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1.
Performance of NO x 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 NO x trap, Pt–Ba/Al 2O 3. These were done at 950 °C for 3 h, in air and in 1% H 2/N 2, respectively. Lean aging had a severe impact on NO x trap performance, including HC and CO oxidation, and NH 3 and N 2O 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 NO x trap and two model NO x traps, Pt–Ba/Al 2O 3 and Pt–Ba–Ce/Al 2O 3. Pretreatments were done at 600 °C for 10 min, and used feed gas that simulated diesel exhaust under several conditions. Lean pretreatment severely suppressed NO x, HC, CO, NH 3 and N 2O activities for the ceria-containing NO x traps, but had no impact on Pt–Ba/Al 2O 3. 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 NO x traps for light-duty diesel vehicles. 相似文献
2.
The NO x storage and reduction functions of a Pt–Ba/Al 2O 3 “NO x storage–reduction” catalyst has been investigated in the present work by applying the transient response and the temperature programmed reaction methods, by using propylene as the reducing agent. It is found that: (i) the storage of NO x occurs first at BaO and then at BaCO 3, which are the most abundant sites following regeneration of catalyst with propylene; (ii) the overall storage process at BaCO 3 is slower than at BaO; (iii) CO 2 inhibits the NO x storage at low temperatures; (iv) the amount of NO x stored up to catalyst saturation at 350 °C corresponds to 17.6% of Ba; (v) the reduction of stored NO x groups is fast and is limited by the concentration of propylene in the investigated T range (250–400 °C); (vi) selectivity to N 2 is almost complete at 400 °C but is significantly lower at 300 °C due to the formation of NO which can be tentatively ascribed to the presence of unselective Pt–O species. 相似文献
3.
A Pt–Re/Al 2O 3 reforming catalyst with different levels of chlorine content prior to reduction has been studied by various techniques such as combined STEM/EDX, TPR, H 2 chemisorption and model reactions in order to investigate the effect of the chlorine content on the bimetallic particle formation. TPR, H 2 chemisorption and model reactions show that chlorine inhibits the formation of bimetallic particles in the Pt–Re/Al 2O 3 catalyst. The effect of chlorine is, however, limited. Direct measurements by STEM/EDX analysis could not reveal any significant differences in alloy formation by varying the chlorine content from 0.6 to 1.5 wt.%. In comparison, the effect of adding water during reduction has a greater impact on the final state of the metal particles. 相似文献
4.
The effect of Al 2O 3 levels on the properties of NiO in coprecipitated NiO–Al 2O 3 samples were investigated, using samples with up to 60.7 wt.% Al 2O 3 that had been calcined in the range 300–700°C. Characterization techniques included BET surface area of fresh and reduced catalysts, X-ray diffraction analysis of structure and crystallite size, magnetic susceptibility measurements, oxidizing power, and reducibility in H 2. Only NiO was detected in samples with up to 4.1 wt.% Al 2O 3 for all sample calcination temperatures. Surface areas were similar for all fresh samples but decreased rapidly after calcination at high temperatures. The surface area loss was less for the higher Al 2O 3-containing samples. Nickel oxide crystallite sizes increased at higher calcination temperatures, but remained approximately the same for each Al 2O 3 level. The NiO was nonstoichiometric (NiO1+x), with x decreasing at higher calcination temperatures and increasing with small amounts of added Al2O3 through a maximum at about 3 wt.% Al2O3. However, this did not correlate well with microstrain in the NiO crystallites nor with reducibility, which decreased with Al2O3 addition. At higher levels of Al2O3 (13.6 wt.% and above), surface areas increased with higher Al2O3 loadings, but NiO crystallite sizes remained approximately the same, independent of both Al2O3 content and calcination temperature. X-ray diffraction patterns were very diffuse, and it was not possible to rule out the presence of pseudo-spinel combinations of NiO and Al2O3. Reducibility was more difficult than with low Al2O3 levels, and nonstoichiometry was low and independent of Al2O3 content. Reducibilities of all samples calcined at 300°C correlated well with the final BET surface area of the reduced samples, indicating that more dispersed NiO crystallites are more difficult to reduce, a conclusion that supports a model for reduction proposed previously. 相似文献
5.
Effect of additives, In 2O 3, SnO 2, CoO, CuO and Ag, on the catalytic performance of Ga 2O 3–Al 2O 3 prepared by sol–gel method for the selective reduction of NO with propene in the presence of oxygen was studied. As for the reaction in the absence of H 2O, CoO, CuO and Ag showed good additive effect. When H 2O was added to the reaction gas, the activity of CoO-, CuO- and Ag-doped Ga 2O 3–Al 2O 3 was depressed considerably, while an intensifying effect of H 2O was observed for In 2O 3- and SnO 2-doped Ga 2O 3–Al 2O 3. Of several metal oxide additives, In 2O 3-doped Ga 2O 3–Al 2O 3 showed the highest activity for NO reduction by propene in the presence of H 2O. Kinetic studies on NO reduction over In 2O 3–Ga 2O 3–Al 2O 3 revealed that the rate-determining step in the absence of H 2O is the reaction of NO 2 formed on Ga 2O 3–Al 2O 3 with C 3H 6-derived species, whereas that in the presence of H 2O is the formation of C 3H 6-derived species. We presumed the reason for the promotional effect of H 2O as follows: the rate for the formation of C 3H 6-derived species in the presence of H 2O is sufficiently fast compared with that for the reaction of NO 2 with C 3H 6-derived species in the absence of H 2O. Although the retarding effect of SO 2 on the activity was observed for all of the catalysts, SnO 2–Ga 2O 3–Al 2O 3 showed still relatively high activity in the lower temperature region. 相似文献
6.
The Pd–Pt/Al 2O 3 bimetallic catalysts showed high activities toward the wet oxidation of the reactive dyes in the presence of 1% H 2 together with excess oxygen. Palladium was believed to act as a co-catalyst to spillover the adsorbed H 2 onto the surface of the oxidized Pt surface, and thereby the reducibility of the Pt increased greatly. The organic dye molecule adsorbed on the reduced Pt surface more easily than the oxidized Pt surface under the competition with excess oxygen, which is an essential step for the catalytic wet oxidation (CWO). The Pd–Pt/Al 2O 3 catalysts also produced H 2O 2 from H 2/O 2 mixture, and the hydroxyl radical was formed through the subsequent decomposition of H 2O 2. Additional oxidation of the reactive dyes was obtained with hydroxyl radical. The high activities of the Pd–Pt/Al 2O 3 catalysts were believed to be due to the combined effects of the faster redox cycle resulting from the increased reducibility of Pt surface and the additional oxidation of the reactive dyes with hydroxyl radical. 相似文献
7.
Ag-based catalysts supported on various metal oxides, Al 2O 3, TiO 2, and TiO 2–Al 2O 3, were prepared by the sol–gel method. The effect of SO 2 on catalytic activity was investigated for NO reduction with propene under lean burn condition. The results showed the catalytic activities were greatly enhanced on Ag/TiO 2–Al 2O 3 in comparison to Ag/Al 2O 3 and Ag/TiO 2, especially in the low temperature region. Application of different characterization techniques revealed that the activity enhancement was correlated with the properties of the support material. Silver was highly dispersed over the amorphous system of TiO 2–Al 2O 3. NO 3− rather than NO 2− or NO x reacted with the carboxylate species to form CN or NCO. NO 2 was the predominant desorption species in the temperature programmed desorption (TPD) of NO on Ag/TiO 2–Al 2O 3. More amount of formate (HCOO −) and CN were generated on the Ag/TiO 2–Al 2O 3 catalyst than the Ag/Al 2O 3 catalyst, due to an increased number of Lewis acid sites. Sulfate species, resulted from SO 2 oxidation, played dual roles on catalytic activity. On aged samples, the slow decomposition of accumulated sulfate species on catalyst surface led to poor NO conversion due to the blockage of these species on active sites. On the other hand, catalytic activity was greatly enhanced in the low temperature region because of the enhanced intensity of Lewis acid site caused by the adsorbed sulfate species. The rate of sulfate accumulation on the Ag/TiO 2–Al 2O 3 system was relatively slow. As a consequence, the system showed superior capability for selective adsorption of NO and SO 2 toleration to the Ag/Al 2O 3 catalyst. 相似文献
8.
In this paper, the effect of CO 2 and H 2O on NO x storage and reduction over a Pt–Ba/γ-Al 2O 3 (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO 2 and H 2O, NO x is stored on BaCO 3 sites only. Moreover, H 2O inhibits the NO oxidation capability of the catalyst and no NO 2 formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N 2 as well as complete regeneration of stored NO. In the presence of CO 2, NO is oxidized into NO 2 and more NO x is stored as in the presence of H 2O, resulting in 30% barium utilization. Bulk barium sites are inactive in NO x trapping in the presence of CO 2·NH 3 formation is seen in the rich phase and the selectivity towards N 2 is 83%. Ba(NO 3) 2 is always completely regenerated during the subsequent rich phase. In the absence of CO 2 and H 2O, both surface and bulk barium sites are active in NO x storage. As lean/rich cycling proceeds, the selectivity towards N 2 in the rich phase decreases from 82% to 47% and the N balance for successive lean/rich cycles shows incomplete regeneration of the catalyst. This incomplete regeneration along with a 40% decrease in the Pt dispersion and BET surface area, explains the observed decrease in NO x storage. 相似文献
9.
In order to make clear the coordinatively unsaturated sites (CUS) of Co–Mo/Al 2O 3 sulfided at high pressure, the temperature programmed desorption of NO adsorbed on Co–Mo/Al 2O 3 sulfided at high pressure was studied by DRIFT method. The intensity of two IR bands (1835 and 1785 cm −1) of adsorbed NO on Co–Mo/Al 2O 3decreased simultaneously up to 393 K. The higher frequency band disappeared at 393 K, while the lower frequency band remained even at 403 K. In the case of Mo/Al 2O 3, the intensities of two bands appeared at 298 K decreased monotonously with increasing temperature, and disappeared simultaneously over 433 K. In the case of Co/Al 2O 3, two bands disappeared simultaneously over 393 K. These results suggest that two kinds of nitrosyl species are formed on Co–Mo/Al 2O 3. One is dinitrosyl species adsorbed on CUS of Co, and the other is unidentified nitrosyl species. Comparing DRIFT spectra of Co–Mo/Al 2O 3 with those of a physical mixture of Mo/Al 2O 3 and Co/Al 2O 3, it is also suggested that the formation of the latter one correlates with the interaction between Co and Mo in Co–Mo/Al 2O 3. The unidentified nitrosyl species might be the key to explain the dependency of DRIFT spectrum of adsorbed NO on the pressure of sulfiding. 相似文献
10.
Mixed oxides of alumina and zirconia having a relative composition of 50, 80 and 100% Zr 2O were synthesized by means of sol–gel methods. The catalysts were sulfated with H 2SO 4 1N, and were loaded with 0.3% Pt metal using the incipient wetness technique. The characterization of the physicochemical properties was carried out using XRD, N 2-adsorption at 78 K, and SEM. The catalytic properties of the Al 2O 3–ZrO 2 series were studied by means of dehydration of 2-propanol at 180°C and isomerization of n-hexane at 250°C, 1 atm. The sulfated solids presented a high surface acidity and a limited crystallinity, together with high activity for alcohol dehydration (i.e. 2-propanol). On the other hand, the Al 2O 3–ZrO 2 solid solutions (i.e. those having a 20–80% composition) turned out to be the most active ones for the isomerization of n-hexane. 相似文献
11.
Alumina–titania supports containing 5–50 wt.% of TiO 2 were prepared by coprecipitation method using inorganic precursors (sodium aluminate and titanium chloride). DTA-TGA, XRD, SEM, TPD NH3, and IR spectroscopy were used to characterise these materials. The study shows that the promoting effect of nickel on the HDS activity of molybdenum catalysts supported on Al 2O 3TiO 2 is significantly lower than that for molybdenum catalyst supported on Al 2O 3, and depends on the TiO 2 content. The SEM results show that in the case of rich Al support (20 wt.% of TiO 2) molybdenum was aggregated on the external surface of the catalyst, whereas it was uniformly dispersed on the external surface of alumina. Results also show that molybdenum is preferably supported on aluminum oxide. Application of Al 2O 3TiO 2 oxides enhances the HDN activity of nickel–molybdenum catalysts. The highest HDN efficiency was obtained for the NiMo/Al 2O 3TiO 2 catalyst containing 50 wt.% of TiO 2. HDN activity was found to depend on protonic acidity and anatase content. 相似文献
12.
Characteristics of MnO y–ZrO 2 and Pt–ZrO 2–Al 2O 3 as reversible sorbents of NO x were investigated under dynamic changes in atmosphere. These sorbents can be used reversibly with a change of C 3H 8 concentration in the reaction gases. Catalytic reduction of NO occurred in the presence of propane, which was more pronounced on Pt–ZrO 2–Al 2O 3 than on MnO y-ZrO 2 due to high activity of Pt surface for this reaction on MnO y in MnO y–ZrO 2. The sorption was observed as soon as the atmosphere changed from a reducing to an oxidizing one. This implies that a high equilibrium partial pressure of O 2 is necessary for NO uptake since the sorbed NO−3 species becomes stable. The beginning of NO x desorption atmospheres was somewhat dependent on the amount of stored NO x. The presence of propane in the gas phase strongly affected the characteristic sorption and desorption properties of MnO y–ZrO 2 and Pt–ZrO 2–Al 2O 3. The sorption and desorption properties are different for MnO y–ZrO 2 and Pt–ZrO 2–Al 2O 3, since the noble metal or metal oxide possesses unique activity for the NO reaction with C 3H 8 and the amount of oxygen available for oxidative sorption of NO. 相似文献
13.
The behaviour of a Pt(1 wt.%) supported on CeO 2–ZrO 2(20 wt.%)/Al 2O 3(64 wt.%)–BaO(16 wt.%) as a novel NO x 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 NO x storage–reduction behaviour comparable with that of Pt–BaO on alumina, although Ba ions result much better dispersed. 相似文献
14.
The phase diagram of the Al 2O 3–ZrO 2–Nd 2O 3 system was constructed in the temperature range 1250–2800 °C. The liquidus surface of the phase diagram reflects the preferentially eutectic interaction in the system. Two new ternary and one new binary eutectics were found. The minimum melting temperature is 1675 °C and it corresponds to the ternary eutectic Nd 2O 3·11Al 2O 3 + F-ZrO 2 + NdAlO 3. The solidus surface projection and the schematic of the alloy crystallization path confirm the preferentially congruent character of phase interaction in the ternary system. The polythermal sections present the complete phase diagram of the Al 2O 3–ZrO 2–Nd 2O 3 system. No ternary compounds or regions of remarkable solid solution were found in the components or binaries in this ternary system. 相似文献
15.
The oxidation of CH 4 over Pt–NiO/δ-Al 2O 3 has been studied in a fluidised bed reactor as part of a major project on an autothermal (combined oxidation–steam reforming) system for CH 4 conversion. The kinetic data were collected between 773 and 893 K and 101 kPa total pressure using CH 4 and O 2 compositions of 10–35% and 8–30%, respectively. Rate–temperature data were also obtained over alumina-supported monometallic catalysts, Pt and NiO. The bimetallic Pt–NiO system has a lower activation energy (80.8 kJ mol −1) than either Pt (86.45 kJ mol −1) and NiO (103.73 kJ mol −1). The superior performance of the bimetallic catalyst was attributed to chemical synergy. The reaction rate over the Pt–NiO catalyst increased monotonically with CH 4 partial pressure but was inhibited by O 2. At low partial pressures (<30 kPa), H 2O has a detrimental effect on CH 4 conversion, whilst above 30 kPa, the rate increased dramatically with water content. 相似文献
16.
The importance of the hydrodearomatisation (HDA) is increasing together with tightening legislation of fuel quality and exhaust emissions. The present study focuses on hydrogenation (HYD) kinetics of the model aromatic compound naphthalene, found in typical diesel fraction, in n-hexadecane over a NiMo (nickel molybdenum), Ni (nickel) and Ru (ruthenium) supported on trilobe alumina (Al 2O 3) catalysts. Kinetic reaction expressions based on the mechanistic Langmuir–Hinshelwood (L–H) model were derived and tested by regressing the experimental data that translated the effect of both naphthalene and hydrogen concentration at a constant temperature (523.15 and 573.15 K over the NiMo catalyst and at 373.15 K over the Ni and Ru/Al 2O 3 catalysts) on the initial reaction rate. The L–H equation, giving an adequate fit to the experimental data with physically meaningful parameters, suggested a competitive adsorption between hydrogen and naphthalene over the presulphided NiMo catalyst and a non-competitive adsorption between these two reactants over the prereduced Ni and Ru/Al 2O 3 catalysts. In addition, the adsorption constant values indicated that the prereduced Ru catalyst was a much more active catalyst towards naphthalene HYD than the prereduced Ni/Al 2O 3 or the presulphided NiMo/Al 2O 3 catalyst. 相似文献
17.
In the present study, both t-phase zirconia and m-phase zirconia particles are incorporated into an alumina matrix. Dense Al 2O 3/(t-ZrO 2+m-ZrO 2) composites were prepared by sintering pressurelessly at 1600 °C. The microstructure of the composites are characterized, the elastic modulus, strength and toughness determined. Because the ZrO 2 inclusions are close to each other in the Al 2O 3 matrix, the yttrium ion originally in t-ZrO 2 particles can diffuse to nearby m-ZrO 2 particles during sintering, and the m-phase zirconia is thus stabilized after sintering. The strength of the Al 2O 3/(t-ZrO 2+m-ZrO 2) composites after surface grinding can reach values as high as 940 MPa, which is roughly three times that of Al 2O 3 alone. The strengthening effect is contributed by microstructural refinement together with the surface compressive stresses induced by grinding. The toughness of alumina is also enhanced by adding both t-phase and m-phase zirconia, which can reach values as high as two times that of Al 2O 3 alone. The toughening effect is attributed mainly to the zirconia t–m phase transformation. 相似文献
18.
The current work is devoted to study of CO interaction with PdO/Al 2O 3–(Ce x–Zr 1−x)O 2 catalysts. Ceria–zirconia–alumina supports with different Ce/Zr ratio were prepared by sol–gel technique. The FT-IR characterization of CO adsorbed at −120 and 25 °C on oxidized and reduced samples revealed that Ce/Zr ratio modifies the surface properties of support and oxidation state of palladium. The catalyst with Ce/Zr molar ratio 0.5/0.5 was characterized with the highest ability to stabilize palladium in oxide state and the highest activity to oxidize CO. Redox treatment of catalysts improves their catalytic activity. 相似文献
19.
The catalytic performance of mono- and bimetallic Pd (0.6, 1.0 wt.%)–Pt (0.3 wt.%) catalysts supported on ZrO 2 (70, 85 wt.%)–Al 2O 3 (15, 0 wt.%)–WO x (15 wt.%) prepared by sol–gel was studied in the hydroisomerization of n-hexane. The catalysts were characterized by N 2 physisorption, XRD, TPR, XPS, Raman, NMR, and FT-IR of adsorbed pyridine. The preparation of ZrW and ZrAlW mixed oxides by sol–gel favored the high dispersion of WO x and the stabilization of zirconia in the tetragonal phase. The Al incorporation avoided the formation of monoclinic-WO 3 bulk phase. The catalysts increased their SBET for about 15% promoted by Al 2O 3 addition. Various oxidation states of WO x species coexist on the surface of the catalysts after calcination. The structure of the highly dispersed surface WO x species is constituted mainly of isolated monotungstate and two-dimensional mono-oxotungstate species in tetrahedral coordination. The activity of Pd/ZrW catalysts in the hydroisomerization of n-hexane is promoted both with the addition of Al to the ZrW mixed oxide and the addition of Pt to Pd/ZrAlW catalysts. The improvement in the activity of Pd/ZrAlW catalysts is ascribed to a moderated acid strength and acidity, which can be correlated to the coexistence of W 6+ and reduced-state WO x species (either W 4+ or W 0). The addition of Pt to the Pd/ZrAlW catalyst does not modify significantly its acidic character. Selectivity results showed that the catalyst produced 2MP, 3MP and the high octane 2,3-dimethylbutane (2,3-DMB) and 2,2-dimethylbutane (2,2-DMB) isomers. 相似文献
20.
Ni–Mo/Al 2O 3–MCM-41 supported catalysts have been investigated for modification of MCM-41 by using sol–gel alumina incorporation method. Different catalysts were synthesized with variation of Si/Al molar ratios of 10, 50, 100 and 200. High specific surface area ordered meso-porous solid (MCM-41) was synthesized by using organic template method. In order to modify the low acidity of silica solid, the surface of MCM-41 was modified by incorporation of alumina. The surface acidity of solids modified significantly with variation of alumina content in the supports. The sol–gel method of alumina incorporation was used, which does not modify extensively the pore characteristics of MCM-41 material during the preparation of Al 2O 3–MCM-41. The X-ray diffraction intensities indicated that alumina as well as MCM-41 were present in the synthesized supports. Additionally, the hydrothermal stability of the Al 2O 3–MCM-41 materials was maintained up to 873 K using sever conditions like 100% water vapor stream. The catalytic activity of the catalysts was tested in the hydrodesulfurization (HDS) of dibenzothiophene (DBT). Selectivity was oriented mainly to the production of biphenyl (BP) and for high Si/Al ratios toward cyclohexylbenzene (CHB) and showed a higher conversion and better selectivity to hydrogenation (cyclohexylbenzene). 相似文献
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