Analysis by DSC of the drying and sintering processes of alkoxide-derived SiO2–ZrO2 gels

The drying and sintering processes of SiO₂–ZrO₂ alkoxide-derived gels have been studied by means of DSC technique. In the drying process, most part of water and alcohols are removed from the gels. For the SiO₂ gel such elimination occurs at the end of the drying process, however for the ZrO₂ gel this elimination occurs during the whole drying time. An intermediate behavior is observed for the binary system SiO₂–ZrO₂ gels. In the sintering process, the DSC technique allows to determine the elimination of water and alcohols retained within the structure (open or close pores) and the well-known hydroxyl condensation of silica gel between 700° and 800°C is also observed. The ZrO₂ gel shows the final hydroxyl condensation at the heating temperature of 600°C. For the binary SiO₂–ZrO₂ gels, the hydroxyl condensation has been associated to the activation energy needed for the dissociation of silica hydroxyls. This energy decreases with the ZrO₂ concentration in the gel resulting in a sintering treatment of 500°C leading to the entire hydroxyl condensation for the gel with 75% ZrO₂–25% SiO₂.

By studying the temperature of the DSC peaks, it is possible to know the temperature at which most part of water and alcohols are leaving the gel, and these results can be used in order to select the corresponding drying or sintering schedules for obtaining a well-fabricated material.     

ZrO2修饰Cu催化剂还原以及反应过程中表面组成的变化

通过热分解法制备Cu模型催化剂,然后经浸渍制备ZrO₂/Cu催化剂,采用SEM、XPS考察了催化剂表面形态和组成,并采用in-situ Raman考察了催化剂在还原和吸附CO和水的过程中随时间的变化。结果表明,还原前Cu催化剂表面主要存在CuO物种,而在ZrO₂/Cu表面,除了CuO物种,还存在着大量的表面羟基物种。ZrO₂/Cu相对Cu更加容易还原为Cu⁰,同时,ZrO₂在催化剂表面聚集形成絮状态,而Cu催化剂还原后主要形成Cu₂O物种。Cu催化剂表面吸附CO后,除了形成Cu-CO外,Cu₂O物种均会迅速消失形成CO₂。Cu催化剂对水的作用比较弱,但是ZrO₂/Cu催化剂和水作用较强,并且通过Cu-OH中间物形成Cu₂O物种。     

Dynamic Simulation on Surface Hydration and Dehydration of Monoclinic Zirconia

The commonly used oxide-supported metal catalysts are usually prepared in aqueous phase, which then often need to undergo calcination before usage. Therefore, the surface hydration and dehydration of oxide supports are critical for the realistic modeling of supported metal catalysts. In this work, by ab initio molecular dynamics (AIMD) simulations, the initial anhydrous monoclinic ZrO\begin{document}$_2$\end{document}(111) surfaces are evaluated within explicit solvents in aqueous phase at mild temperatures. During the simulations, all the two-fold-coordinated O sites will soon be protonated to form the acidic hydroxyls (HO\begin{document}$_{\rm{L}}$\end{document}), remaining the basic hydroxyls (HO^*) on Zr. The basic hydroxyls (HO^*) can easily diffuse on surfaces via the active proton exchange with the undissociated adsorption water (H\begin{document}$_2$\end{document}O^*). Within the temperatures ranging from 273 K to 373 K, in aqueous phase a certain representative equilibrium hydrated m-ZrO\begin{document}$_2$\end{document}(111) surface is obtained with the coverage (\begin{document}$\theta$\end{document}) of 0.75 on surface Zr atoms. Later, free energies on the stepwise surface water desorption are calculated by density functional theory to mimic the surface dehydration under the mild calcination temperatures lower than 800 K. By obtaining the phase diagrams of surface dehydration, the representative partially hydrated m-ZrO\begin{document}$_2$\end{document}(111) surfaces (0.25\begin{document}$\leq$\end{document}\begin{document}$\theta$\end{document} < 0.75) at various calcination temperatures are illustrated. These hydrated m-ZrO\begin{document}$_2$\end{document}(111) surfaces can be crucial and readily applied for more realistic modeling of ZrO\begin{document}$_2$\end{document} catalysts and ZrO\begin{document}$_2$\end{document}-supported metal catalysts.     

MoO3和Nb2O5助剂对Pt/ZrO2催化剂上短链烷烃燃烧反应的不同促进作用

贵金属Pt催化剂具有高活性和热稳定性,广泛应用于催化挥发性有机物的完全氧化反应(燃烧反应).短链烷烃(甲烷、乙烷、丙烷等)化学性质稳定,是最难氧化的一类有机物,常用作考察燃烧反应催化剂性能的模型反应物.然而,目前报道的研究工作通常仅限于针对某一种烷烃底物的催化燃烧,系统考察催化剂以及助剂对不同短链烷烃的催化燃烧活性鲜有报道.在短链烷烃中,甲烷只有C–H键;而其它烷烃除了C–H键;还有C–C键.因此,研究催化剂对甲烷、乙烷和丙烷燃烧反应催化性能的差异性,对于认识催化剂上C–H键和C–C键的活化具有非常重要的意义.本文制备了MoO3或Nb2O5修饰的Pt/ZrO2催化剂并用于短链烷烃的燃烧反应.研究发现,MoO3助剂对甲烷燃烧有明显的抑制作用,但对乙烷,丙烷和正己烷燃烧反应具有促进作用,促进作用随着烷烃碳链的增长逐渐增加;Nb2O5助剂对甲烷、乙烷、丙烷和正己烷燃烧反应均具有促进作用,然而促进作用随着碳链的增长而逐渐减弱.MoO3和Nb2O5助剂的不同促进作用与助剂影响催化剂表面酸性以及Pt物种的氧化或还原态有关.NH3-TPD结果表明,MoO3助剂可以显著增加Pt/ZrO2催化剂表面强酸位点数量,而Nb2O5助剂可以显著增加Pt/ZrO2催化剂表面中强酸位点数量.HTEM结果表明,两种助剂的添加都不会明显改变Pt物种的颗粒尺寸.在Pt-Mo/ZrO2催化剂上,MoO3覆盖部分Pt物种形成丰富的Pt-MoO3界面,促进了金属Pt物种和强表面酸性位点的生成,提高了丙烷燃烧反应活性;Pt-Nb/ZrO2催化剂上载体表面的部分Nb2O5被Pt物种包覆,使得生成的表面Pt-Nb2O5界面低于Pt-Mo/ZrO2催化剂,但由于催化剂表面酸性位的提升,也促进了丙烷燃烧反应活性的提高.XPS结果表明,在甲烷燃烧反应中,Pt-Nb/ZrO2催化剂上Ptn+物种能够更加稳定地存在,这可能是Nb2O5助剂提高Pt-Nb/ZrO2催化剂上甲烷燃烧活性的关键.而Pt-Mo/ZrO2催化剂上Ptn+物种在甲烷反应中可以更容易地被还原,并且由于MoO3的包裹导致暴露的Pt位点数量降低,使催化剂催化甲烷燃烧的活性受到抑制.可见,MoO3助剂更有利于C–C键活化,而Nb2O5助剂更有利于高键能的C–H键活化.综上,本文系统性地研究MoO3助剂和Nb2O5助剂对Pt/ZrO2催化剂上不同短链烷烃的燃烧反应的影响,证实了两种助剂的促进作用与碳链长度的关系是截然不同的.     

Improvement of resistive switching fluctuations by using one step lift‐off process

Bipolar resistive switching characteristics are investigated in ZrO₂ containing Cu thin layer devices, particularly for the self‐isolated‐structure device fabricated by one step lift‐off process. Compared with the traditional‐structure device, the self‐isolated‐structure device shows more uniform resistive switching characteristics. This is because the isolation of each device cell has negligible influence on each other and thus mitigates possible crosstalk between each cell. These results suggest that the feasibility of good stabilization of the resistive switching parameters can be obtained through one step lift‐off process. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Preparation of P(MBA‐co‐MAA)/Zr(OH)4/P(EGDMA‐co‐MAA)/TiO2 Tetra‐layer Hybrid Microspheres and the Corresponding ZrO2/TiO2 Double‐shelled Hollow Microspheres

Double‐shelled zirconia/titania (ZrO₂/TiO₂) hollow microspheres were prepared by the selective removal of the polymer components via the calcination of the corresponding tetra‐layer poly(N,N′‐methylenebisacryl amide‐co‐methacrylic acid) (P(MBA‐co‐MAA))/Zr(OH)₄/poly(ethyleneglycol dimethacrylate‐co‐methacrylic acid) (P(EGDMA‐co‐MAA))/TiO₂ hybrid microspheres. These tetra‐layer microspheres were synthesized by the combination of the distillation copolymerization of N,N(‐methylenebisacryl amide‐co‐methacrylic acid (MBA) or ethyleneglycol dimethacrylate (EGDMA) crosslinker and methacrylic acid (MAA) for the preparation of polymer core and third‐layer as well as the controlled sol‐gel hydrolysis of inorganic precursors for the construction of zirconium hydroxide (Zr(OH)₄) and titania (TiO₂) layers. The thicknesses of zirconia and titania shell‐layers were conveniently controlled via varying the feed of zirconium n‐butoxide (Zr(OBu)₄) and titanium tetrabutoxide (TBOT) during the sol‐gel hydrolysis, while the sizes of polymer layers were tuned through a multi‐stage distillation precipitation copolymerization. The structure and morphology of the resultant microspheres were characterized by transmission electron microscopy (TEM), X‐ray diffractometer (XRD), X‐ray photoelectronic spectroscopy (XPS), and thermogrametric analysis (TGA).     

Coking-resistant Ni-ZrO2/A1203 catalyst for CO methanation

Highly coke-resisting ZrO2-decorated Ni/A1203 catalysts for CO methanation were prepared by a two-step process. The support was first loaded with NiO by impregnating method and then modified with ZrO2 by deposition-precipitation method （IM-DP）. Nitrogen adsorption- desorption, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetdc analysis, H2 temperature- programmed reduction and desorption, NH3 temperature-programmed desorption, and zeta potential analysis were employed to characterize the samples. The results revealed that, compared with the catalysts with the same composition prepared by co-impregnation （CI） and sequential impregnation （SI） methods, the Ni/A1203 catalyst prepared by IM-DP showed much enhanced catalytic performance for syngas methanation under the condition of atmospheric pressure and a high weight hourly space velocity of 120000 mL.g-1 .h-1. In a 80 h life time test under the condition of 300-600 ~C and 3.0 MPa, this catalyst showed high stability and resistance to coking, and the amount of deposited carbon was only 0.4 wt%. On the contrary, the deposited carbon over the catalyst without ZrO2 reached 1.5 wt% after a 60 h life time test. The improved catalytic performance was attributed to the selective deposition of ZrO2 nanoparticles on the surface of NiO rather than A1203, which could he well controlled via changing the electrostatic interaction in the DP procedure. This unique structure could enhance the dissociation of CO2 and generate surface oxygen intermediates, thus preventing carbon deposition on the Ni particles in syngas methanation.     

F-T组分改性KCuZrO2催化剂上CO加氢合成异丁醇的性能研究

采用共沉淀法分别制备了不同F-T组分(Fe、Co、Ni)改性的KCuZrO_2催化剂,并用于催化CO加氢合成异丁醇。通过BET、XRD、TEM、XPS、H_2-TPR、CO-TPD以及in-situ DRIFTS对催化剂进行了表征。结果显示,F-T组分的加入促进了乙醇和丙醇的形成,但是对异丁醇选择性影响不同。结果表明,Fe促进了催化剂中各组分的分散,活性组分Cu在催化剂表面发生了富集,提高了H_2/CO活化吸附;另外,KFeCuZrO_2的催化剂表面含有较多的C1物种,有利于乙醇和丙醇进一步发生β-加成反应得到异丁醇,而Co和Ni改性的催化剂上缺少足够的C1物种,因此,异丁醇的选择性并未明显增加。Co的引入对催化剂结构以及Cu的分散影响不大,但是Co改性后催化剂性能有所下降,其原因是催化剂发生了失活; Ni添加后催化剂比表面积有所减小,且催化剂表面Cu/Zr物质的量比也降低到0. 19,催化剂粒径增大,Cu-Zr之间相互作用减弱,异丁醇选择性降低。     

MnOx在ZrO2上分散状态及对CO氧化的影响

Mn-O/ZrO2 catalysts were prepared by impregnation method and were characterized by XRD, TPR and TPD-MS. The oxidation activity of CO on these catalysts was also studied. The results indicated that the interaction between MnOx and ZrO2 would restrained the phase transformation of ZrO2, and raised the dispersion of MnOx species on ZrO2. The highly-dispersed MnOx on ZrO2 is predominant contributors in catalytic activity of the Mn-O/ZrO2 catalysts.     

以ZrO_2-Al_2O_3为载体Co-Mo-K耐硫变换催化剂的TPR研究

利用程序升温还原 (TPR)技术 ,研究了ZrO2 对Co/Al2 O3、Mo/Al2 O3、Mo -K/Al2 O3以及Co -Mo -K耐硫变换催化剂氧化还原性能的影响。结果表明 ,ZrO2 的引入 ,使活性组分在载体表面分散的更好 ,促进了Mo -K活性相的形成 ,使Co和Mo的还原变得容易 ,并起到抑制催化剂在反应中被重新氧化的作用。