Preparation and characterization of Ce1-xFexO2 complex oxides and its catalytic activity for methane selective oxidation

A series of Ce1-xFexO2 （x=0, 0.2, 0.4, 0.6, 0.8, 1） complex oxide catalysts were prepared using the coprecipitation method. The catalysts were characterized by means of XRD and H2-TPR. The reactions between methane and lattice oxygen from the complex oxides were investigated. The characteristic results revealed that the combination of Ce and Fe oxide in the catalysts could lower the temperature necessary to reduce the cerium oxide. The catalytic activity for selective CH4 oxidation was strongly influenced by dropped Fe species. Adding the appropriate amount of Fe2O3 to CeO2 could promote the action between CH4 and CeO2. Dispersed Fe2O3 first returned to the original state and would then virtually form the Fe species on the catalyst, which could be considered as the active site for selective CH4 oxidation. The appearance of carbon formation was significant and the oxidation of carbon appeared to be the rate-determining step; the amounts of surface reducible oxygen species in CeO2 were also relevant to the activity. Among all the catalysts, Ce0.6Fe0.402 exhibited the best activity, which converted 94.52% of CH4 at 900 ℃.     

Chemical interaction of Ce-Fe mixed oxides for methane selective oxidation

Chemical interaction of Ce-Fe mixed oxides was investigated in methane selective oxidation via methane temperature programmed reduction and methane isothermal reaction tests over Ce-Fe oxygen carriers. In methane temperature programmed reduction test, Ce-Fe oxide behaved complete oxidation at the lower temperature and selective oxidation at higher temperatures. Ce-Fe mixed oxides with the Fe content in the range of 0.1–0.5 was able to produce syngas with high selectivity in high-temperature range(800–900 °C), and a higher Fe amount over 0.5 seemed to depress the CO formation. In isothermal reaction, complete oxidation occurred at beginning following with selective oxidation later. Ce1–xFexO2–δ oxygen carriers(x≤0.5) were proved to be suitable for the selective oxidation of methane. Ce-Fe mixed oxides had the well-pleasing reducibility with high oxygen releasing rate and CO selectivity due to the interaction between Ce and Fe species. Strong chemical interaction of Ce-Fe mixed oxides originated from both Fe*activated CeO2 and Ce3+ activated iron oxides(FeOm), and those chemical interaction greatly enhanced the oxygen mobility and selectivity.     

Catalytic performance for methane combustion of supported Mn-Ce mixed oxides

A series of supported Mn-Ce mixed oxide catalysts were prepared by the impregnation method and used for the oxidation of methane. The catalysts were characterized by N2 adsorption （BET）, X-ray diffraction （XRD）, laser Raman spectrum （LRS）, and temperature programmed reduction （TPR） techniques. The XRD and LRS results confirmed the high dispersion of active components or formation of solid solution between manganese and cerium oxides in the bulk and on the surface of mixed oxide catalysts. The reducibility was remarkably promoted by the stronger synergistic interaction between the two oxides from H2-TPR measurements. As expected, all the experimental mixed oxide catalysts showed excellent activity for methane combustion at low temperature. Especially, for the catalyst with Mn-Ce ratio 3：7, methane conversion reached 92% at a temperature as low as 470 ℃.     

Preparation and characterization of Ce-Fe-Zr-O(x)/MgO complex oxides for selective oxidation of methane to synthesize gas

A series of Ce-Fe-Zr-O(x)/MgO(x denotes the mass fraction of Ce-Fe-Zr-O,x=10%,15%,20%,25%,30%) complex oxide oxygen carriers for selective oxidation of methane to synthesis gas were prepared by the co-precipitation method.The catalysts were characterized by means of X-ray diffraction and H2-TPR.The XRD measurements showed that MgFeO4 particles were formed and Fe2O3 particles well dispersed on the oxygen carriers.The reactions between methane diluted by argon(10% CH4) and oxygen carriers were investigated.Suitable content of CeO2/Fe2O3/ZrO2 mixed oxides could promote the reaction between methane and oxygen carriers.There are mainly two kinds of oxygen of carriers:surface lattice oxygen which had higher activity but lower selectivity,and bulk lattice oxygen which had lower activity but higher selectivity.Among all the catalysts,Ce-Fe-Zr-O(20%)/MgO exhibited the best catalytic performance.The conversion of the methane was above 56%,and the selectivity of the H2 and CO were both above 93%,the ratio of H2/CO was stable and approached to 2 for a long time.     

稀土在甲烷部分氧化制取合成气中的应用研究

稀土（La,Ce,Pr,Nd等）通常可以作为催化剂载体、助剂或与其它元素形成固溶体,成为催化剂的重要组成部分。文中系统地介绍了稀土在甲烷部分氧化（CPOM）制取合成气中的相关应用,重点分析了稀土作为催化剂载体、助剂、固溶体等在催化部分氧化制取合成气中的应用,并对其在CPOM中未来前景做出展望。     

Preparation and characterization of Ce_1-xNi_xO₂ as oxygen carrier for selective oxidation methane to syngas in absence of gaseous oxygen

A citric acid complex method was employed to prepare Ce/Ni mixed oxides with various Ce/Ni ratios useful for selective oxidation methane to syngas in the absence of gaseous oxygen,and the catalytic activity measurement was investigated in a fixed bed reactor at 800 oC.The prepared oxygen carriers were characterized by various characterization techniques such as TG-DSC,XRD and TPR.The results of TG-DSC indicated that the Ce1-xNixO2 precursor generated a stable phase after the heat-treatment at temperatures above 800 oC.The XRD characterization suggested that some Ce-Ni solid solution was formed when Ni2+ ions was incorporated into the lattice of CeO2,and it led to the generation of O-vacancy which could improve the oxygen mobility in the lattice of oxygen carriers.It was found that Ce0.8Ni0.2O2 gave the highest activity in the selective oxidation methane to syngas reaction,and the average methane conversion,CO and H2 selectivity reached to 82.31%,82.41% and 87.64%,respectively.The reason could be not only attributed to the fitting amount of NiO dispersed on the CeO2 surface and bulk but also to actual lattice oxygen amount increased in oxygen carrier.     

La-Hexaaluminate Catalyst Preparation and Its Performance for Methane Catalytic Combustion

The La-hexaaluminate catalysts with high performance ration method with the buffer solution of NH4HCO3 and NH4OH were synthesized by the modified controllable co-precipimixture as the precipitation agent. The physicochemical properties of catalysts were characterized by the means of BET, XRD, and TPR techniques. With methane catalytic combustion as the probe reaction, the catalytic performances were also tested on a fixed bed, continual flow system. The resuits show that it is a good method to obtain chemical homogeneous hexaaluminate materials by the buffer solution as the precipitation agent. The La-hexaaluminate can be formed at low temperatures ranging from 1050 to 1200 ℃. The cerium introduction plays a great role in the methane catalytic combustion on La-Mn hexaaluminate because of its high oxygen storage capacity property and the well synergic effect between Ce and Mn. However, the CeO2 appears in hexaaluminate through the XRD pattern, which reveals that Ce can not enter the crystal lattice position. Mn introduction improves the methane catalytic activity to a large extent due to its high redox property. When Mn atomic substitution amount for A1 is 2, the hexaaluminate shows the highest activity, and the catalyst possesses good H2 consumption and redox performance. Mn can easily occupy the hexaaluminate crystal position, which reveals that the Mn substitute La-hexaaluminate is a promising high temperature methane combustion catalyst with high activity and good stability.     

Effect of cerium addition on oxidation behavior of 25Cr20Ni alloy under low oxygen partial pressure

The influence of Ce addition on the oxidation behavior of 25Cr20Ni alloy at 950 oC under low oxygen partial pressure was inves-tigated. The oxidized samples were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and scratch tester to obtain the oxide phases, morphology, thickness, composition and adhesion property of the oxide scales. The experiment results indicated that a small amount of Ce addition (0.02 wt.% or 0.05 wt.%) promoted oxidation resistance and inhibited the growth of the needlelike oxide. The Ce addition also decreased the formation of MnCr2O4 but promoted the SiO2 formation un-derneath the Cr2O3, which largely contributed to the improvement of oxide scale spallation resistance. For the sample with 0.3 wt.% Ce addi-tion, the oxidation rate significantly increased and the spallation resistance of the oxide scale decreased.     

Pseudo core-shell LaCoO_3@MgO perovskite oxides for high performance methane catalytic oxidation

Magnesia modified LaCoO_3 was prepared by a facile one-step sol-gel method and used for removal of dilute methane.Compared with the conventional doping technique,the obtained LaCoO_3@MgO-x exhibits pseudo core-shell structure and shows superior catalytic activity.The methane conversion exceeds90% at 532℃ on LaCoO_3@MgO-0.1,while only 60% of methane is conversed using the doped perovskite LaCo_(0.9)Mg_(0.1)O_3.The high catalytic performance of LaCoO_3@MgO-0.1 is mainly attributed to the adjustment of surface acid-base properties by the MgO shell structure.According to density functional theory(DFT) calculation,the methane is more likely to be adsorbed and cracked on LaCoO_3@MgO-0.1.The in situ DRIFTS shows that CH_3-O-CH_3 intermediate specie is formed.The pseudo core-shell structure also enhances the stability and the LaCoO_3@MgO-0.1 maintains high activity after working for 100 h.The above results demonstrate that surface modification by magnesia is an effective strategy for improving LaCoO_3 catalytic performance.     

甲烷部分氧化催化剂上的积碳研究进展

积碳是影响甲烷转化制合成气的催化剂性能的重要因素.在阐述甲烷催化部分氧化过程反应机理和催化剂体系的基础上,重点论述了甲烷催化部分氧化的催化剂失活与积碳的关系及影响催化剂的积碳因素.认为提高催化剂抗积碳性能的关键在于活性组分和负载量的改变、助剂的添加、载体的调控和制备方法的改进和储氧材料的使用,并对晶格氧催化甲烷部分氧化制合成气技术提出了展望,认为该技术具有良好的工业化应用前景.     

Catalytic performance of Zr-doped CuO-CeO_2 oxides for CO selective oxidation in H_2-rich stream

Zr-doped CuO-CeO_2 catalysts for CO selective oxidation were designed and prepared by the hydrothermal method and coprecipitation. The experimental samples were characterized by means of N_2 adsorption-desorption isotherms, powder X-ray diffraction, temperature-programmed reduction and Xray photoelectron spectroscopy. It is observed that the catalyst prepared by hydrothermal method exhibits larger specific surface area, smaller crystalline size and higher dispersion of active components compared with those of the catalyst obtained by coprecipitation. Meanwhile, redox properties of copper oxide are improved significantly and highly dispersed copper species providing CO oxidation sites are present on the surface. Furthermore, adsorptive centers of CO and active oxygen species increase on the copper-ceria interfaces. The Zr-doped CuO-CeO_2 catalyst prepared by hydrothermal method possesses superior catalytic activity and selectivity for selective oxidation of CO at low temperature compared with those of the sample prepared by coprecipitation. The temperature corresponding to 50% CO conversion is only 73 ℃ and the temperature span of total CO conversion is expanded from 120 to 160 ℃.     

Effect of calcination temperature and reaction conditions on methane partial oxidation using lanthanum-based perovskite as oxygen donor

We investigated the effect of calcination temperature, reaction temperature, and different amounts of replenished lattice oxygen on the partial oxidation of methane （POM） to synthesis gas using perovskite-type LaFeO3 oxide as oxygen donor instead of gaseous oxygen, which was prepared by the sol-gel method, and the oxides were characterized by XRD, TG/DTA, and BET. The results indicated that the particle size increased with the calcination temperature increasing, while BET and CH4 conversion declined with the calcination temperature increasing using LaFeO3 oxide as oxygen donor in the absence of gaseous oxygen. CO selectivity remained at a high level such as above 92%, and increased slightly as the calcination temperature increased. Exposure of LaFeO3 oxides to methane atmosphere enhanced the oxygen migration of in the bulk with time online owing to the loss of lattice oxygen and reduction of the oxidative stated Fe ion simultaneously, The high reaction temperature was favorable to the migration of oxygen species from the bulk toward the surface for the synthesis gas production with high CO selectivity. The product distribution and evolution for POM by sequential redox reaction was determined by amounts of replenished lattice oxygen with gaseous oxygen. The optimal process should decline the total oxidation of methane, and increase the selectivity of partial oxidation of methane.     

Characteristic of macroporous CeO2-ZrO2 oxygen carrier for chemical-looping steam methane reforming

Chemical-looping steam methane reforming （CL-SMR） is a novel process towards the production of pure hydrogen and syngas, consisting ofa syngas production reaction and a hydrogen production reaction. Macroporous CeQ-ZrO2 oxygen carders with different pore sizes prepared by colloidal crystal templating method and characterized by techniques of scalming electron microscopy （SEM）, transmission electron microscopy （TEM）, Brunauer-Emmett-Teller （BET）, X-ray diffraction （XRD） and temperature pro- grammed reduction （H2-TPR） were tested in CL-SMR process. For comparison, nonporous CeO2-ZrO2 oxygen carrier prepared by precipitation method was also investigated. It was found that macroporous CeO2-ZrO2 oxygen carriers owned higher reducibility and reactivity in CL-SMR process than nonporous samples. For the macroporous CeO2-ZrO2 sample, the decline of pore size could im- prove the reducibility and reactivity. The macroporous sample with a pore size of 100 nm （labeled as Ce-Zr-100） showed the highest performance for the co-production of syngas and hydrogen during the successive CL-SMR redox cycles. After 10 redox cycles, it still retained good porous structure and reducibility. It was found that the porous structure could accelerate the oxygen release from bulk to surface, leading to a good mobility of oxygen and higher reducibility. In addition, it was also favorable for diffusion and penetration of methane and water steam into the sample particles to accelerate the reaction rate.     

Synthesis of mesoporous CeO2-MnOx binary oxides and their catalytic performances for CO oxidation

Mesoporous CeO2-MnOx binary oxides with different Mn/Ce molar ratios were prepared by hydrothermal synthesis and characterized by scanning electron microscopy （SEM）, N2 sorption, X-ray diffraction （XRD）, X-ray photoelectron spectroscopy （XPS） and H2 temperature-programmed reduction （H2-TPR）. The characterization results indicated that the CeO2-MnOx catalysts exhibited flower-like microspheres with high specific surface areas, and partial Mn cations could be incorporated into CeO2 lattice to form solid solution. The CeO2-MnOx catalysts showed better catalytic activity for CO oxidation than that prepared by the coprecipitation method. Furthermore, the CeO2-MnOx catalyst with Mn/Ce molar ratio of 1 in the synthesis gel （Ce-Mn-1） exhibited the best catalytic activity, over which the conversion of CO could achieve 90% at 135 ℃. This was ascribed to presence of more Mn species with higher oxida- tion state on the surface and the better reducibility over the Ce-Mn-I catalyst than other CeO2-MnOx catalysts.     

Oxides Catalysts of Rare Earth and Transient Metal for Catalytic Oxidation of Benzene

The catalysts of CeO2 and the mixture of CeO2 and CuO were prepared, and the activities of these catalysts for completely oxidizing benzene were studied.The results show that the optimal proportion of CeO2/CuO is 6: 4.The highest temperature at which benzene was completely oxidized on these catalysts at different airspeed was measured.Compared these catalysts with the noble metal used, our catalysts had superiority in the resources and the industrial cost besides good activities.     

Partial Oxidation of Methane to Syngas over Monolithic Ni/γ-Al2O3 Catalyst——Effects of Rare Earths and Other Basic Promoters

A series of monolithic Ni/γ-Al2O3 catalysts with and without basic promoters (Na, Sr, La, Ce) were prepared. Partial oxidation of methane (POM) to syngas was carried out in a continuous-flow, fixed-bed reactor. The influences of reaction conditions, including temperature, CH4/O2 ratio and space velocity, on the performance of the catalyst were investigated. The results show that at a high space velocity of 1×105 h-1, optimal CH4 conversion can be obtained. Effects of promoters such as Na, Sr, Ce, La were also investigated, and the catalyst samples were characterized by means of temperature-programmed reduction and XRD techniques. XRD suggests that the addition of promoters has no influence on the crystal structure of Ni/γ-Al2O3 catalyst. The results show that the addition of a small amount of promoters improves the reducibility and activity of the catalyst. The side reaction CH4 2 O2→CO2 H2O, is fully restrained and 100% H2 selectivity is achieved when Ce and La are used as promoters, respectively.     

Effects of cerium doping position on physicochemical properties and catalytic performance in methanol total oxidation

The physicochemical properties of Pd and PdPt catalysts which possess different Ce doping position were investigated by techniques of TEM, XRD, N₂ adsorption-desorption, XPS and FT-IR. The catalytic performance for methanol total oxidation was examined to study the effects of Ce adding position. CeO₂Al₂O₃TiO₂ (CAT) catalysts that Ce is directly introduced into support show higher reactivity and CO₂ selectivity than CeO₂/Al₂O₃TiO₂ (Ce/AT) samples in which Ce is loaded by impregnation method. The characterization results reveal that the Ce doping position does not cause obvious otherness of basic crystalline phase and mesoporous structure of support. However, the Ce doping position affects the pore shapes of support and then influences the pore diameter. CAT catalysts possess more abundant adsorbed oxygen and more Ti³⁺ can transform the more gaseous oxygen into the active oxygen species on the catalyst surface, which is beneficial to the reaction. The AlOTi bridges in CAT facilitate the cooperation of Al and Ti species, which further speeds up the reaction rate.     

Partial Oxidation of Methane to Syngas over Monolithic Ni/γ-Al2O3 Catalyst——Effects of Rare Earths and Other Basic Promoters

Overthe last fewyears ,partial oxidation of meth-ane (POM)tosyngas has attractedincreasing attentioninthe field of catalysis[1 ,2].Compared withtraditionalgranular catalysts , monolithic catalysts are more suit-able for practical application of heterogeneously cata-lyzed gas/liquid reactions ,with many advantages suchas lowpressure drops ,excellent heat and gas conduc-tivity ,and larger external surface areas .The active components in the catalysts of POMmainlyinclude noble metals andtransit…     

Effect of different mixing ways in palladium/ceria-zirconia/alumina preparation on partial oxidation of methane

The effect of the mixing ways of Ce0.7Zr0.3O2-Al2O3 mixed oxides on the partial oxidations of methane （POM） was investigated over Pd/Ce0.7Zr0.3O2-Al2O3 catalysts, the mixing ways including salt precursor mixing （ATOM）, precipitator mixing （MOL）, and powder mechanically mixing （MECH）, respectively. The test results indicated that among the three samples, Pd/ATOM had the best catalytic activity while Pd/MOL had the best stability in the stability test. Both the activity sequences of the fresh and used samples were consistent with the order of Pd dispersion. According to the X-ray diffraction （XRD） and BET characterization, the interaction of Ce^4＋, Zr^4＋, and Al^3＋ in the ATOM mixed oxide was in favor of performing higher catalytic activity and thermal stability. The stability test indicated that Pd/MOL had the highest Pd dispersion and least coke formation on the active sites calculated by the Hz-chemisorption and TG results, which was considered to relate to its superior activity of POM to other catalysts.     

LaCo_(0.9)Cu_(0.1)O_3的葡萄糖溶胶—凝胶法合成及二甲苯催化氧化性能研究

本文以葡萄糖溶胶—凝胶法合成了稀土钙钛矿型复合氧化物 L a Co0 .9Cu0 .1 O3,并对其进行了 XRD、BET、TPR表征和二甲苯催化氧化性能测试 ,同时与柠檬酸溶胶—凝胶法作了比较。结果表明 ,葡萄糖作为络合剂成胶和干胶容易 ,粒径较小 ,具有进一步降低 L a Co0 .9Cu0 .1 O3形成温度的作用 ,以葡萄糖作为络合剂在 70 0℃所制备的催化剂 L a Co0 .9Cu0 .1 O3低温活性较好