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【催化基础知识普及】催化剂表面积碳以及碳物种表征

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发表于 2008-1-17 16:11:27 | 显示全部楼层 |阅读模式


【催化基础知识普及】催化剂表面积碳以及碳物种表征


何谓积碳

      以含碳化合物为原料的催化反应的第一步通常是在催化剂表面上进行吸附形成含碳物种,如含碳物种经过分解、聚合等反应生成碳或焦炭沉积在催化剂表面,既所谓的积碳或结焦。积碳会堵塞催化剂的活性位或催化剂的孔道,从而导致催化剂的失活!

常发生积碳的催化反应
  • 催化裂化
  • Pt重整
  • 加氢精制
  • 轻油制氢
积碳形态
     积碳的形态一般分为无定形碳、层状石墨碳、管须状结晶碳以及粘稠状液态碳或焦油。

积碳原因
  • 热裂解
  • 催化裂解
  • 深度脱氢
  • 烯烃聚合
热裂解积碳是气相有机原料在高温下热裂解形成烟炱、焦油,它们在催化剂表面上生成有序的或无序的碳;
催化积碳是在催化剂的作用下,烃类化合物发生催化积碳反应,它与催化剂的性质密切相关。
  • 氧化物、硫化物上主要是酸性积碳,积碳速率与催化剂的表面酸碱性有关;
  • 金属上的积碳是通过烃类深度脱氢和脱氢环化聚合产生积碳,金属颗粒大小、分散度、合金化影响积碳;
  • 金属负载酸性载体催化剂的催化积碳则两者同时存在。
积碳防止
  • 合金化,如Pt重整催化剂中添加第二组分金属形成合金,可减缓催化剂的积碳;
  • 添加助剂,Pt重整催化剂采用少量Cl改性,可以减缓酸性积碳;
  • 利用载体活性组分的相互作用,降低积碳;
  • 对于酸性催化剂或载体,添加碱性助剂(K2O、MgO),减弱酸性积碳
  • 通入水蒸气,消碳;
  • 添加其他元素如稀土,加速消碳,防止积碳。
积碳失活再生——烧碳
     催化剂因为积碳而失活,其催化剂再生通常采用烧碳的方法可以部分恢复催化剂的活性。
     烧碳条件如气氛、温度、气体组分等都会影响到再生后催化剂的结构、物化性质以及催化性能,因此必须严加控制。几条注意事项(三步骤):
  1. 除催化剂上的烃类和挥发物,必须在惰性气氛下进行;
  2. 除去硫和轻质碳,必须在低温250-350度下先使金属氧化除去硫,同时某些聚合物和轻质碳也随之除去;
  3. 最后除碳,根据碳的量以及类型在较高温度下进行。
积碳表征

 催化剂积碳表征手段一览表





表征手段具体介绍
(1)Temperature-Programmed Techniques
Temperature-Programmed Oxidation (TPO)
可获取信息:

  • provides direct information regarding coke oxidation rate
  • obtain useful information such as: location, composition (hydrogen/carbon ratio) and morphology (highly dispersed or multidimensional particles).
具体如:
  • Determination of H/C Ratio
  • Determination of Coke Location.
  • Determination of Oxidation Kinetics
  • Determination of Morphology
  • Determination of Coke Amount
方法:
  • detection of CO2 by a thermal conductivity detector (TCD) after it is separated from oxygen and water in a GC column;
  • detection of CO, CO2 and hydrocarbonaceous compounds after methanation;
  • quantification of CO2 with a mass spectrometer;
  • monitoring temperature increment above a reference sample, in differentialthermal analysis (DTA) equipment;
  • measurement of weight loss in thermal gravimetric analysis (TGA) equipment.
  • detection of CO2 by FTIR
Temperature-Programmed Hydrogenation (TPHy).
   尤其适用于H2为反应物的催化反应——Characterization of coke by TPHy is of special interest when hydrogen is one of the reactants, 如:
  • reforming,
  • dehydrogenation
  •  methanol synthesis
  •  Fischer-Tropsch synthesis
Temperature-Programmed Gasification
与TPO相似,采用载气如He、Ar,也可以采用CO2代替O2

(2)Electron Microscopy
The localization, nature and structure of coke deposits have been examined with electron microscopy。Typically, the electron microscopy alone does not provide much information, and is generally used in combination with related spectroscopies

(3)Electron Energy Loss Spectrocopy (EELS)
EELS能够提供的信息:
  • provides analytical and structural information, similar to that given by X-ray absorption spectroscopy;
  • detect the location of the coke(由于EELS具有高分辨率——1nm2);
  • provides qualitative information regarding the type of coke present on the catalyst(与参照样品对比).
局限性:
used only in few cases to characterize coke deposits on heterogeneus catalysts


(4)Infrared Techniques (FTIR, DRIFTS)
能够提供的信息:
  • obtained with these techniques is the chemical identity of compounds that form the coke, such as olefinic, saturated or aromatic.
  • information regarding the location of coke can be obtained by following the signal of certain catalyst surface groups, such as Bronsted OH.
  • observe the deposition of carbonaceous materials on the working catalyst.
局限性:
    limited information on the nature of carbonaceous deposits because of their complexity and of the difficulty to assign unambiguously an IR band to particular species。

波数归属情况


(5)Laser Raman Spectroscopy

Classical Laser Raman Spectroscopy (LRS)

提供信息:
provide information regarding coke structure (pregraphitic or highly organized) and on the average dimension of the crystallite, as long as a monophasic carbon is produced

优点:
  • the high sensitivity that allows the analysis of catalysts with low coke content (0.3 — 0.5 wt%)
  • the possibility of following the graphitization of amorphous carbon.
缺点:
  • 结果解析困难;
  • surface fluorescence干扰
UV-Raman Spectrometry (UV-RS).
解决了传统Raman的缺点

(6)Dissolution of the Support and Solvent Extraction
原理:
after the dissolution of the support with a strong acid, the coke is extracted with different solvents,然后采用GC、MS等进行分析洗脱物。


缺点:

the coke could be modified during this procedure
常采用的溶剂


(7)Neutron Scattering and Attenuation
可提供信息:
measure coke content and C/H ratio

优点:
This technique has the advantage that the coke content and therefore the coke profile along the catalyst bed, can be measured in-situ.


(8)Nuclear Magnetic Resonance (NMR)
提供信息:
  • The NMR technique is a powerful technique to investigate the nature of carbonaceous deposits
  • detect bindings between aromatic rings, alkyl fragments, and even tertiary carbenium ion-like species.
缺点:
The information provided by NMR is not quantitative.

种类:
  • 13CCP/MAS – NMR.

  • 1H NMR
also used to study HY zeolites coked with n-heptane
  • 129Xe NMR
This technique has been used to study coke location on zeolite catalysts
  • 29Si MAS NMR.
(9)Auger Electron Spectroscopy (AES)
Auger electron spectroscopy (AES) was used in combination with secondary ion mass spectrometry (SIMS) to distinguish between four types of carbonaceous deposits, on metal foils (rhodium, iridium and platinum). The foils were coked by exposing to ethylene at low pressure. Auger spectroscopy can distinguish between molecular or carbidic on the one hand, and graphitic or amorphous carbon on the other.

(10)X-Ray diffraction (XRD)
Coke structure can be characterized by X-Ray diffraction analysis. This technique makes it possible to determine if there is coke with crystalline structure on the catalyst.

缺点:
the sensitivity of this type of determination is rather low,being it difficult to determine the fraction and/or amount of coke in the crystalline form.

(11)Secondary Ion Mass Spectrometry (SIMS)
SIMS is among the few surface sensitive techniques which are capable of detecting hydrogen content of the deposits. It has been used in combination with AES to analyse coke on metal foils

缺点:
However, it was not possible to obtain the real H/C ratio of the carbon deposit

(12)Sorption Capacity: Surface Area and Pore Volume
原理:
Adsorption measurements allow the determination of coke location. When the volume occupied by coke is much smaller than the volume inaccessible to adsorbates, it means that there is a pore blockage.

缺点:
However, in many cases the adsorption study is carried out at a different temperature than the reaction, and therefore diffusivity could be quite different. Another aspect that should be taken into account is that if the pretreatment for adsorption measurement requires temperatures higher than the reaction temperature, an important fraction of carbonaceous deposits could be stripped off the catalyst and, therefore, the pore volume measured in this way will be higher than the actual volume under reaction conditions.

(13)X-Ray Photo-electron Spectroscopy (XPS)

(14)Ultra Violet-Visible Spectroscopy (UV-VIS)
The UV-VIS spectroscopy can be used to determine the chemical identity of the coke componentes. UV-VIS, unlike NMR and IR, can easily detect alkyl and alkenyl carbenium ions, essentially due to its much higher sensitivity55. This is usually carried out under vaccum, and therefore, the more volatile compounds could be lost under these conditions.

(15)Electron Paramagnetic Resonance (EPR)
The EPR (or ESR) technique allows the study of the radicals that accompany the coke formation, and thus estimate roughly the amount of coke and obtain information regarding its nature.
One of the advantages of this technique, is that it can be used both under static or under on-stream conditions.

(16)Coke Formation Rate

The amount of coke that is being deposited on a catalyst has been traditionally followed with conventional microbalances. However, due to the inherent limitations of this equipment, in which it is almost impossible to avoid feed by-pass effect and diffusional effects, this technique has not been very useful to determine coking kinetics as a function of feed composition. A recycle electrobalance reactor has been designed to avoid this undesirable effect。

(17)其他
特殊表征手段,只针对某些特殊催化剂、反应有效
Some of the techniques limited to single crystals or polycrystalline foils, such as low energy electron diffraction (LEED), He scattering, core electron energy loss spectroscopy (CEELS), and metastable deactivation spectroscopy (MDS)


而通常情况下,催化剂若积炭后在烧炭过程中都会伴随重量的变化,鉴于热分析特别是热重法可以原位定量检测,所以采用热分析技术来研究催化剂的积炭行为不仅可行而且也十分方便。


来源: http://www.sciencenet.cn/blog/catachina.htm

评分

参与人数 1金币 +3 活跃度 +20 收起 理由
gaoxufeng + 3 + 20 好东西,值得推荐

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发表于 2011-10-11 13:37:41 | 显示全部楼层
虽然是基础,但是很全很详细,谢谢楼主。 赞!
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发表于 2011-10-23 16:17:33 | 显示全部楼层
楼主真是强啊,很详细,谢谢:):):):)
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发表于 2011-11-30 10:10:32 | 显示全部楼层
谢谢楼主!楼主费心了
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发表于 2012-2-21 08:49:58 | 显示全部楼层
谢谢,学习一下!
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发表于 2014-6-4 08:53:50 | 显示全部楼层
楼主,您好,我看完之后有点迷茫,我想表征一下一个黑色粉末(92%+的组成是C)到底是什么形态?您有啥好的建议~ :)谢谢谢谢~
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发表于 2014-6-4 09:02:15 | 显示全部楼层
回复 ZhongshanRR 的帖子

XRD有峰,但是基本看不出来是啥东西,比表面积在140多,拉曼看看C的杂化形式?但是不知道怎么判断~
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发表于 2015-11-2 22:35:52 | 显示全部楼层
{:4_100:}{:4_100:}{:4_100:}
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发表于 2015-12-6 13:59:50 | 显示全部楼层
总结的很好。
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头像被屏蔽
发表于 2016-6-30 01:13:06 | 显示全部楼层
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发表于 2016-10-13 12:35:04 | 显示全部楼层
谢谢,全面学习一下!
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发表于 2017-1-9 09:35:06 | 显示全部楼层
谢谢,学到了更多知识!
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