements. The catalyst after calcination at 450 °C gave highest activity for this low-temperature CO oxidation, and XPS measurements yielded that a 780.2-eV Co 2p3/2 main peak appeared with this catalyst sample and this binding energy was similar to that measured with pure Co3O4. After calcination at 570 °C, the catalyst, which had possessed practically no activity in the oxidation reaction, gave a Co 2p3/2 main structure peak at 781.3 eV which was very similar to those obtained for synthesized ConTiOn+2 compounds (CoTiO3 and Co2TiO4), and this catalyst sample had relatively negligible CO chemisorption as observed by TPD spectra. XRD peaks indicating only the formation of Co3O4 particles on titania surface were developed in the catalyst samples after calcination at temperatures ≥350 °C. Based on these characterization results, five types of Co species could be modeled to exist with the catalyst calcined at different temperatures. Among these surface Co species, the Type A clean Co3O4 particles were predominant on a sample of the catalyst after calcination at 450 °C and highly active for CO oxidation at 100 °C, and the calcination at 570 °C gave the Type B Co3O4 particles with complete ConTiOn+2 overlayers inactive for this oxidation reaction. 相似文献
18.
Yun Guo Guanzhong Lu Zhigang Zhang Shunhai Zhang Yan Qi Yi Liu 《Catalysis Today》2007,126(3-4):296-302
Nanoparticles of CexZr1−xO2 (x = 0.75, 0.62) were prepared by the oxidation-coprecipitation method using H2O2 as an oxidant, and characterized by N2 adsorption, XRD and H2-TPR. CexZr1−xO2 prepared had single fluorite cubic structure, good thermal stability and reduction property. With the increasing of Ce/Zr ratio, the surface area of CexZr1−xO2 increased, but thermal stability of CexZr1−xO2 decreased. The surface area of Ce0.62Zr0.38O2 was 41.2 m2/g after calcination in air at 900 °C for 6 h. TPR results showed the formation of solid solution promoted the reduction of CeO2, and the reduction properties of CexZr1−xO2 were enhanced by the cycle of TPR-reoxidation. The Pd-only three-way catalysts (TWC) were prepared by the impregnation method, in which Ce0.75Zr0.25O2 was used as the active washcoat and Pd loading was 0.7 g/L. In the test of Air/Fuel, the conversion of C3H8 was close to 100% and NO was completely converted at λ < 1.025. The high conversion of C3H8 was induced by the steam reform and dissociation adsorption reaction of C3H8. Pd-only catalyst using Ce0.75Zr0.25O2 as active washcoat showed high light off activity, the reaction temperatures (T50) of 50% conversion of CO, C3H8 and NO were 180, 200 and 205 °C, respectively. However, the conversions of C3H8 and NO showed oscillation with continuously increasing the reaction temperature. The presence of La2O3 in washcoat decreased the light off activity and suppressed the oscillation of C3H8 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 La2O3 in active washcoat can enhance the thermal stability of catalyst significantly. 相似文献
19.
Performance of NOx traps after high-temperature treatments in different redox environments was studied. Two types of treatments were considered: aging and pretreatment. Lean and rich agings were examined for a model NOx trap, Pt–Ba/Al2O3. These were done at 950 °C for 3 h, in air and in 1% H2/N2, respectively. Lean aging had a severe impact on NOx trap performance, including HC and CO oxidation, and NH3 and N2O formation. Rich aging had minimal impact on performance, compared to fresh/degreened performance. Deactivation from lean aging was essentially irreversible due to Pt sintering, but Pt remained dispersed with the rich aging. Pretreatments were examined for a commercially feasible fully formulated NOx trap and two model NOx traps, Pt–Ba/Al2O3 and Pt–Ba–Ce/Al2O3. Pretreatments were done at 600 °C for 10 min, and used feed gas that simulated diesel exhaust under several conditions. Lean pretreatment severely suppressed NOx, HC, CO, NH3 and N2O activities for the ceria-containing NOx traps, but had no impact on Pt–Ba/Al2O3. Subsequently, a relatively mild rich pretreatment reversed this deactivation, which appears to be due to a form of Pt–ceria interaction, an effect that is well known from early work on three-way catalysts. Practical applications of results of this work are discussed with respect to NOx traps for light-duty diesel vehicles. 相似文献
20.
A small single-cylinder diesel engine is used to thermally age model (Pt + Rh/Ba/γ-Al2O3) lean NOx traps (LNTs) under lean/rich cycling at target temperatures of 600 °C, 700 °C, and 800 °C. During an aging cycle, fuel is injected into the exhaust to achieve reproducible exotherms under lean and rich conditions with the average temperature approximating the target temperature. Aging is performed until the cycle-average NOx conversion measured at 400 °C is approximately constant. Engine-based NOx conversion decreased by 42% after 60 cycles at 600 °C, 36% after 76 cycles at 700 °C and 57% after 46 cycles at 800 °C. The catalyst samples were removed and characterized by XRD and using a microreactor that allowed controlled measurements of surface area, precious metal size, NOx storage, and reaction rates. Three aging mechanisms responsible for the deactivation of LNTs have been identified: (i) loss of dispersion of the precious metals, (ii) phase transitions in the washcoat materials, and (iii) loss of surface area of the storage component and support. These three mechanisms are accelerated when the aging temperature exceeds 850 °C—the γ to δ transition temperature of Al2O3. Normalization of rates of NO reacted at 400 °C to total surface area demonstrates the biggest impact on performance stems from surface area losses rather than from precious metal sintering. 相似文献