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【催化基础知识普及】氢溢流——H-Spillover

已有 12245 次阅读 2007-11-17 18:20 |个人分类:催化基础知识

氢溢流——H-Spillover

    所谓溢流(Spillover) 现象,是指固体催化剂表面的活性中心(原有的活性中心)经吸附产生出一种离子的或者自由基的活性物种,它们迁移到别的活性中心处(次级活性中心)的现象。 它们可以化学吸附诱导出新的活性或进行某种化学反应。如果没有原有活性中心,这种次级活性中心不可能产生出有意义的活性物种,这就是溢流现象。它的发生至 少两个必要的条件:
(A)溢流物种发生的主源;
(B)接受新物种的受体,它是次级活性中心。

 前者是Pt、Pd、Ru、Rh和Cu等金属原子。 催化剂在使用中是处于连续变化状态,这种状态是温度、催化剂组成,吸附物种和催化环境的综合函数。据此可以认为,传统的Langmuir- Hinshelwood动力学模型,应基于溢流现象重新加以审定。因为从溢流现象中知道,催化加氢的活性物种不只是H,而应该是H0、H+、H2、H-等 的平衡组成;催化氧化的活性物种不只是O,而应该是O0、O-、O=和O2等的平衡组成。
   溢流现象是50年代初研究H2在Pt/Al2O3上的解离吸附时 发现的,现在发现O2、CO、NO和某些烃分子吸附时都可能发生这种溢流现象。溢流现象的研究是近二十多年来催化领域中最有意义的进展之一。


    氢溢流可以看作是吸附物种在表面(甚至浅体相中)的迁移或运动的形式之一,或者可以看作质子传递的一种特殊形式。其大小通过H2吸附量来衡量。

 

 

氢溢流现象是Khoobier1964年首次观察到的,后被SierfeltTeicher试验验证——检测气体中氢气组分的一个传统方法是将该气体通673K以上的WO3粉末(黄色),如果该粉末变成蓝色,则说明有氢气组分存在,这时反应形成了氢与WO3的非化学计量配合物,HxWO3x=0.35)。他们发现,在室温下,用H2和纯WO3WO3/Al2O3时,没有反应发生,但若用H2+WO3/Pt-Al2O3则反应迅速发生,黄色的粉末变成蓝色。他认为:H2Pt上被解离化学吸附成活性的原子态氢,而后通过表面迁移与WO3反应。

 

 

 

氢溢流发生的条件:

 

1)能够产生原子态氢(如要求催化剂能够解离吸附氢)

 

2)原子态氢能够顺利迁移运动(如固体粒子的间隙和通道,或质子传递链)。

 
 
 

也存在氢反溢流现象,即氢溢流的逆过程,储存在载体上的原子态氢迁移到表面金属粒子然后以分子态氢排出。
 

氢溢流现象的研究,发现了另一类重要的作用,即金属、载体间的强相互作用,常简称之为SMSI(Strong-Metal-Support- Interaction)效应。当金属负载于可还原的金属氧化物载体上,如在TiO2上时,在高温下还原导致降低金属对H2的化学吸附和反应能力。这是由 于可还原的载体与金属间发生了强相互作用,载体将部分电子传递给金属,从而减小对H2的化学吸附能力。
  受此作用的影响,金属催化剂可以分为两 类:一类是烃类的加氢、脱氢反应,其活性受到很大的抑制;另一类是有CO参加的反应,如CO + H2反应,CO + NO反应,其活性得到很大提高,选择性也增强。这后面一类反应的结果,从实际应用来说,利用SMSI解决能源及环保等问题有潜在意义。研究的金属主要是 Pt、Pd、Rh贵金属,目前研究工作仍很活跃,多偏重于基础研究,对工业催化剂的应用尚待开发。

溢流及其相关过程图解:












关于氢溢流, 推荐几本参考资料:

(1)NEW ASPECTS OF SPILLOVER EFFECT IN CATALYSISStudies in Surface Science and Catalysis, 77

 


For Development of Highly Active Catalysts

Proceedings of the Third International Conference on Spillover,  Kyoto,  Japan,  17-20 August 1993

 

Description

The spillover effect is one of the most promising factors in the enhancement of catalytic reaction rates. The spillover phenomenon was found in the 1960s by several research groups independently. It was defined at the previous International Symposium on Spillover as follows “Spillover involves the transport of active species which are adsorbed or formed on a first phase onto another phase that does not adsorb or form the species under the same condition.” However, this effect appears typically in the dynamic state, i.e., during reactions, which advance with a considerable reaction rate. This makes it difficult to elucidate the spillover effect through static methods. Therefore, more fundamental investigations are necessary. The development of alternative energy sources and protection of the environment demands highly selective and very rapid catalytic reactions at deviating conditions. To achieve these goals new concepts, such as the spillover effect, are needed. This proceedings book, which comprises 80 papers, contains methods for preparing catalysts in order to realize spillover effects and suggests the most probable mechanisms to explain these non-linear phenomena. It also contains studies on applications of the spillover effect to improve industrial catalytic processes.

 

(2) 辛勤同李灿合作主编催化丛书《Spillover and Migration of Surface Species on Catalysts---Studies in Surface Science and Catalysis112》


Proceedings of the 4th International Conference on Spillover, Dalian, China, 15-18 September 1997

Edited by
Can Li

Qin Xin, State Key Laboratory of Catalysis, Dalian Institute of Catalysis, Chinese Academy of Sciences, Dalian 116023, China

Included in series
Studies in Surface Science and Catalysis, 112

Description
This fourth spillover conference in Dalian exhibits advances and new results in spillover studies since 1993.

In the proceedings, 5 invited lectures, 23 oral and 28 poster presentations are collected. In the 5 invited lectures, the active research areas are reviewed by well recognized scientists in the relevant fields. The first lecture highlights the fact that spillover plays a significant role in controlling the dynamic state of catalysts when it occurs in the catalytic process. The second lecture studies the surface mobility of oxygen and hydrogen using isotopic exchange method, and the impact of surface mobility on many catalytic processes is discussed. In the third paper the acidic functions of spilt-over hydrogen in a number of acid catalyzed reactions are well clarified. Subsequently, the fourth lecture explores electrochemically controlled spillover phenomenon over mm distances directly observed on metal catalysts. Finally, the last lecture reports the spontaneous dispersion of compounds onto supports and its importance in catalysis, and concludes that surface migration is a quite common process.
The conference papers present a number of aspects of spillover studies, such as new spillover phenomena, new methods for spillover characterization, spillover effects in catalysis, etc. Studies on migration and mobility of surface species are also included in the conference topics, as it is believed that the migration of species is a more general phenomenon, and spillover can be considered as a special case of migration. The spillover of hydrogen and oxygen is the major issue at the conference, while the spillover of larger molecules are reported as well in some papers. Simulation and modeling of spillover and surface mobility are also a very interesting subject in the meeting.


可惜的是上面两篇参考文献我暂时都不能拿到电子版的全文,以后有了及时补上。非常抱歉!

(3)两篇中文相关材料(催化书籍某一章)

吸附与催化——吸附和催化反应过程中的溢流现象

物种的溢流现象 Stiles 催化剂载体与负载催化剂之第九章

 

 



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