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1.
The preparation of CeO 2–ZrO 2 mixed oxides preparation was studied by evaluating the influence of several conditions. Coprecipitation was taken as the standard method and the effects brought about by the cerium salt precursor ((NH 4) 2Ce(NO 3) 6 or Ce(NO 3) 3), the introduction of drying and aging steps as well as pH controlling upon precipitation were analyzed. The samples were characterized by X-ray diffraction, Raman spectroscopy, temperature-programmed reduction, infrared spectroscopy, oxygen storage capacity and surface area. The use of Ce(NO 3) 3 leads to the formation of c-CeO 2 and t-ZrO 2 mixed oxide whereas a solid solution is achieved by using (NH 4) 2Ce(NO 3) 6. It was observed that the cerium precursor is the most significant parameter of preparation procedure since it defines the crystalline phases and consequently the reducibility behavior of the CeO 2–ZrO 2 system. 相似文献
2.
ZrO 2–TiO 2 mixed oxide (30–70 mol/mol) was prepared by low-temperature sol–gel followed by solvo-thermal treatment (1 day) at various temperatures (40, 80, 120, 160 and 200 °C). Selected samples of the corresponding single oxides were also prepared. Materials characterization was carried out by N 2 physisorption, XRD, thermal analysis (TG-DTA) and UV–vis DRS, infra-red and Laser-Raman spectroscopies. Binary solids of enhanced pore volume and pore size diameter were obtained by increasing the post-treatment severity. Anatase TiO 2 micro-segregation was evidenced by Raman spectroscopy for the mixed oxide solvo-treated at the highest temperature. This solid also showed the highest crystallization temperature to ZrTiO 4 (702 °C). Mo impregnated (2.8 atom nm −2) on various mixed oxides was sulfided under H 2S/H 2 (400 °C, 1 h), the catalysts being tested in the dibenzothiophene hydrodesulfurization (HDS, T = 320 °C, P = 5.59 MPa). By increasing the severity of the solvo-treatment improved supports for MoS 2 phase were obtained. The HDS activity of the catalyst with carrier post-treated at 200 °C was 40% higher (in per total mass basis) than that of sulfided Mo supported on the binary oxide solvo-treated at 80 °C. The ZrO 2–TiO 2-supported catalysts showed higher selectivity to products from the hydrogenation route than their counterparts supported on either single oxide. 相似文献
3.
The local structure and the photoactivity of B 2O 3–SiO 2/TiO 2 ternary mixed oxides (SiO 2 content was fixed as 30 at.% with respect to TiO 2) was investigated by using XRD, FT-IR, BET, UV-vis spectra, and electron paramagnetic resonance (EPR) measurement. In FT-IR analysis, boron was incorporated into the framework of titania matrix with replacing Ti---O---Si with Si---O---B or Ti---O---B bonds. Also, paramagnetic species such as O − and Ti 3+ defects were formed by the boron incorporation. In SiO 2/TiO 2 mixed oxides, a blue shift in the light absorption band was observed due to the quantization of band structure. All B 2O 3–SiO 2/TiO 2 samples had pure anatase phase and no rutile phase was formed even though the calcination temperature was over 900 °C. Incorporating boron oxides of more than 10% enlarges the grain size of anatase phase and causes a red shift of the light absorption spectrum. The surface area was monotonically decreased with increasing the content of boron content. As a result, the photoactivity of B 2O 3–SiO 2/TiO 2 ternary mixed oxides was greatly influenced by the content of boron oxide. The highest photoactivity (g moles/min l) was obtained when the boron content was 5% and seven times higher than that of silica/titania binary mixed oxide. In addition, the specific photoactivity (g moles/m 2 l) was maximum still at 5%. It was concluded that the large reduction of surface area, the change of band structure, and more formation of bulk Ti 3+ sites are responsible for the deterioration in the photoactivity of B 2O 3–SiO 2/TiO 2 ternary mixed oxides when the content of boron is over 10%, although their crystallinity was enhanced by increasing the calcination temperature with keeping anatase phase. 相似文献
4.
CeO 2–ZrO 2–La 2O 3 (CZL) mixed oxides were prepared by citric acid sol–gel method. The as-received gel was calcined at 500, 700, 900 and 1050 °C to obtain the so-called C5, C7, C9 and CK, respectively. The C5, C7 and C9 powders were impregnated with H 2PtCl 6 and then calcined at 500 °C to prepare P5C5, P5C7 and P5C9, respectively. The impregnated CK powders were calcined at 500, 700 and 900 °C to prepare P5CK, P7CK and P9CK, respectively. The XRD and XPS analyses show that the surface distribution of Pt is evidently influenced by the structural and textural properties of the support. The CO adsorption followed by FTIR reveals that the dispersion and the chemisorption sites of Pt are reduced as the calcination temperature of CZL support increases. The chemisorption ability of the CK samples is even completely deactivated. The encapsulation mechanism, which has been applied to explain the so-called strong metal–support interaction (SMSI) after reductive treatment, is introduced here to demonstrate the abnormal observations though the samples were prepared in oxidative atmosphere. The HRTEM results also confirm this explanation. The effects of oxygen vacancies, the chemisorption sites on the Pt surface and Pt/Ce interfacial sites on the three-way catalytic activities are discussed. 相似文献
5.
Direct synthesis route was developed to support TiO 2–ZrO 2 binary metal oxide onto the carbon templated mesoporous silicalite-1 (CS-1). Metal hydroxide modified carbon particles could play a role as hard template and simultaneously support metal components on the mesopores during the crystallization of zeolites. Such supported TiO 2–ZrO 2 binary metal oxides (TZ/CS-1) showed better resistance to deactivation in the oxidative dehydrogenation of ethylbenzene (ODHEB) in the presence of CO 2. These catalysts were found to be active, selective and catalytically stable (10 h of time-on-stream) at 600 °C for the dehydrogenation of ethylbenzene (EB) to styrene (Sty). 相似文献
6.
This work aims at exploring the thermal ageing mechanism of Pt on ceria-based mixed oxides and the corresponding effect on the oxygen storage capacity (OSC) performance of the support material. Pt was supported on low-surface-area CeO 2–ZrO 2–La 2O 3 mixed oxides (CK) by impregnation method and subsequently calcined in static air at 500, 700 and 900 °C, respectively. The evolutions of textural, microstructural and redox properties of catalysts after the thermal treatments were identified by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and high-resolution transmission electron microscope (HRTEM). The results reveal that, besides the sintering of Pt, encapsulation of metal by the mixed oxides occurs at the calcination temperature of 700 °C and above. The burial of Pt crystallites by support particles is proposed as a potential mechanism for the encapsulation. Further, the HRTEM images show that the distortion of the mixed oxides lattice and other crystal defects are distributed at the metal/oxides interface, probably indicating the interdiffusion/interaction between the metal and mixed oxide. In this way, encapsulation of Pt is capable to promote the formation of Ce 3+ or oxygen vacancy on the surface and in the bulk of support. The OSC results show that the reducibility and oxygen release behavior of catalysts are related to both the metal dispersion and metal/oxides interface, and the latter seems to be more crucial for those supported on low-surface-area mixed oxides. Judging by the dynamic oxygen storage capacity (DOSC), oxygen storage capacity complete (OSCC) and oxygen releasing rate, the catalyst calcined at 700 °C shows the best OSC performance. This evident promotion of OSC performance is believed to benefit from the partial encapsulation of Pt species, which leads to the increment of Ce 3+ or oxygen vacancies both on the surface and in the bulk of oxides despite a loss of chemisorption sites on the surface of metal particles. 相似文献
7.
A series of CeO 2/SiO 2 mesostructured composite materials was synthesized by sol–gel process using Pluronic P123 as template, tetraethylorthosilicate as silica source and hexahydrated cerium nitrate as precursor under acid condition. The as-synthesized materials with Ce/Si molar ratio ranging from 0.03 to 0.3 were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), laser Raman spectroscopy (LRS), and N 2 adsorption. Characterization revealed that all samples possess ordered hexagonal mesoporous structure similar to SBA-15 and possess high surface area, large pore volume and uniform pore size. The fact that cerium species are present as highly dispersed CeO 2 nanocrystals in hexagonal matrix was confirmed by XRD combined with high-resolution TEM and selected area electron diffraction (SAED) analysis. Introduction of ceria to silica matrix can cause a distortion of hexagonal ordering structure and decrease pore diameter and increase the wall thickness of mesopores. Moreover, it can be found that this sol–gel route is a feasible, effective and simple method for templating synthesis of CeO 2/SiO 2 composite materials. 相似文献
8.
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
9.
High surface area CeO 2–ZrO 2 mixed oxides were treated at 900–950°C either under wet air or under successive reducing and oxidizing atmospheres in order to study the evolution of the oxygen storage capacity (OSC) of these solids after different aging treatments. Several complementary methods were used to characterize the redox behavior: temperature programmed reduction (TPR) by H 2, TPO, magnetic susceptibility measurements to obtain the Ce 3+ content, FT-IR spectroscopy of adsorbed methanol and a method to compare the oxygen buffering capacity (OBC) of the oxides. All the results confirm that the mixed oxides exhibit better redox properties than pure ceria, particularly after aging. The enhancement in the OSC at moderate temperature has to be related to a deeper penetration of the reduction process from the surface into the under-layers. Redox cycling aging promotes the reduction at low temperature of all the mixed oxides, the improvement being much more important for low surface area aged samples. The magnitude of this effect does not depend on the BET surface areas which have similar values after cycling. This underlines the critical influence that the preparation and activation procedure have on the final OSC behaviors of the ceria–zirconia mixed oxides. 相似文献
10.
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
11.
Catalytic light-off of a stream of NO, H 2, CO in an excess O 2 has been studied over various metal oxides loading 1 wt% Pt. Because a low-surface area Y 2O 3 (<5 m 2 g −1) was found to exhibit the highest de-NO x activity, a mesoporous Y 2O 3 was then synthesized from an yttrium-based surfactant mesophase templated by dodecyl sulfate , which was anion-exchanged by acetate (AcO = CH 3COO −). The product showed a 3-D mesoporosity with a large surface area (396 m 2 g −1) and the Pt-supported catalyst achieved much improved light-off characteristics suitable for the low-temperature de-NO x in the presence of CO and excess O 2. 相似文献
12.
Sulfated and non-sulfated TiO 2–ZrO 2 were promoted with platinum and the activity and selectivity of the resulting catalysts for the hydrolytic decomposition chlorodifluoromethane in air was investigated. The addition of platinum reduced the specific surface area of the catalyst slightly and lowered the catalytic activity. On the other hand, the selectivity of the catalysts towards CO 2 formation was much improved. Metallic platinum was formed by the reduction of the platinum precursor with carbon monoxide produced during the hydrolysis of chlorodifluoromethane over the acidic mixed oxide and promoted the oxidation of CO. At the same time, platinum suppressed completely the formation of the fluorinated byproduct CHF 3, possibly by anchoring itself on the active sites responsible for the fluorination reaction. The platinum promoted non-sulfated TiO 2–ZrO 2 was stable and gave more than 90% of CHClF 2 conversion and 95% selectivity to CO 2 for over 60 h. 相似文献
13.
Two series of Ta 2O 5–TiO 2 photocatalysts (Ta:Ti = 4:1, 1:1 and 1:4) were prepared by sol–gel technique applying triblock copolymer of Pluronic P123 and were tested in platinized form (0.3 wt.%) in photodecomposition of water under ultraviolet and visible light ( λ > 300 nm). It was found the mesoporous character of tantalum containing catalysts with relatively high surface area (100–130 m 2 g −1) of these samples. However, higher concentration of TiO 2 in mixed oxides leads to the destruction of mesoporous character of synthesized photocatalysts. All samples were characterized with thermogravimetry, XRD, N 2 physisorption, DR-UV–vis and FTIR spectroscopy. The mixed oxides of Ta 2O 5–TiO 2 system showed much lower band-gap than pure Ta 2O 5 and relatively high activity in platinized state in photocatalytic hydrogen generation under visible. Doping of pure oxides and mixed systems with sulfur resulted in lowering of the band-gap values below 3 eV and much better activity in H 2 evolution reaction. Non-platinized photocatalysts showed activity in liquid phase cyclohexene photooxidation at 305 K. 相似文献
14.
The solubilities of Sn in ZrO 2 and Zr in SnO 2 are investigated. The X-ray diffraction (XRD) studies show the solubility limit for Sn in zirconia to be 20 mol% whereas Zr in SnO 2 is around 25 mol%. All the compositions were prepared by the coprecipitation method. The average particle size for a typical composition was 25 nm as revealed by the transmission electron microscopy (TEM). 相似文献
16.
The catalytic behaviour of multiphasic catalysts based on -bismuth pyrostannate, Bi 2Sn 2O 7, was investigated in the selective oxidation of isobutene into methacrolein. When -Bi 2Sn 2O 7 is mixed with MoO 3, strong cooperation effects on the yield and selectivity in methacrolein occur. However, XRD analyses performed on samples after test revealed the formation of a low quantity of -bismuth molybdate, -Bi 2Mo 3O 12, when the reaction temperature exceeded 673 K. Additional experiments were therefore carried out on the “Bi–Sn–Mo–O” catalysts in order to shed light on the role of Bi 2Mo 3O 12 in the synergetic effects observed in the Bi 2Sn 2O 7–MoO 3 system. The experimental results are discussed in terms of several hypotheses. First, the intrinsic activity of Bi 2Mo 3 O 12 is probably the simplest explanation for the synergetic effects, although experiments have shown that this phase present in a low quantity is only poorly active. Second, catalytic tests made on Bi 2Sn 2O 7–Bi 2Mo 3O 12 mechanical mixtures have evidenced a cooperation between these two ternary oxides, particularly when Bi 2Sn 2O 7 was the major component of the mixture. Consequently, it is likely that a synergy between Bi 2Sn 2O 7 and the in situ generated Bi 2Mo 3O 12 might play a role in the synergy observed in the Bi 2Sn 2O 7–MoO 3 association. Third, as bismuth pyrostannate was previously shown to behave as an oxygen donor phase with respect to WO 3, a remote control mechanism could therefore occur between Bi 2Sn 2O 7 and MoO 3, independently from the formation of -Bi 2Mo 3O 12. 相似文献
17.
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
18.
The system Al 2O 3–ZrO 2 was studied by differential thermal analysis in inert atmosphere and in vacuum. The eutectic was located at 1866°C and 40% mass of ZrO 2. Zirconia solid solution at the eutectic temperature is up to 1.1±0.3% mass of Al 2O 3. Enthalpy of melting of this eutectic is 1080±90 J/g. Pure ZrO 2 transforms from monoclinic to tetragonal at 1162±7°C, but the saturated solid solution of ZrO 2, with 0.7±0.2% mass Al 2O 3 at this temperature, transforms at 1085±5°C. Inverse transitions occur with hysteresis correspondingly at 1055±5 and 995±5°C. Enthalpy of transformation of pure ZrO 2 from monoclinic to tetragonal phase is 42±5 J/g (5.2±0.6 J/mol) but only 30±5 J/g for a ZrO 2 saturated solid solution. 相似文献
19.
The conversion of C3 organic compounds (propane, propene, 1- and 2-propanol, allyl alcohol, propanal, acrolein, acetone and 1- and 2-chloropropane) in the presence of excess oxygen has been investigated over two V–W–TiO 2 commercial SCR catalysts differing in the V content and over Mn–TiO 2 alternative SCR catalysts. V–W–Ti catalysts show poor activity in the oxidation of hydrocarbons and oxygenates and give significant amounts of partial oxidation products. Moreover they give rise to CO in excess of CO 2. The sample higher in V is more active. Mn–TiO 2 is definitely more active in oxidation of hydrocarbons and oxygenates, and produces, at total conversion, CO 2 as the only detectable product. V–W–Ti catalysts are very active in dehydrochlorination of the two 2-chloropropane isomers and retain the same oxidation activity also in the presence of HCl. On the contrary, Mn-based catalysts in the presence of chlorocarbons convert into dehydrochlorination catalysts but lose their catalytic activity in oxidation. V–W–Ti catalysts can be used in Cl-containing atmospheres while Mn–TiO2 can be proposed for DeNOx and VOC abatement in Cl-free atmospheres such as for diesel engine exhaust gas purification. 相似文献
20.
The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for purification of motor exhaust gases have been widely investigated in recent years. This work shows the cooperative effects of CeO 2–Pd on the kinetics of CO oxidation over Pd/CeO 2–ZrO 2. Under reducing-to-moderately oxidizing conditions, a zero-order O 2 pressure dependence is found which can be interpreted on the basis of a mechanism involving a reaction between CO adsorbed on Pd and surface oxygen from the support. The high oxygen-exchange capability of the CeO 2–ZrO 2 support, as determined from temperature-programmed reduction/oxygen uptake measurements is suggested as being responsible for such a catalytic behavior. 相似文献
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