In Situ Raman Spectroscopy of Supported Chromium Oxide ...

Spectra of Supported Chromium Oxide Catalysts J. Phys. Chem., Vol. 100, No. 34, 1996 14441Figure 9. In situ Raman spectra of the 1% CrO 3/SiO 2/TiO 2 catalystafter calcination (A) and during butane dehydrogenation as a functionof temperature (B, 200 °C; C, 250 °C; D, 300 °C; E, 350 °C; and F,400 °C).DiscussionSpectroscopic techniques, like Raman spectroscopy, have tobe developed which allow measurements under conditions asclose as possible to “real” catalytic conditions. Thus, in general,in situ spectroscopic studies are required to fully understandthe catalytic action of supported metal oxide catalysts. In thiswork, the extent of reduction of supported chromium oxidecatalysts in methanol oxidation and butane dehydrogenation/oxidation has been probed using in situ Raman spectroscopy.The extent of reduction of surface chromium oxides duringcatalysis is strongly dependent on the support type and meritsa more detailed discussion, which will cover the followingtopics: (1) the potential of in situ Raman spectroscopy to probesurface chromium oxide species during catalysis and (2) theredox behavior of supported chromium oxides.1. Evaluation of in Situ Raman Spectroscopy. Ramanspectroscopy is an optical technique and, therefore, is applicablefor studying the structural changes of the surface chromiumoxide species under in situ conditions where the temperatureand gas composition of the catalyst can be controlled. In thepresent characterization study, the reaction temperature wasvaried between 200 and 500 °C and various gas compositionwere examined. Although Raman spectroscopy is a bulktechnique, it provides detailed information about the surfacechromium oxide species during catalytic reactions since thesupported chromium oxides are all located on the surface ofthe inorganic oxides (100% dispersed). The same band patternswere obtained under catalytic reaction conditions as underdehydrated conditions and only small shifts to lower energyvalues were observed. These small Raman band shifts are dueto the relatively high temperatures (CrdO vibrations changeslightly with temperature) and to some hydrogen bonding ofthe probe molecules during catalysis.The biggest advantage of RS is that it is able to distinguishbetween different polymerization degrees of Cr 6+ . The molecularvibrations of surface Cr 6+ are always easy to measurebecause of the large change in polarizability of the CrdO andCr-O-Cr bands during vibration. This results in relativelystrong Raman bands, but this is not the case for the loweroxidation states of Cr (i.e., Cr 3+ and Cr 2+ ). Thus, the decreaseof intensity of the Cr 6+ Raman bands during butane dehydrogenationand methanol oxidation is not compensated by anincrease in intensity of the RS bands of reduced Cr species.Consequently, information about redox behavior can only beobtained by following the relative intensities of the Cr 6+ RSbands as a function of reaction temperature. However, suchreduction studies enable us to clearly distinguish for the firsttime two surfaces Cr 6+ species differing in redox behavior, i.e.a difficult to reduce monochromate species characterized by aRS band around 1030 cm -1 and a more easily reduciblepolychromate species with RS bands at 1010 and 880 cm -1 .The copresence of chromate and polychromate surface specieswas already proposed by one of us from diffuse reflectancespectroscopy, 28,29 and the present results complete the pictureof surface chemistry and spectroscopy of chromium oxides.One possible limitation of Raman spectroscopy for thecharacterization of supported chromium oxide catalysts occurswhen the vibrations of the oxide support overlap with thevibrations arising of the surface chromium oxide phase.However, most oxide supports give only weak Raman signals(i.e. weak backgrounds) and in this work some minor problemswere encountered with niobia and to a lesser extent with titania.The weak support Raman signals make this characterizationpreferable for supported metal oxide catalysts than infraredspectroscopy, which gives rise to strong support IR signals.Raman spectroscopy can suffer from fluorescence duringcatalytic action. This is especially true for silica- and aluminabasedcatalysts and at high reaction temperatures. Suchfluorescence complicates the quantitative interpretation of theobtained RS spectra, especially in the absence of oxygen in thegas phase.2. Redox Behavior of Supported Chromium Oxides. Theredox behavior of supported chromium oxides is stronglydependent on the specific support and can be evaluated byfollowing the relative intensities of the Cr 6+ RS bands as afunction of the reaction temperature during butane dehydrogenation.The following sequence in redox behavior can bededuced from the reported Raman spectra: 3% TiO 2 /SiO 2 ≈3% SiO 2 /TiO 2 ≈ (SiO 2 + TiO 2 ) ≈ TiO 2 > ZrO 2 . The SiO 2and Al 2 O 3 supports cannot be evaluated because of thefluorescence background. The differences in redox behaviorof surface chromium oxides between different supports havebeen previously observed by diffuse reflectance spectroscopyand were explained in terms of hardness-softness concepts,first introduced by Pearson. 30 The present investigation,however, gives an important extension to other support typesand preparation methods. The obtained sequence also revealsthat the order of magnitude variation in turnover frequencies(TOF, the number of methanol molecules converted to redoxproducts per surface chromium species per second) previouslyobserved for methanol oxidation does not correlate with thereducibility of the supported chromium oxide catalysts. 12,13Although an inorganic support acts as an important ligandinfluencing the stability of supported chromium oxide species,the catalytic action of supported chromium oxide catalysts inredox reactions is determined by its reduction and reoxidationproperties. Thus, supported metal oxides need to behave as areversible redox couple to be active in gas phase catalysis.ConclusionsIn situ Raman spectroscopy is a valuable tool for thecharacterization of supported chromium oxide catalysts under