催化劑表面碳物種及表征

1、何謂積碳    以含碳化合物為原料的催化反應(yīng)的第一步通常是在催化劑表面上進(jìn)行吸附形成含碳物種，如含碳物種經(jīng)過分解、聚合等反應(yīng)生成碳或焦炭沉積在催化劑表面，既所謂的積碳或結(jié)焦。積碳會(huì)堵塞催化劑的活性位或催化劑的孔道，從而導(dǎo)致催化劑的失活！常發(fā)生積碳的催化反應(yīng)· 催化裂化 · Pt重整 · 加氫精制 · 輕油制氫 積碳形態(tài)     積碳的形態(tài)一般分為無定形碳、層狀石墨碳、管須狀結(jié)晶碳以及粘稠狀液態(tài)碳或焦油。積碳原因· 熱裂解 · 催化裂解 · 深度脫氫

2、83; 烯烴聚合 熱裂解積碳是氣相有機(jī)原料在高溫下熱裂解形成煙炱、焦油，它們?cè)诖呋瘎┍砻嫔仙捎行虻幕驘o序的碳；催化積碳是在催化劑的作用下，烴類化合物發(fā)生催化積碳反應(yīng)，它與催化劑的性質(zhì)密切相關(guān)。· 氧化物、硫化物上主要是酸性積碳，積碳速率與催化劑的表面酸堿性有關(guān)； · 金屬上的積碳是通過烴類深度脫氫和脫氫環(huán)化聚合產(chǎn)生積碳，金屬顆粒大小、分散度、合金化影響積碳； · 金屬負(fù)載酸性載體催化劑的催化積碳則兩者同時(shí)存在。 積碳防止· 合金化，如Pt重整催化劑中添加第二組分金屬形成合金，可減緩催化劑的積碳； · 添加助劑，Pt重整催化劑采用少量Cl改性，

3、可以減緩酸性積碳； · 利用載體活性組分的相互作用，降低積碳； · 對(duì)于酸性催化劑或載體，添加堿性助劑（K2O、MgO），減弱酸性積碳 · 通入水蒸氣，消碳； · 添加其他元素如稀土，加速消碳，防止積碳。 積碳失活再生燒碳     催化劑因?yàn)榉e碳而失活，其催化劑再生通常采用燒碳的方法可以部分恢復(fù)催化劑的活性。     燒碳條件如氣氛、溫度、氣體組分等都會(huì)影響到再生后催化劑的結(jié)構(gòu)、物化性質(zhì)以及催化性能，因此必須嚴(yán)加控制。幾條注意事項(xiàng)（三步驟）：1. 除催化劑上的烴類和揮發(fā)物，必

4、須在惰性氣氛下進(jìn)行； 2. 除去硫和輕質(zhì)碳，必須在低溫250-350度下先使金屬氧化除去硫，同時(shí)某些聚合物和輕質(zhì)碳也隨之除去； 3. 最后除碳，根據(jù)碳的量以及類型在較高溫度下進(jìn)行。 積碳表征 催化劑積碳表征手段一覽表表征手段具體介紹（1）Temperature-Programmed TechniquesTemperature-Programmed Oxidation (TPO)可獲取信息：· provides direct information regarding coke oxidation rate · obtain useful information s

5、uch as: location, composition (hydrogen/carbon ratio) and morphology (highly dispersed or multidimensional particles). 具體如：· Determination of H/C Ratio · Determination of Coke Location. · Determination of Oxidation Kinetics · Determination of Morphology · Determination of Co

6、ke 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 temperatur

7、e 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).   尤其適用于H2為反應(yīng)物的催化反應(yīng)Characterization of coke

8、by TPHy is of special interest when hydrogen is one of the reactants, 如：· reforming, · dehydrogenation ·  methanol synthesis ·  Fischer-Tropsch synthesis Temperature-Programmed Gasification與TPO相似，采用載氣如He、Ar，也可以采用CO2代替O2（2）Electron MicroscopyThe localization, nature and

9、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能夠提供的信息：· provides analytical and stru

10、ctural 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（與參照樣品對(duì)比）. 局限性：used only in few cases to characterize coke deposits on heter

11、ogeneus catalysts（4）Infrared Techniques (FTIR, DRIFTS)能夠提供的信息：· 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

12、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 par

13、ticular species。波數(shù)歸屬情況（5）Laser Raman SpectroscopyClassical 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優(yōu)點(diǎn)：· the high sensitivity that

14、 allows the analysis of catalysts with low coke content (0.3 0.5 wt%) · the possibility of following the graphitization of amorphous carbon. 缺點(diǎn)：· 結(jié)果解析困難； · surface fluorescence干擾 UV-Raman Spectrometry (UV-RS). 解決了傳統(tǒng)Raman的缺點(diǎn)（6）Dissolution of the Support and Solvent Extraction原理：after t

15、he dissolution of the support with a strong acid, the coke is extracted with different solvents，然后采用GC、MS等進(jìn)行分析洗脫物。缺點(diǎn)：the coke could be modified during this procedure常采用的溶劑（7）Neutron Scattering and Attenuation可提供信息：measure coke content and C/H ratio優(yōu)點(diǎn)：This technique has the advantage that the coke co

16、ntent 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 te

17、rtiary carbenium ion-like species. 缺點(diǎn)：The information provided by NMR is not quantitative.種類：· 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

18、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. A

19、uger 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 crystallin

20、e structure on the catalyst.缺點(diǎn)：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 dete

21、cting hydrogen content of the deposits. It has been used in combination with AES to analyse coke on metal foils缺點(diǎn)：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 co

22、ke location. When the volume occupied by coke is much smaller than the volume inaccessible to adsorbates, it means that there is a pore blockage. 缺點(diǎn)：However, in many cases the adsorption study is carried out at a different temperature than the reaction, and therefore diffusivity could be quite diffe

23、rent. 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

24、 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 a

25、lkyl 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 stat