分散阈值—催化剂研究中的一个重要参数

分散阈值是指催化剂活性组分在载体表面上分散量的最大值,其大小与活性组分和载体相互作用有关,是催化剂研究中的一个重要实验参数。分散阈值的测定不仅为单层分散原理广泛的应用奠定了实验基础,亦为催化剂的研制提供一种定量研究手段。     

高分散负载型Rh簇催化剂的研究

利用化学吸附及原位红外吸附态的测定装置和连续流动微反研究了用非传统方法制备的Rh_4/Al_2O_3和Rh_2/Al_2O_3。结果表明:催化剂具有高的金属分散度和均一的金属粒度;CO在催化剂上只有一种吸附方式,孪生吸附态;CO和H_2共吸附时,观测到的2035cm~(-1)谱带,归属为吸附态;在CO加氢反应中,催化剂具有高活性和高选择性,归结于高的金属分散度和均一的金属粒度。     

X射线相定量法测单层分散阈值

本文简要介绍了自发单层分散现象及单层分散阈值效应 ,重点介绍了XRD(X光衍射 )相定量法测单层分散阈值的原理及简化方法     

盐类和氧化物在载体上自发单层分散研究新进展

 盐类和氧化物在载体上自发单层分散的现象和原理自被发现以来在国内外得到了广泛认同，国内外这方面的工作越来越多．我们从国内外同行近年发表的上百篇有关论文中，选择一部分内容加上我们的部分工作做一个简单的综述，介绍有关自发单层分散原理在基础研究和应用两个方面的新进展．     

CuO调配SnO2表面用于碳烟颗粒燃烧:单层分散阈值效应对催化性能的影响

柴油发动机由于具有良好的动力和经济性等优势而得到广泛的应用,但排放的尾气中碳烟颗粒物(PM)却给人类健康和环境带来了严重的危害.目前PM污染的治理引起了广大科研工作者的关注.催化燃烧成为PM污染消除技术中最有效的方法.金属氧化物负载到载体上时,通常会出现单层分散现象,其单层分散容量可以通过XRD, XPS和Raman等外推法测定.此外,当负载体系用作某种反应的催化剂时,往往会表现出单层分散阈值效应,即阈值附近的催化剂通常具有较高的活性和选择性等.目前单层分散理论已被广泛接受,成为设计高性能催化材料的重要指导思想.在过去八年本课题组将SnO_2基催化材料用于多种环保和能源反应,并系统地探究了其催化性能.结果表明, SnO_2含有丰富的表面活泼氧和晶格氧,且热稳定性高,是一种优良的催化剂载体.基于单层分散理论,为获得更实用的PM燃烧催化剂,我们利用浸渍法设计制备了系列不同Cu O负载量的Cu O/SnO_2催化剂,并使用XRD, XPS, Raman, TEM, STEM-mapping,H2-TPR, soot-TPR和O2-TPD等手段深入研究了其构效关系.利用XRD和XPS外推法测得的Cu O高度分散在SnO_2载体表面,其单层分散阈值为2.09 mmol100m~(-2),即相当于4.8%CuO负载量.低于该负载量时, CuO以亚单层分散态存在;而高于该负载量时,形成的Cu O微晶和单层分散态的Cu O共存.在单层分散阈值之前,催化剂的碳烟燃烧活性随Cu O负载量的升高而上升;进一步提高CuO负载量对其活性无明显的影响.Raman结果表明,在单层分散阈值之前, CuO负载到SnO_2载体上可有效形成表面活泼氧中心,且其含量随Cu O负载量升高而增加;进一步提高Cu O负载量则对其含量无明显改变.因此,Cu O/SnO_2催化剂用于碳烟颗粒燃烧具有明显的阈值效应.以上结果表明,表面活泼氧中心含量是决定该催化剂活性的主要因素.     

Ni/AC催化剂的分散阈值及阈值效应

 采用浸渍法制备了以活性炭为载体的镍催化剂,用X射线衍射(XRD)和热重(TG)手段分别考察了前驱体及浸渍液溶剂对活性组分Ni在载体上分散度的影响. XRD测试结果表明,以水为浸渍液溶剂制得的催化剂,经550 ℃焙烧后,前驱体为Ni(NO3)2时, Ni的分散阈值为0.065 g/g, 而前驱体为NiAc2时, Ni的分散阈值为0.058 g/g. 对于前驱体为Ni(NO3)2的催化剂,若浸渍液溶剂用甲醇, Ni的分散阈值则为0.082 g/g, 比水作溶剂时的阈值大. 说明前驱体镍盐中的阴离子和浸渍液溶剂均对活性物种与载体之间的相互作用有影响,从而导致镍的分散容量有差异. TG的阈值分析结果略小于XRD的测试值. 活性评价结果显示,负载Ni催化剂在甲醇气相羰基合成醋酸反应中存在较为明显的阈值效应.     

蔗糖在MCM-48表面的自发单层分散

采用液相浸渍法研究了蔗糖在中孔MCM-48表面的自发单层分散. 用X射线衍射(XRD)相定量分析法, 测定了蔗糖在MCM-48上的最大分散容量, 并用差热分析、N2吸附技术对制备的样品进行表征. 结果表明, 在蔗糖与MCM-48的质量比小于1.1时, 蔗糖在MCM-48表面能自发分散成单层. 当蔗糖/MCM-48 质量比为0.8时, MCM-48的比表面积由初始的998 m2·g-1降至114 m2·g-1, 孔径由原来的2.7 nm降至2.0 nm, 孔容由0.76 cm3·g-1降至0.11 cm3·g-1. 当蔗糖与MCM-48的质量比超过1.1时, MCM-48的比表面积迅速减小至10 m2·g-1以下. 这是因为蔗糖分子大小和MCM-48孔径相差不大, 导致蔗糖在MCM-48表面分散时出现孔道堵塞.     

Monolayer dispersion thresholds and threshold effect displayed by supported catalysts

The principle of spontaneous monolayer dispersion holds that active components of many supported catalysts will disperse spontaneously onto the surface of the carrier. The monolayer dispersion threshold of the active component on the surface of the carrier can be measured by X-ray diffraction phase-quantitative extrapolation method, etc. By measuring the monolayer dispersion threshold, beneficial information on the surface structure and dispersion of supported catalysts can be obtained, and the optimal preparative processing conditions of the catalysts can be chosen. The proportion of the active component of many supported catalysts can be optimized while its monolayer dispersion threshold is observed. Mutation values of many physicochemical properties of supported catalysts are related to monolayer dispersion thresholds; the threshold effect on catalysts is apparent, and the proposal regarding the threshold effect provides instruction for the research on catalysts. __________ Translated from Chemistry, 2007, 70(12): 900–907     

某些有机物在氧化物载体表面的自发单层分散

自发单层分散原理已在载负型催化剂制备、再生等方面得到越来越广泛的应用[1，2]．许多氧化物和盐类可以在载体表面形成单层分散或亚单层分散．有些分散物与载体混合后在低于其熔点的温度下处理，就可以自发分散到载体表面[1，3]．这一现象通过XRD、LRS、XPS、SIMS、ISS、EXAFS     

氧化物载负型沸石催化剂的阈值效应

用一系列氧化物，如Ｂ２Ｏ３，Ｓｂ２Ｏ３和ＭｇＯ，对ＨＺＳＭ－５沸石以及用Ｓｂ２Ｏ３对Ｈβ沸石进行改性。实验结果表明，这些氧化物可在沸石上自发单层分散且大部分分散的氧化物处于沸石孔内。用ＸＲＤ及ＸＰＳ方法测得它们的分散阈值。实验中发现，在甲苯烷基化反应中，对二甲苯选择性随氧化物含量增加而迅速提高；当氧化物在ＨＺＳＭ－５上的载量接近其分散阈值时，对二甲葳选择性达到最大，对某些体系可高达９５％左右，百在     

MgO-ZrO_2共沉淀体系的结构表征及单层分散现象

用共沉淀法制备了一系列MgO-ZrO2复合氧化物,用XRD、XPS、BET、DTA、EXAFS和TEM等进行了表征.结果表明,500℃与600℃下焙烧得到的MgO-ZrO2复合氧化物,在ZrO2高含量一端,MgO倾向于固溶到ZrO2中,最大固溶量可达到1:1(摩尔比);固溶达到饱和后,MgO开始在固溶体表面分散,并析出单独的MgO晶粒;在MgO高含量端,少量的ZrO2以分散态存在于MgO的表面,并使其表面性质发生较明显的变化.这些结果表明,在共沉淀法制备的MgO-ZrO2样品中,在一定相对含量范围内,会出现一种组分在另一种组分或由两种组分形成的固溶体上的单层分散现象.     

Effect of Chlorine Released during Hydrodechlorination of Chlorobenzene over Pd, Pt and Rh Supported Catalysts

Chlorobenzene hydrodechlorination in liquid phase has been studied. Chlorine released during the reaction inhibits conversion in large metallic particles. It was found comparing the behavior of Pd/SiO₂, Pd/Al₂O₃ and Pd/C catalysts that the support plays the role of a chlorine trap. The activity for Pt, Rh and Pd at similar dispersions corresponds to the chlorine adsorption affinity of the metals.     

Highly Dispersed and Stable Supported Metal Catalysts Prepared by Solid Phase Crystallization Method

Starting from the precursors containing active metal species in the crystal structure, highly dispersed supported metal catalysts have been prepared by the calcination-reduction treatment. This method is named as solid phase crystallization (spc) method and was compared with the conventional impregnation (imp) method. As the precursors, both perovskite-type and hydrotalcite-type oxides have been utilized for preparing various supported metal catalysts, which were successfully applied for the reforming of CH₄ and the decomposition of methanol. Especially spc-Ni/Mg–Al catalyst prepared from the hydrotalcite-type precursors showed high activity as well as high sustainability for the hydrogen production from methane.     

担载Fe2（CO）9络合物及其分散型Fe催化剂的CO吸脱附和催化作用

担载于Ａｌ２Ｏ３和ＺｒＯ２上的Ｆｅ（ＣＯ）９络合物的羰基在真空中极易脱附，在Ａｒ或Ｈ２中２５０℃左右也可完全脱羰而成分散型催化剂。以ＺｒＯ２为载体者在Ａｒ中低温下易发生表面岐化反应而生成ＣＯ２，高温下生成少量ＣＨ４。吸附于分散型催化剂上的ＣＯ在Ａｒ或Ｈ２中均易发生岐化反应，以ＺｒＯ２为载体者在Ｈ２中发生加氢反应而生成ＣＨ４，在ＣＯ加氢中其反应物除了大量ＣＯ２外还有少量ＣＨ物。原位ＦＴ－ＩＲ谱表明以     

Ethylene Polymerization over Supported Titanium-Magnesium Catalysts: Heterogeneity of Active Centers and Effect of Catalyst Composition on the Molecular Mass Distribution of Polymer

New experimental approach was used for analysis of molecular weight distribution (MWD) of polymers produced over titanium-magnesium catalysts (TMC). Polymers were fractionated on to fractions with narrow MWD (polydispersity (PD) values Mw/Mn ≤ 2). Then some of these fractions were combined to get the minimal quantity of fractions with PD values close to 2 (Flory components). It was found that three fractions corresponding to three groups of active centers are sufficient for proper fitting experimental MWD curve for PE obtained over TMC with different Ti content and with different hydrogen concentration in polymerization.     

Mössbauer Spectroscopic Studies of Supported α-Fe2O3 and Fe Catalysts I: Particle Size Determination and Support Effect

The chemical state of particles of 5wt% Fe in α-Fe₂O₃ and the subsequently reduced iron particles supported on different particle size (50–200 mesh) of silica (SiO₂), alumina (Al₂O₃), magnesium oxide (MgO) and carbon (C) was examined by Mössbauer spectroscopy at various stages of calcination and reduction. The particle size of the α-Fe₂O₃ supported on different mesh sizes (50, 100, 140, 200 mesh) of SiO₂ has been determined. The strength of metal-support interaction with respect to the kind of support was found to be MgO>SiO₂>Al₂O₃>C.