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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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【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）： 除催化剂上的烃类和挥发物，必须在惰性气氛下进行； 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； 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 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 obtainuseful information such as: location, composition (hydrogen/carbonratio) 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 Thelocalization, nature and structure of coke deposits have been examinedwith electron microscopy。Typically, the electron microscopy alone doesnot provide much information, and is generally used in combination withrelated 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 thechemical identity of compounds that form the coke, such as olefinic,saturated or aromatic. informationregarding the location of coke can be obtained by following the signalof 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 oftheir complexity and of the difficulty to assign unambiguously an IRband to particular species。 （5）Laser Raman Spectroscopy Classical Laser Raman Spectroscopy (LRS) 提供信息： provideinformation regarding coke structure (pregraphitic or highly organized)and on the average dimension of the crystallite, as long as amonophasic 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). （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 （7）Neutron Scattering and Attenuation 可提供信息： measure coke content and C/H ratio 优点： Thistechnique has the advantage that the coke content and therefore thecoke 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 electronspectroscopy (AES) was used in combination with secondary ion massspectrometry (SIMS) to distinguish between four types of carbonaceousdeposits, on metal foils (rhodium, iridium and platinum). The foilswere coked by exposing to ethylene at low pressure. Auger spectroscopycan distinguish between molecular or carbidic on the one hand, andgraphitic 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： thesensitivity of this type of determination is rather low，being itdifficult to determine the fraction and/or amount of coke in thecrystalline 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： Adsorptionmeasurements allow the determination of coke location. When the volumeoccupied by coke is much smaller than the volume inaccessible toadsorbates, it means that there is a pore blockage. 缺点： However,in many cases the adsorption study is carried out at a differenttemperature than the reaction, and therefore diffusivity could be quitedifferent. Another aspect that should be taken into account is that ifthe pretreatment for adsorption measurement requires temperatureshigher than the reaction temperature, an important fraction ofcarbonaceous deposits could be stripped off the catalyst and,therefore, the pore volume measured in this way will be higher than theactual volume under reaction conditions. （13）X-Ray Photo-electron Spectroscopy (XPS) （14）Ultra Violet-Visible Spectroscopy (UV-VIS) The UV-VISspectroscopy can be used to determine the chemical identity of the cokecomponentes. UV-VIS, unlike NMR and IR, can easily detect alkyl andalkenyl carbenium ions, essentially due to its much highersensitivity55. 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 cokeformation, and thus estimate roughly the amount of coke and obtaininformation 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 Theamount of coke that is being deposited on a catalyst has beentraditionally followed with conventional microbalances. However, due tothe inherent limitations of this equipment, in which it is almostimpossible to avoid feed by-pass effect and diffusional effects, thistechnique has not been very useful to determine coking kinetics as afunction of feed composition. A recycle electrobalance reactor has beendesigned 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 Someof the techniques limited to single crystals or polycrystalline foils,such as low energy electron diffraction (LEED), He scattering, coreelectron energy loss spectroscopy (CEELS), and metastable deactivationspectroscopy (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。 [ Last edited by 小博士 on 2009-4-22 at 12:21 ] |
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