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1.
γ-Al 2O 3 supported vanadium oxides were modified by tungsten and molybdenum oxides in order to improve dispersion and selectivity towards olefins in propane oxidative dehydrogenation (ODH). Both vanadium–tungsten and vanadium–molybdenum catalysts were obtained by adsorption of mixed isopolyanions (VW 5O 195−, V 2W 4O 194−, VMo 5O 195− and V 2Mo 4O 194−) from aqueous solutions. The isopolyanion solutions were characterized by UV-Vis and 51V NMR spectroscopy. Vanadium, vanadium–tungsten and vanadium–molybdenum precursors and catalysts were also characterized by UV-Vis (diffuse reflectance) and solid state 51V NMR spectroscopy. An improved selectivity to propene in the presence of tungsten and molybdenum in VO x/γ-Al 2O 3 was observed and attributed to dilution of vanadium by tungsten or molybdenum oxides on the γ-Al 2O 3 surface. 相似文献
2.
We have studied the activity and selectivity of Pd/γ-Al 2O 3, VO x/γ-Al 2O 3 and Pd–VO x/γ-Al 2O 3 catalysts for the decomposition of NO and the reduction of NO with CO. Pd–VO x/γ-Al 2O 3 catalysts were prepared by anchoring Pd(AcAc) 2 on VO x/γ-Al 2O 3. Characterization of the binary samples by hydrogen chemisorption and TPR measurements indicated that the reduction of VO x is enhanced by a close contact with palladium and that partially reduced vanadia decorate noble metal particles. This palladium–vanadium interaction alters the catalytic properties of palladium: the activity for NO decomposition is higher for the binary sample and, for the NO–CO reaction, both the activity and the selectivity to N 2 increase when vanadium is in contact with palladium. 相似文献
3.
Combustion of CO, ethyl acetate and ethanol was studied over CuO x/Al 2O 3, CuO x–CeO 2/Al 2O 3, CuMn 2O 4/Al 2O 3 and Mn 2O 3/Al 2O 3 catalysts. It was found that modification of the alumina with ceria before subsequent copper oxide deposition increases the activity for combustion of CO substantially, but the effect of ceria was small on the combustion of ethyl acetate and ethanol. The activity increases with the CuO x loading until crystalline CuO particles are formed, which contribute little to the total active surface. The CuO x–CeO 2/Al 2O 3 catalyst is more active than the CuMn 2O 4/Al 2O 3 catalyst for the oxidation of CO but the CuMn 2O 4/Al 2O 3 catalyst is more active for the combustion of ethyl acetate and ethanol. Thermal ageing and water vapour in the feed caused a modest decrease in activity and did not affect the CuOx–CeO2/Al2O3 and CuMn2O4/Al2O3 catalysts differently. In addition, no difference in intermediates formed over the two catalysts was observed. Characterisation with XRD, FT-Raman and TPR indicates that the copper oxide is present as a copper aluminate surface phase on alumina at low loading. At high loading, bulk CuO crystallites are present as well. Modification of the alumina with ceria before the copper oxide deposition gives well dispersed copper oxide species and bulk CuO crystallites associated to the ceria, in addition to the two copper oxide species on the bare alumina. The distribution of copper species depends on the ceria and copper oxide loading. The alumina supported copper manganese oxide and manganese oxide catalysts consist mainly of crystalline CuMn2O4 and Mn2O3, respectively, on Al2O3. 相似文献
4.
The oxidation of perchloroethylene (PCE) was investigated over chromium oxide catalysts supported on SiO 2, SiO 2–Al 2O 3, activated carbon, mordenite type zeolites, MgO, TiO 2 and Al 2O 3. Supported chromium oxide catalysts were more active than any other metal oxide catalysts including noble metal examined in the present study. PCE removal activity of chromium oxide catalysts mainly depended on the type of supports and the content of metal loaded on the catalyst surface. TiO 2 and Al 2O 3 containing high surface areas were effective for the high performance of PCE removal, since the formation of well dispersed Cr(VI) active reaction sites for the present reaction system, was enhanced even for the high Cr loading on the catalyst surface. CrO x catalysts supported on TiO 2 and Al 2O 3 also exhibited stable PCE removal activity at a low feed concentration of PCE of 30 ppm up to 100 h at 350°C. However, significant catalyst deactivation was observed at high PCE concentration of 10 000 ppm. CrO x/TiO 2 revealed stronger water tolerance than CrO x/Al 2O 3 due to the surface hydrophobicity. 相似文献
5.
The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium–antimony mixed oxide catalysts. The investigation was focused on the phase cooperation between V–Sb–O and Bi 2O 3 in this reaction. Strong synergistic phenomenon in catalytic activity was observed for the mechanically mixed catalysts of V–Sb–O and Bi 2O 3. Temperature-programmed reduction (TPR) and oxidation (TPO), two separated bed reaction tests, and XPS analyses were carried out to explain this synergistic effect by the reoxidation ability of Bi 2O 3. 相似文献
6.
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
7.
The surface properties of a series of V 2O 5 catalysts supported on different oxides (Al 2O 3, H–Na/Y zeolite, MgO, SiO 2, TiO 2 and ZrO 2) were investigated by transmission electron microscopy and FTIR spectroscopy augmented by CO and NH 3 adsorption. In the case of the V 2O 5/SiO 2 system TEM images evidenced the presence of V 2O 5 crystallites, whereas such segregated phase was not observed for the other samples. VO x species resulted widely spread on the surface of Al 2O 3, H–Na/Y zeolite, MgO and SiO 2, whereas on TiO 2 and ZrO 2 they are assembled in a layer covering almost completely the support. Furthermore, evidences for the presence in this layer of V–OH Brønsted acid sites close to the active centres were found. It is proposed that propene molecules primarily produced by oxydehydrogenation of propane can be adsorbed on this acid centres and then undergo an overoxidation by reaction with redox centres in the neighbourhood. This features could account for the low selectivity of V 2O 5/TiO 2 and V 2O 5/ZrO 2 catalysts. 相似文献
8.
The effect of the nature and distribution of VO x species over amorphous and well-ordered (MCM-41) SiO 2 as well as over γ-Al 2O 3 on their performance in the oxidative dehydrogenation of propane with O 2 and N 2O was studied using in situ UV–vis, ex situ XRD and H 2-TPR analysis in combination with steady-state catalytic tests. As compared to the alumina support, differently structured SiO 2 supports stabilise highly dispersed surface VO x species at higher vanadium loading. These species are more selective over the latter materials than over V/γ-Al 2O 3 catalysts. This finding was explained by the difference in acidic properties of silica- and alumina-based supports. C 3H 6 selectivity over V/γ-Al 2O 3 materials is improved by covering the support fully with well-dispersed VO x species. Additionally, C 3H 6 selectivity over all materials studied can be tuned by using an alternative oxidising agent (N 2O). The improving effect of N 2O on C 3H 6 selectivity is related to the lower ability of N 2O for catalyst reoxidation resulting in an increase in the degree of catalyst reduction, i.e. spatial separation of active lattice oxygen in surface VO x species. Such separation favours selective oxidation over CO x formation. 相似文献
9.
Reticular oxygen of Al 2O 3 or CeO x supported on Al 2O 3 was used for the epoxidation of propene without any double bond cleavage. In batch reaction, Al 2O 3 alone was able to convert propene into propene oxide (PO) with 100% selectivity and 2% conversion of propene with a close to 3:1 ratio with respect to the number of Al(III) reduced to elemental Al. When Ce 2O 3/Al 2O 3 or CeO 2/Al 2O 3 was used, Al remained in its +3 oxidation state, while the Ce oxide was the oxidant as demonstrated by XPS analyses. CeO x/Al 2O 3 was more active (propene conversion yield of 4–5%) but the selectivity was lower (70%) as PO was isomerized into acetone and propionaldehyde. Interestingly the use of reticular oxygen very much improves the selectivity with respect to the use of pure O2. In fact, while propene was more efficiently oxidized (10%) with O2 in presence of Al2O3 or CeOx/Al2O3, the selectivity was as low as 40% because C1 and C2 products were formed. However, the use of reticular oxygen represents a selective two-step technique for the use of molecular oxygen as oxidant of propene. The used oxides can be re-oxidized and the whole process can be further improved towards higher yields. PO is quantitatively converted into propene carbonate by reaction with CO2 in presence of Nb2O5. 相似文献
10.
The vapor-phase selective oxidation of propylene (H 2CCHCH 3) to acrolein (H 2CCHCHO) was investigated over supported V 2O 5/Nb 2O 5 catalysts. The catalysts were synthesized by incipient wetness impregnation of V-isopropoxide/isopropanol solutions and calcination at 450 °C. The catalytic active vanadia component was shown by in situ Raman spectroscopy to be 100% dispersed as surface VO x species on the Nb 2O 5 support in the sub-monolayer region (<8.4 V/nm 2). Surface allyl species (H 2CCHCH 2*) were observed with in situ FT-IR to be the most abundant reaction intermediates. The acrolein formation kinetics and selectivity were strongly dependent on the surface VO x coverage. Two surface VO x sites were found to participate in the selective oxidation of propylene to acrolein. The reaction kinetics followed a Langmuir–Hinshelwood mechanism with first-order in propylene and half-order in O 2 partial pressures. C 3H 6-TPSR spectroscopy studies also revealed that the lattice oxygen from the catalyst was not capable of selectively oxidizing propylene to acrolein and that the presence of gas phase molecular O 2 was critical for maintaining the surface VO x species in the fully oxidized state. The catalytic active site for this selective oxidation reaction involves the bridging VONb support bond. 相似文献
11.
Various vanadium-based binary and multi-metallic oxides were prepared and their catalytic activities for the selective oxidation of H 2S to elemental sulfur were tested. Because the deactivation of vanadium-based catalysts originated from a relatively slow rate of reoxidation of the reduced vanadium oxide [PhD thesis, Pohang University of Science and Technology, 2000], the focus was given to increase the redox ability, especially in the reoxidation step. Stable and improved activity was observed in BiVO x, TiVO x, and ZrV 2O 7 at 250°C, but TiVO x was the only catalyst that could maintain its activity below 250°C. Much higher activity was observed when VO x/TiO 2 became multi-metallic by the incorporation of Fe, Cr, and Mo. TPR–TPO, microbalance, and XPS techniques were used to explain the redox properties of VO x/SiO 2, VO x/TiO 2, and V-Fe-Cr-Mo-O x/TiO 2 catalysts in the reoxidation step. 相似文献
12.
Catalytic reduction of NO by propene in the presence of oxygen was studied over SnO 2-doped Ga 2O 3–Al 2O 3 prepared by sol–gel method. Although SnO 2-doped Ga 2O 3–Al 2O 3 gave lower NO conversion than Ga 2O 3–Al 2O 3 in the absence of H 2O, the activity was enhanced considerably by the presence of H 2O and much higher than that of Ga 2O 3–Al 2O 3. The presence of SnO 2 and Ga 2O 3–Al 2O 3 species having intimate Ga–O–Al bondings was found to be essential for the promotional effect of H 2O. The promotional effect of H 2O was interpreted by the following two reasons. The first one is the removal of carbonaceous materials deposited on the catalyst surface by H 2O. The other is the selective inhibition by H 2O of the reaction steps resulting in propene oxidation to CO x without reducing NO. 相似文献
13.
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
14.
A series of the Ce 1−xCu xO 2−x/Al 2O 3/FeCrAl catalysts ( x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H 2-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce 1−xCu xO 2−x/Al 2O 3/FeCrAl catalysts the surface phase structure were the Ce 1−xCu xO 2−x solid solution, -Al 2O 3 and γ-Al 2O 3. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce 1−xCu xO 2−x solid solution. The Cu component of the Ce 1−xCu xO 2−x/Al 2O 3/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce 1−xCu xO 2−x solid solution and the Al 2O 3 washcoats and the FeCrAl support. 相似文献
15.
Chromium oxide catalysts supported on TiO 2 and Al 2O 3 were examined in a fixed-bed flow reactor system for the removal of PCE (perchloroethylene), a simulant of 2,3,7,8-TCDD (2,3,7,8-tetrachlorodibenzo- p-dioxin), and in a pilot plant employing actual flue gas from a sintering plant for the removal of PCDDs/PCDFs (poly-chlorinated dibenzo-dioxin/poly-chlorinated dibenzo-furan). The 12.5 wt.% chromium oxides supported on TiO 2 and Al 2O 3 revealed excellent stability and performance of PCE removal in the feed gas stream containing water vapor. In a pilot plant study, the catalysts washcoated on the honeycomb reactor revealed 93–95% of PCDDs/PCDFs removal activity over CrO x/Al 2O 3-HC20 (CrO x/Al 2O 3 catalyst washcoated on 20 cell-honeycomb), and more than 99% of the decomposition activity over CrO x/TiO 2-HC20 (CrO x/TiO 2 catalyst washcoated on 20 cell-honeycomb) at 325 °C and 5000 h −1 of reactor space velocity without the de novo synthesis of PCDDs/PCDFs. In particular, CrO x/TiO 2-HC20 showed 94% of PCDDs/PCDFs decomposition activity even at 280 °C reaction temperature. The catalyst also exhibited significant NO removal activity. The chromium oxide seems to be a promising catalyst for the removal of PCDDs/PCDFs and NO x contained in the flue gas. 相似文献
16.
Vanadium oxide nanotubes (VO x-NT) have been synthesized in high yield by adding hexadecylamine to V 2O 5· nH 2O gels, followed by a hydrothermal treatment (150–180 °C, 2–7 days). Scanning electron microscopy (SEM) and X-ray diffraction analysis have been performed to optimize the temperature and reaction time required for formation of VO x-NT and the morphology of the nanotubes investigated by transmission electron microscopy (TEM). 相似文献
17.
Selective catalytic reduction of NO x by C 3H 6 in the presence of H 2 over Ag/Al 2O 3 was investigated using in situ DRIFTS and GC–MS measurements. The addition of H 2 promoted the partial oxidation of C 3H 6 to enolic species, the formation of –NCO and the reactions of enolic species and –NCO with NO x on Ag/Al 2O 3 surface at low temperatures. Based on the results, we proposed reaction mechanism to explain the promotional effect of H 2 on the SCR of NO x by C 3H 6 over Ag/Al 2O 3 catalyst. 相似文献
18.
VO x catalysts supported on SBA-15 with and without MgO modification were prepared and characterized by N 2 adsorption–desorption, XRD, HRTEM, H 2-TPR, NH 3-TPD and XPS. Compared to the VO x/SBA-15 catalyst, the VO x/MgO/SBA-15 ones exhibit much higher C 4-olefins selectivity and yield in the oxidative dehydrogenation of n-butane. The enhanced performance can be attributed to the rise in VO x reducibility as well as to the relatively lower acidity of the MgO-modified SBA-15 materials. 相似文献
19.
Four series of cobalt-based catalysts, such as bare Co 3O 4 and CoO, CoO x–CeO 2 mixed oxides, CoO x supported over alumina and alumina–baria and CoMgAl and CoNiAl hydrotalcites have been synthesized and investigated for the oxidative degradation of phenol in the presence of ozone. Characterizations were obtained by several techniques in order to investigate the nature of cobalt species and their morphological properties, depending on the system. Analyses by XRD, BET, TPR, UV–visible diffuse reflectance spectroscopy and TG/DT were performed. The CoNiAl hydrotalcite exhibits, after 4 h of reaction, the highest phenol ozonation activity followed by Co(3 wt%)/Al2O3–BaO and CoMgAl. The samples Co(1 wt%)/Al2O3–BaO and Co(1 and 3 wt%)/Al2O3 show a comparable medium activity, while the oxidation properties of bare oxides Co3O4, CoO and CoOx–CeO2 are really low. Leaching of cobalt ions in the water solution was detected during the reaction, the amount varied depending on the nature of catalysts. A massive release was observed for the CoMgAl and CoNiAl hydrotalcites, while cobalt catalysts over alumina and alumina–baria look much more stable. The recycle of CoOx/Al2O3 and CoOx/Al2O3–BaO was studied by performing three consecutive cycles in the phenol oxidation. Because of the potential interest of the cobalt-supported catalysts in the ozonation process, the oxidative degradation of naphtol blue black was also investigated. On the basis of TPR and UV–visible results it appears that highly dispersed Co2+ ions especially present over Co(3 wt%)/Al2O3–BaO are the main active sites for phenol and naphtol blue black oxidative degradation by ozone. 相似文献
20.
Pt–Ba–Al 2O 3 active and selective for NO x storage and selective reduction to N 2 has been prepared and tested. Characterization of the parent Al 2O 3, Pt–Al 2O 3 and Ba–Al 2O 3 materials, as well as of Pt–Ba–Al 2O 3 catalyst in the oxidized, reduced and sulphated state has been performed by FT-IR spectroscopy of low-temperature adsorbed carbon monoxide and of adsorbed acetonitrile. XRD, TEM and XPS analyses have also been performed. Evidence for the predominance of Ba species, which are highly dispersed on the alumina support surface, and may be carbonated or sulphated, has been provided. Competitive interaction of Pt and Ba species with the surface sites of alumina has also been found. 相似文献
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