Growth mechanism of palladium clusters on rutile TiO_2 (110) surface

Oxide-supported transition metal systems have been the subject of enormous interest due to the improvement of catalytic properties relative to the separate metal.Thus in this paper,we embark on a systematic study for Pd n （n=1-5） clusters adsorbed on TiO2 （110） surface based on DFT-GGA calculations utilizing periodic supercell models.A single Pd adatom on the defect-free surface prefers to adsorb at a hollow site bridging a protruded oxygen and a five-fold titanium atom along the [110] direction,while Pd dimer is located on the channels with the Pd-Pd bond parallel to the surface.According to the transition states （TSs） search,the adsorbed Pd trimer tends to triangular growth mode,rather than linear mode,while the Pd4 and Pd5 clusters prefer three-dimensional （3D） models.However,the oxygen vacancy has almost no influence on the promotion of Pd n cluster nucleation.Additionally,of particular significance is that the Pd-TiO2 interaction is the main driving force at the beginning of Pd nucleation,whereas the Pd-Pd interaction gets down to control the growth process of Pd cluster as the cluster gets larger.It is hoped that our theoretical study would shed light on further designing high-performance TiO2 supported Pd-based catalysts.

Role of Surface Adsorption in Fast Oxygen Storage/Release of CeO_2-ZrO_2 Mixed Oxides

Four kinds of CeO2-ZrO2 mixed oxides, i.e., a physical mixture of ceria and zirconia （CZP）, zirconia-coated ceria （ZCC）, ceria-coated zirconia （CCZ） and a chemical mixture of celia and zirconia （CZC）, were prepared. The oxygen storage capacity （OSC） measurements at 500℃ were performed under transient and stationary reaction conditions. All the curves of CO2 evolution during CO-O2 cycles presented a bimodal shape. The fast peak was primarily the result of the reaction of CO with the oxygen from the oxides, which was mainly determined by the nature of the material The sec- ond peak was mostly related to the CO2 adsorption behavior and was highly influenced by the surface area and the number of surface active sites. As a result, OSC activity of the samples followed in the order of CZC 〉 CCZ 〉 ZCC=CZP.

Theoretical Study on the Structural and Electronic Properties of the Reduced SnO_2 (110) Surface

The reduced SnO2（110） surface has been investigated by using first-principles method with a slab model. By examining the vacancy formation energy of three kinds of reduced SnO2（110） surfaces, the most energetically favorable defect surface is confirmed to be the surface with the coexistence of bridging and in-plane oxygen vacancies, which is different with the traditional model by only removing bridging oxygen. The results of band structure calculations indicate that the electronic structure of this defect surface is similar to the SnO surface.

The Ab Initio Studies of NO Chemisorption on TiO_2(110) Surface

The studies of NO chemisorption on TiO2(110) surface are the base of research to NO decomposed to N2O on TiO2 surface. In this paper, 12 kinds of possible models of NO adsorbed on TiO2 perfect and defect surface were calculated by use of ab initio cluster method. We carried out optimization of the geometry, calculation of the chemisorption energy and analysis of the Mulliken population to those adsorption models. According to the calculation results, it can be got that the adsorbed decomposition of NO on defect surface is more advantageous and M6 and M12 are the important models to NO chemisorption and decomposition on TiO2 surface.

甲醛在CeO_2(110)表面的吸附

利用密度泛函理论计算了甲醛在洁净CeO_2(110)表面的吸附行为,发现甲醛在表面存在化学与物理吸附。甲醛化学吸附时,甲醛的碳、氧原子分别与表面氧、铈原子发生作用,生成CH_2O_2,平面结构变为四面体构型;吸附能随覆盖度的减小而增大;电子结构分析表明,其最高占据轨道nO的电子进入表面铈原子的空轨道,形成Ce-Os化学键;而表面氧原子的电子填充到甲醛的C-Of反键轨道,形成新的C-Os键,甲醛的C-Of键长伸长。而物理吸附的甲醛,平面结构不变,吸附能较小,电子结构无明显变化。

硬脂酸对CeO_2纳米粒子的表面修饰研究

利用表面修饰法合成了硬脂酸修饰的 CeO2 纳米粒子,采用透射电子显微镜(TEM)观察了经表面修饰的CeO2 纳米粒子的形貌及分散性,并采用红外光谱(IR)、紫外可见分光光度计等对修饰的CeO2 纳米粒子进行了表征。结果表明:表面修饰剂硬脂酸与 CeO2 纳米粒子表面之间发生了化学键合作用;修饰后的CeO2 纳米粒子表面存在疏水有机基团,阻隔了 CeO2 纳米粒子的团聚,起到了分散作用;同时,修饰后的CeO2 纳米粒子在苯乙烯中的稳定性得到了提高。并且获得了硬脂酸的修饰量与CeO2 纳米粒子的最佳配比。

TiO_2(110)面的弛豫结构及吸附O_2的密度泛函研究

采用DFT/B3LYP方法研究了TiO2 ( 110 )的完整和氧缺陷表面的弛豫构型 ,并对O2 在氧缺陷表面的三种可能吸附构型进行了优化 ,计算了它们的吸附能、振动频率和重叠布居 .分析并预测了吸附后可能产生的物种 .本文的计算结果与XPS 。

CeO_2纳米薄膜的光声光谱研究

本文以Ｃｅ（ＮＯ３）３·６Ｈ２Ｏ为前驱物，火棉胶为添加剂，用ｓｏｌ－ｇｅｌ提拉法制备ＣｅＯ２透明薄膜。本方法成膜均匀，重复性好。薄膜的结晶性质和光谱性质研究表明，薄膜中ＣｅＯ２为纳米粒径，具有强烈的量子尺寸效应和表面效应。

表面掺杂CeO_2对2Y-TZP陶瓷抗低温水热腐蚀的影响

比较了对２Ｙ－ＴＺＰ陶瓷采用表面掺杂ＣｅＯ２和体掺杂ＣｅＯ２两种不同方式处理后，２Ｙ－ＴＺＰ陶瓷低温水热腐蚀前后的抗弯强度的变化。结果表明，对Ｙ－ＴＺＰ陶瓷进行表面掺杂ＣｅＯ２，不仅可以克服体掺杂ＣｅＯ２时抗弯强度大幅度下降的问题，而且可以明显改善低温水热腐蚀对Ｙ－ＴＺＰ陶瓷力学性能的影响。表面掺杂ＣｅＯ２对２Ｙ－ＴＺＰ陶瓷抗低温水热腐蚀的原因，是既保持了Ｙ－ＴＺＰ陶瓷整体对等温相变的约束，又阻止了Ｙ－ＴＺＰ陶瓷产生氧空位的综合作用。

Characterization and catalytic performance of CeO_2-Co/SiO_2 catalyst for Fischer-Tropsch synthesis using nitrogen-diluted synthesis gas over a laboratory scale fixed-bed reactor

The surface species of CO hydrogenation on CeO2-Co/SiO2 catalyst were investigated using the techniques of temperature programmed reaction and transient response method. The results indicated that the formation of H2O and CO2 was the competitive reaction for the surface oxygen species, CH4 was produced via the hydrogenation of carbon species step by step, and C2 products were formed by the polymerization of surface-active carbon species （-CH2-）. Hydrogen assisted the dissociation of CO. The hydrogenation of surface carbon species was the rate-limiting step in the hydrogenation of CO over CeO2-Co/SiO2 catalyst. The investigation of total pressure, gas hourly space velocity （GHSV）, and product distribution using nitrogen-rich synthesis gas as feedstock over a laboratory scale fixed-bed reactor indicated that total pressure and GHSV had a significant effect on the catalytic performance of CeO2-Co/SiO2 catalyst. The removal of heat and control of the reaction temperature were extremely critical steps, which required lower GHSV and appropriate CO conversion to avoid the deactivation of the catalyst. The feedstock of nitrogen-rich synthesis gas was favorable to increase the conversion of CO, but there was a shift of product distribution toward the light hydrocarbon. The nitrogen-rich synthesis gas was feasible for F-T synthesis for the utilization of remote natural gas.

表面缺陷对一氧化碳和甲基基团在金红石TiO_(2)(110)表面吸附的影响

本文利用程序升温脱附技术研究了氧空位浓度对甲基基团和CO在R-TiO_(2)(110)表面吸附的影响.结果表明,随着氧空位浓度的变化,吸附在桥氧位的甲基基团和吸附在五配位Ti^(4+)位点上的CO分子的脱附温度呈现了不同的趋势,揭示了表面缺陷可能对RTiO_(2)(110)不同位点上的物质吸附具有重要影响.

Experimental design for optimization of 4-nitrophenol reduction by green synthesized CeO2/g-C3N4/Ag catalyst using response surface methodology

In this study,the enhancement of catalytic activity of ceria when modified with co-catalysts such as graphitic carbon nitride and silver was establishe d.The material was synthe sized using phytogenic combustion method,a green alternative to the traditional preparative routes.The catalyst was characterized using XRD,FTIR,SEM,EDX,XPS and TEM techniques.The synergistic effect of the composite CeO2/g-C3 N4/Ag was tested for catalytic reduction of 4-nitrophenol in the prese nce of sodium borohydride.The reaction was carried out at room tempe rature without any light source or exte rnal stirring.The individual and combined effects of four parameters,viz.,concentration of 4-NP,amount of catalyst,amount of NaBH4 and time for the reduction of reduction 4-NP were investigated using Box-Behnken design of response surface methodology(RSM).This statistical model was used to optimize the reaction conditions for maximum reduction of 4-NP.The optimum conditions for the reduction reaction are found to be 0.01 mmol/L 4-NP,15 mg catalyst,20 mg NaBH4 and 13.7 min time interval.

CeO_2(110)表面上CO氧化反应的密度泛函理论研究

利用密度泛函理论系统研究了O2与CO在CeO2(110)表面的吸附反应行为.研究表明,O2在清洁的CeO2(110)表面吸附热力学不利,而在氧空位表面为强化学吸附,O2分子被活化,可能是重要的氧化反应物种.CO在清洁的CeO2(110)表面有化学吸附与物理吸附两种构型,前者形成二齿碳酸盐物种,后者与表面仅存在弱相互作用.在氧空位表面,CO可分子吸附或形成碳酸盐物种,相应吸附能均较低.当表面氧空位吸附O2后(O2/Ov),CO可吸附生成碳酸盐或直接生成CO2,与原位红外光谱结果相一致.过渡态计算发现,O2/Ov/CeO2(110)表面的三齿碳酸盐物种经两齿、单齿过渡态脱附生成CO2.利用扩展休克尔分子轨道理论分析了典型吸附构型的电子结构,说明表面碳酸盐物种三个氧原子电子存在离域作用,物理吸附的CO及生成的CO2电子结构与相应自由分子相似.

CeO_2用于柴油机尾气NO还原的DFT研究

采用密度泛函理论广义梯度近似的RPBE方法结合周期性平板模型研究了NO分子分别以N端和O端两种吸附形式在洁净的以及具有氧空穴的CeO_2(110)表面的吸附行为。对比了不同的吸附位及不同覆盖度下的几何构型参数与吸附能。研究结果表明:CeO_2(110)表面O原子上的吸附构型为较稳定的吸附构型,且N端吸附较O端有利;NO在洁净CeO_2(110)表面为物理吸附;当CeO_2(110)表面存在氧空穴时,吸附能明显增大且均大于40 k J·mol^(-1),为化学吸附;覆盖度为0.25 m L时的吸附比较稳定。计算了NO分子吸附前后的态密度以及电荷密度,研究发现:NO分子与底物之间具有相互作用,且整个吸附体系发生了电荷从Ce原子向NO分子的转移,O端吸附时转移的电荷较N端的多。

Introduction manner of sulfate acid for improving the performance of SO_4^(2–)/CeO_2 on selective catalytic reduction of NO by NH_3

A series of sulfated CeO2 catalysts were synthesized by impregnation and sol-gel methods and used for selective catalytic reduction （SCR） of NOx by NH3. The results showed that the sulfated CeO2 catalysts prepared by sol-gel method showed excellent catalytic activity at 150-50 ℃, and more than 90% NOx conversion was obtained at 232-450 ℃ with a gas hourly space velocity of 60000 h-1. The catalysts were characterized by X-ray diffraction （XRD）, N2 adsorption, Raman, thermogravimetry （TG）, H2-tem- perature-programmed reduction （H2-TPR） and Py-infrared spectroscopy （Py-IR）. The excellent SCR performance was associated with the surface acidity and the micro-structure. The introduction of sulfate acid into CeO2 could increase the amount of BrOnsted and Lewis acid sites over the catalysts, resulting in the improvement of the low temperature activity. The sulfated CeO2 catalysts prepared by sol-gel method possessed lower crystallization degree, excellent redox property and larger specific surface areas, which were responsible for the superior SCR performance.

Stochastic resonance in surface catalytic oxidation of carbon monoxide induced by colored noise

The dynamical behavior of surface catalytic oxidation reaction of Pt(110)/CO+O2 modulated by colored noise, under the condition of specific temperature, has been investigated when the partial pressure of CO gas is near the supercritical Hopf bifurcation point. By computer simulation the oscillation and stochastic resonance induced by colored noise are observed. The influences of the intensity and correlation time of colored noise on stochastic resonance are discussed. The range of sensitivity of the system to the environmental fluctuation is analyzed.

Thickness modulation effect of CeO_2 layer for YBCO films grown by pulsed laser deposition

CeO2 film plays an essential role in nucleation and growth of YBa2 Cu3 O（7-x）（YBCO） films. In this work,the dependence of superconducting properties of YBCO on CeO2 films with different thicknesses was investigated,in order to achieve fabrication of high-performance YBCO coated conductors in industrial scale. The crystalline structure and morphology of CeO2 films with thickness ranging from 21 to 563 nm were systematically characterized by means of X-ray diffraction（XRD）, atomic force microscope（AFM） and reflection high-energy electron diffraction（RHEED）. Additional focus was addressed on evolution of the surface quality of CeO2 films with thickness increasing. The results show that at the optimal thickness of 221 nm, CeO2 film exhibits sharp in-plane and out-of-plane texture with full width of half maximum（FWHM） values of 5.9° and 1.8°, respectively, and smooth surface with a mean root-mean-square（RMS） roughness value as low as 0.6 nm. Combing RHEED and transmission electron microscope（TEM） cross-sectional analysis, it is found that nucleation and growth of CeO2 films at early stage remain in island growth mode with rougher surface,while further increasing the thickness beyond the optimal thickness leads to weak surface quality, consequently resulting in degradation of superconductor layers deposited subsequently. Eventually, a critical current density（Jc） as high as 4.6×10-6 A·cm-（-2）（77 K, self-field） is achieved on a YBCO film on a thickness-modulated CeO2/MgO/Y2 O3/Al2 O3/C276 architecture, demonstrating the advantages of CeO2 films as buffer layer in high-throughput manufacture of coated conductors.

Unique adsorption behaviors of NO and O_2 at hydrogenated anatase TiO_2(101)

Titanium dioxide（TiO2） is one of the most widely studied transition metal oxides, especially for its unique performances in heterogeneous photocatalysis. Different phases of TiO2 have been found to exhibit different photo-activities, though the origins are still not fully understood. In this work, we use the density functional theory（DFT） calculations, corrected by on-site Coulomb and long-range dispersion interactions, to study the adsorptions of nitric oxide（NO） and oxygen（O2） molecules on the clean and hydrogenated anatase TiO2（101） surfaces. We also compare the detailed calculated results regarding their structural, energetic and electronic properties with those obtained at rutile TiO2（110）. It has been found that the behaviors of the surface localized electrons being transferred from adsorbed H, as well as the adsorption behaviors of NO and O2 are quite different at the two surfaces, which can be attributed to their characteristic local bonding structures around the surface hydroxyl. These results may also help explain the different photocatalytic activities of these two main facets of anatase and rutile TiO2