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1.
In situ time-resolved FTIR spectroscopy was used to study the reaction mechanism of partial oxidation of methane to synthesis gas and the interaction of CH 4/O 2/He (2/1/45) gas mixture with adsorbed CO species over SiO 2 and γ-Al 2O 3 supported Rh and Ru catalysts at 500–600°C. It was found that CO is the primary product for the reaction of CH 4/O 2/He (2/1/45) gas mixture over H 2 reduced and working state Rh/SiO 2 catalyst. Direct oxidation of methane is the main pathway of synthesis gas formation over Rh/SiO 2 catalyst. CO 2 is the primary product for the reaction of CH 4/O 2/He (2/1/45) gas mixture over Ru/γ-Al 2O 3 and Ru/SiO 2 catalysts. The dominant reaction pathway of CO formation over Ru/γ-Al 2O 3 and Ru/SiO 2 catalysts is via the reforming reactions of CH 4 with CO 2 and H 2O. The effect of space velocity on the partial oxidation of methane over SiO 2 and γ-Al 2O 3 supported Rh and Ru catalysts is consistent with the above mechanisms. It is also found that consecutive oxidation of surface CO species is an important pathway of CO 2 formation during the partial oxidation of methane to synthesis gas over Rh/SiO 2 and Ru/γ-Al 2O 3 catalysts. 相似文献
2.
Dispersing La 2O 3 on δ- or γ-Al 2O 3 significantly enhances the rate of NO reduction by CH 4 in 1% O 2, compared to unsupported La 2O 3. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La 2O 3 precursor used, the pretreatment, and the La 2O 3 loading. The most active family of catalysts consisted of La 2O 3 on γ-Al 2O 3 prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m 2) occurred between the addition of one and two theoretical monolayers of La 2O 3 on the γ-Al 2O 3 surface. The best catalyst, 40% La 2O 3/γ-Al 2O 3, had a turnover frequency at 700°C of 0.05 s −1, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La 2O 3/Al 2O 3 catalysts exhibited stable activity under high conversion conditions as well as high CH 4 selectivity (CH 4 + NO vs. CH 4 + O 2). The addition of Sr to a 20% La 2O 3/γ-Al 2O 3 sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO =4 to the latter Sr-promoted La 2O 3/Al 2O 3 catalyst decreased activity although sulfate increased the activity of Sr-promoted La 2O 3. Dispersing La 2O 3 on SiO 2 produced catalysts with extremely low specific activities, and rates were even lower than with pure La 2O 3. This is presumably due to water sensitivity and silicate formation. The La 2O 3/Al 2O 3 catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications. 相似文献
3.
Lanthanum-doped Pd/γ-Al 2O 3 and Pd/γ-Al 2O 3 membranes were prepared by sol-gel methods. The thermal stability of the unsupported Pd/γ-Al 2O 3 and La/Pd/γ-Al 2O 3 membranes was investigated with BET (including average pore size, pore volume and BET surface area), XRD, and DTA techniques. The average pore size of the Pd/γ-Al 2O 3 membranes increased sharply after sintering at temperatures higher than 1000°C. Addition of 3 mol% lanthanum can raise the temperature of the γ-Al 2O 3 to-Al 2O 3 phase transformation significantly. This improves the thermal stability of the Pd/γ-Al 2O 3 catalytic membranes. 相似文献
4.
This paper presents an investigation into the complex interactions between catalytic combustion and CH 4 steam reforming in a co-flow heat exchanger where the surface combustion drives the endothermic steam reforming on opposite sides of separating plates in alternating channel flows. To this end, a simplified transient model was established to assess the stability of a system combining H 2 or CH 4 combustion over a supported Pd catalyst and CH 4 steam reforming over a supported Rh catalyst. The model uses previously reported detailed surface chemistry mechanisms, and results compared favorably with experiments using a flat-plate reactor with simultaneous H 2 combustion over a γ-Al 2O 3-supported Pd catalyst and CH 4 steam reforming over a γ-Al 2O 3-supported Rh catalyst. Results indicate that stable reactor operation is achievable at relatively low inlet temperatures (400 °C) with H 2 combustion. Model results for a reactor with CH 4 combustion indicated that stable reactor operation with reforming fuel conversion to H 2 requires higher inlet temperatures. The results indicate that slow transient decay of conversion, on the order of minutes, can arise due to loss of combustion activity from high-temperature reduction of the Pd catalyst near the reactor entrance. However, model results also show that under preferred conditions, the endothermic reforming can be sustained with adequate conversion to maintain combustion catalyst temperatures within the range where activity is high. A parametric study of combustion inlet stoichiometry, temperature, and velocity reveals that higher combustion fuel/air ratios are preferred with lower inlet temperatures (≤500 °C) while lower fuel/air ratios are necessary at higher inlet temperatures (600 °C). 相似文献
5.
Monodispersed nano-Au/γ-Al 2O 3 catalysts for low-temperature oxidation of CO have been prepared via a modified colloidal deposition route, which involves the deposition of dodecanethiolate self-assembled monolayer (SAM)-protected gold nanoparticles (C 12 nano-Au) in hexane on γ-Al 2O 3 at room temperature. The diameter of the gold nanoparticles deposited on the support is 2.5 ± 0.8 nm after thermal treatment, and their valence states comprise both the metallic and oxidized states. It is found that the thermal treatment temperature affects significantly the catalytic activity of the catalysts in the processing steps. The catalyst treated at 190 °C exhibits considerably higher activity as compared to catalysts treated at 165 and 250 °C. A 2.0-wt.% nano-Au/γ-Al 2O 3 catalyst treated at 190 °C for 15 h maintains the catalytic activity at nearly 100% CO oxidation for at least 800 h at 15 °C, at least 600 h at 0 °C, and even longer than 450 h at −5 °C. Evidently, the catalysts obtained using this preparation route show high catalytic activity, particularly at low temperatures, and a good long-term stability. 相似文献
6.
Polychlorinated benzenes (PhCl x) are formed as byproducts in the combustion of chlorobenzene on Pt supported on γ-Al 2O 3, SiO 2, SiO 2–Al 2O 3, or ZrO 2. The congener and isomer distribution of the PhCl x differs for the various supports. The amounts of PhCl x correlate with the dispersion of platinum. Thus, a Pt/γ-Al 2O 3 catalyst calcined at 500°C to yield very small Pt crystallites was more active in PhCl x formation than Pt/γ-Al 2O 3 calcined at 800°C. In all cases T50% for chlorobenzene conversion is close to 300°C and appears to be independent of the crystallite size of the platinum. Replacing platinum by palladium led to lower rates of combustion and to more byproducts. These results lead us to propose that, in the presence of Cl and higher oxygen concentrations, small Pt crystallites are converted more easily into Pt(IV) species. These are less efficient in combustion, but can be more active in chlorination. 相似文献
7.
The interactions NO—CO and O 2—NO—CO have been studied onCuCo 2O 4γ-Al 2O 3 and on γ-Al 2O 3- and CuCo 2O 4γ-Al 2O 3-supported Pt, Rh and Pt—Rh catalysts. The deposition of noble metals (Pt, Rh and Pt—Rh) on CuCo 2O 4γ-Al 2O 3 instead of γ-Al 2O 3 is beneficial in: lowering the temperature at which maximum N 2O is formed and decreasing the maximum N 2O concentration attained; lowering the onset temperature of NO to N 2 reduction, and increasing the N 2 selectivity; preserving the activity towards NO to N 2 reduction on a higher level following the concentration step NO + COO 2+ NO + CO and changing the conditions from stoichiometric to oxidizing (50% excess of oxidants). The reason for this behaviour of the CuCo 2O 4γ-Al 2O 3-based noble metal catalysts is the formation (reversible) of a reduced surface layer on the CuCo 2O 4 supported spinel under the conditions of a stoichiometric NO + CO mixture. 相似文献
8.
The capability of flame-made Rh/Ce 0.5Zr 0.5O 2 nanoparticles catalyzing the production of H 2- and CO-rich syngas from butane was investigated for different Rh loadings (0–2.0 wt% Rh) and two different ceramic fibers (Al 2O 3/SiO 2 and SiO 2) as plugging material in a packed bed reactor for a temperature range from 225 to 750 °C. The main goal of this study was the efficient processing of butane at temperatures between 500 and 600 °C for a micro-intermediate-temperature SOFC system. Our results showed that Rh/Ce 0.5Zr 0.5O 2 nanoparticles offer a very promising material for butane-to-syngas conversion with complete butane conversion and a hydrogen yield of 77% at 600 °C. The catalytic performance of packed beds strongly depended on the use of either Al 2O 3/SiO 2 or SiO 2 fiber plugs. This astonishing effect could be attributed to the interplay of homogeneous and heterogeneous chemical reactions during the high-temperatures within the reactor. 相似文献
9.
The reaction between methane and cerium oxide to produce syngas has been studied at 700°C in a pulse apparatus. The cerium oxide was supported on γ-Al 2O 3 and promoted by re-impregnation with Pt or Rh. The promoters drastically enhanced the conversion of methane. TPR with hydrogen shows that Pt and Rh also lowered the temperature necessary to reduce the cerium oxide. Studies of the reaction between methane and promoted cerium oxide showed that the selectivity to syngas depends on the degree of reduction of the cerium oxide. The promoters also led to some carbon formation. Regeneration of the reduced oxide was studied both with oxygen and carbon dioxide. 相似文献
10.
Raman scattering is used to analyze a series of model catalysts consisting of γ-alumina powder with up to 20 wt.-% Pd, and two monolithic catalyst samples with palladium concentrations typically employed in automotive emission control. By using the 514.5 nm Ar laser line to take advantage of a resonant Raman effect, the 651-cm −1 Raman line in PdO can be detected in the oxidized catalysts with palladium loadings as small as 0.2 wt.-%. A strong fluorescence signal is observed in reduced pure γ-Al 2O 3. This signal is quenched by the addition of palladium, admixed either from aqueous solution or from Pd black, provided the palladium was allowed to interact with the γ-Al 2O 3 during at least one oxidation treatment at 600°C, followed by reduction in hydrogen at 300°C. Raman spectroscopy is found to be applicable for a quantitative, nondestructive analysis of PdO on γ-alumina, including automotive formulations, within certain limitations. At low concentrations, some palladium may interact with the support and thus may not contribute to the signal associated with crystalline PdO. The analysis is also suited for studying sintering and reduction/oxidation kinetics in situ. 相似文献
11.
Addition of Na to Rh/γ-Al 2O 3 and Pd/γ-Al 2O 3 catalysts operated under identical simulated TWC conditions has dramatically different effects in the two cases (although the two metals respond similarly to Na promotion of NO x reduction in the absence of gaseous oxygen). Na addition to rhodium has a detrimental effect as manifested by severe poisoning and decreased nitrogen selectivity over the greater part of the temperature range studied. In contrast, Na promotion significantly improves the overall performance of Pd/γ-Al 2O 3 catalysts under simulated TWC conditions. This is manifested by a considerable widening of the gas composition window over which palladium delivers high NO x conversion, whilst at the same time exhibiting markedly improved selectivity towards N 2 formation. The very different behaviour of the two metals may be understood in terms of a single underlying effect, namely, the electronic influence of sodium ions on the adsorption strength (and hence relative coverages) of the various reactants on the metal surface. 相似文献
12.
Zirconium sulfate supported on γ-Al 2O 3 catalysts were prepared by impregnation of powdered γ-Al 2O 3 with zirconium sulfate aqueous solution followed by calcining in air at high temperature. For Zr(SO 4) 2/γ-Al 2O 3 samples, no diffraction line of zirconium sulfate was observed up to 50 wt.%, indicating good dispersion of Zr(SO 4) 2 on the surface of γ-Al 2O 3. The acidity of catalysts increased in proportion to the zirconium sulfate content up to 40 wt.% of Zr(SO 4) 2. 40-Zr(SO 4) 2/γ-Al 2O 3 calcined at 400 °C exhibited maximum catalytic activities for 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation were correlated with the acidity of catalysts measured by ammonia chemisorption method. 相似文献
13.
Operando DRIFTS was applied to the study of the evolution of surface species formed on a Pd (2 wt.%)/γ-Al 2O 3 catalyst in various conditions. No differences were observed as a function of the initial oxidation state of palladium. Formates/carbonates species were identified at low temperature (<400 °C) and disappeared when CO 2 production started. These species come from the Pd-catalyzed interaction of CO with the alumina support, while CO 2 induces hydrogenocarbonates formation at low temperature (<300 °C). Their presence does not explain the inhibiting effect of CO 2 observed in CCM on Pd/γ-Al 2O 3 catalysts. 相似文献
14.
Two types of NiO/γ-Al 2O 3 catalysts prepared by the impregnation and the sol–gel method were used for the partial oxidation of methane to syngas at 850°C (GHSV1.8×10 5 lkg −1 h −1). The effects of the carbon deposition, the loss and sintering of nickel and the phase transformation of γ-Al 2O 3 support on the catalytic performance during 80 h POM reaction were investigated with a series of characterization such as XRD, BET, AAS, TG, and XPS. The results indicated that the carbon deposition and the loss and sintering of nickel could not cause the serious decrease of catalytic performance over NiO/γ-Al 2O 3 catalyst during the short-time reaction. However, the slow process of the support γ-Al 2O 3 phase transforming into -Al 2O 3 could slowly decrease the performance of NiO/γ-Al 2O 3 catalysts. Aimed at the reasons of the deactivation, an improved catalyst was obtained by the complexing agent-assisted sol–gel method. 相似文献
15.
A novel catalytic combustion concept for zero emissions power generation has been investigated. Catalysts consisting of Rh supported on ZrO 2, Ce-ZrO 2 or -Al 2O 3 were prepared and tested under fuel-rich conditions, i.e. for catalytic partial oxidation (CPO) of methane. The experiments were performed in a subscale gas-turbine reactor operating at 5 bar with exhaust gas-diluted feed mixtures. The catalyst support material was found to influence the light-off temperature significantly, which increased in the following order Rh/Ce-ZrO2 < Rh/ZrO2 < Rh/-Al2O3. The Rh loading, however, only had a minor influence. The high activity of Rh/Ce-ZrO2 is probably related to the high dispersion of Rh on Ce-ZrO2 and the high oxygen mobility of this support compared to pure ZrO2. The formation of hydrogen was also found to increase over the catalyst containing ceria in the support material. 相似文献
16.
The reduction of NO by CO over Rb-promoted Pt/γ-Al 2O 3 catalysts has been investigated over a wide range of temperature (ca. 200–500°C), partial pressures of reactants and promoter loadings. For purposes of comparison, K- and Cs-promoted Pt/γ-Al 2O 3 catalysts were tested under the same conditions. Rubidium strongly enhanced both catalytic activity and N 2-selectivity. Rate increases by factors as high as 110 and 45 for the production of N 2 and CO 2, respectively, relative to unpromoted Pt were obtained, accompanied by substantial increase in N 2-selectivity (e.g. from 24 to 82% at 350°C and [CO]=0.5%, [NO]=1%). Under stoichiometric conditions, Rb-promoted catalysts gave 100% conversion of both reactants with 100% selectivity towards N 2 at T350°C and at an effective reactant contact time of only 0.5 s. In contrast, under the same conditions unpromoted Pt delivered <30% conversion and poor N 2-selectivity (approximately <40%); even at 480°C the conversion was only 60%. The observed promotional effects are ascribed to alkali-induced changes in the chemisorption bond strengths of CO, NO and NO dissociation products which lead to the observed activity enhancement and dependence of N 2-selectivity on promoter loading. The effects of K-promotion mirror those of Rb-promotion, but are significantly less pronounced. Rb is the best alkali promoter. 相似文献
17.
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
18.
通过制备高纯度的前驱体湃铝石获得了η-Al 2O 3材料,采用XRD验证了η-Al 2O 3与γ-Al 2O 3在晶相结构上的差异,比较了两者的表面形貌、织构及酸碱性能,结果显示,η-Al 2O 3与γ-Al 2O 3的比表面积相当,但η-Al 2O 3具有更弱的弱碱位和较少的强碱位,并拥有丰富的中等强度酸性位。将η-Al 2O 3与γ-Al 2O 3作为催化剂应用于CS 2水解反应,结果表明,在(200~450) ℃测试温度范围内,η-Al 2O 3催化剂对CS 2的水解活性始终优于γ-Al 2O 3,两种催化剂上CS 2反应的浓度效应也明显不同,推测与它们的酸碱性质影响了对CS 2的吸附能力有关,导致两者催化CS 2水解反应遵循了不同的机制。 相似文献
19.
Past research in this laboratory on catalytic steam reforming of chlorinated hydrocarbons demonstrated extremely high levels of destruction (0.99999+) at 600–750 °C, with GHSVs as high as 2.5 × 10 5 h −1. Feasible operation was demonstrated with chlorinated alkanes, alkenes, aromatics and PCBs using Pt/γ-Al 2O 3 catalysts. The major mechanism for deactivation with trichloroethylene was sintering of the γ-Al 2O 3 support and encapsulation of Pt crystallites. Evidence is presented here that ZrO2 is a superior support for steam reforming of trichloroethylene (TCE), due to its low acidity and ability to store oxygen. Formulations of 0.8 wt.% Pt/ZrO2 tested at a GHSV of 20,000 h−1 and a H2O/C ratio of 20 operated for 42 days at 750 °C, with only slight carbon deposits in the first 15% of the catalyst bed. No pyrolysis was found, and the product CO/CO2 ratio was at equilibrium, indicating high water gas shift activity with very low CO concentrations. Kinetic measurements revealed a pseudo-first order rate equation, sintering of the support and Pt was much less than with γ-Al2O3 supports, and no encapsulation was detected. Slow deactivation occurred due to deposition of catalytic carbon. This carbon was removed by combustion with air, and the rate of deactivation indicated the 42-day run would have lasted seven months. 相似文献
20.
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
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