A Comparison of Ultraviolet and Visible Raman Spectra of Supported Metal Oxide Catalysts
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8600 J. Phys. Chem. B 2001, 105, 8600-8606 A Comparison of Ultraviolet and Visible Raman Spectra of Supported Metal Oxide Catalysts Yek Tann Chua,† Peter C. Stair,*,† and Israel E. Wachs*,‡ Department of Chemistry, Center for Catalysis and Surface Science and Institute of EnVironmental Catalysis, Northwestern UniVersity, EVanston, Illinois 60208, and Zettlemoyer Center for Surface Studies and Department of Chemical Engineering, Lehigh UniVersity, Bethlehem, PennsylVania 18015 ReceiVed: April 11, 2001 The recent emergence of ultraviolet-wavelength-excited Raman spectroscopy as a tool for catalyst characterization has motivated the question of how UV Raman spectra compare to visible-wavelength-excited Raman spectra on the same catalyst system. Measurements of Raman spectra from five supported metal oxide systems (Al2O3-supported Cr2O3,V2O5, and MoO3 as well as TiO2-supported MoO3 and Re2O7), using visible (514.5 nm) and ultraviolet (244 nm) wavelength excitation have been compared to determine the similarities and differences in Raman spectra produced at the two wavelengths. The samples were in the form of self-supporting disks. Spectra from the oxides, both hydrated as a result of contact with ambient air and dehydrated as a result of calcination or laser-induced heating, were recorded. A combination of sample spinning and translation to produce a spiral pattern of laser beam exposure to the catalyst disk was found to be most effective in minimizing dehydration caused by laser-induced heating. Strong absorption by the samples in the ultraviolet significantly reduced the number of scatterers contributing to the Raman spectrum while producing only modest increases in the Raman scattering cross section due to resonance enhancement. The result was much lower signal levels with ultraviolet excitation compared to visible wavelength excited spectra. The absence of strong resonance enhancement effects in the ultraviolet also resulted in Raman spectra that were remarkably similar in terms of the vibrational bands observed, their Raman shift, and their pattern of intensities. Generally, the UV Raman spectra appear to be more sensitive to the out-of-plane bending and symmetric stretching vibrations of bridging oxygen species (M-O-M), whereas the visible Raman spectra are more sensitive to terminal oxygen vibrations (M)O). These differences suggest that a more complete characterization of the supported metal oxide species can be obtained by Raman measurements at several excitation wavelengths. Subtle differences in the spectra due to the extent of dehydration were also evident. These results indicate both that the Raman spectra are very sensitive to the nature of the supported metal oxide species and their environments, which requires that substantial care must be taken in the method of sample treatment during the measurements to obtain meaningful spectra. Introduction cm-1.6 Consequently, the IR bands of the surface metal oxide Supported metal oxide catalysts have wide applications in species are usually masked by the strong IR bands of the oxide the chemical and petroleum industries.1 Complete understanding support. In contrast, the Raman bands of these oxide supports 6 of the catalytic properties of these metal oxides requires the are weak or Raman inactive in the same region. Because of determination of the structure-activity relationship of these this, Raman spectroscopy is preferred over IR spectroscopy for materials. Vibrational spectroscopy has become a very powerful measuring the vibrational spectra of supported metal oxides. tool for characterizing the molecular structures of these sup- Since the Raman spectrum of each molecular structure is unique, ported metal oxides. Infrared spectroscopy has been used to Raman spectroscopy can discriminate between different mo- study the interactions of the surface metal oxide species with lecular structures of the supported metal oxides. In the past 25 the surface hydroxyls of the support.2 It can also be used to years numerous successful experiments, pertaining to the measure the distribution of surface Bro¨nsted and Lewis acid structural elucidation of the supported metal oxide species under - sites by adsorption of pyridine.3 The number of surface sites ambient and in situ conditions, have been performed.6 9 More on these catalysts can also be estimated by measuring the recently, success has also been achieved when Raman spec- 4 amount of chemisorbed CO2. IR can also be used to detect the troscopy was used to conduct experiments under reaction terminal metal-oxygen stretches (MdO) on supported metal conditions.10-12 oxides.5 Occasionally sample fluorescence can mask the Raman Surface metal oxide species typically vibrate in the 100- spectrum of catalytic materials when using excitation by visible - 1100 cm 1 region. Common oxide supports, such as γ-alumina wavelengths. This phenomenon may be caused by the presence and amorphous silica, absorb strongly below approximately 1000 of carbon impurities or deposits, which are inevitably produced during some chemical reactions.13 It may also be due to traces * Authors to whom correspondence should be addressed. 13 † Northwestern University. of highly fluorescent transition metal ions or the presence of ‡ Lehigh University. certain hydroxyl groups.14 Laser excitation in the near-IR15 or 10.1021/jp011366g CCC: $20.00 © 2001 American Chemical Society Published on Web 08/09/2001 Raman Spectra of Supported Metal Oxide Catalysts J. Phys. Chem. B, Vol. 105, No. 36, 2001 8601 UV16 has been used to avoid fluorescence. UV excitation offers certain advantages over IR excitation. The frequency dependence in normal Raman scattering ensures that a Raman peak will be more intense under UV excitation than under IR irradiation. For chemical species that absorb UV photons, the resonance Raman effect may enhance the peak intensity further. In the past few years, UV Raman spectroscopy has been successfully employed to study various catalytic materials such as zeolites (fresh or heavily coked) or sulfated zirconia (fresh or heavily coked), under either ambient or reaction conditions.17-23 Fluorescence, in all cases, was completely avoided. In the present paper, UV and visible Raman spectroscopy were employed to study the molecular structures of supported metal oxide species. The main objective of this work is to Figure 1. (a) UV Raman spectra of hydrated and dehydrated 0.5% compare the Raman spectra taken under UV and visible CrO3/Al2O3. (b) Visible Raman spectra of hydrated and dehydrated 0.5% CrO /Al O . radiation. We will also discuss possible scenarios where it would 3 2 3 be more advantageous to use UV excitation than visible the laser power used was 5-20 mW. For alumina-supported irradiation and vice versa for recording Raman spectra of metal oxides, the collection time for a UV Raman spectrum is supported metal oxide catalysts. from 1800 to 6000 s. For titania-supported samples, the Raman signals are relatively weaker and the collection time ranges from Experimental Section 10 800 to 51 600 s. A 2400 g/mm grating was used in second A. Sample Preparation. The supported metal oxides were order to disperse the UV Raman spectra. Typical resolution - -1 prepared at Lehigh University. They were prepared by incipient achieved was 10 12 cm . Details of the instrument have been 21 wetness impregnation using the appropriate precursors: chro- previously published. mium(III) nitrate for surface chromium oxides, vanadium The visible Raman spectra of supported metal oxides were triethoxide oxide or vanadium triisopropoxide oxide for surface measured at Lehigh University. The excitation source was the vanadium oxide, ammonium heptamolybdate for surface mo- 514.5 nm line from an argon ion laser (Spectra Physics, model - lybdenum oxides, and perrhenic acid for surface rhenium oxides. 171) delivering 10 20 mW of laser power at the sample. Each The impregnated samples were subsequently dried at room Raman spectrum was recorded in 900 s, significantly shorter temperature, dried at 110-120 °C, and then calcined at high collection than for the UV Raman spectra. The scattered temperatures. The temperature of calcination depended on the radiation was directed into a Spex triplemate spectrometer specific metal oxides. Details of the preparations for each (model 1877) coupled to a Princeton Applied Research OMA catalyst can be found in previous publications.24-28 III optical multichannel analyzer (model 1463) equipped with B. Raman Spectra. The UV Raman spectra of the supported an intensified photodiode array detector. The visible Raman metal oxides were measured using the UV Raman instrument spectra were collected and recorded using an OMA III (PAR) ∼ built at Northwestern University. The excitation source was the directed computer and software. Typical resolution was 2 -1 244 nm line from a Lexel 95 SHG (Second Harmonic Genera- cm . The Raman spectra of dehydrated and hydrated samples tion) laser equipped with an intracavity nonlinear crystal, BBO were recorded with stationary (under in situ conditions after ° (Beta Barium Borate: BaB O ) that frequency doubled visible dehydration at 450 C) and spinning disks (under ambient 2 4 ∼ radiation into mid-ultraviolet. The scattered photons were conditions) ( 2000 rpm), respectively. - collected by an ellipsoidal mirror and focused into a Spex The UV visible diffuse reflectance spectra of the supported Triplemate (model 1877) triple monochromator. The photons metal oxides were measured under ambient conditions using a were detected by an imaging photomultipier tube (ITT, model Cary 1-E spectrophotometer from Varian Inc. F4146). The signal from the detector was subsequently analyzed Results and Discussion by a position computer (Surface Science Laboratories, model 240) and presented in the form of a spectrum with the PCA3 A. Alumina-Supported Metal Oxides. γ-Alumina is Raman program from Oxford Instrument Inc.. All UV Raman spectra inactive in the 100-1200 cm-1 region.6 All Raman peaks are were recorded under ambient conditions on samples in the form assigned to surface metal oxide vibrations unless stated other- of 1.5 cm diameter, self-supporting disks. The UV Raman wise. spectra were recorded from samples under three sets of 1a.