DFT Study of Methanol Adsorption on Defect-Free CeO2 Low-Index Surfaces

Methanol decomposition is a promising method for hydrogen production. However, the performance of current catalysts for this process is not sufficient for commercial applications. In this work, methanol adsorption on the CeO₂ low-index surfaces is studied by density functional theory (DFT). The results show that methanol always dissociates spontaneously on the (100) surface, whereas dissociation on the (110) surface is site-selective; dissociation does not occur at all on the (111) surface, where only weak physisorption is found. The results confirm that surfaces with higher energies are more catalytically active. Analysis of the surface geometries shows that the dominant factors for the dissociation of methanol are the degree of undercoordination and the charges of the surface ions. The adsorption energy of each methanol molecule decreases with increasing coverage and there is a transition threshold between dissociative and associative adsorption. The present work indicates that a strategy to design catalysts with high activity is to maximize exposure of surfaces on which the ions have a high degree of undercoordination and a strong tendency to donate/accept electrons. The results demonstrate the importance of appropriately selecting and controlling exposed facets and particle morphology for optimizing catalyst performance.     

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%。     

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.     

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。     

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.     

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.     

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.     

以六次甲基四胺为沉淀剂制备纳米ZrO2的研究

以ZrOCl2·8H2O和(CH2)6N4为原料,利用均匀沉淀法制得了纳米ZrO2,考察了不同反应条件如:反应物原始浓度、反应物配比、不同陈化方式、不同干燥方式、表面活性剂等对粒径大小、粒子形状、均匀度的影响,采用Malvern 粒度分布仪、透射电子显微镜和X射线衍射仪等对产品进行了表征,并得出结论:以六次甲基四胺为沉淀剂制备纳米氧化锆,所得粒子粒度较小、均匀性好,而且操作简便,反应速度快.     

不同添加剂对纳米氧化锆粉体影响的探讨

研究了不同添加剂乙醇、丙酮、硬脂酸铝、聚乙二醇、聚乙烯醇、溴代十六烷基吡啶在化学沉淀法制备纳米级氧化锆粉体的过程中对产物粒径、形状的影响,结果表明:乙醇、丙酮、聚乙烯醇对改善粉体团聚、使粒径减小有明显的作用,硬脂酸铝和溴代十六烷基吡啶可使产物粒径增大,聚乙二醇可以改变产物形状,得到纺锤形氧化锆,并对其影响机理进行了探讨.