Unveren Ercan, Kemnitz Erhard, Lippitz Andreas, Unger Wolfgang E S
Humboldt Universität zu Berlin, Institut für Chemie, Brook-Taylor Strasse 2, D-12489 Berlin, Germany.
J Phys Chem B. 2005 Feb 10;109(5):1903-13. doi: 10.1021/jp045902r.
The dismutation of CCl(2)F(2) was used to probe the effect of halogenation of chromia by Cl/F exchange reactions to find out the difference between the halogenated inactive and active catalysts. The heterogeneous reactions were performed in a continuous flow Ni reactor and also under simulated reaction conditions in a reactor where after the reaction X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES) analyses are possible without air exposure of the catalyst, i.e., under so-called "in situ" conditions. The Cr(III) 2p XP spectra, which revealed multiplet splitting features and satellite emission, were used for chemical analysis by using a simple evaluation procedure which neglects this inherent complexity. Chemical analysis was also applied by using chemical state plots for Cr 3s in order to cross-check Cr 2p related results. Both ex and in situ XPS show that as soon as Cr(2)O(3) is exposed to CCl(2)F(2) at 390 degrees C fluorination as well as chlorination takes place at the catalyst surface. When the XPS surface composition reaches approximately 4 at. % fluorination and 6 at. % chlorination, maximum catalytic activity was obtained. Application of longer reaction times did not change significantly the obtained surface composition of the activated chromia. The fluorination and chlorination of chromia was further investigated by various HF and HCl treatments. The activated chromia samples and the Cr(2)O(3), Cr(OH)(3), CrF(2)OH, CrF(3) x H(2)O, alpha-CrF(3), beta-CrF(3), and CrCl(3) reference samples with well-known chemical structures were also characterized by X-ray absorption near edge structure (XANES), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), pyridine-FTIR, wet chemical (F and Cl) analysis, X-ray powder diffraction (XRD), and surface area (BET) analysis. The results suggest that the formation of chromium oxide chloride fluoride species, e.g., chromium oxide halides, at the surface is sufficient to provide catalytic activity. The presence of any CrF(3) and/or CrCl(3) phases on the activated chromia samples was not found.
利用二氯二氟甲烷的歧化反应来探究通过氯/氟交换反应对氧化铬进行卤化的效果，以找出卤化后的惰性催化剂和活性催化剂之间的差异。多相反应在连续流动的镍反应器中进行，并且在模拟反应条件下于一个反应器中进行，在该反应器中反应后无需使催化剂暴露于空气中即可进行X射线光电子能谱（XPS）和X射线激发俄歇电子能谱（XAES）分析，即所谓的“原位”条件下。揭示了多重分裂特征和卫星发射的Cr(III) 2p XP光谱通过使用一种忽略这种固有复杂性的简单评估程序用于化学分析。还通过使用Cr 3s的化学状态图进行化学分析，以交叉核对与Cr 2p相关的结果。原位和非原位XPS均表明，一旦Cr₂O₃在390℃下暴露于二氯二氟甲烷，催化剂表面就会发生氟化以及氯化。当XPS表面组成达到约4原子％的氟化和6原子％的氯化时，获得了最大催化活性。延长反应时间并没有显著改变所获得的活化氧化铬的表面组成。通过各种HF和HCl处理进一步研究了氧化铬的氟化和氯化。具有已知化学结构的活化氧化铬样品以及Cr₂O₃、Cr(OH)₃、CrF₂OH、CrF₃·xH₂O、α-CrF₃、β-CrF₃和CrCl₃参考样品也通过X射线吸收近边结构（XANES）、飞行时间二次离子质谱（TOF-SIMS）、吡啶-FTIR、湿化学（氟和氯）分析、X射线粉末衍射（XRD）和表面积（BET）分析进行了表征。结果表明，在表面形成氧化铬氯氟化物物种，例如卤化氧化铬，足以提供催化活性。在活化氧化铬样品上未发现任何CrF₃和/或CrCl₃相的存在。
