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1.
This study focuses on the loading of catalytic materials, e.g., palladium on the surface of supporting materials, with the aim to obtain catalysts with high activity for methane combustion. The catalyst PdO/CeO 2-Al 2O 3 was prepared by impregnation under ultrasonic condition. The effect of different activation methods on the activity of catalysts for methane catalytic combustion was tested. The properties of reaction and adsorption of oxygen species on catalyst surface were characterized by H 2-temperature programmed reduction (H 2-TPR), and O 2-temperature programmed desorption (O 2-TPD). Furthermore, the sulfur tolerance and sulfur poisoning mode were investigated. The results indicate that the catalyst PdO/CeO 2-Al 2O 3 activated with rapid activation shows higher activity for methane combustion and better sulfur tolerance. The result of sulfur content analysis shows that there is a large number of sulfur species on the catalyst’s surface after reactivation at high temperature. It proves that the activity of catalysts cannot be fully restored by high-temperature reactivation. 相似文献
2.
This work investigates the effect of treatments under different CH 4-containing atmospheres on the reactivity of fresh and S-poisoned 2% w/w Pd/Al 2O 3/CeO 2 catalysts for methane combustion. Over the fresh catalyst the decomposition/reformation processes of PdO occurring during cycles of CH4-reducing/lean combustion pulses allowed the complete recovery of activity losses possibly associated with H2O poisoning which were observed during prolonged exposure under lean combustion conditions. The presence of CeO2 markedly enhances both the activity losses under lean combustion conditions and the rate of PdO reoxidation/reactivation upon Pd redox cycle. Under lean combustion conditions, regeneration of catalyst deactivated by exposure to SO2-containing atmosphere required very high temperatures (above 750 °C) in order to decompose stable sulphate species adsorbed on the support. Treatments consisting of alternate CH4-reducing/lean combustion pulses allowed a complete recovery of activity at much lower temperatures (550–600 °C) due to the reduction of sulphates by CH4 activated on the surface of Pd metal. A protecting role of CeO2 on Pd poisoning due either to exposure to SO2-containing atmosphere or to spill-back of support sulphates species was also evidenced. 相似文献
3.
以CeO_2修饰多孔NaY分子筛作为载体,采用高温液相还原法制备纳米晶PdO催化剂,用于低浓度苯催化氧化反应。采用XRD、N2吸附-脱附、透射电镜-能谱(HRTEM-EDS)、H2程序升温还原(H_2-TPR)、O_2程序升温脱附(O_2-TPD)和程序升温表面反应(TPSR)等对载体和催化剂进行表征。结果表明,NaY分子筛结构稳定,比表面积651 m~2·g~(-1)和孔容0. 326 cm~3·g~(-1),纳米晶PdO能够较均匀地分散在NaY载体上,颗粒尺寸约(3~5) nm。加入一定量CeO_2后,Pd O以较小的纳米晶颗粒形式分散在CeO_2周围,活性组分与助剂协同作用促进了催化剂中晶格氧的流动性,明显改善了0. 2%Pd/NaY的氧化性能。0. 2%Pd/8%Ce/NaY表现出最佳催化活性和良好稳定性,250℃可完全催化降解1000×10~(-6)的苯,并且230℃连续反应100 h,催化剂转化率稳定在86%。 相似文献
4.
The influence of catalyst pre-treatment temperature (650 and 750 °C) and oxygen concentration ( λ = 8 and 1) on the light-off temperature of methane combustion has been investigated over two composite oxides, Co 3O 4/CeO 2 and Co 3O 4/CeO 2–ZrO 2 containing 30 wt.% of Co 3O 4. The catalytic materials prepared by the co-precipitation method were calcined at 650 °C for 5 h (fresh samples); a portion of them was further treated at 750 °C for 7 h, in a furnace in static air (aged samples). Tests of methane combustion were carried out on fresh and aged catalysts at two different WHSV values (12 000 and 60 000 mL g−1 h−1). The catalytic performance of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 were compared with those of two pure Co3O4 oxides, a sample obtained by the precipitation method and a commercial reference. Characterization studies by X-ray diffraction (XRD), BET and temperature-programmed reduction (TPR) show that the catalytic activity is related to the dispersion of crystalline phases, Co3O4/CeO2 and Co3O4/CeO2–ZrO2 as well as to their reducibility. Particular attention was paid to the thermal stability of the Co3O4 phase in the temperature range of 750–800 °C, in both static (in a furnace) and dynamic conditions (continuous flow). The results indicate that the thermal stability of the phase Co3O4 heated up to 800 °C depends on the size of the cobalt oxide crystallites (fresh or aged samples) and on the oxygen content (excess λ = 8, stoichiometric λ = 1) in the reaction mixture. A stabilizing effect due to the presence of ceria or ceria–zirconia against Co3O4 decomposition into CoO was observed. Moreover, the role of ceria and ceria–zirconia is to maintain a good combustion activity of the cobalt composite oxides by dispersing the active phase Co3O4 and by promoting the reduction at low temperature. 相似文献
5.
Alkali halide added transition metal oxides produced ethylene selectively in oxidative coupling of methane. The role of alkali halides has been investigated for LiCl-added NiO (LiCl/NiO). In the absence of LiCl the reaction over NiO produced only carbon oxides (CO 2 + CO). However, addition of LiCl drastically improved the yield of C 2 compounds (C 2H 6 + C 2H 4). One of the roles of LiCl is to inhibit the catalytic activity of the host NiO for deep oxidation of CH 4. The reaction catalyzed by the LiCl/NiO proceeds stepwise from CH 4 to C 2H 4 through C 2H 6 (2CH 4 → C 2H 6 → C 2H 4). The study on the oxidation of C 2H 6 over the LiCl/NiO showed that the oxidative dehydrogenation of C 2H 6 to C 2H 4 occurs very selectively, which is the main reason why partial oxidation of CH 4 over LiCl/NiO gives C 2H 4 quite selectively. The other role of LiCl is to prevent the host oxide (NiO) from being reduced by CH 4. The catalyst model under working conditions was suggested to be the NiO covered with molten LiCl. XPS studies suggested that the catalytically active species on the LiCl/NiO is a surface compound oxide which has higher valent nickel cations (Ni (2+δ)+ or Ni 3+). The catalyst was deactivated at the temperatures>973 K due to vaporization of LiCl and consumption of chlorine during reaction. The kinetic and CH 4---CD 4 exchange studies suggested that the rate-determining step of the reaction is the abstraction of H from the vibrationally excited methane by the molecular oxygen adsorbed on the surface compound oxide. 相似文献
6.
Zirconia supported on alumina was prepared and characterized by BET surface area, X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), temperature programmed desorption (TPD), and pulse reaction. 0.2% Pd/ZrO 2/Al 2O 3 catalyst were prepared by incipient wetness impregnation of supports with aqueous solution of Pd(NO 3) 2. The effects of support properties on catalytic activity for methane combustion and CO oxidation were investigated. The results show that ZrO 2 is highly dispersed on the surface of Al 2O 3 up to 10 wt.% ZrO 2, beyond this value tetragonal ZrO 2 is formed. The presence of a small amount of ZrO 2 can increase the surface area, pore volume and acidity of support. CO–TPD results show that the increase of CO adsorption capacity and the activation of CO bond after the presence of ZrO 2 lead to the increase of catalytic activity of Pd catalyst for CO oxidation. CO pulse reaction results indicate that the lattice oxygen of support can be activated at lower temperature following the presence of ZrO 2, but it does not accelerate the activity of 0.2% Pd/ZrO 2/Al 2O 3 for methane combustion. 0.2% Pd/ZrO 2/Al 2O 3 dried at 120 °C shows highest activity for CH 4 combustion, and the activity can be further enhanced following the repeat run. The increase of treatment temperature and pre-reduction can decrease the activity of catalyst for CH 4 combustion. 相似文献
7.
Ni catalysts supported on various solid solutions of ZrO 2 with alkaline earth oxide and/or rare earth oxide were synthesized. The catalytic activities were compared for partial oxidation of methane and autothermal reforming of methane. For partial oxidation of methane, the Ni catalyst supported on a CaO–ZrO 2 solid solution showed a high activity. Incorporation of CaO in the ZrO 2 matrix was effective for increasing the reduction rate of the NiO particles and for decreasing the coke formation. On the other hand, the Ni particles supported on the CaO–CeO 2–ZrO 2 solid solution had a strong interaction with the support, and the Ni particles showed high activity and stability for autothermal reforming of methane. 相似文献
8.
A new catalyst composed of nickel oxide and cerium oxide was studied with respect to its activity for NO reduction by CO under stoichiometric conditions in the absence as well as the presence of oxygen. Activity measurements of the NO/CO reaction were also conducted over NiO/γ-Al 2O 3, NiO/TiO 2, and NiO/CeO 2 catalysts for comparison purposes. The results showed that the conversion of NO and CO are dependent on the nature of supports, and the catalysts decreased in activity in the order of NiO/CeO 2 > NiO/γ-Al 2O 3 > NiO/TiO 2. Three kinds of CeO 2 were prepared and used as support for NiO. They are the CeO 2 prepared by (i) homogeneous precipitation (HP), (ii) precipitation (PC), and (iii) direct decomposition (DP) method. We found that the NiO/CeO 2(HP) catalyst was the most active, and complete conversion of NO and CO occurred at 210 °C at a space velocity of 120,000 h −1. Based on the results of surface analysis, a reaction model for NO/CO interaction over NiO/CeO 2 has been proposed: (i) CO reduces surface oxygen to create vacant sites; (ii) on the vacant sites, NO dissociates to produce N 2; and (iii) the oxygen originated from NO dissociation is removed by CO. 相似文献
9.
Ethanol steam reforming was studied over Ni/Al 2O 3 catalysts. The effect of support (- and γ-Al 2O 3), metal loading and a comparison between conventional H 2 reduction with an activation method employing a CH 4/O 2 mixture was investigated. The properties of catalysts were studied by N 2 physisorption, X-ray diffraction (XRD) and temperature programmed reduction (TPR). After activity tests, the catalysts were analyzed by scanning electron microscopy (SEM) and thermogravimetric analysis (TG/DTA). Ni supported on γ-Al 2O 3 was more active for H 2 production than the catalyst supported on -Al 2O 3. Metal loading did not affect the catalytic performance. The alternative activation method with CH 4/O 2 mixture affected differently the activity and stability of the Ni/γ-Al 2O 3 and the Ni/-Al 2O 3 catalyst. This activation method increased significantly the stability of Ni/-Al 2O 3 compared to H 2 reduction. SEM and TG/DTA analysis indicate the formation of filamentous carbon during the CH 4/O 2 activation step, which is associated with the increasing catalyst activity and stability. The effect of temperature on the type of carbon formed was investigated; indicating that filamentous coke increased activity while encapsulating coke promoted deactivation. A discussion about carbon formation and the influence on the activity is presented. 相似文献
10.
采用前驱体转化法将四氧化三锰分别与硝酸、硫酸、醋酸进行歧化反应,制备了3组 γ-二氧化锰催化剂(Mn-N、Mn-S、Mn-A),研究了不同酸对产物结构和形貌的影响。通过X射线衍射(XRD)、扫描电镜(FE-SEM)、透射电镜(HR-TEM)、氮气吸附-脱附、X射线光电子能谱(XPS)、氢气程序升温还原(H 2-TPR)和氧气程序升温脱附(O 2-TPD)等表征方法系统分析了各产物的物化性质,并采用甲苯催化燃烧作为探针反应用于评估 γ-二氧化锰催化剂的活性。结果表明,经硝酸处理得到的催化剂Mn-N具有最佳的甲苯催化燃烧活性,在质量空速为40 000 mL/(g·h)条件下,起燃温度 T10(对应转化率为10%)为190 ℃,完全燃烧温度 T90(对应转化率为90%)为229 ℃。这可能与其具有较大比表面积、较高的锰(Ⅲ)和吸附氧物种含量、较好的可还原性和晶格氧流动能力有关。此外,经过连续30 h的稳定性实验,Mn-N仍可保持良好的催化活性。 相似文献
11.
The effect of CeO 2 loading (1–20 wt.%) on the properties and catalytic behaviors of CeO 2–Al 2O 3-supported Pt catalysts on the partial oxidation of methane was studied. The catalysts were characterized by SBET, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and oxygen storage capacity (OSC). XRD and TPR results showed that the pretreatment temperature of the support influences on the amount of CeO 2 with fluorite structure. The pretreatment temperature of the support and CeO 2 loading influenced the morphology of Pt. OSC analysis showed a significant increase in the oxygen storage capacity per weight of CeO 2 for samples with high CeO 2 loading (12 and 20 wt.%). TPR analyses showed that the addition of Pt promotes the reduction of CeO 2. This effect was more significant for the catalysts with high CeO 2 loading (≥12 wt.%). The dispersion of Pt, measured by the rate of cyclohexane dehydrogenation, increases with increasing of the pretreatment temperature of the support. It was shown that the kind of the support is very important for obtaining of catalysts resistant to carbon formation. The catalysts with high CeO 2 loading (≥12 wt.%) showed the highest catalytic activity and stability in the reaction of partial oxidation of methane due to a higher Pt–CeO 2 interface. 相似文献
12.
采用水热法制备CeO 2纳米颗粒(W-CeO 2)、CeO 2纳米片(S-CeO 2)、CeO 2纳米棒(B-CeO 2)及CeO 2纳米八面体(O-CeO 2),用浸渍法负载相同质量分数的铜形成CuO/CeO 2催化剂。通过扫描电镜(SEM)、高分辨透射电子显微镜(TEM)、X射线衍射(XRD)、拉曼光谱(Raman)、自动吸附分析仪(BET)、H 2程序升温还原(H 2-TPR)、N 2O滴定等表征技术对催化剂进行表征,并在可控温控压的固定床石英管反应器中对催化剂的催化性能进行评价。研究了不同形貌CuO/CeO 2催化剂对CO 2加氢制备甲醇的影响;结果表明,CuO/CeO 2催化剂的催化活性存在明显的形貌依赖性,催化剂的暴露晶面、比表面积、表面碱性位点、表面氧缺陷的差异均会对CO 2转化率、甲醇选择性和产率产生影响。其中,不同形貌CeO 2优先暴露晶面的活性顺序为S-CeO 2({100}+{110})>W-CeO 2{100}>B-CeO 2{111}≈O-CeO 2{111},暴露晶面活性越高,催化剂表面氧缺陷越多,CuO-CeO 2间相互作用越强,则催化活性越好。当为CuO/S-CeO 2时,催化剂表面中碱性位点最多,催化剂比表面积为88.8m 2/g,铜分散度为19.2%,CO 2转化率为6.56%,甲醇选择性和收率为96.3%和0.063g/(g cat·h),催化活性最好,由活性评价试验得转化率由高到低依次为S-CeO 2>B-CeO 2>W-CeO 2>O-CeO 2,可知CeO 2形貌差异会决定CuO/CeO 2催化剂的物化性能和催化活性,从而提升对不同形貌CuO/CeO 2催化剂催化CO 2加氢制甲醇的基础认识。 相似文献
13.
The reactions of HCOOH and CH 3COOH on CeO 2(111) and CeO 2(100) were studied using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The carboxylic acids were found to dissociate on both surfaces to form carboxylates. Adsorbed formates and acetates decomposed near 600 K to produce primarily the dehydration products CO and CH 2CO, respectively. This result is consistent with previous studies which have indicated that the ease of reduction of the oxide is the primary factor in determining the selectivity for dehydration versus dehydrogenation during carboxylate decomposition. In addition to CO and CO 2, small amounts of formaldehyde were produced during formate decomposition on both CeO 2(111) and CeO 2(100). In contrast, acetone was observed as a product during acetate decomposition only on the CeO 2(111) surface. 相似文献
14.
化学链甲烷重整耦合CO 2还原技术既能生产合成气还可以还原CO 2生成CO。采用共沉淀法制备不同Ce/Ni摩尔比的系列Ce 1-xNi xO y( x = 0 ,0.2 ,0.4 ,0.6 ,0.8 ,1)氧载体。通过XRD、BET、XPS及CH 4-TPR等表征对氧载体的理化性质进行了研究。系统考察了Ce 1-xNi xO y氧载体在化学链甲烷重整耦合CO 2还原反应中的反应性能。与单一金属氧化物NiO和CeO 2相比,Ce 1-xNi xO y复合氧载体在该反应中具有更高的活性和热稳定性。在甲烷部分氧化阶段,Ce 0.2Ni 0.8O y和Ce 0.4Ni 0.6O y氧载体具有较高的CH 4转化率。经历了20次redox循环实验,Ce 0.2Ni 0.8O y氧载体的CO 2转化率几乎保持不变,表明Ce 0.2Ni 0.8O y氧载体具有较高的热稳定性。 相似文献
15.
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
16.
The effect of the addition of a second fuel such as CO, C 3H 8 or H 2 on the catalytic combustion of methane was investigated over ceramic monoliths coated with LaMnO 3/La-γAl 2O 3 catalyst. Results of autothermal ignition of different binary fuel mixtures characterised by the same overall heating value show that the presence of a more reactive compound reduces the minimum pre-heating temperature necessary to burn methane. The effect is more pronounced for the addition of CO and very similar for C 3H 8 and H 2. Order of reactivity of the different fuels established in isothermal activity measurements was: CO>H 2≥C 3H 8>CH 4. Under autothermal conditions, nearly complete methane conversion is obtained with catalyst temperatures around 800 °C mainly through heterogeneous reactions, with about 60–70 ppm of unburned CH 4 when pure methane or CO/CH 4 mixtures are used. For H 2/CH 4 and C 3H 8/CH 4 mixtures, emissions of unburned methane are lower, probably due to the proceeding of CH 4 homogeneous oxidation promoted by H and OH radicals generated by propane and hydrogen pyrolysis at such relatively high temperatures. Finally, a steady state multiplicity is found by decreasing the pre-heating temperature from the ignited state. This occurrence can be successfully employed to pilot the catalytic ignition of methane at temperatures close to compressor discharge or easily achieved in regenerative burners. 相似文献
17.
采用等体积浸渍法制备HZSM-5分子筛负载Mn(9Mn/Z),Ce(9Ce/Z)和同时负载Mn、Ce(4.5Mn4.5Ce/Z)催化剂,并考察了它们的催化碳烟颗粒燃烧的性能。结果表明,同时负载Mn、Ce的催化剂(4.5Mn4.5Ce/Z)具有更好的催化碳烟燃烧的活性,碳烟转化率为50%时所对应的温度(T 50)为400℃,低于9Mn/Z(T 50=414℃)和9Ce/Z(T 50=447℃)催化剂。相比于纯碳烟的燃烧,T 50降低了128℃。通过氢气程序升温还原(H 2-TPR)测试,相比较催化剂9Mn/Z和9Ce/Z,催化剂4.5Mn4.5Ce/Z中混合氧化物的氢气的还原温度向低温方向产生了偏移,具有更好的氧化还原性,说明MnO x和CeO 2产生了一定的相互作用;X射线光电子能谱(XPS)测试表明,样品4.5Mn4.5Ce/Z具有更多的表面化学吸附氧物种,这些氧物种有助于碳烟氧化燃烧,因而表现出较高的催化碳烟燃烧活性。 相似文献
18.
Palladium (Pd) supported on CeO 2-promoted γ-Al 2O 3 with various CeO 2 (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO 2 structure and dispersion on the γ-Al 2O 3 support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO 2 structure. Pd is present on the oxidized CeO 2-promoted catalysts as Pd 0, Pd + and Pd 2+, at ratios strongly dependent on CeO 2 structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO 2 and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO 2. FTIR spectra of adsorbed CO revealed that after pre-treatment under H 2 or in the presence of amorphous CeO 2, partial encapsulation of Pd particles occurs. CeO 2 structure influences the CH 4 steam reforming reaction rates. Crystalline CeO 2 and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO 2 as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al 2O 3 or on amorphous (reduced) CeO 2/Al 2O 3 catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO 2/γ-Al 2O 3 than on Pd/γ-Al 2O 3, which is attributed to a catalytic synergism between Pd and CeO 2. The low rates on the reduced Pd/CeO 2/Al 2O 3 catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration. 相似文献
19.
During the reactions related to oxidative steam reforming and combustion of methane over -alumina-supported Ni catalysts, the temperature profiles of the catalyst bed were studied using an infrared (IR) thermograph. IR thermographical images revealed an interesting result: that the temperature at the catalyst bed inlet is much higher under CH 4/H 2O/O 2/Ar = 20/10/20/50 than under CH 4/H 2O/O 2/Ar = 10/0/20/70; the former temperature is comparable to that over noble metal catalysts such as Pt and Pd. Based on the temperature-programmed reduction and oxidation measurements over fresh and used catalysts, the metallic Ni is recognized at the catalyst bed inlet under CH 4/H 2O/O 2/Ar = 20/10/20/50, although it is mainly oxidized to NiAl 2O 4 under CH 4/H 2O/O 2/Ar = 10/0/20/70. This result indicates that the addition of reforming gas (CH 4/H 2O = 10/10) to the combustion gas (CH 4/O 2 = 10/20) can stabilize Ni species in the metallic state even under the presence of oxygen in the gas phase. This would account for its extremely high combustion activity. 相似文献
20.
The glow discharge plasma treated Ni/Al 2O 3 catalyst showed an excellent anti-coke property for CO 2 reforming of methane. Characterizations using X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), transmission electron microscopy (TEM), and CO adsorbed infrared spectroscopy (IR) were conducted to investigate the structure and reactivity of the plasma treated Ni/Al 2O 3 catalyst for CO 2 reforming of methane. It confirms that the plasma treatment of Ni precursor at room temperature followed by calcination thermally has a significant influence on the surface characteristics of the active phase. The plasma treated catalyst contains high concentration of close packed plane with improved Ni dispersion and enhanced Ni-alumina interaction, which lead to high catalytic activity and excellent resistance to formations of filamentous carbon and encapsulating carbon. 相似文献
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