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1.
Spectra of the adsorbed species arising from contact of a V 2O 5-MoO 3-TiO 2 model SCR catalyst with ortho-dichlorobenzene ( o-DCB) and dibenzofuran (DBF) and their evolution with the temperature are presented and discussed. Dichlorobenzene adsorbs weakly probably on Lewis acid sites through the chlorine atom. A very fast nucleophilic substitution on dichlorobenzene to a chlorophenate species occurs already at RT. On the contrary, adsorption of dibenzofuran is molecular, probably through the oxygen atom on Lewis sites. The aromatic rings of both molecules tend to be later oxidized to give carboxylate species. Parallel experiments with chloropropane show that dehydrochlorination occurs readily, hydrochloric acid is adsorbed quite weakly and that propene can be further oxidized. However, heavier oxidized species like cyclic anhydrides are also formed from chloropropane. The data suggest that the dechloration step of the phenyl ring could be not critical. On the contrary, the building-up of aromatic compounds from smaller molecules is possible and the oxidation of phenyl ring can be slow near reaction conditions. The possibility that the reaction between phenate species and chlorobenzenes give the “de novo” synthesis of dioxins is envisaged. 相似文献
2.
Zirconium-containing mesoporous silica of nominal Si/Zr ratio in the range 40–5 have been synthesized using dodecylamine as a structure-directing surfactant. The samples were characterized with PXRD, SEM/EDX, BET, CEC measurement and chemical analysis. The materials were used as supports for chromium species incorporated by means of cation exchange procedure. Gradual substitution of Si by Zr in the mesoporous framework results in an increasing structural disorder, increasing cation exchange capacity and increasing capacity for incorporation of Cr. All Cr-doped ZrMMS x catalysts are active in the deep oxidation of methylene chloride reaching 100% conversion at temperature ≥400°C. The catalytic performance depends strongly on the catalyst composition. The role of particular components in determining the catalytic activity and selectivities to various products is discussed. 相似文献
3.
The deactivation by sulfur and regeneration of a model Pt/Ba/Al 2O 3 NO x trap catalyst is studied by hydrogen temperature programmed reduction (TPR), X-ray diffraction (XRD), and NO x storage capacity measurements. The TPR profile of the sulfated catalyst in lean conditions at 400 °C reveals three main peaks corresponding to aluminum sulfates (550 °C), “surface” barium sulfates (650 °C) and “bulk” barium sulfates (750 °C). Platinum plays a role in the reduction of the two former types of sulfates while the reduction of “bulk” barium sulfates is not influenced by the metallic phase. The thermal treatment of the sulfated catalyst in oxidizing conditions until 800 °C leads to a stabilization of sulfates which become less reducible. Stable barium sulfides are formed during the regeneration under hydrogen at 800 °C. However, the presence of carbon dioxide and water in the rich mixture allows eliminating more or less sulfides and sulfates, depending on the temperature and time. The regeneration in the former mixture at 650 °C leads to the total recovery of the NO x storage capacity even if “bulk” barium sulfates are still present on the catalyst. 相似文献
4.
The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al 2O 3 and 1 wt.% Pd/20 wt.% BaO/Al 2O 3 NO x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al 2O 3 catalyst exhibits higher overall NO x reduction activity. Limited amounts of N 2O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al 2O 3 and Pd/Al 2O 3 catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al 2O 3. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity ( i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity ( i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375 °C. 相似文献
5.
The performance of the active catalyst 5%V 2O 5-1.9%MgO/TiO 2 in propane oxidative dehydrogenation is investigated under various reactant contact modes: co-feed and redox decoupling using fixed bed and co-feed using fluid bed. Using fixed bed reactor under co-feed conditions, propane is activated easily on the catalyst surface with selectivities ranging from 30 to 75% depending on the degree of conversion. Under varying oxygen partial pressures, especially for higher than the stoichiometric ratio O 2/C 3H 8 = 1/2, nor the propane conversion or the selectivities to propene and COx are affected. The performance of the catalyst in the absence of gas phase oxygen was tested at 400 °C. It was confirmed that the catalyst surface oxygen participates to the activation of propane forming propene and oxidation products with similar selectivities as those obtained under co-feed conditions. The ability of the catalyst to fully restore its activity by oxygen treatment was checked in repetitive reduction–oxidation cycles. Fluid bed reactor using premixed propane–oxygen mixtures was also employed in the study. The catalyst was proved to be very active in the temperature range 300–450 °C attaining selectivities comparable to those of fixed bed. 相似文献
6.
The catalytic combustion of chlorobenzene on a 2 wt.% Pt/γ-Al 2O 3 catalyst in binary mixtures with various hydrocarbons (toluene, benzene, cyclohexane, cyclohexene, 1,4-cyclohexadiene, 2-butene, and ethene) and with carbon monoxide has been explored. For all binary mixtures used the (excess of) added hydrocarbon increased the rate of conversion of chlorobenzene. With 2-butene, T50% and T100% for chlorobenzene were reduced by 100 and 200°C, respectively. Toluene and ethene were almost equally efficient as 2-butene. Co-feeding benzene or carbon monoxide resulted in a much smaller decrease of the T50%. The additional heat and water production in hydrocarbon combustion may contribute to some extent to the observed rate acceleration, but removal of Cl from the surface due to the hydrocarbon appears to be the major factor. The co-feeding of hydrocarbons invariably reduced the output of polychlorinated benzenes, which are formed as byproducts in the combustion of chlorobenzene on Pt/γ-Al2O3. Again, especially toluene, ethene, and 2-butene were very efficient. Benzene — as well as cyclohexane, cyclohexene, and 1,4-cyclohexadiene, which were converted in situ into benzene — was much less effective, due to chlorination of the aromatic nucleus. In chlorobenzene–CO mixtures the levels of polychlorinated benzenes were almost as high as with chlorobenzene per se. Removal of Cl from the surface (mainly in the form of HCl) by (non-aromatic) hydrocarbons is responsible for reducing the formation of byproducts. 相似文献
7.
Catalytic hydrodechlorination (HDCl) of 2,4,4′,6-tetrachlorobiphenyl and 2,3,4,5-tetrachlorobiphenyl has been studied in the presence of a sulphided Ni–Mo/γ-Al 2O 3 catalyst. The reaction runs were carried out at constant temperature and pressure in a stirred batch reactor using hexadecane as the reaction medium. Temperature levels of T=250°C and T=300°C at a hydrogen pressure of PH2=20 bar were tested. The results demonstrate that the HDCl process proceeds via an irreversible stepwise pattern. The kinetic constants of all observed intermediate HDCl steps were evaluated. The selectivity of HDCl reactions has been interpreted on the basis of electrophilic aromatic substitution mechanism. It shows that the HDCl rate is slower when carbon atoms adjacent to a substituted chlorine atom are bonded to other chlorine atoms. This depends on the inductive effect of chlorine atoms. It also shows that ortho-substitution is slower than meta- and para-substitution. In this case, both steric and electronic effects are probably relevant. 相似文献
8.
In the present work, the mechanism of charging/discharging at the amorphous manganese oxide electrode was investigated in 0.1 M Na 2SO 4 solution with respect to amount of hydrates and valence (oxidation) states of manganese using a.c.-impedance spectroscopy, anodic current transient technique and cyclic voltammetry. For this purpose, first the amorphous manganese oxide film was potentiostatically electrodeposited, followed by heat-treatment at 25–400 °C to prepare the electrode specimen with different amounts of hydrates and oxidation states of manganese. For as-electrodeposited electrode with the most hydrates, the anodic current transient clearly exhibited a linear relationship between the logarithm of current density and the logarithm of time, with a slope of −0.5, indicating that the charging/discharging is purely limited by Na +/H + ion diffusion. From the analyses of the impedance spectra combined with anodic current transients measured on the hydrated electrode heat-treated at 25–150 °C, it was found that as the amount of hydrates decreases, the depth of cation diffusion in the electrode becomes shallower and the ratio of charge-transfer resistance to diffusion resistance also increases. This suggests that a transition occurs of pure diffusion control to a mixed diffusion and charge-transfer reaction control. For the dehydrated electrode heat-treated at 200–400 °C, the charging/discharging purely proceeds by the charge-transfer reaction. The reversibility of the redox reaction increases with increasing amount of hydrates and oxidation states of manganese, which provides us the higher power density. On the other hand, the pseudocapacitance decreases in value with increasing heat-treatment temperature, thus causing the lower energy density. 相似文献
9.
Chromium spinels M IICr 2O 4 (M II = Co, Mn, Fe, Mg, Cu) of the same structure but different reducibility, were found new, promising supports for Au nanoparticles in oxidation of CO, including selective (preferential) oxidation in the presence of hydrogen. Oxidation of CO on Au/M IICr 2O 4 occurred already at room temperature, the activity depends on the nature of M II (100% conversion of CO at 50 °C on Au/CoCr 2O 4 was observed). The activity in CO oxidation increases with the increasing reducibility of the supports and the catalysts, measured by H 2TPR, and with the increasing potential of the M II/M III redox couple (for Co, Mn, and Fe spinels). This shows the role of the support redox properties in Au-based catalysts. In the presence of hydrogen (selective oxidation of CO), the catalysts are more active and show high (90–100%) selectivity to CO 2. 相似文献
10.
A 5 wt% CoO x/TiO 2 catalyst has been used to study the effect of calcination temperature on the activity of this catalyst for CO oxidation at 100 °C under a net oxidizing condition in a continuous flow type fixed-bed reactor system, and the catalyst samples have been characterized using TPD, XPS and XRD measurements. The catalyst after calcination at 450 °C gave highest activity for this low-temperature CO oxidation, and XPS measurements yielded that a 780.2-eV Co 2p 3/2 main peak appeared with this catalyst sample and this binding energy was similar to that measured with pure Co 3O 4. After calcination at 570 °C, the catalyst, which had possessed practically no activity in the oxidation reaction, gave a Co 2p 3/2 main structure peak at 781.3 eV which was very similar to those obtained for synthesized Co nTiO n+2 compounds (CoTiO 3 and Co 2TiO 4), and this catalyst sample had relatively negligible CO chemisorption as observed by TPD spectra. XRD peaks indicating only the formation of Co 3O 4 particles on titania surface were developed in the catalyst samples after calcination at temperatures ≥350 °C. Based on these characterization results, five types of Co species could be modeled to exist with the catalyst calcined at different temperatures. Among these surface Co species, the Type A clean Co 3O 4 particles were predominant on a sample of the catalyst after calcination at 450 °C and highly active for CO oxidation at 100 °C, and the calcination at 570 °C gave the Type B Co 3O 4 particles with complete Co nTiO n+2 overlayers inactive for this oxidation reaction. 相似文献
11.
Residual chlorine ions on a Pt/Al 2O 3 catalyst surface prepared from chlorine-containing precursors appear to inhibit the total oxidation of methane. At 450°C, as chlorine is eliminated with time on stream, the reaction rate increases despite the sintering of the platinum particles. The steady state reaction rate which is reached after 60 h is identical to that obtained with a catalyst prepared from a precursor containing no chlorine. Whether chlorine is present or not in the initial state of the catalyst does not appear to have an influence on the evolution of the platinum particle size. 相似文献
12.
The sulphur tolerance and thermal stability of a 2 wt% Ag/γ-Al 2O 3 catalyst was investigated for the H 2-promoted SCR of NO x with octane and toluene. The aged catalyst was characterised by XRD and EXAFS analysis. It was found that the effect of ageing was a function of the gas mix and temperature of ageing. At high temperatures (800 °C) the catalyst deactivated regardless of the reaction mix. EXAFS analysis showed that this was associated with the Ag particles on the surface of the catalyst becoming more ordered. At 600 and 700 °C, the deactivating effect of ageing was much less pronounced for the catalyst in the H 2-promoted octane-SCR reaction and ageing at 600 °C resulted in an enhancement in activity for the reaction in the absence of H 2. For the toluene + H 2-SCR reaction the catalyst deactivated at each ageing temperature. The effect of addition of low levels of sulphur (1 ppm SO 2) to the feed was very much dependent on the reaction temperature. There was little deactivation of the catalyst at low temperatures (≤235 °C), severe deactivation at intermediate temperatures (305 and 400 °C) and activation of the catalyst at high temperatures (>500 °C). The results can be explained by the activity of the catalyst for the oxidation of SO 2 to SO 3 and the relative stability of silver and aluminium sulphates. The catalyst could be almost fully regenerated by a combination of heating and the presence of hydrogen in the regeneration mix. The catalyst could not be regenerated in the absence of hydrogen. 相似文献
13.
Ammonia, a well-known by-product of chemical, fertiliser and metallurgy industries, is also the most refractory product of nitrogen-containing compound oxidation. Consequently, NH 4+ is a key component of waste disposal of conventional processes like anaerobic digestion or nitrification/denitrification. Catalytic wet air oxidation (CWAO) process, able to eliminate organic matter with non toxic by-product formation, was investigated for ammonium ions removal from wastewater. Oxidation of aniline and of ammonia were carried out on mono- and bimetallic noble metal catalysts (Pt, Ru, Pd, etc.) prepared by impregnation and supported on cerium oxides. In liquid phase, at high temperature (150–250 °C) and high pressure of oxygen (20 bar), a Ru/CeO 2 catalyst is able to achieve the elimination of refractory nitrogenous organic products like aniline. The greatest interest of CWAO compared to the classical biological one, is that the selectivity towards molecular nitrogen is much higher (>90%). Indeed, in this process, ammonium ions give essentially N 2, via hydroxylamine and below 200 °C. At higher temperatures the rate of conversion is extremely high but nitrite and nitrate ions appear in the effluent. On a RuPd/CeO 2 catalyst, the optimal temperature for ammonia conversion is then 200 °C. In these conditions, the N 2 selectivity is up to 90%. 相似文献
14.
The kinetics of the reaction of NO, N 2O and CO 2 with activated carbon without catalyst and impregnated with a precursor salt of vanadium (ammonium monovanadate) was investigated. The conversion of NO, N 2O and CO 2 was studied (450–900°C) using a TGA apparatus and a fixed bed reactor. The reactor effluents were analysed using a GC/MS on line. The addition of vanadium increased carbon reactivity and adsorption at lower temperatures. For NO and N 2O conversion the main products obtained were N 2, N 2O, CO and CO 2 but for CO 2 conversion only CO was detected. In situ XRD was a useful tool for interpreting catalyst behaviour and identifying phases present during reaction conditions. The catalytic effect of vanadium can be explained by the occurrence of redox processes in which the catalyst is reduced to lower oxidation states such as V 2O 5/V 6O 13. 相似文献
15.
The effect of a commercial Pt/Al 2O 3 catalyst on the oxidation by NO 2 and O 2 of a model soot (carbon black) in conditions close to automotive exhaust gas aftertreatment is investigated. Isothermal oxidations of a physical mixture of carbon black and catalyst in a fixed bed reactor were performed in the temperature range 300–450 °C. The experimental results indicate that no significant effect of the Pt catalyst on the direct oxidation of carbon by O 2 and NO 2 is observed. However, in presence of NO 2–O 2 mixture, it is found that besides the well established catalytic reoxidation of NO into NO 2, Pt also exerts a catalytic effect on the cooperative carbon–NO 2–O 2 oxidation reaction. An overall mechanism involving the formation of atomic oxygen over Pt sites followed by its transfer to the carbon surface is established. Thus, the presence of Pt catalyst increases the surface concentration of –C(O) complexes which then react with NO 2 leading to an enhanced carbon consumption. The resulting kinetic equation allows to model more precisely the catalytic regeneration of soot traps for automotive applications. 相似文献
16.
In this work, different procedures, namely carbonate coprecipitation and modified solid–solid diffusion, were used to prepare hexaaluminate samples, unsupported or supported onto θ-Al 2O 3. These samples were used as catalyst for the methane total oxidation as synthesized or after impregnation of 1 wt% Pd. It was observed that the modified solid–solid diffusion procedure is an efficient method to obtain the hexaaluminate structure. At a theoretical ratio x of hexaaluminate onto Al 2O 3 less than 0.6 ( xLa 0.2Sr 0.3Ba 0.5MnAl 11O 19 + (1− x)·Al 2O 3, with x = 0.25, 0.60), samples with high specific surface area and θ-Al 2O 3 structure are then obtained. Large differences in catalytic activity can be observed among the series of sample synthesized. All the pure oxide samples (i.e. without palladium) present low catalytic activity for methane total oxidation compared to a reference Pd/Al 2O 3 catalyst. The highest activity was obtained for the samples presenting a θ-Al 2O 3 structure (with x = 0.60) and a high surface area. Impregnation of 1 wt% palladium resulted in an increase in catalytic activity, for all the solids synthesized in this work. Even if the lowest light-off temperature was obtained on the reference sample, similar methane conversions at high temperature (700 °C) were obtained on the stabilized θ-Al 2O 3 solids ( x = 0.25, 0.60). Moreover, the reference sample is found to strongly deactivate with reaction time at the temperature of test (700 °C), due to a progressive reduction of the PdO x active phase into the less active Pd° phase, whereas excellent stabilities in reaction were obtained on the pure and palladium-doped hexaaluminate and supported θ-Al 2O 3 samples. This clearly showed the beneficial effect of the support for the stabilization of the PdO x active phase at high reaction temperature. These properties are discussed in term of oxygen transfer from the support to the palladium particle. Oxygen transfer is directly related to the Mn 3+/Mn 2+ redox properties (in the case of the hexaaluminate and stabilized θ-Al 2O 3 samples), that allows a fast reoxidation of the metal palladium sites since palladium sites reoxidation cannot occur directly by gaseous dioxygen adsorption and dissociation on the surface. 相似文献
17.
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
18.
CeO 2 and CeReO x_ y catalysts are prepared by the calcination at different temperatures ( y = 500–1000 °C) and having a different composition (Re = La 3+ or Pr 3+/4+, 0–90 wt.%). The catalysts are characterised by XRD, H 2-TPR, Raman, and BET surface area. The soot oxidation is studied with O 2 and NO + O 2 in the tight and loose contact conditions, respectively. CeO 2 sinters between 800–900 °C due to a grain growth, leading to an increased crystallite size and a decreased BET surface area. La 3+ or Pr 3+/4+ hinders the grain growth of CeO 2 and, thereby, improving the surface catalytic properties. Using O 2 as an oxidant, an improved soot oxidation is observed over CeLaO x_ y and CePrO x_ y in the whole dopant weight loading and calcination temperature range studied, compared with CeO 2. Using NO + O 2, the soot conversion decreased over CeLaO x_ y catalysts calcined below 800 °C compared with the soot oxidation over CeO 2_ y. CePrO x_ y, on the other hand, showed a superior soot oxidation activity in the whole composition and calcination temperature range using NO + O 2. The improvement in the soot oxidation activity over the various catalysts with O 2 can be explained based on an improvement in the external surface area. The superior soot oxidation activity of CePrO x_ y with NO + O 2 is explained by the changes in the redox properties of the catalyst as well as surface area. CePrO x_ y, having 50 wt.% of dopant, is found to be the best catalyst due to synergism between cerium and praseodymium compared to pure components. NO into NO 2 oxidation activity, that determines soot oxidation activity, is improved over all CePrO x catalysts. 相似文献
19.
Catalytic oxidation of p-xylene (PX) to terephthalic acid (TA) was studied with catalysts containing cobalt acetate, manganese acetate, CoBr 2 and MnBr 2. The catalysts contain neither highly corrosive hydrogen bromide nor other metal ions, and have the advantage of easy catalyst recovery. The effects of Br/Co atomic ratio, reaction time and temperature, PX concentration, oxygen pressure, and catalyst concentration on PX conversion and product/intermediate yields were investigated. The catalyst system had a suitable reaction temperature of 100 °C, which was much lower than the commercial process temperature (175–225 °C). The maximum product (TA) yield was 93.5%, obtained at a Br/Co atomic ratio of three. Higher Br concentration resulted in the lower TA yield, which was ascribed to the benzylic bromide formation. The synthesis of TA could be adequately described as four reaction steps in series (PX → p-tolualdehyde → p-toluic acid → 4-carboxybenzaldehyde → TA), with a pseudo-first-order rate equation for each step, and the third step was rate-limiting. The rate constant ratios ( kj/ k3, j = 1 → 4) obtained at 100 °C were similar to the kj/ k3 values reported earlier for cobalt acetate/manganese acetate/HBr catalysts in a range of 185–191 °C. 相似文献
20.
A 1% Pd catalyst (38% dispersion) was prepared by impregnating a γ-alumina with palladium acetylacetonate dissolved in acetone. The behaviour of this catalyst in oxidation and steam reforming (SR) of propane was investigated. Temperature-programmed reactions of C 3H 8 with O 2 or with O 2 + H 2O were carried out with different stoichiometric ratios S( S =[O 2]/5[C 3H 8]). The conversion profiles of C 3H 8 for the reaction carried out in substoichiometry of O 2 ( S < 1) showed two discrete domains of conversion: oxidation at temperatures below 350°C and SR at temperatures above 350°C. The presence of steam in the inlet gases is not necessary for SR to occur: there is sufficient water produced in the oxidation to form H 2 and carbon oxides by this reaction. Contrary to what was observed with Pt, an apparent deactivation between 310 and 385°C could be observed with Pd in oxidation. This is due to a reduction of PdO x into Pd 0, which is much less active than the oxide in propane oxidation. Steam added to the reactants inhibits oxidation while it prevents the reduction of PdO x into Pd 0. Compared to Pt and to Rh, Pd has a higher thermal resistance: no deactivation occurred after treatment up to 700°C and limited deactivation after treatment up to 900°C, provided that the catalyst is maintained in an oxygen-rich atmosphere during the cooling. 相似文献
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