J. Phys. Chem. B 2005, 109, 1903-1913
1903
Surface Characterization of Chromia for Chlorine/Fluorine Exchange Reactions Ercan U 2 nveren,†,‡ Erhard Kemnitz,*,† Andreas Lippitz,‡ and Wolfgang E. S. Unger‡ Humboldt UniVersita¨t zu Berlin, Institut fu¨r Chemie, Brook-Taylor Strasse 2, D-12489 Berlin, Germany, and Bundesanstalt fu¨r Materialforschung und -pru¨fung, Laboratorium VIII.23, D-12200 Berlin, Germany ReceiVed: September 10, 2004; In Final Form: NoVember 16, 2004
The dismutation of CCl2F2 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 Cr2O3 is exposed to CCl2F2 at 390 °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 Cr2O3, Cr(OH)3, CrF2OH, CrF3‚H2O, R-CrF3, β-CrF3, and CrCl3 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 CrF3 and/or CrCl3 phases on the activated chromia samples was not found.
1. Introduction The Cr2O3 is one of the most important catalysts in the chlorine/fluorine (Cl/F) exchange reactions; for example in the production of hydrofluorocarbons (HFC). Most of the reaction types in the synthesis of HFCs are basically halogen exchange reactions, e.g. fluorination (1), isomerization (2), and dismutation (3):
CHClCCl2 + 4HF f CH2FCF3 + 3HCl
(1)
CHF2CHF2 f CH2FCF3
(2)
2CCl2F2 f CCl3F + CClF3
(3)
Chromia is established as an excellent heterogeneous catalyst for fluorination reactions.1-6 Although chromium(III) oxide is an important catalyst, the chemical nature and the active sites of the catalyst surface have not been completely investigated during these reactions. To study the chromia catalyst surface in detail, as a probe reaction for the catalytic halogen exchange, dismutation of CCl2F2, was chosen because this has the advantage that the formation of HF in the course of catalytic reaction can be ruled out. It is well-known that activity of the oxide catalysts is achieved by exposure to fluorine containing * Corresponding author. E-mail:
[email protected]. Fax: +49 30 2093 7277. Telephone: +49 30 2093 7555. † Humboldt Universita ¨ t zu Berlin. ‡ Bundesanstalt fu ¨ r Materialforschung und -pru¨fung.
gases, e.g., fluoroalkanes, HF, SF4, etc.7-9 Drastic changes occur in the solid surface region due to the chemical reactions with the gas phase. Therefore, the main interest of this study was to find out which kind of modifications take place on the catalyst surface during conditioning and formation processes. Moreover, we were interested whether there are differences between the halogenated inactive and active catalysts and if there are separate phases or species formed like with Al2O3 catalyst samples activated by dismutation reactions of CCl2F210 or if there are instead oxide-fluorides or mixed oxide-halides to be expected. This investigation was performed with samples which were activated in a continuous flow Ni reactor and also under simulated reaction conditions in a reactor with subsequent XPS analysis without air contact, providing so-called “in situ” conditions. Various analytical methods were applied and the results were compared with those of well-defined reference samples. Finally, it was the aim of this work to provide a better understanding on the basis of deeper inside in the conviction of the catalytically active fluorinated chromia phase or species as a result of activation of chromia by gaseous fluorine containing reactants. 2. Experimental Section 2.1. Sample Preparation. Reference Samples. Cr2O3 was synthesized by the volcano reaction of (NH4)2Cr2O7. To get rid of impurities the primary reaction product was boiled with distilled water for 4 h, filtered, washed with distilled water and
10.1021/jp045902r CCC: $30.25 © 2005 American Chemical Society Published on Web 01/13/2005
1904 J. Phys. Chem. B, Vol. 109, No. 5, 2005 dried in air. Specific surface area (BET): 45.6 m2/g. XRD structure: PDF 38-1479.11 Cr(OH)3 was prepared by the addition of 0.1 M chromium nitrate to 0.25 M ammonia until a pH of 10.5 was reached. The gel obtained was allowed to settle. It was separated by centrifugation and then washed twice with distilled water and three times with acetone, slurried with diethyl ether, and again separated by centrifugation. It was dried for 24 h at room temperature in air.12 Specific surface area (BET) was not detected, since Cr(OH)3 decomposes above 100 °C. XRD structure: see ref 12. CrF3‚H2O was prepared from chromium(III) nitrate which was dissolved in slightly heated ethanol and then added dropwise to a stirred 40 wt % hydrofluoric acid solution. After 1 h the precipitate was separated, washed with water and ethanol and dried in air. Specific surface area (BET) was not detected, since CrF3‚H2O decomposes above 100 °C. XRD structure: PDF 17316.11 CrF2OH was synthesized by CrF3‚H2O which was covered by aluminum foil to allow a self-produced atmosphere. Then, it was heated to 390 °C with a rate of 2 °C/min and kept at this temperature for 1 h under ∼ 20 mL/min Ar flow. Specific surface area (BET): 1.5 m2/g. XRD structure: see refs 13 and 14. β-CrF3 was synthesized from (NH4)3CrF6 which was covered by aluminum foil to allow a self-produced atmosphere. Then, it was heated to 490 °C with a rate of 3 °C/min and kept at this temperature for 2 h under ∼ 20 mL/min Ar flow. Specific surface area (BET): 24.7 m2/g. XRD structure: PDF 80-555.11 R-CrF3 was obtained as a commercial product (Aldrich) received under Ar in a closed ampule. Specific surface area (BET): 0.4 m2/g. XRD structure: PDF 16-44.11 CrCl3 was obtained as a commercial product (Aldrich) received under Ar in a closed ampule. Specific surface area (BET): below the detection limit (
