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1.
Vanadium oxides supported on γ-Al 2O 3, SiO 2, TiO 2, and ZrO 2 were studied on their molecular structures and reactive performances for soot combustion. To investigate the effect of different alkali metals on the structures and reactivities of supported-vanadium oxide catalysts, they were doped into the V 4/TiO 2 catalyst which had the best intrinsic activity for soot combustion in the selected supported vanadium oxide catalysts. The experimental results demonstrated that the catalytic properties of these catalysts depended on the vanadium loading amount, support nature, and the presence or the absence of alkali metals. The spectroscopic analysis (FT-IR and UV–vis) and H 2-TPR results revealed that the higher activity of alkali-promoted vanadium oxide catalysts could be related to the ability of alkali metal promoting the redox cycle of the active vanadyl species. TG results showed that adding alkali to V m/TiO 2 catalyst was beneficial to lowering their melting points. Low melting points could ensure the good surface atom migration ability, which would improve the contact between the catalyst and soot. Due to the alkali metal components promoting the redox ability and the mobility of the catalysts, alkali-modified vanadium oxide catalysts could remarkably improve their catalytic activities for soot combustion. The catalytic activity order for soot combustion followed Li > Na > K > Rb > Cs in the catalyst system of alkali-V 4/TiO 2, and the reason why it followed this sequence was discussed. 相似文献
2.
FeO x/ZrO 2 samples, prepared by impregnation with Fe(NO 3) 3, were characterised by means of DRS, XRD, FTIR, redox cycles and volumetric CO adsorption. Volumetric CO adsorption, combined with FTIR, showed that 45% of iron in the sample containing 2.8 Fe atoms nm −2 was capable of forming iron carbonyls. DRS evidenced Fe 2O 3 on samples with Fe-content≥2.8 atoms nm −2. The selective catalytic reduction of NO with C 3H 6 in the presence of O 2 was studied with a reactant mixture containing NO=4000 ppm, C 3H 6=4000 ppm, O 2=2%. The dependence on iron-content suggests that only isolated iron, prevailing in dilute FeO x/ZrO 2, is active for NO reduction, whereas iron on the surface of small oxide particles, prevailing in concentrated FeO x/ZrO 2, is active for C 3H 6 combustion. 相似文献
3.
The nanometer particles of two Fe xO y/TiO 2’s with high photocatalytic activities were obtained through hydrothermal treatment and impregnation method. The XRD result did not show the peaks assigned to the Fe components (for example Fe 2O 3, Fe 3O 4, FeO 3, and Fe metal) on the external surface of the anatase structure in the Fe xO y/TiO 2 attained through hydrothermal treatment. This meant that Fe components were well incorporated into the TiO 2 anatase structure. In addition, it exhibited uniform anatase structure with particle size of below 50 nm. The FeO 3 component on the external surface of the TiO 2 anatase structure was identified in the Fe-loaded TiO 2 prepared through the impregnation method. In particular, the FT-IR spectroscopy revealed that the Fe xO y/TiO 2 particle attained through hydrothermal treatment had higher hydrophilic property compared to the other catalysts. Together with the Fe component, they absorbed wavelength of above 370 nm. The band slightly shifted to the right without tail broadness, which was the UV absorption of Fe oxide in the Fe xO y/TiO 2 particle attained through hydrothermal method. This meant that Fe components were well inserted into the framework of the TiO 2 anatase structure. Despite the red shift in UV-Vis absorption, however, CHCl 3 decomposition on the Fe xO y/TiO 2 catalyst was not largely enhanced compared to pure TiO 2. 相似文献
4.
Correlations between the electronic and chemical properties of perovskites, molybdates, and metal-doped MgO or CeO 2 are examined. Simple models based on band-orbital mixing can explain trends found for the interaction of these catalytic materials with adsorbates: the less stable the occupied levels of a mixed-metal oxide, the higher its chemical reactivity. Metal↔oxygen↔metal interactions are common in mixed-metal oxides and can lead to substantial changes in the electronic and chemical properties of the cations. This is particularly true in the case of ABO 3 perovskites (A=Pb, Ca, Sr, Li, K, Na; B=Ti, Zr, Nb), and it is an important phenomenon that has to be considered when mixing AO and BO 2 oxides for catalytic applications. In systems like Ce 1−xZr xO 2 and Ce 1−xCa xO 2, the structural stress induced by the dopant (Zr or Ca) leads to perturbations in the electronic properties of the Ce cations. The trends in the behavior of metal-doped MgO illustrate a basic principle in the design of mixed-metal oxide catalysts for DeNO x and DeSO x operations. The general idea is to find metal dopants that upon hybridization within an oxide matrix remain in a relatively low oxidation state and at the same time induce occupied electronic states located well above the valence band of the host oxide. Electronic effects should not be neglected a priori when explaining the behavior or dealing with the design of mixed-metal oxide catalysts. 相似文献
5.
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
6.
The catalytic reduction of NO x with hydrocarbons (butane or methane) on CoMOR washcoated monolithic catalysts was studied in the presence of steam and excess oxygen. The significant changes observed in the catalytic behavior of CoMOR powder and monoliths depended essentially on the hydrocarbon nature (carbon number) and the concentration of water in the feed. When the reducing agent was methane, a low concentration of water (2%) decreased the NO to N 2 conversion. However, when butane was used instead of methane, the maximum NO x conversions increased from 50 to 58% and from 52 to 64% for the CoMOR powder and monolith, respectively. The presence of water inhibited the NO adsorption when the reducing agent was methane but when butane was used, water helped to remove the surface-carbon deposits as indicated by TPO and XPS results. This fact explains the increase observed in the NO x conversion. The characterization with TPR and UV–vis spectroscopy showed that the main Co species present in the selective catalysts were the Co(II) ions exchanged at different sites of the mordenite and highly dispersed Co xO y moieties. More rigorous reaction conditions, i.e. 10% of water, led to the irreversible deactivation with both reductants. The Co 3O 4 phase was detected in all the deactivated powder and monolithic catalysts. The Co 3O 4 spinel was formed from the cobalt ion migration, which was promoted in wet atmosphere. In addition, for monolithic catalysts washcoated with CoMOR, the silica binder inhibited the water deactivation effect probably due to the silica–cobalt interaction, as a Co xO ySi silicate. 相似文献
7.
Nine different metal oxide catalysts were prepared by impregnating alumina washcoats with water solutions containing La 3+, Sr 2+, Cu 2+ and Ru 3+ ions and calcining them at 900°C. The produced samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) studies combined with energy-dispersive spectroscopy (EDS) analysis, X-ray powder diffraction and specific surface area measurements. A perovskite phase of the nominal composition La 1-xSr xAl 1-2yCu yRu yO 3 was found in all samples, in increasing amount in the samples with increasing contents of strontium and ruthenium. The catalysts were evaluated with respect to light-off temperatures and redox characteristics using two gas mixtures, one containing NO/CO/C 3H 6/O 2/N 2 and the other NO/CO/N 2. The light-off temperatures for nitric oxide reduction decreased from 534 to 333°C for the catalysts without and with strontium and ruthenium, respectively. In the presence of oxygen the conversion of nitric oxide declined rapidly under oxidative conditions whereas in absence of oxygen this decline was less pronounced and found to be linear over the entire redox interval studied. These studies suggest that the perovskite phase takes an active part in the conversion of nitric oxide and carbon monoxide to nitrogen and carbon dioxide. 相似文献
8.
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
9.
Catalytic performance for partial oxidation of methane (POM) to synthesis gas was studied over the Rh/Al 2O 3 catalysts with Rh loadings between 0.1 and 3 wt%. It was found that the ignition temperature of POM reaction increased with the decreasing of the Rh loadings in the catalysts. For the POM reaction over the catalysts with high (≥1 wt%) Rh loadings, steady-state reactivity was observed. For the reaction over the catalysts with low (≤0.25 wt%) Rh loadings, however, oscillations in CH 4 and reaction products (CO, H 2, and CO 2) were observed. Comparative studies using H 2-TPR, O 2-TPD and high temperature in situ Raman spectroscopy techniques were carried out in order to elucidate the relation between the redox property of the Rh species in the Rh/Al 2O 3 with different Rh loadings and the performance of the catalysts for the reaction. Three kinds of oxidized rhodium species, i.e. the rhodium oxide species insignificantly affected by the support (RhO x), that intimately interacting with the Al 2O 3 surface (Rh iO x) and the Rh(AlO 2) y species formed by diffusion of rhodium oxides in to sublayers of Al 2O 3 [C.P. Hwang, C.T. Yeh, Q.M. Zhu, Catal. Today, 51 (1999) 93.], were identified by H 2-TPR and O 2-TPD experiments. Among them, the first two species can be easily reduced by H 2 at temperature below 350 °C, while the last one can only be reduced by H 2 at temperature above 500 °C. The ignition temperatures of POM reaction over the catalysts are closely related to the temperature at which most of the RhO x and Rh iO x species can be reduced by CH 4 in the reaction mixture. Compared to the Rh/Al 2O 3 with high Rh loadings, the catalysts with low Rh loadings contain more Rh iO x species which possess stronger RhO bond strength and are more difficult to be reduced than RhO x by the reaction mixture. Higher temperature is therefore required to ignite the POM reaction over the catalysts with lower Rh loadings. The oscillation during the POM reaction over the Rh/Al 2O 3 with low Rh loadings can be related to the behaviour of Rh(AlO 2) y species in the catalyst switching cyclically from the oxidized state to the reduced state during the reaction. 相似文献
10.
Structural, redox and catalytic deep oxidation properties of LaAl 1−xMn xO 3 ( x=0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073 K were investigated. XRD analysis showed that all the LaAl 1−xMn xO 3 samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280–1180 Å and 4–33 m 2 g −1 ranges, respectively. Redox properties and the content of Mn 4+ were derived from temperature programmed reduction (TPR) with H 2. Two reduction steps are observed by TPR for pure LaMnO 3, the first attributed to the reduction of Mn 4+ to Mn 3+ and the second due to complete reduction of Mn 3+ to Mn 2+. The presence of Al in the LaAl 1−xMn xO 3 solid solutions produces a strong promoting effect on the Mn 4+→Mn 3+ reducibility and inhibits the further reduction to Mn 2+. Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO 3, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion. 相似文献
11.
The activity of cobalt-containing catalysts based on sulfated zirconium dioxide in selective catalytic reduction of NO by methane depends on the amount of sulfur and the preparation method. Modification by Fe and Mn improves the catalytic behavior of SO 42−/ZrO 2 as a result of the increase in the concentration of active sites. The SCR-activity of the oxide catalysts based on zirconium dioxide depends on the chemical nature and strength of redox and acid centers of the catalysts. 相似文献
12.
The water-gas shift (WGS) activity of platinum catalysts dispersed on a variety of single metal oxides as well as on composite MO x/Al 2O 3 and MO x/TiO 2 supports (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Ho, Er, Tm) has been investigated in the temperature range of 150–500 °C, using a feed composition consisting of 3% CO an 10% H 2O. For Pt catalysts supported on single metal oxides, it has been found that both the apparent activation energy of the reaction and the intrinsic rate depend strongly on the nature of the support. In particular, specific activity of Pt at 250 °C is 1–2 orders of magnitude higher when supported on “reducible” compared to “irreducible” metal oxides. For composite Pt/MO x/Al 2O 3 and Pt/MO x/TiO 2 catalysts, it is shown that the presence of MO x results in a shift of the CO conversion curve toward lower reaction temperatures, compared to that obtained for Pt/Al 2O 3 or Pt/TiO 2, respectively. The specific reaction rate is in most cases higher for composite catalysts and varies in a manner which depends on the nature, loading, and primary crystallite size of dispersed MO x. Results are explained by considering that reducibility of small oxide particles increases with decreasing crystallite size, thereby resulting in enhanced WGS activity. Therefore, evidence is provided that the metal oxide support is directly involved in the WGS reaction mechanism and determines to a significant extent the catalytic performance of supported noble metal catalysts. Results of catalytic performance tests obtained under realistic feed composition, consisting of 3% CO, 10% H 2O, 20% H 2 and 6% CO 2, showed that certain composite Pt/MO x/Al 2O 3 and Pt/MO x/TiO 2 catalysts are promising candidates for the development of active WGS catalysts suitable for fuel cell applications. 相似文献
13.
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
14.
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
15.
The selective catalytic reduction (SCR) of NO x (NO + NO 2) by NH 3 in O 2 rich atmosphere has been studied on Cu-FAU catalysts with Cu nominal exchange degree from 25 to 195%. NO 2 promotes the NO conversion at NO/NO 2 = 1 and low Cu content. This is in agreement with next-nearest-neighbor (NNN) Cu ions as the most active sites and with N xO y adsorbed species formed between NO and NO 2 as a key intermediate. Special attention was paid to the origin of N 2O formation. CuO aggregates form 40–50% of N 2O at ca. 550 K and become inactive for the SCR above 650 K. NNN Cu ions located within the sodalite cages are active for N 2O formation above 600 K. This formation is greatly enhanced when NO 2 is present in the feed, and originated from the interaction between NO (or NO 2) and NH 3. The introduction of selected co-cations, e.g. Ba, reduces very significantly this N 2O formation. 相似文献
16.
The effects of ZrO 2 content on the CO oxidation activity in a series of CuO x/Ce xZr 1−xO 2 ( x = 0, 0.15, 0.5, 0.7 and 1) catalysts were investigated, both in the absence and in the presence of H 2, i.e. preferential CO oxidation—PROX. The investigation was performed under light-off conditions to focus the effects of transients and shut-down/start-up cycles on the performance; such phenomena are expected to affect the activity of PROX catalysts in small/delocalised fuel reformers. Evidence has been obtained for a transition from an “oxidized” towards a “reduced” state of the catalyst under the simulated PROX reaction conditions as a function of the reaction temperature, leading to different active species under the reaction conditions. Both CO oxidation activity and PROX selectivity appear to be affected by this process. IR characterisation of the surface copper species suggests an important role of reduced cerium sites in close contact with copper clusters on the CO oxidation activity at low temperatures. 相似文献
17.
Manganese–cerium mixed oxide catalysts with different molar ratio Mn/(Mn + Ce) (0, 0.25, 0.50, 0.75, 1) were prepared by citric acid method and investigated concerning their adsorption behavior, redox properties and behavior in the selective catalytic reduction of NO x by NH 3. The studies based on pulse thermal analysis combined with mass spectroscopy and FT-IR spectroscopy uncovered a clear correlation between the dependence of these properties and the mixed oxide composition. Highest activity to nitrogen formation was found for catalysts with a molar ratio Mn/(Mn + Ce) of 0.25, whereas the activity was much lower for the pure constituent oxides. Measurements of adsorption uptake of reactants, NO x (NO, NO 2) and NH 3, and reducibility showed similar dependence on the mixed oxide composition indicating a clear correlation of these properties with catalytic activity. The adsorption studies indicated that NO x and NH 3 are adsorbed on separate sites. Consecutive adsorption measurements of the reactants showed similar uptakes as separate measurements indicating that there was no interference between adsorbed reactants. Mechanistic investigations by changing the sequence of admittance of reactants (NO x, NH 3) indicated that at 100–150 °C nitrogen formation follows an Eley–Rideal type mechanism, where adsorbed ammonia reacts with NO x in the gas phase, whereas adsorbed NO x showed no significant reactivity under conditions used. 相似文献
18.
In the off-gases of internal combustion engines running with oxygen excess, non-thermal plasmas (NTPs) have an oxidative potential, which results in an effective conversion of NO to NO 2. In combination with appropriate catalysts and ammonia (NH 3-SCR) or hydrocarbons (HC-SCR) as a reducing agent, this can be utilized to reduce nitric oxides (NO and NO 2) synergistically to molecular nitrogen. The combination of SCR and cold plasma enhanced the overall reaction rate and allowed an effective removal of NOX at low temperatures. Using NH3 as a reducing agent, NOX was converted to N2 on zeolites or NH3-SCR catalysts like V2O5–WO3/TiO2 at temperatures as low as 100–200 °C. Significant synergetic effects of plasma and catalyst treatment were observed both for NH3 stored by ion exchange on the zeolite and for continuous NH3 supply. Certain modifications of Al2O3 and ZrO2 have been found to be effective as catalysts in the plasma-assisted HC-SCR in oxygen excess. With an energy supply of about 30 eV/NO-molecule, 500 ppm NO was reduced by more than half at a temperature of 300 °C and a space velocity of 20 000 h−1 at the catalyst. The synergistic combinations of NTP and both NH3- and HC-SCR have been verified under real diesel engine exhaust conditions. 相似文献
19.
Atomic layer epitaxy (ALE), a technique relying on saturating gas–solid reactions, was applied in the preparation of CrO x/Al 2O 3 catalysts using Cr(acac) 3 vapor and air as source materials for CrO x. Vaporized Cr(acac) 3 was reacted with preheated Al 2O 3, and the surface complex formed was treated with air to remove the ligand residues. The Cr loading increased from 1.3 to 12.5 wt.% as the number of saturating Cr(acac) 3 and air reactions was increased from one to 10. CrO x/Al 2O 3 catalysts were also prepared from solution by incipient wetness impregnation (0.3–21 wt.%). XPS and UV–VIS measurements of the catalysts revealed the presence of both Cr 6+ and Cr 3+. Although the oxidation state distribution was similar, H 2-temperature programmed reduction (TPR) and solubility measurements indicated that Cr 6+ surface sites were in stronger interaction with Al 2O 3 and more uniformly distributed in the catalysts prepared by ALE than by impregnation. On the basis of the activity of the catalysts in the dehydrogenation of i-butane, we propose that the dehydrogenation reaction uses both reduced Cr 6+, i.e. redox Cr 3+, and exposed non-redox Cr 3+ sites. Furthermore, the dehydrogenation reaction must be insensitive to the size of the CrO x ensembles since activities were similar for the catalysts prepared by ALE and impregnation. The decay of the dehydrogenation activity in successive prereduction–reaction–regeneration cycles was attributed to a decrease in the number of redox Cr 3+ sites. 相似文献
20.
The effect of treatment with different mineral acids (H 2SO 4, H 3PO 4, HNO 3 and HCl) on the activity of monolithic CoO x/γ-Al 2O 3 catalysts in the reduction of nitric oxide with methane in the presence of oxygen (CH 4-SCR of NO x) was studied. Their behaviour in the methane oxidation reaction in both the presence and absence of NO x was determined in order to interpret the results in terms of intrinsic activity and competition between both processes. Depending on the nature of the acid used, significant differences were observed in the catalytic activities which were related to the textural states, surface acidities and the nature of the detected species. The best results were obtained after treatment with H 2SO 4, which increased the activity towards NO x elimination compared to the other catalysts. This behaviour was attributed not only to an increase in surface acidity but also to the stabilisation of the active Co 2+ species, thus avoiding the formation of Co 3O 4 spinel that is responsible for the strongly adsorbed NO x species that lead to NO 2 formation which increase the rate of the undesired methane oxidation reaction at high temperatures. 相似文献
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