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1.
Temperature-programmed reduction (TPR), oxidation (TPO), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to characterise catalysts based on manganese oxides, copper oxides or one of them mixed with platinum or palladium-supported on γ-alumina. The catalysts were characterised before and after they had been exposed either to high temperature in the presence of steam or to sulphur dioxide. Raman spectroscopy, XRD, XPS and TPR performed on the fresh samples of MnO x, mixed MnO x–Pt and MnO x–Pd revealed the presence of a mixture of manganese oxides, particularly Mn 2O 3. In the fresh mixed MnO x–Pd and CuO x–Pd samples, Pd catalysed the reduction of both MnO x and CuO x, whereas Pt only catalysed the reduction of MnO x. After hydrothermal treatment at 900°C of the MnO x, mixed MnO x–Pt and MnO x–Pd samples, there was a formation of new manganese oxide phase, Mn 3O 4 detected by Raman spectroscopy. TPR revealed increasing interaction between the metal oxides and the noble metals in the hydrothermally treated mixed MnO x–Pd and CuO x–Pd samples, and also the appearance of interaction in the treated mixed CuO x–Pt sample. The sulphur adsorbed in all the MnO x samples formed sulphate, which was more difficult to reduce than the oxides. Also, the reduction temperature of sulphates was lowered when noble metals are present. 相似文献
2.
Deactivation of catalysts based on either manganese oxides, copper oxides, platinum, palladium or combinations of these metal oxides and noble metals supported on γ-alumina was studied. The activity of the catalysts for the oxidation of carbon monoxide, naphthalene and methane, in a mixture resembling the flue gases from wood combustion, was measured before and after exposure of the catalysts either to a temperature of 900°C in the presence of steam or to sulphur dioxide. Most of the mixed catalysts were more resistant to hydrothermal and sulphur treatments than the catalysts with a single active component. After the hydrothermal treatment the activity of the MnO x catalyst was enhanced. When Pt is combined with MnO x or CuO x, the loss of activity of Pt was decreased during the hydrothermal treatment. Also, the hydrotreated mixed MnO x–Pd and CuO x–Pd catalysts were more active than the treated Pd catalyst for the oxidation of methane. After sulphur treatment, the activities of the mixed MnO x–Pt (Pt: 0.05 mol%), MnO x–Pd and CuO x–Pd catalysts were improved for the oxidation of carbon monoxide and naphthalene. Among the catalysts studied, the MnO x–Pt, CuO x–Pt and CuO x–Pd catalysts, with a metal oxide and a noble metal loading of 10 and 0.1 mol%/γ-alumina, respectively, had the best combination of activity, thermal stability and resistance to sulphur treatment. 相似文献
3.
A study of CO oxidation by O 2 over Pt catalysts, promoted by MnO x and CoO x, is described. The activities of Pt/SiO 2, Pt/MnO x/SiO 2 and Pt/CoO x/SiO 2 are compared with commercial Pt/Al 2O 3, Pt/Rh/Al 2O 3 and Pt/CeO x/Al 2O 3 catalysts. Since these catalysts differ in dispersion and weight loading of platinum, the turnover frequencies are also compared. The following order in activity in CO oxidation after a reductive pretreatment is found: Pt/CoO x/SiO 2 > Pt/MnO x/SiO 2, Pt/CeO x/Al 2O 3 > Pt/Al 2O 3, Pt/Rh/Al 2O 3, Pt/SiO 2. Over Pt/CoO x/SiO 2 CO is already oxidised at room temperature. Possible models to account for the high activity of Pt/CoO x/SiO 2 in the CO/O 2 reaction are presented and discussed. Partially reduced metal oxides are necessary to increase the activity of the Pt/CoO x/SiO 2, Pt/MnO x/SiO 2 or Pt/CeO x/Al 2O 3 catalysts. It was shown that mild ageing treatments did not affect the activity of the Pt/CoO x/SiO 2 catalyst in CO oxidation. 相似文献
4.
MnO x–CeO 2 mixed oxides prepared by sol–gel method, coprecipitation method and modified coprecipitation method were investigated for the complete oxidation of formaldehyde. Structure analysis by H 2-TPR and XPS revealed that there were more Mn 4+ species and richer lattice oxygen on the surface of the catalyst prepared by the modified coprecipitation method than those of the catalysts prepared by sol–gel and coprecipitation methods, resulting in much higher catalytic activity toward complete oxidation of formaldehyde. The effect of calcination temperature on the structural features and catalytic behavior of the MnO x–CeO 2 mixed oxides prepared by the modified coprecipitation was further examined, and the catalyst calcined at 773 K showed 100% formaldehyde conversion at a temperature as low as 373 K. For the samples calcined below 773 K, no any diffraction peak corresponding to manganese oxides could be detected by XRD measurement due to the formation of MnO x–CeO 2 solid solution. While the diffraction peaks corresponding to MnO 2 phase in the samples calcined above 773 K were clearly observed, indicating the occurrence of phase segregation between MnO 2 and CeO 2. Accordingly, it was supposed that the strong interaction between MnO x and CeO 2, which depends on the preparation route and the calcination temperature, played a crucial role in determining the catalytic activity toward the complete oxidation of formaldehyde. 相似文献
5.
The direct decomposition of nitric oxide (NO) over barium catalysts supported on various metal oxides was examined in the absence and presence of O 2. Among the Ba catalysts supported on single-component metal oxides, Ba/Co 3O 4 and Ba/CeO 2 showed high NO decomposition activities, while Ba/Al 2O 3, Ba/SiO 2, and Ba/TiO 2 exhibited quite low activities. The effect of an addition of second components to Co and Ce oxides was further examined, and it was found that the activities were significantly enhanced using Ce–Mn mixed oxides as support materials. XRD results indicated the formation of CeO 2–MnO x solid solutions with the cubic fluorite structure. O 2-TPD of the CeO 2–MnO x solid solutions showed a large desorption peak in a range of relatively low temperature. The BET surface areas of the CeO 2–MnO x solid solutions were larger than those of pure CeO 2 and Mn 2O 3. These effects caused by the addition of Mn are responsible for the enhanced activities of the Ba catalysts supported on Ce–Mn mixed oxides. 相似文献
6.
MnO x–CeO 2 mixed oxides with a Mn/(Mn + Ce) molar ratios of 0–1 were prepared by a modified coprecipitation method and investigated for the complete oxidation of formaldehyde. The MnO x–CeO 2 with Mn/(Mn + Ce) molar ratio of 0.5 exhibited the highest catalytic activity among the MnO x–CeO 2 mixed oxides. Structure analysis by X-ray powder diffraction and temperature-programmed reduction of hydrogen revealed that the formation of MnO x–CeO 2 solid solution greatly improved the low-temperature reducibility, resulting in a higher catalytic activity for the oxidation of formaldehyde. Promoting effect of Pt on the MnO x–CeO 2 mixed oxide indicated that both the Pt precursors and the reduction temperature greatly affected the catalytic performance. Pt/MnO x–CeO 2 catalyst prepared from chlorine-free precursor showed extremely high activity and stability after pretreatment with hydrogen at 473 K. 100% conversion of formaldehyde was achieved at ambient temperature and no deactivation was observed for 120 h time-on-stream. The promoting effect of Pt was ascribed to enhance the effective activation of oxygen molecule on the MnO x–CeO 2 support. 相似文献
7.
Kinetic study of CO oxidation in combination with experiments of temperature-programmed oxidation (TPO) and reduction (TPR) have been performed on various unsupported crystalline manganese oxides (MnO x); while the reactivity shows an order of MnO ≤ MnO 2 < Mn 2O 3 in a mixture of unit ratio of O 2/CO at/below 523 K. We propose that under the current conditions the interaction of adsorbed CO and O is mainly responsible for CO 2 formation on Mn 2O 3 and MnO 2 catalysts, following either the Langmuir–Hinshelwood mechanism or Eley–Rideal mechanism. Meanwhile, direct evidence from transient CO oxidation suggests that the Mars-van-Krevelen mechanism may occur for all catalysts simultaneously, especially, it is predominant for the MnO catalyst. The evidence of structural modifications during reaction was confirmed by Raman spectra obtained from used MnO. 相似文献
8.
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
9.
The catalytic activity of a series of CeO 2–ZrO 2 mixed oxides in the total oxidation of methane and light hydrocarbons has been investigated. The influence of dopants like Mn and Cu has also been studied. It is shown that both MnO x and CuO at low loading dissolve within the ceria–zirconia lattice. This strongly influences the redox behaviour of the catalysts by promoting low-temperature reduction of Ce 4+. In addition, the ternary oxides show better stability to repeated redox cycles, which is attributed to the presence of ZrO 2. The catalytic activity of pure CeO 2 is also enhanced in the presence of ZrO 2, reaching a maximum with Ce 0.92Zr 0.08O 2; a further promotion of activity is observed with the introduction of MnO x and CuO dissolved into CeO 2–ZrO 2 lattice. 相似文献
10.
Surface properties of supported MoV heteropoly compounds and their activities in hydrodesulfurization of thiophene and oxidation of ethanol were studied. Vanadium incorporated into the phosphomolybdic acid anion increased extent of oxidative dehydrogenation of ethanol to acetaldehyde, whereas the catalyst with (VO) 2+ increased complete ethanol oxidation to CO 2. The catalysts with (VO) 2+ or one V atom in the anion showed an increase in HDS activity. Vanadium in anion of phosphomolybdic acid also increased hydrogenation ability. Acidic sites of medium strength were proved to be the most suitable for ethanol oxidation. Such explicit dependence of thiophene hydrodesulfurization on any type of acidic sites was not confirmed. Activity order of supported MoV heteropoly compounds in HDS of thiophene correlated well with the amount of hydrogen consumed during TPR. 相似文献
11.
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
12.
Gas-phase oxidation of benzene using a mixture of oxygen and hydrogen has been carried out on silica-supported vanadium oxide catalysts modified with platinum or palladium. Catalyst activity and phenol selectivity were studied as a function of the precious metal used, the vanadium oxide loading as well as of temperature. The binary catalysts have been characterized by TPR and TEM. Pt-VO x/SiO 2 catalysts were more active than Pd-VO x/SiO 2 catalysts. By using platinum catalysts benzene conversion amounted to 1.0% ( Sphenol=97%) at 413 K, whereas palladium catalysts reached a conversion of only 0.2% ( Sphenol=86%) for the same contact time and temperature. The most active catalyst for the oxidation of benzene to phenol was a low vanadium loaded 0.5 wt.% Pt–3 wt.% V on silica catalyst. At temperatures above 413 K phenol selectivity decreased strongly because of enhanced total oxidation. Active catalysts need both components: a dispersed transition metal oxide such as VO x as well as small precious metal particles such as platinum. The activity of the catalysts arises from a close interaction between the redox-active compound VO x and the electron mediator and hydrogen activator platinum as was confirmed by correlation of catalytic results and catalyst properties. Highly dispersed platinum particles are exclusively located on the vanadium oxide covered surface as demonstrated by TEM investigations. TPR studies showed and enhanced reducibility of a part of vanadium(V) oxide indication a close neighborhood of VO x and platinum. 相似文献
13.
The catalytic activity of Pt on alumina catalysts, with and without MnO x incorporated to the catalyst formulation, for CO oxidation in H 2-free as well as in H 2-rich stream (PROX) has been studied in the temperature range of 25–250 °C. The effect of catalyst preparation (by successive impregnation or by co-impregnation of Mn and Pt) and Mn content in the catalyst performance has been studied. A low Mn content (2 wt.%) has been found not to improve the catalyst activity compared to the base catalyst. However, catalysts prepared by successive impregnation with 8 and 15 wt.% Mn have shown a lower operation temperature for maximum CO conversion than the base catalyst with an enhanced catalyst activity at low temperatures with respect to Pt/Al 2O 3. A maximum CO conversion of 89.8%, with selectivity of 44.9% and CO yield of 40.3% could be reached over a catalyst with 15 wt.% Mn operating at 139 °C and λ = 2. The effect of the presence of 5 vol.% CO 2 and 5 vol.% H 2O in the feedstream on catalysts performance has also been studied and discussed. The presence of CO 2 in the feedstream enhances the catalytic performance of all the studied catalysts at high temperature, whereas the presence of steam inhibits catalysts with higher MnO x content. 相似文献
14.
Catalytic methane combustion and CO oxidation were investigated over AFeO 3 (A=La, Nd, Sm) and LaFe 1−xMg xO 3 ( x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe 4+ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m 2 g −1 range. XRD analysis showed that LaFeO 3, NdFeO 3, SmFeO 3 and LaFe 1−xMg xO 3 ( x·0.3) are single phase perovskite-type oxides. Traces of La 2O 3, in addition to the perovskite phase, were detected in the LaFe 1−xMg xO 3 catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO 3 of a very small fraction of Fe 4+ which reduces to Fe 3+. The fraction of Fe 4+ in the LaFe 1−xMg xO 3 samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO 2 below 973 and 773 K, respectively. For the AFeO 3 materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe 1−xMg xO 3 catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase ( x=0.3). Concerning the CO oxidation, the order of activity for the AFeO 3 materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe 1−xMg xO 3 catalysts decreases at high magnesium content. 相似文献
15.
Ce xTi 1−xO 2 oxides have been synthesised by sol–gel method with x varying from 0 to 0.3 and characterised by XRD and TPR. The structure of oxides changes with the Ce/Ti molar ratio. The presence of ceria in Ce-Ti oxides inhibits the phase transition from anatase to rutile. When x = 0.3 (Ce 0.3Ti 0.7O 2 sample), the solid presents an amorphous state. The TPR results indicate that the presence of Ti enhances the reducibility of cerium oxide species. Catalytic oxidation of propene is investigated on Ce-Ti oxides and the better conversion is obtained with Ce 0.3Ti 0.7O 2 but the CO 2 selectivity reaches 63% at 400 °C. Gold is then deposited on theses oxides to improve the catalytic activity. On the basis of characterisation data (H 2 TPR), it has been suggested that gold influences the reduction of the Ce-Ti oxide support and the catalytic activity to the propene oxidation. Thus, Au/Ce-Ti-O system catalysts are promising catalysts for propene oxidation. 相似文献
16.
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
17.
CrO x and CrO x supported on SiO 2 have been found to be active for the selective oxidation of hydrogen sulfide to elemental sulfur. The catalysts show maximum sulfur yield at a stoichiometric ratio of O 2/H 2S, 0.5. Amorphous Cr 2O 3 exhibits higher yield of sulfur and has stronger resistance against water than supported Cr/SiO 2, especially at low temperatures. At high temperatures above 300°C, the sulfur yield over the supported catalyst becomes similar to amorphous Cr 2O 3 because the Claus reaction occurring on the silica support removes SO 2 to increase the sulfur yield. Active sites are the amorphous monochromate species that can be detected as a strong temperature programmed reduction (TPR) peak at 470°C. Catalytic activity can be correlated with the amount of labile lattice oxygen and the strength of Cr–O bonding. The reaction proceeds via the redox mechanism with participation of lattice oxygen. 相似文献
18.
Various vanadium-containing catalysts were searched for the commercial application in the selective oxidation of H 2S to elemental sulfur at low temperatures (less than 250°C) in the presence of excess (more than 35 vol.%) water. In the test of binary oxides, it was found that TiVO x was the only catalyst that could sustain its activity without deactivation at 230°C. The best catalytic activity (85–90% sulfur yield) was obtained when VO x/TiO 2 was incorporated with other metals such as Fe, Cr and Mo. Reaction occurred via redox mechanism and the reoxidation of reduced vanadium was the rate-limiting step. A long-term deactivation observed during the reaction was due to slower reoxidation of reduced vanadium by oxygen than the reduction by H 2S. Catalytic activities of VO x/SiO 2, VO x/TiO 2 and V–Fe–Cr–Mo–O x/TiO 2 were well correlated with their redox properties that were observed by TPR/TPO and XPS measurements. 相似文献
19.
The performance of unpromoted and MO x-(M: alkali (earth), transition metal and cerium) promoted Au/Al 2O 3 catalysts have been studied for combustion of the saturated hydrocarbons methane and propane. As expected, higher temperatures are required to oxidize CH 4 (above 400 °C), compared with C 3H 8 (above 250 °C). The addition of various MO x to Au/Al 2O 3 improves the catalytic activity in both methane and propane oxidation. For methane oxidation, the most efficient promoters to enhance the catalytic performance of Au/Al 2O 3 are FeO x and MnO x. For C 3H 8 oxidation a direct relationship is found between the catalytic performance and the average size of the gold particles in the presence of alkali (earth) metal oxides. The effect of the gold particle size becomes less important for additives of the type of transition metal oxides and ceria. The results suggest that the role of the alkali (earth) metal oxides is related to the stabilization of the gold nanoparticles, whereas transition metal oxide and ceria additives may be involved in oxygen activation. 相似文献
20.
Ce- or Sr-doped LaCoO 3 bulk perovskites were prepared by citric acid method as well as 10 wt.% of LaCoO 3 was deposited on alumina carrier stabilized with lanthanum. Properties of prepared materials were characterized by determination of surface area, acid-basic properties and XRD, XPS, TPDO 2, H 2-TPR measurements as well as catalytic activity and selectivity for ethanol combustion was tested. It was found that substitution of La in LaCoO 3 with either Sr or Ce has only small effect on its activity in ethanol combustion. Strontium inserted into LaCoO 3 structure increases basic character of the perovskite surface as well as selectivity to acetaldehyde (ACA). Substitution of La with cerium has no effect on the concentration of basic sites and does not affect the selectivity to ACA. Activity of LaCoO 3-based catalysts in ethanol combustion and their selectivity to ACA formation can be explained on the basis of the presence of both -oxygen species and sites with basic character on the material surface. Acid-basic properties of supported LaCoO3 are dominated by acidic character of the carrier. Results of XPS and H2-TPR measurements of LaCoO3 supported on La–Al2O3 suggest that perovskite remains in strong interaction with carrier and probably is partially decomposed. Deposition of perovskite on stabilized carrier significantly increases the rate of ethanol combustion. 相似文献
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