氧化物气凝胶催化剂的超临界CO2干燥法制备及其应用

超临界CO2干燥技术是一项绿色安全,制品结构可控的新的制备特殊材料的技术,综述了国内外探索其工艺和干燥机理的研究现状,阐述了利用其制备超细ZrO2气凝胶的工艺实验研究,并展望了氧化物气凝胶催化剂的应用前景.     

SiO2与ZrO2-SiO2气凝胶表面酸性的研究

采用溶胶-凝胶法结合乙醇超临界流体干燥法制备了SiO2气凝胶和ZrO2-SiO2复合氧化物气凝胶,采用氨-程序升温脱附法(NH3-TPD)和吡啶吸脱附-原位红外法(Py-IR)测定了其表面酸性.结果表明,SiO2气凝胶只有弱的L型酸位,ZrO2-SiO2复合氧化物气凝胶有L型酸位和B型酸位,且酸性和酸量比SiO2气凝胶都有较大增强.     

纳米锑掺杂氧化锡制备中超临界CO_2干燥的工艺优化及动力学

纳米锑掺杂氧化锡(ATO)具有优越的光电性能,在制备过程中前驱体干燥处理至关重要。为掌握ATO前驱体的超临界CO2干燥工艺特性,研究了超临界干燥的时间、温度、压力和CO2流量等工艺参数对纳米ATO粒径、比表面积等的影响,进一步探讨了超临界CO2流量对干燥动力学的影响规律,并拟合了干燥曲线方程。结果表明:温度35~40℃、压力10~14 MPa、CO2流量1.2~1.8 L·h-1、干燥5~6 h,最终能够获得平均粒径20~30 nm、高比表面积的纳米ATO材料;流量对干燥速率有明显影响,醇分比0.4为干燥速率由恒速转为降速的临界点,醇凝胶干燥动力学方程能够很好地描述ATO醇凝胶干燥工艺过程。研究结果可为湿法制备纳米材料的湿凝胶干燥工业化生产和控制提供参考。     

纳米多孔材料的超临界干燥新技术

论述了超临界干燥的基本原理和实现凝胶超临界干燥操作的途径。讨论凝胶的超临界干燥操作步骤及时间。分析评述了高温超临界有机溶剂干燥、低温超临界CO2 干燥、低温超临界CO2 萃取干燥、惰性气体预加压超临界干燥和高温快速超临界反应干燥等几种新型超临界干燥操作方法及其特点。     

ZrO2负载铜镍双金属对CO低温催化活化研究

用两种方法合成载体ZrO2,分别负载镍单金属和铜镍双金属制备催化剂,对其做CO催化,确定载体最佳制备方法。用最佳方法制备的载体负载铜镍金属做催化剂,考察催化剂焙烧温度和负载比例不同对CO催化影响。实验结果表明,用溶胶凝胶超临界干燥法制备的ZrO2为载体负载铜镍制备的催化剂其最佳焙烧温度为250℃,Zr与Cu和Ni的最佳物质的量比是10：8：4时,获得催化剂起始催化活性温度为22℃,169℃时C0的转化率达到50％,较好地实现了ZrO2负载铜镍在较低温度下对CO的催化。     

疏水性二氧化硅气凝胶的合成及表征

本文以正硅酸乙酯为原料,经溶胶-凝胶过程制备二氧化硅醇凝胶,再利用二甲基二氯硅烷对醇凝胶进行化学表面修饰,最后用超临界干燥方法制备了疏水性二氧化硅气凝胶.运用红外光谱、BET、扫描电镜、X射线衍射等测试方法对制得样品的结构、形貌及化学组成进行了分析.结果表明该样品是表面存在疏水基团-CH3 的疏水性SiO2 气凝胶,是呈连续网络结构的球状纳米粒子,孔径分布主要集中在2～10 nm,是热稳定性较高的非晶、多孔、轻质介孔材料.     

纳米ZrO2的制备

本实验研究了利用ZrOCl2·8H2O为原料,根据不同的醇水配比及氨水配比,采用溶胶-凝胶法,并以超临界干燥法（SCFD）制备纳米ZrO2。得出在一个大气压,温度在15～25℃,采用溶胶-凝胶法,在不同的醇水配比及氨水配比下,对ZrO2颗粒将有一定的影响。     

柔韧性二氧化硅气凝胶的研究进展

本文综述了柔韧性二氧化硅气凝胶制备的最新研究进展,介绍了最常用的制备方法和干燥工艺,并对所制得样品的结构与性能的关系进行了阐述.目前,柔韧性二氧化硅气凝胶的制备方法主要有两种:一种是采用衍生法,用来制备纯的气凝胶;另一种是交联法,可以制备聚合物交联的气凝胶.超临界流体干燥是目前最常用的干燥工艺.     

载体制备工艺对Pt/CA催化剂甲醇氧化催化性能的影响

比较研究了炭气凝胶(CA)的制备工艺条件对其表面微观结构及以其为载体的催化剂Pt/CA甲醇氧化催化活性的影响.结果表明,常压干燥制得的CA表面以微孔为主,而超临界CO2干燥制得的CA表面主要以中孔为主,而且比表面积、表面孔容和平均孔径更大;超临界CO2干燥比常压干燥更适合制备高活性甲醇氧化Pt/CA催化剂的载体材料;CA制备过程中催化剂Na2CO3的用量(常用R/C表示,其中R代表制备CA的原料间苯二酚,C代表制备CA的催化剂Na2CO3)为200至1000的范围内,R/C的增大会引起超临界CO2干燥制得CA的表面平均孔径随之增加,R/C为300时制得的CA具有最大的BET比表面积和表面孔容,以其为载体制得的催化剂具有最好的甲醇氧化催化性能.     

稀土氧化物RE2O3(RE=La,Ce,Gd,Yb)掺杂氧化钇稳定氧化锆气凝胶相稳定性与晶粒生长分析

本文采用超临界干燥技术与溶胶-凝胶法制备了无裂纹、块状RE2 O3(RE=La,Ce,Gd,Yb)掺杂Y2 O3稳定ZrO2(RE-YSZ)气凝胶.采用X射线衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和比表面积分析仪等工具研究了RE-YSZ气凝胶的相稳定性和晶体生长行为.结果表明,具有较大阳离子半径稀土元素的掺杂可阻碍热处理过程中的原子扩散,因此所得的YSZ气凝胶具有较高的热稳定性、较小的晶粒尺寸和较大的比表面积.     

对苯二酚-甲醛有机气凝胶的结构测试及性能研究

首次用溶胶-凝胶法和CO2超临界干燥技术制备了对苯二酚-甲醛有机气凝胶,并经高温碳化处理得到其碳气凝胶,测得气凝胶的比表面积由碳化前的375.28 M2/G增大到碳化后的468.66 M2/G,碳气凝胶的孔径集中分布在15NM以内。观察到其表观形貌,以及碳气凝胶中存在微晶结构。研究了不同反应物配比对凝胶性能的影响,并发现碳气凝胶具有良好导电性。     

Synthesis of Alumina Aerogels by Ambient Drying Method and Control of Their Structures

A new method to synthesize alumina aerogels was introduced in this article. Alumina aerogels were firstly prepared by sol-gel process, using Al(β) inorganic compound as a precursor, 1,2-epoxy propane as a gelation inducing agent (GIA) and methane amide as drying control chemical additive (DCCA), followed by ambient drying method (ADM). The bulk alumina aerogels were examined by TEM, SEM and BET method, which have similar qualities to those prepared by supercritical fluid drying method (SCFM). Propane oxide was introduced to accelerate the gelation rate of Al(β) ions and improve the homogeneity of pore distributions of aerogels, formamide as drying control chemical additive (DCCA) to improve the homogeneity of oxide particles in the gel texture. Alumina gels were aged and reinforced through a solvent exchanging of alcohol, the immersing of tetraethyl orthosilicate (TEOS)/alcohol and absolute alcohol at a certain temperature. The aged gels were dried at the ambient pressure and the bulk alumina aerogels with high surface areas and higher porosity were obtained. On the base of the relationship of the variation of alumina aerogels, structures with molar ratio of propylene oxide/Al, the structure of alumina aerogels can be controlled by adjusting molar ratio of propylene oxide/Al.     

Formation and Pore Structure of Boron Nitride Aerogels

Gels containing a poly(borazinyl amine) and tetrahydrofuran were processed by CO₂ supercritical drying techniques followed by pyrolysis. The resulting BN ceramic aerogels are highly porous, and the microstructure, porosity, and surface area characteristics have been examined. The aerogels show excellent thermal stability exhibiting surface areas in excess of 350 m²/g and porosities greater than 0.8 even when heated in argon at 1500°C for 8 h. By removing solvent via evaporation before supercritical drying, the mean pore radius can be varied between 3.6 and 10 nm.     

A novel preparation of superhydrophobic silica aerogels via the combustion drying method

Silica aerogels with prominent physical, thermal, optical, and acoustic properties are considered to be highly promising materials. However, owing to the high cost and the complex production processes associated with existing drying technologies, the application of silica aerogels is limited in many fields. In this study, a novel combustion drying method (CDM) was successfully used in the synthesis of superhydrophobic silica aerogels for the first time. It was confirmed that silica aerogel prepared by CDM has a typical aerogel structure with low density, high porosity, high specific surface area, high total pore volume, superhydrophobicity and high thermal stability. Compared with supercritical fluid drying, freeze drying and ambient pressure drying, CDM possesses significant advantages in the drying efficiency and low-cost production due to its active drying mode. Finally, the mechanism of the combustion drying method is proposed based on the combustion of organic solvents. The results will be meaningful for the design and production of aerogel materials in the future.     

Synthesis and Characterization of Mesoporous Hybrid Silica-Polyacrylamide Aerogels and Xerogels

We report the synthesis of highly porous hybrid silica-polyacrylamide aerogels where the inorganic network was obtained through the hydrolysis and poly-condensation of tetramethoxysilane via a two-step sol–gel process while the polyacrylamide polymer was made by photo-copolymerization of two organic monomers, the acrylamide and the bis-acrylamide. These aerogels were obtained after a carbon dioxide supercritical drying while the corresponding xerogels were dried by simple evaporation. These materials, as well as pure silica and polyacrylamide aerogels and xerogels, were characterized by FTIR spectroscopy, solid-state ²⁹Si and ¹³C NMR spectroscopy, Thermogravimetric Analysis, a nitrogen adsorption-desorption technique, and Scanning Electron Microscopy. The FTIR and NMR spectra and the TGA/DTA analyses confirm the coexistence of highly branched silica and polyacrylamide networks reflecting the hybrid nature of the materials obtained. Nitrogen adsorption measurements reveal high specific surface areas and pore size distributions disclosing the mesoporous character of these hybrid materials. Hybrid silica-polyacrylamide aerogels having a specific surface area equal to 572 m²/g and a pore volume 1.92 cm³/g were successfully prepared for the first time in this study. The high porosity of these aerogels is due to a better resistance of the silica network to capillary forces during the supercritical drying when silica coexists with a polyacrylamide network.     

Preparation of freestanding and crack-free titania–silica aerogels and their performance for gas phase, photocatalytic oxidation of VOCs

Freestanding and crack-free titania–silica aerogels with high titanium content (i.e., Ti/Si = 1) were successfully prepared by adjusting the hydrolysis of the two alkoxide precursors to a comparable rate during the sol–gel processing. Two titania–silica aerogels were prepared by ethanol and CO₂ supercritical drying methods. Well-dispersed, nanometer-sized anatase crystal domains (ca. 10 nm) were crystallized by high temperature, ethanol supercritical drying. The crystalline domains were solidly anchored to the aerogel network by Ti–O–Si bonds. Titania–silica aerogels prepared by CO₂ supercritical drying method were devoid of TiO₂ crystals. A molecular-level mixing was achieved and anatase TiO₂ was only crystallized with difficulty by high temperature calcination (1073 K). Both aerogels were mesoporous and displayed similar open pore structure that is readily accessible to reactant molecules. However, only the titania–silica aerogel with anatase TiO₂ prepared by ethanol supercritical drying was active for the gas phase, photocatalytic oxidation of volatile organic compounds (i.e., isopropanol and trichloroethylene). Catalysts prepared from Degussa P25 TiO₂ displayed lower activity under similar reaction conditions.     

ZrO_2负载Cu催化剂对CO低温催化氧化研究

用溶胶-凝胶-超临界干燥法制备了纳米氧化锆。采用沉淀法制备氧化锆负载铜催化剂。制备的催化剂用X-ray射线衍射(XRD),透射电镜(TEM),比表面积(BET)和H2-TPR等进行了表征。研究了催化剂的焙烧温度和负载比例对CO转化效率的影响,其最佳焙烧温度为650℃,Zr与Cu的最佳物质的量比是10:8。获得催化剂在温度为68℃具有催化活性,176℃时CO的转化率达到50%,较好地实现了ZrO2负载Cu在较低温度下对CO的催化。     

Influence of aging conditions on textural properties of water-glass-based silica aerogels prepared at ambient pressure

The experimental results of aging time and temperature on the textural properties of water-glass (sodium silicate)-based silica aerogels are reported and discussed. Aging of the hydrogel for different times and temperatures led to an ability to increase the stiffness and strength of the networks. These improvements enabled the gel to withstand ambient pressure drying (APD) and, consequently, preserve the highly porous silica network without collapse. The pore size and volume increased with increasing aging temperature and time, while the specific surface area decreased. Monolithic aerogels with extremely low bulk density (～0.069 g/cm³), high specific surface area (820 m²g^?1), large cumulative pore volume (3.8 cm³g^?1), and high porosity (～96%) were obtained by aging at 60 °C for 18 hours. Therefore, easy synthesis of monolithic silica aerogels at ambient pressure is achievable using a relatively inexpensive silica precursor (sodium silicate).     

应用超临界流体干燥技术制备气凝胶的研究进展

超临界流体干燥技术广泛应用于气凝胶的制备中，是制备具有高比表面积、低堆积密度及大孔体积气凝胶的重要途径之一。综述了超临界流体干燥技术原理、历史背景及其研究现状，并对超临界流体干燥技术中的工艺条件进行了简要评述。