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Raman Spectroscopic Study of V/θ-Al2O3 Catalysts: Quantification of Surface Vanadia Species and Their Structure Reduced by Hydrogen Zili Wu,†,| Peter C. Stair,*,†,‡ Sreekala Rugmini,§ and S. David Jackson§ Department of Chemistry, Center for Catalysis and Surface Science and Institute for Catalysis and Energy Processing, Northwestern UniVersity, EVanston, Illinois 60208, Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439, and Department of Chemistry, Joseph Black Building, UniVersity of Glasgow, Glasgow G12 8QQ, Scotland, U.K. ReceiVed: May 31, 2007; In Final Form: August 3, 2007
Two interesting topics about supported vanadia catalysts were studied using in situ UV and visible Raman and UV-vis diffuse reflectance spectroscopy (DRS): The quantification of different surface vanadia species and the hydrogen reduction of these vanadia species. Using the diffuse reflectance value as an external standard, we could correct the Raman intensity measurements of V/θ-Al2O3 for the self-absorption effect. On the basis of the ability to selectively detect monovanadate (UV-excited), polyvanadate (visible-excited), and V2O5 (visibleexcited) in the Raman measurements, the distribution of monovanadate, polyvanadate, and V2O5 present on dehydrated V/θ-Al2O3 samples was successfully quantified as a function of surface VOx density. It is shown that monovanadate species are present at all surface VOx densities studied but are the dominant species at low surface VOx density. Polyvanadate and V2O5 are also present and predominate on the surface at intermediate and high surface VOx density. The UV- and visible-excited Raman studies of the V/θ-Al2O3 samples reduced in hydrogen show that polyvanadate and V2O5 are more easily reduced than monovanadate species. UV Raman is better able to obtain information on reduced vanadia species than visible Raman, mainly because of a decrease in self-absorption and resonance enhancement in the UV region. Comparison of the UV Raman spectra from reduced V/θ-Al2O3 with bulk vanadium oxide compounds suggests that reduced VOx species can assume a V2O3-like form. The reduced VOx species redisperse on the support surface upon reoxidation.
1. Introduction Supported vanadium oxide catalysts are widely used in a variety of industrial applications and show great potential in a number of redox reactions.1 Extensive structural characterization of supported vanadia catalysts has been done which significantly advanced our understanding of their catalytic performance.1-7 However, a number of interesting and important issues remain unclear about supported vanadia catalysts. For example, the structure of supported vanadia species is widely agreed to consist of monovanadate, polyvanadate, and crystalline V2O5 forms, but quantification of the amounts of the three types of surface vanadia species has rarely been approached.8,9 As another example, the state of supported vanadia catalysts under reaction conditions, especially in a reducing environment, is not well understood. These two intriguing issues will be addressed in this article. For supported vanadia catalysts, the distribution of different vanadia species, monovanadate, polyvanadate, and V2O5, on the support surface depends on vanadia coverage and the method of preparation. This distribution makes it difficult to identify the active and selective sites for catalytic reactions. Quantitative information on the speciation of surface vanadia species is an important tool for understanding structure/function relationships * Corresponding author. E-mail:
[email protected]. † Northwestern University. ‡ Argonne National Laboratory. § University of Glasgow. | Current address: Chemical Science Division and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 378316493.
for these catalysts. Qualitative determination of the three vanadia species is accessible by several techniques such as Raman, UVvis, NMR, and so forth.1 However, quantification of the individual vanadia species is more difficult because these techniques only provide information about the mixture as a whole. Our recently reported multiwavelength Raman spectroscopic study of V/Al2O3 catalysts10 shows that UV-excited (244 nm) Raman selectively detects primarily monovanadate species while visible-excited (488 nm) Raman detects primarily polyvanadate and V2O5. The ability to selectively detect different vanadia species makes it possible to quantify the individual species using multiwavelength Raman spectroscopy. Because supported vanadia is highly absorbing in the UV region and sometimes in the visible region, the self-absorption effect11 greatly affects the measured Raman intensity. Consequently, quantification of surface vanadia species requires a correction to the measured intensities to account for self-absorption. Either internal or external standards can be employed for this purpose.11-13 The external standardization method for absorbing solid samples makes use of the diffuse reflectance of the samples to correct the measured Raman intensity.11-13 Thus, the combination of multiwavelength Raman spectroscopy and UVvis diffuse reflectance spectroscopy of V/Al2O3 samples can be applied for quantification of the distribution of different vanadia species. This will be shown in the first part of this article. Vanadia catalysts are usually reduced to some extent during redox reactions, and the reduced vanadia species have been proposed as the active sites.1,14-18 Therefore, information on the valence state and molecular structure of the reduced vanadia
10.1021/jp074223o CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007
Quantification and Reduction of Surface Vanadia/Alumina catalysts is of great interest. A number of techniques have been applied to investigate the reduction of supported vanadia catalysts, such as temperature-programmed reduction (TPR),10,19,20 X-ray photoelectron spectroscopy,21 electron spin resonance (ESR),22 UV-vis diffuse reflectance spectroscopy (UV-vis DRS),15-17,23-25 X-ray absorption fine structure spectroscopy (XAFS),26 and Raman spectroscopy.5,18,19,27-34 These techniques give information primarily on the oxidation state of vanadia species, although ESR and XAFS can provide additional information on local environment. Although Raman spectroscopy is a powerful tool for characterization of the molecular structure of supported vanadia,1,2,24 it has been very difficult to detect reduced supported vanadia species with conventional (visible) Raman measurements.19,27-33 A widely accepted explanation for this phenomenon is that the Raman cross section of reduced vanadium oxide species is very small or near zero.6 Thus, it remains challenging to use Raman spectroscopy for obtaining information on the molecular state of reduced vanadia species. Our recent UV Raman study of V/Al2O3 catalysts18 shows that Raman bands due to surface VOx species are still observable for catalysts with low VOx density even under reducing conditions of butane dehydrogenation at 873 K. The UV Raman investigation of Cr/Al2O3 also gives structural information for hydrogen-reduced CrOx species.35 These UV Raman observations are quite different from previous results for reduced metal oxides where typically no Raman bands are detectable. In addition to the inherent advantages of UV Raman over visible Raman such as increased sensitivity and potential resonant enhancement effects,36 the possibility for decreased selfabsorption effects in the UV region indicated by in situ UVvis DRS studies of reduced VOx and CrOx15,16,23-25,37,38 suggests UV Raman may be capable of detecting reduced, supported metal oxides. On the basis of the advantages associated with multiwavelength Raman spectroscopy (selective detection of different vanadia species) and UV Raman spectroscopy (potential for detecting reduced metal oxide species), quantitative determination of the speciation of surface vanadia species and their structure under reducing condition will be addressed in the present report using a combination of UV- and visible-excited Raman spectroscopy. It is shown that the amount of different vanadia species is well quantified as a function of surface vanadia coverage. The UV Raman results also suggest that the surface VOx species have a V2O3-like structure after V/Al2O3 is reduced by hydrogen at high temperatures. 2. Experimental Section 2.1. Catalyst Preparation. All V/θ-Al2O3 samples with surface VOx densities in the range 0.06-14.2 V/ nm2 were prepared via incipient wetness impregnation of θ-Al2O3 (Johnson Matthey, SBET ) 101 m2/g) with aqueous NH4VO3 (99+%, Aldrich) solutions. Oxalic acid (99%, Aldrich) (NH4VO3/oxalic acid ) 0.5 M) was added into the solutions for high VOx loadings to ensure the dissolution of NH4VO3. A V/θ-Al2O3 sample with surface VOx density of Y V/nm2 will be denoted as YV in the following text. After impregnation, the samples were dried at room temperature by purging with air and then heated at 393 K overnight. Finally, the samples were calcined at 823 K for 6 h in air. 2.2. Raman Studies. In situ UV (244 nm) Raman studies of the reduction of various V/θ-Al2O3 catalysts by hydrogen were conducted on a Raman instrument built at Northwestern University.39,40 The 244-nm excitation is produced by a Lexel
J. Phys. Chem. C, Vol. 111, No. 44, 2007 16461 95 SHG (Second Harmonic Generation) laser equipped with an intracavity nonlinear crystal, BBO (Beta Barium Borate: BaB2O4) that frequency doubles visible radiation into the midultraviolet region. Raman spectra were collected under a controlled atmosphere using a fluidized bed reactor described previously.41 The laser power and spot size at the sample position were, respectively, ca. 2 mW and 50 µm. Under these conditions, the contribution of laser-induced sample changes to the measured spectra was negligible.41 Data collection times varied from 1 to 4 h depending on the signal level. Visible (488 nm) Raman spectra of the reduced samples were also collected on the same Raman instrument. The 488-nm line was obtained by removing the BBO crystal and using a visible output coupler in the Lexel 95 SHG laser. Because the grating settings in the spectrometer are optimized for Raman scattering near the 244nm region, the spectral range for visible Raman (∼260 cm-1) measurements on this spectrometer is much narrower than that of UV Raman (∼1500 cm-1). Consequently, the spectra obtained using 488-nm excitation are displayed in the range 820-1070 cm-1. This region contains the strongest and most informative Raman bands from supported VOx and provides the best comparison to UV Raman spectra. In the Raman study of hydrogen-reduced V/θ-Al2O3 catalysts, the sample was first calcined (5% O2/N2, 60 mL/min) at 823 K for 2 h and cooled to room temperature in helium. The reduction was then carried out in flowing 5% H2/N2 (60 mL/min) at different temperatures (473-973 K) for 1 h. Raman spectra were collected at both reduction temperature and room temperature in flowing He (150 mL/min). The spectra collected at reduction temperature were nearly identical to those collected at room temperature except for the difference in band intensities due to thermal effects.42,43 All the reported spectra were collected at room temperature. Each experiment was conducted in a sequence of measurements after progressively higher temperature treatments. After reduction at 873 K, the catalyst was reoxidized in a 5% O2/N2 flow (60 mL/min) in the fluidized bed reactor at a series of temperatures, from 473 to 873 K, for 1 h. Raman spectra of the reoxidized catalysts were then collected at room temperature in flowing He. The Raman shift was calibrated by measuring several liquid standards including cyclohexane, acetonitrile, acetone, chloroform, ethyl acetate, toluene, and benzene. A mathematical procedure involving a quadratic fit of the observed to the actual wavenumbers of the standards was employed for the calibration. The band positions and intensities in the Raman spectra were determined using the program PeakFit v4.11. 2.3. UV-Visible Diffuse Reflectance Spectroscopy Study. DRS spectra of dehydrated V/θ-Al2O3 samples were taken in the range of 200-800 nm on a Varian Cary 1E UV-vis spectrophotometer equipped with a diffuse-reflectance attachment, using MgO as a reference. The samples were first calcined in an oven at 723 K for 2 h, transferred into a desiccator and later into a glovebox. The sample was sealed into a small vial that was transparent in the range 200-800 nm, and the UVvis DRS spectrum was measured at room temperature. In situ DRS spectra of V/θ-Al2O3 samples reduced by H2 at different temperatures were measured at Glasgow University on a Varian Cary 5000 Win UV-vis-NIR spectrophotometer equipped with a specially designed Praying Mantis diffuse reflectance attachment (Harrick) with full environmental control. The samples were preheated at 373 K for 1 h in 2%O2/Ar and cooled to room temperature in a flow of argon before reducing with H2 at a flow rate of 10 mL/min at a programmed heating
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Figure 1. (A) Uncorrected (O) and UV-vis diffuse reflectance-corrected (b, inset) Raman band intensity of surface vanadia species (band at ∼1015 cm-1) as a function of surface VOx concentration in V/θ-Al2O3 system. (B) Amount of monovanadate as a function of surface VOx density. (C) Distribution of monovanadate, polyvanadate, and crystalline V2O5 as a function of surface VOx density.
rate of 10 K/min. The overlay spectra were recorded at regular intervals of 10 min. 2.4. Temperature-Programmed Reduction. H2-TPR of V/θAl2O3 was carried out in a H2/Ar (5%) flow of 40 mL/min from room temperature to 1023 K with a ramp of 8 K/min. Before this, the samples (ca. 100 mg) were treated at 773 K for 1 h in pure O2, cooled to room temperature, and purged with Ar for at least 30 min. The H2 consumption was determined by a TCD, with H2O being trapped in a dry ice-cooled trap. CuO/SiO2 was used as a standard to calibrate the H2 consumption. 3. Results 3.1. Quantification of Surface Vanadia Species. The UV and visible Raman spectra and UV-vis DRS spectra of dehydrated V/θ-Al2O3 samples were reported in previous studies.10,11 The measured Raman intensity ψ∞ can be corrected using the formula:11
Ic ) ψ∞‚(1 - R∞)/[R∞(1 + R∞)]
(1)
where Ic is the corrected Raman intensity, and R∞ is the diffuse reflectance at the wavelength of the Raman scattered radiation. Several restrictions apply when using eq 1, as outlined in a previous study.11 Briefly, the scattering coefficient of the sample must not change among a series of sample measurements; the sample must not be too strongly absorbing; and the diffuse reflectance R∞ must be identical at the excitation wavelength and the wavelength of the Raman bands.
Since only the UV Raman spectra give measurable information for all V/θ-Al2O3 samples (0.01-14.2 V/nm2), the diffuse reflectance spectra are combined with UV Raman spectra for the correction of self-absorption. Figure 1A presents the corrected and uncorrected Raman intensities of VOx (band at ca. 1015-1026 cm-1) as a function of surface VOx concentration. At VOx concentrations V-O-Al > VdO. The presence of a broad feature at 870 cm-1, observed on both reduced V/θ-Al2O3 and bulk V2O3 in the UV Raman spectra, suggests that the reduced, supported VOx species may have a V2O3-like structure. The reduced VOx species can redisperse on the support surface upon reoxidation. Acknowledgment. This work was financially supported by ATHENA and the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Grant No. DE-FG0297ER14789. The Athena project is funded by the Engineering & Physical Sciences Research Council (EPSRC) of the U.K. and Johnson Matthey plc. References and Notes (1) Weckhuysen, B. M.; Keller, D. E. Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis. Catal. Today 2003, 78 (1-4), 25-46. (2) Weckhuysen, B. M. Snapshots of a working catalyst: Possibilities and limitations of in situ spectroscopy in the field of heterogeneous catalysis. Chem. Commun. 2002, No. 2, 97-110. (3) Wachs, I. E. Recent conceptual advances in the catalysis science of mixed metal oxide catalytic materials. Catal. Today 2005, 100 (1-2), 79-94. (4) Wachs, I. E.; Chen, Y.; Jehng, J. M.; Briand, L. E.; Tanaka, T. Molecular structure and reactivity of the Group V metal oxides. Catal. Today 2003, 78 (1-4), 13-24. (5) Ban˜ares, M. A.; Wachs, I. E. Molecular structures of supported metal oxide catalysts under different environments. J. Raman Spectrosc. 2002, 33 (5), 359-380. (6) Wachs, I. E.; Weckhuysen, B. M. Structure and reactivity of surface vanadium oxide species on oxide supports. Appl. Catal., A 1997, 157 (12), 67-90. (7) Deo, G.; Wachs, I. E.; Haber, J. Supported vanadium-oxide catalysts - molecular structural characterization and reactivity properties. Crit. ReV. Surf. Chem. 1994, 4 (3-4), 141-187. (8) Went, G. T.; Leu, L. J.; Bell, A. T. Quantitative structural analysis of dispersed vanadia species in TiO2(anatase)-supported V2O5. J. Catal. 1992, 134 (2), 479-491. (9) Miyata, H.; Tokuda, S.; Yoshida, T. Quantitative characterization of Raman spectra of vanadium oxides layered on SiO2. Appl. Spectrosc. 1989, 43 (3), 522-526. (10) Wu, Z. L.; Kim, H.-S.; Stair, P. C.; Rugmini, S.; Jackson, S. D. On the structure of vanadium oxide supported on aluminas: UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy, and temperature-programmed reduction studies. J. Phys. Chem. B 2005, 109 (7), 2793-2800. (11) Wu, Z. L.; Zhang, C.; Stair, P. C. Influence of absorption on quantitative analysis in Raman spectroscopy. Catal. Today 2006, 113 (12), 40-47. (12) Kuba, S.; Knozinger, H. Time-resolved in situ Raman spectroscopy of working catalysts: Sulfated and tungstated zirconia. J. Raman Spectrosc. 2002, 33 (5), 325-332. (13) Tinnemans, S. J.; Kox, M. H. F.; Nijhuis, T. A.; Visser, T.; Weckhuysen, B. M. Real time quantitative Raman spectroscopy of supported metal oxide catalysts without the need of an internal standard. Phys. Chem. Chem. Phys. 2005, 7 (1), 211-216. (14) Mamedov, E. A.; Corbera´n, V. C. Oxidative dehydrogenation of lower alkanes on vanadium oxide-based catalysts. The present state of the art and outlooks. Appl. Catal., A 1995, 127 (1-2), 1-40. (15) Argyle, M. D.; Chen, K. D.; Iglesia, E.; Bell, A. T. In situ UVvisible spectroscopic measurements of kinetic parameters and active sites for catalytic oxidation of alkanes on vanadium oxides. J. Phys. Chem. B 2005, 109 (6), 2414-2420. (16) Argyle, M. D.; Chen, K.; Resini, C.; Krebs, C.; Bell, A. T.; Iglesia, E. Extent of reduction of vanadium oxides during catalytic oxidation of alkanes measured by in-situ UV-visible spectroscopy. J. Phys. Chem. B 2004, 108 (7), 2345-2353.
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