CeO2-MnOx催化剂形貌对低浓度甲烷催化燃烧反应性能的影响

采用水热合成法制备了船形、扁球形及纳米片CeO₂-MnO_x复合氧化物。并运用低温N₂吸脱附、XRD、SEM、TEM、H₂-TPR、拉曼光谱、XPS等表征技术对不同形貌CeO₂-MnO_x复合氧化物的结构与其低浓度CH₄催化燃烧反应性能之间的关系进行了关联。结果表明,CeO₂-MnO_x复合氧化物的形貌与其催化性能密切相关。其中,扁球形CeO₂-MnO_x复合氧化物的氧空位、Ce³⁺含量及表面吸附活性氧物种最多,其CH₄催化燃烧反应活性最高,540℃时,可将CH₄完全转化;其次是船形CeO₂-MnO_x复合氧化物催化剂,540℃时其CH₄转化率为94.05%;与前两者相比,纳米片CeO₂-MnO_x复合氧化物催化剂的氧空位及表面吸附活性氧物种较少,活性较差,相同反应温度下,其CH₄转化率仅为89.68%。     

金的化学状态对Au／CoCeOx催化剂CO氧化性能的影响

以CoCeOx复合氧化物为载体,采用沉积沉淀法制备了负载型的金催化剂,并通过不同温度的预处理控制Au的化学状态. 使用粉末X射线衍射、高分辨透射电子显微镜、程序升温还原和X射线光电子能谱对催化剂进行了表征,考察了在室温条件下该系列催化剂的一氧化碳氧化性能. 结果表明, Au/CoCeOx催化剂的CO氧化性能与催化剂表面Au 的含量成正比, Au 可能是反应的主要活性物种. 添加水汽对反应有一定的促进作用,但由于Au 不能稳定存在,特别是当催化剂表面Au 的含量过高时,在水汽的作用下Au 迅速发生歧化反应,使得催化剂的性能下降.     

介孔Ce1-xZrxO2负载的Cu基催化剂在富氢条件下催化CO选择性氧化

采用多元醇为模板剂合成了介孔Ce1-xZrxO2(x=0.2,0.35,0.5)固溶体材料,并以其为载体负载CuO制备了Cu基催化剂.应用透射电子显微镜、X射线衍射、程序升温还原、程序升温脱附和N2吸附-脱附等技术对载体及催化剂进行了表征,并研究了Zr的取代比例x值对载体和Cu基催化剂性能的影响.结果表明,所有Ce1-xZrxO2样品均为介孔材料,其中Ce0.5-Zr0.5O2载体样品有较大的比表面积(181m2/g),CuO/Ce0.5Zr0.5O2催化剂样品在富氢条件下有较高的催化CO选择性氧化反应的活性和选择性.与其他催化剂样品相比,CuO/Ce0.5Zr0.5O2催化剂样品中形成的活性中心更多,分散性更好,对CO的吸附量更大,CO脱附温度更低,活性组分与载体的相互作用更强。     

氧化铈形貌对Au/CeO2催化剂催化氧化CO反应活性的影响

采用水热合成法制备了形貌规则的纳米氧化铈颗粒,分别为棒状、立方体和多面体,通过溶胶沉积法将金颗粒沉积到不同形貌氧化铈表面制得了Au/CeO2催化剂.考察了催化剂载体的不同形貌对CO催化氧化反应活性的影响.实验结果表明,棒状(110 100)和多面体(111 100)氧化铈作为载体时的催化剂活性比立方体(100)作为载体时的活性高.在低温段,多面体氧化铈作为载体的催化剂表现出较高活性,而在高温范围,棒状氧化铈作为载体的催化剂的催化活性最好.     

Influence of Calcination on the Properties of Nickel Oxide-Samarium Doped Ceria Carbonate (NiO-SDCC) Composite Anodes

Apart from its composition, the starting powder properties such as particle size potentially affect the triple phase boundary and the electrochemical performance. Calcination process has been identified as one of the factors that influence the particle size of the composite anode powders. This study investigates the correlation between calcination temperature and properties (i.e., chemical, physical, and thermal) of NiO–samarium-doped ceria carbonate (SDCC) composite anodes. NiO–SDCC composite anode powder was prepared with NiO and SDCC through high-energy ball milling. The resultant composite powder was subjected to calcination at various temperatures ranging from 600 °C to 800 °C. Characterizations of the composite anode were performed through X-ray diffraction (XRD), Fourier transform infrared spectroscopy, energy dispersive spectroscopy, field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), dilatometry, and porosity measurements. The composite anodes exhibited good chemical compatibility during XRD after calcination and sintering. The FTIR result verified the existence of carbonates in all the composite anodes. The increment in calcination temperature from 600 °C to 800 °C resulted in the growth of nanoscale particles, as evidenced by the FESEM micrographs and crystallite size. Nonetheless, the porosity obtained remained within the acceptable range for a good anodic reaction (20% to 40%). The TGA results showed gradual mass loss in the range of 400 °C to 600 °C (within the low-temperature solid oxide fuel cell region). The composite anodes calcined at 600 °C and 700 °C revealed a good thermal expansion coefficient that matches that of the SDCC electrolyte.     

Dry Reforming of Methane on a Highly‐Active Ni‐CeO2 Catalyst: Effects of Metal‐Support Interactions on C−H Bond Breaking

Ni‐CeO₂ is a highly efficient, stable and non‐expensive catalyst for methane dry reforming at relative low temperatures (700 K). The active phase of the catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria. Experiments of ambient pressure XPS indicate that methane dissociates on Ni/CeO₂ at temperatures as low as 300 K, generating CH_x and CO_x species on the surface of the catalyst. Strong metal–support interactions activate Ni for the dissociation of methane. The results of density‐functional calculations show a drop in the effective barrier for methane activation from 0.9 eV on Ni(111) to only 0.15 eV on Ni/CeO_2?x(111). At 700 K, under methane dry reforming conditions, no signals for adsorbed CH_x or C species are detected in the C 1s XPS region. The reforming of methane proceeds in a clean and efficient way.     

Water splitting as a tool for obtaining insight into metal–support interactions in catalysis

This review is focused on the use of the water splitting reaction for characterizing oxygen vacancies in supported metal catalysts and more generally to get insight into the high-temperature modifications of metal–support interactions. Three supports widely used in catalysis are considered, namely alumina, silica and ceria. The catalysts were reduced at temperatures T_R ranging from 200 to 1000 °C. The reaction with water was carried out at temperatures T_OX ranging from 100 to 1000 °C. In every case, the metal (Rh or Pt) was chosen among those which are not oxidizable by water. Extensive investigations of the reactivity of water with unsupported metals and films confirmed this choice. The reaction is then selective for the titration of O vacancies, generally associated with reduced cations of the support. On alumina-supported catalysts, reduction at T_R > 600 °C leads to the formation of oxygen vacancies strictly confined to the periphery of metal particles. The amount of hydrogen produced Q_H is coherent with the peripheral oxygen density. Reduction of silica-supported catalysts at T_R > 600 °C generates metal silicides that can be selectively destroyed by water with reformation of silica and metal nanoparticles. Oxygen vacancies are formed on ceria catalysts at 200 °C. These oxygen vacancies are confined to the surface up to 600 °C. At higher temperatures, oxygen vacancies are formed in the bulk: about 50% of CeO₂ would be reduced at 900 °C. The amount of H₂ produced by reaction with water is thus very high on metal-ceria catalysts. At T_R > 900 °C, metal cerides start to form. Remarkably, a significant reactivity of H₂O on a Rh/CeO₂ catalyst reduced at 850 °C is recorded as of 100 °C. However, the quantitative titration of oxygen vacancies required temperatures T_OX > 500 °C. As a rule, the technique of water splitting allows the detection of 1 μmol g⁻¹ of oxygen vacancies, i.e. a few 0.1% of the surface in the case of reducible oxides of 10–20 m² g⁻¹.     

CO_2 Reforming of CH_4 over Ni/CeO_2-ZrO_2-Al_2O_3 Prepared by Hydrothermal Synthesis Method

The Ni/CeO2-ZrO2-Al2O3 catalyst with different Al2O3 and NiO contents were prepared by hydrothermal synthesis method. The catalytic performance for CO2 reforming of CH4 reaction, the interaction among components and the relation between Ni content and catalyst surface basicity were investigated. Results show that the interaction between NiO and Al2O3 is stronger than that between NiO and CeO2-ZrO2.The addition of Al2O3 can prevent the formation of large metallic Ni ensembles, increase the dispersion of Ni, and improve catalytic activity, but excess Al2O3 causes the catalyst to deactivate easily. The interaction between NiO and CeO2 results in more facile reduction of surface CeO2. The existence of a small amount of metallic Ni can increase the number of basic sites. As metallic Ni may preferentially reside on the strong basic sites, increasing Ni content can weaken the catalyst basicity.     

Chemomechanical properties and microstructural stability of nanocrystalline Pr-doped ceria: An in situ X-ray diffraction investigation

The chemomechanical properties and microstructural stability of nanocrystalline Pr_xCe_1 − xO_2 − δ solid solutions are studied as a function of temperature by in situ X-ray diffraction measurements under oxidizing conditions at P(O₂) ~ 200 mbar. The chemical expansion coefficient of nanocrystalline powder specimens, operative at intermediate temperatures during which Pr⁴⁺ is reduced to Pr³⁺, is found to be similar to that obtained for coarse-grained Pr_xCe_1 − xO_2 − δ. This is contrary to reports regarding variation of physical and chemical properties with crystallite size. The thermal expansion coefficient, measured under conditions for which Pr_xCe_1 − xO_2 − δ is highly oxygen deficient, was found to be greater than that measured for fully oxidized Pr_xCe_1 − xO_2 − δ, with potential sources of these changes discussed. Moreover, the microstructure of nanocrystalline Pr_xCe_1 − xO_2 − δ is observed to have excellent stability at working temperatures below 800 °C, enabled by the inherent microstrain in the structure, highlighting the potential application of this material for solid state electrochemical devices.     

Modifications of interface chemistry of LSM-YSZ composite by ceria nanoparticles

A porous composite electrode LSM-YSZ (lanthanum strontium manganite and yttria stabilized zirconia) was impregnated with different amounts of SDC (samarium substituted ceria) nanoparticles. The materials were investigated with X-ray diffraction, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy to determine the microstructure, the interface chemistry and the surface chemistry of the various impregnated samples. The SDC nanoparticles cover the surface of the LSM-YSZ backbone to a large extent; they are approximately 5-20 nm in diameter and have a cubic crystal structure. Low concentrations of lanthanum and manganese originating from LSM were detected within SDC particles. It was also observed that the relative atomic concentration of strontium increased on the LSM-YSZ surface with increasing amount of SDC nanoparticles. These findings are related to the applied nanoparticle impregnation method. It is indicated that interactions between surfactant, nanoparticles, impregnation solution and the LSM-YSZ composite take place which can locally affect the surface and interface chemistry of the investigated materials.