X-ray Absorption Spectroscopy of Small Chromium Oxide Particles

Nov 15, 1996 - The spectra exhibit a sharp split. White line (WL) at about 533.5 eV near threshold. This structure can be explained in terms of ligand...
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Langmuir 1996, 12, 6377-6381

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X-ray Absorption Spectroscopy of Small Chromium Oxide Particles (Cr2O3, CrO2) Supported on Titanium Dioxide Th. Schedel-Niedrig,* Th. Neisius, C. T. Simmons, and K. Ko¨hler† Fritz-Haber-Insitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin (Dahlem), Germany Received March 18, 1996. In Final Form: September 24, 1996X The oxygen 1s X-ray absorption near-edge structure spectra were obtained for chromium(III) oxide, Cr2O3, and chromium(IV) dioxide, CrO2, supported on titania, TiO2, as well as for the bulk crystalline materials and, for comparison, for a chromium(VI) compound K2Cr2O7. The spectra exhibit a sharp split White line (WL) at about 533.5 eV near threshold. This structure can be explained in terms of ligand-field and exchange splitting. A peak (shoulder) due to the O(1s) f O(2p)Cr(3d) (spin-up t2g) transition could only be observed in the spectra of CrO2. The shape of the WL depends strongly on the oxidation state of the chromium ions in the probed samples suggesting that the WL can be used as an indicator of different environments in the supported chromium oxide films.

Introduction Investigations of chromium and chromium oxides supported on metal oxide surfaces are reported in numerous papers due to their interesting surface chemistry and activity in several catalytic processes.1 For higher chromium contents, mainly the presence of Cr(III) and Cr(VI) oxide phases Cr2O3 and CrO3 or of chromates(VI) is reported, but also oxide particles of mixed valences were proven or proposed. Chromium dioxide, CrO2, supported on a metal oxide surface, however, is generally not discussed in the literature. Very recently, a simple and convenient way to prepare CrO2 supported on titania was proposed for higher loaded (7 wt % Cr) CrOx/TiO2 systems.2 Both phases, CrO2 and Cr2O3, supported on titania exhibited specific activity and selectivity in the selective catalytic reduction (SCR) of nitric oxide by ammonia.3 Finely dispersed Cr2O3 is highly active and selective to N2, whereas the “CrO2” phase is active, but mainly selective (up to 50%) to undesired N2O.3 Whereas the magnetic properties (ferromagnetic resonance, FMR) and thermoanalytical methods clearly indicate the presence of a CrO2 phase in the latter catalyst, other methods like UV-vis diffuse reflectance, infrared, and X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD) did not allow the clear assignment to a chromium oxidation state.2 Especially, a clear-cut identification of the Cr4+/ CrO2 and Cr3+ species or of mixed valence oxides with XRD is rendered difficult or impossible due to the small size or the amorphous state of the oxide particles. X-ray absorption spectroscopy is particularly well suited to the study of supported metal oxides since the absorption process is independent of the electrical conductivity of the sample,4,5 i.e., is independent of charging effects, and * To whom correspondence may be addressed: phone/fax, ++49-30-8413-4460/-4401; e-mail, [email protected]. † Present address: Technische Universita ¨ t Mu¨nchen, Lichtenbergstrasse 4, D-85747 Garching, Germany. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) E.g.: McDaniel, M. P. Adv. Catal. 1985, 33, 47. Clark, A. Catal. Rev. 1970, 3, 145; Yermakov, Y.; Zakharov, V. Adv. Catal. 1975, 24, 173. Krauss, H. L.; Weber, E.; Mo¨vik, N. Z. Anorg. Allg. Chem. 1965, 338, 121. (2) Ko¨hler, K.; Maciejewski, M.; Schneider H.; Baiker, A. J. Catal. 1995, 157, 301. (3) Schneider, H.; Maciejewski, M.; Ko¨hler K.; Baiker, A. J. Catal. 1995, 157, 312. (4) Sto¨hr, J. NEXAFS Spectroscopy; Springer Series in Surface Sciences; Gomer, R., Ed.; Springer-Verlag: Berlin Heidelberg, 1992; Vol. 25, Chapter 5.2.

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has been already successfully applied to supported chromium oxide species.6 The analysis of features in the spectra yields direct information on the local near-neighbor environment of the absorbing atom, and structural information can be obtained for the amorphous and the crystalline state. This paper focuses on the identification and characterization of the chromium oxide phases Cr2O3 and CrO2 on titania model catalysts by their X-ray absorption near edge structure (XANES). In order to check the high dispersion of the chromium oxide on the titania surface determined by FMR (particles of 4 nm diameter were estimated7 ), transmission electron microscopy (TEM) is applied. Experimental Section 1. Instrumentation. Electron microscopy was carried out on a Siemens EM 102 microscope with an instrumental magnification of 360000× and an acceleration voltage of 100 kV. The catalyst powders were suspended in ethanol ultrasonically, and a drop of the resulting suspension was deposited on a carbon film. The remaining solvent was removed after 1 min. The X-ray absorption spectroscopy (XAS) measurements were performed in a Vacuum Generator double-chamber ultrahigh vacuum (UHV) system with a base pressure of less than 1 × 10-10 mbar, which was attached to the SX700-I beamline at the Berliner Synchrotron Radiation source BESSY providing photon energies from 10 to 2000 eV. The monochromator was operated with a resolution of 1.3 eV at the O K-edge and at the Cr L2/3edge. The photon energy was calibrated to an accuracy of (0.5 eV by reference to the La 3d f 4f transition at 836 eV of a LaAl2 sample. A partial-electron-yield detector equipped with two commercial multichannel plates (Hamamatsu) is used for the XAS measurements. All XAS data were collected in the partialelectron yield mode (-450 V retarding voltage) in order to increase the surface sensitivity. The partial yield is proportional to the absorption coefficient, provided that the mean free path of the collected electrons is small compared to the absorption length.4 To avoid possible problems with the spectral artifacts to oxygen and chromium contaminations of optical components, the spectra were divided by the spectrum from a clean Cu(111) surface recorded under the same conditions and the edge jumps were subsequently normalized (see, e.g., ref 4). 2. Preparation. Supported chromium dioxide was prepared according to ref 2 by impregnation of TiO2 (P25, specific surface area 49 m2/g, 30:70 rutile/anatase, supplier Degussa) with (5) de Groot, F. M. F.; Grioni, M.; Fuggle, J. C.; Ghijsen, J.; Sawatzky, G. A.; Petersen, H. Phys. Rev. B 1989, 40, 5715. (6) Ellison, A.; Diakun, G.; Worthington, P. J. Mol. Catal. 1988, 46, 131. (7) Ko¨hler, K.; Mo¨rke, W.; Bieruta, T. Colloids Surf., in press.

© 1996 American Chemical Society

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Figure 1. Transmission electron micrograph of the sample CrO2/TiO2; insert: electron optical Debye-Scherrer pattern. chromium(III) nitrate, Cr(NO3)3‚9H2O (Fluka). The supported precursor was heated to 573 K in a nitrogen stream (50 mL/min), cooled to room temperature, and calcined in an oxygen stream (50 mL/min) at 573 K for 3 or 6 h. A chromium content of 1.46 × 10-3 mol of Cr/g of TiO2 (corresponding to about 11 atom % Cr per Ti) were prepared. Supported Cr2O3 was prepared by reduction of CrO2/TiO2 in hydrogen at 523 K for 1 h followed by thermal decomposition in argon at 773 K (5 h).2 The unsupported (bulk) oxides CrO2 and Cr2O3 were prepared as follows: CrO2 (Aithaca Chemical Corp., USA) was heated in air at 673 K prior to further use, in order to decompose the CrOOH present and to remove the organic impurities. Cr2O3 was prepared by an analogous procedure as described for the supported oxide. XRD analysis confirmed the presence of pure single phases in both samples.8 The purity of the chromium dioxide phase is investigated by thermoanalytical methods (determination of the oxygen evolution during the temperature-programmed decomposition of CrO2(/TiO2) in argon monitored by mass spectrometry) and by chemical methods (determination of the chromium(VI) content in CrO2/TiO22). In good agreement both methods yielded that 95 wt % of Cr is present as CrO2 and 5 wt % as Cr(VI).

Results and Discussion For the supported chromium oxides designated as Cr2O3/ TiO2 and CrO2/TiO2, which were used for the XANES measurements (see below), the following structural information could be obtained by other methods: The ferromagnetism of the sample CrO2/TiO2, which is doubtless due to ferromagnetic/superparamagnetic CrO2, allowed the estimation of the CrO2 particle sizes on titania (thermomagnetic curves7); a diameter of about 4.0 nm (8) Maciejewski, M.; Ko¨hler, K.; Schneider, H.; Baiker, A. J. Solid State Chem. 1995, 119, 13.

was deduced. The particle size is probably still smaller, because the epitaxial growth of CrO2 onto the lattice of TiO2/rutile, which is strongly indicated by the analysis of the magnetic anisotropies, can feign larger particles. Cr2O3/TiO2 exhibits the expected antiferromagnetic behavior. The TEM pictures (Figure 1) do not show any indication of CrO2 particles. They are very similar to those of the pure support.9 This can be a confirmation of the very high distribution of the chromium oxides on the surface of the carrier. However, also the very similar atomic numbers of Ti and Cr (contrast) and the same structure (rutile) of both substrate and support have to be taken into account. Actually, a mechanical mixture of amorphous CrO2 and the titania support did not allow the distinction of both particle types (bright-field image). The electron diffraction pattern of the CrO2/TiO2 sample changed during the electron exposure resulting in pattern with high amorphous background (Figure 1, insert). The oxygen K- and transition-metal (TM) L2,3-edge absorption spectra of oxides can be readily measured using synchrotron radiation, not in a conventional absorption geometry but rather using the electron-yield technique.4 X-ray absorption (XAS) is a local process in which a corelevel electron is promoted to an excited electronic state which can be coupled to the orginal core level by the dipole selection rule. For the oxygen K-edge (L ) 0) this means that only oxygen p character states (L ) 1) can be (9) Schriftenreihe Pigmente no. 56, Degussa AG, Frankfurt/Main, 4th ed., 1989.

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Figure 3. O K-edge XANES spectra of the titania-supported chromium oxide samples as prepared: (a) Cr2O3; (b) CrO2. (c) O K-edge XANES spectrum of K2Cr2O7. All spectra are shown for one angle between the E vector of the light and the surface normal, θe ) 90°. Also shown are the difference spectra (1.0[CrxOy/TiO2] - y[TiO2]; y ) 0.89 for Cr2O3/TiO2 and y ) 0.85 for CrO2/TiO2) below the spectra (a) and (b).

Figure 2. The O K-edge XANES spectra of the transitionmetal (TM) oxide samples as prepared: (a) Cr2O3; (b) CrO2; (c) TiO2. All spectra are shown for one angle between the E vector of the light and the surface normal, θe ) 90°.

reached.10 The resulting spectrum can be interpreted in a first-order approximation as an image of the oxygen p projected unoccupied density of states. Furthermore, the TM L2,3 edges of the oxides probe within this approximation the TM d projected unoccupied denstiy of states. Since the oxygen K-edge XANES spectra reveal significant dependence on the oxidation state of chromium which cannot be observed at the Cr L2,3-edge XANES spectra, we focus here on the discussion of the O K-edge XANES spectra of the chromium oxides. Figure 2 shows the O K-edge absorption spectra of polycrystalline transition-metal oxide films for one angle θe between the E vector of the light and the surface normal, θe ) 90°: in spectra a and b are shown the chromium oxide samples, Cr2O3 (a) and CrO2 (b), and in spectra c is shown the titanium oxide sample TiO2. The O K-edge absorption spectra of the two titania (TiO2) supported chromium oxide film samples, Cr2O3 (a) and CrO2 (b), are shown in Figure 3 together with the difference spectrum between the supported chromium oxide and the titanium dioxide (as shown in Figure 2c; [CrxOy (on TiO2)] - [TiO2]) below each spectrum. The O K-edge spectrum of K2Cr2O7 is also shown in Figure 3c for comparison. A chromium content of 11% and 15% referred to titanium portion was determined from the height of the Ti L2,3- and the Cr L2,3absorption edges for the Cr2O3/TiO2 and CrO2/TiO2 system, respectively. To obtain the remaining portion of chromium oxide in the O K-edge spectra of the supported chromium oxides, we have subtracted the O K-edge spectrum of TiO2 (see Figure 2c) multiplied by a factor of 0.89 and 0.85 from the O K-edge spectrum of the supported Cr2O3/TiO2 and CrO2/TiO2 system, respectively, as shown in the difference spectra of parts a and b of Figure 3. The intensities of the XAS spectra were normalized at ∼560 eV photon energy. The spectra exhibit strong structures up to 20 eV above (10) Zeller, R. In Unoccupied Electronic States; Topics in Applied Physics; Fuggle, J. C., and Inglesfield, J. E., Eds.; Springer-Verlag: Berlin, Heidelberg, 1992; Vol. 69, Chapter 2.

the threshold. Oxygen K-edge data of the supported chromium oxide samples as well as of the CrO2 samples are presented here for the first time. The data for Cr2O3 and TiO2 are in good agreement with the high-resolution data that exist for natural mineral samples of Cr2O3 and TiO2.5 There is also a general agreement with electronenergy-loss spectroscopy (EELS) data for TiO2 thin films.11 One can divide the spectra in Figure 2 into two regions: the first region directly at the threshold is assigned to oxygen 2p weight in the states of mainly transition-metal (TM) 3d character: the TM 3d band. This assignment agrees with molecular orbital calculations.12,13 The second region, typically 5-20 eV above the threshold, is attributed to oxygen 2p states hybridized with metal 4s and 4p states.5,14 The large spread in energy for the oxygen 2p character is an indication of significant covalency in these oxides. In Figure 2 a common structure of the 4sp band at ∼545 eV photon energy is observed for all oxide films prepared in this work. It consists of a double peak separated by ∼4.5 eV for the chromium oxides and ∼3.5 eV for the titania sample. In accordance to the O K-edges of the 3d-TM oxides,5 this structure can be related to the symmetry set up by nearest oxygen neighbors. We observed a different shape of the O(2p)Cr(3d) derived bands (White line (WL)) directly at the threshold at ∼533.5 eV in the spectra of the Figure 2. The O K-edge spectrum of the “d0” TM oxide TiO2 (Figure 2c) exhibits a WL consisting of two peaks at ∼532.4 and 534.8 eV. The O K-edge spectrum of the “d2” TM oxide CrO2 (Figure 2b) reveals a WL consisting of a broader asymmetric peak at ∼533.5 eV preceded by a shoulder at ∼530.0 eV. This latter 530-eV feature can also be observed in the spectrum of the titania-supported CrO2 (Figure 3b indicated by an arrow) and in the difference spectrum. The O K-edge spectrum of the “d3” TM oxide Cr2O3 shows a sharp peak at 533 eV (Figure 2a) near the threshold followed by a weak shoulder at ∼535.3 eV, which can also be observed in the difference spectrum of Figure 2a. (11) Guerlin, Th.; Sauer, H.; Engel, W.; Zeitler, E. Phys. Status. Solidi A 1995, 150, 153. (12) Fisher, D. W. J. Phys. Chem. Solids 1971, 32, 2455. (13) Grunes, L. A.; Leapman, R. D.; Wilker, C. N.; Hoffmann, R.; Kunz, A. B. Phys. Rev. B 1982, 25, 7157. (14) Ghijsen, J.; Tjeng, L. H.; van Elp, J.; Eskes, H.; Westerink, J.; Sawatzky, G. A.; Czyzyk, M. T. Phys. Rev. B 1988, 38, 11322.

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Figure 4. A series of O K-edge spectra of bulk Cr2O3 and CrO2 films is shown: (a) and (c) as prepared (the bulk Cr2O3 film contains small CrO2 contaminations); (d) after heating at 400 K for 15 min in UHV; (b) and (e) after heating at 500 K for 15 min in UHV.

The bulk CrO2 derived 530-eV feature appeared also in the O K-edge spectra of bulk Cr2O3 films, which contain small CrO2 contaminations, as shown in a series of O K-edge spectra of bulk Cr2O3 and CrO2 films (see Figure 4): (a) and (c) as prepared (Cr2O3 film contains small CrO2 contaminations); (d) after heating at 400 K for 15 min in UHV; (b) and (e) after heating at 500 K for 15 min in UHV. The O K-edge spectrum of the as prepared Cr2O3 film (Figure 4a) exhibits the well-known overall shape (see Figure 2a) except that an additional feature at 530 eV preceding the WL at threshold can be observed. After subsequent thermal treatment the 530-eV feature largely vanished. A decrease of this feature in question is also observed after the thermal treatment of the as prepared CrO2 film (see Figure 4d,e), and moreover, another feature above the main peak at 533 eV occurs at ∼535.3 eV after the treatment (see Figure 4e). Additionally, the overall spectral shape of the thermally treated CrO2 film of Figure 4e is in good agreement with the spectrum of Figure 4a of the as prepared Cr2O3 film. This strongly suggests that the as prepared CrO2 film is thermally reduced in UHV to the thermodynamically stable chromium oxide phase of Cr2O3 and, furthermore, that the observation of the 530-eV feature preceding the WL is closely related to the presence of chromium dioxide contaminations in the bulk Cr2O3 films. In a pure ionic model, the oxygen anion would have the configuration O(1s22s22p6) and the 1s f 2p channel would be closed in XAS. Covalency reduces the number of filled states with O(2p) character, so that the strength of the O(1s) signal at the threshold is related to the degree of covalency. It is well-known that the transition-metal oxides are not composed of pure ions but exhibit a considerable covalent contribution to their bonding and that the oxygen 2p-metal 3d hybridization is reduced in the late-transition-metal oxides.5 In a first approximation assuming constant O(1s f 2p) matrix elements, the XAS intensity is proportional to the weight of the O(2p) states in the conduction band. Thus from the observed intensity ratios in the two spectral regions (see Figures 2 and 3) it is evident that in addition to the Cr(3d)/Ti(3d) states also the Cr(4sp)/Ti(4sp) states contribute to the covalent bonding between oxygen and the metal. Furthermore, one has to consider the exchange repulsion between the d electrons, which is due to the requirement that the wavefunction must be antisymmetric with respect to pairwise permutations of the electron coordinates (Pauli exclusion principle) and is a correction term in the Coulomb

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integral. The exchange effect splits the two groups of bands, t2g and eg, into four, if it dominantes over the ligand field interaction: spin-up t2g, spin-up eg, spin-down t2g, and spin-down eg. In the hexagonal corundum structure of Cr2O3, all “Cr3+” ions are located on octahedral sites. In the rutile structure of CrO2, the “Cr4+” ions are located in a slightly distorted octahedral environment, and in the rutile structure of TiO2, all “Ti4+” ions are located on distored octahedral sites. The O(2p) and TM3d orbitals mix to result in the socalled ligand-field split levels t2g and eg, which are separated by the ligand-field splitting energy Do. Furthermore, since the chromium oxides exhibit ground states with high-spin values due to the exchange interaction of the 3d electrons, the exchange splitting has to be considered. The ligand-field splitting energy was found as being somewhat larger for Cr2O3 (“Cr3+”) compared with CrO2 (“Cr4+”) (Do: 2.3 vs 1.5 eV)5 and the exchange splitting was also found to be somewhat larger (P: 2.7 vs 1.8 eV). Since spin pairing requires energy, it is favored if the ligand-field split Do is larger than the averaged spinpairing energy P, as is the case for the low-spin complexes. In high-spin complexes the situation is reversed. It has been found that in the Cr2O3 corundum (high-spin complex; octahedral) the highest occupied orbital is the spin-up t2g containing three electrons and in the CrO2 rutile (highspin complex, octahedral) it is also the spin-up t2g containing two electrons. The O K-edge spectra of the Cr2O3, CrO2, and TiO2 particles (Figure 2) exhibit a sharply split White line at about 533.5 eV and a broader structure with two peaks separated by ∼4.0 eV at about 545 eV. The splitting of the White line in the spectrum of TiO2 (Figure 2c) can be identified as the t2g and eg ligand-field levels separated by the ligand-field splitting of 2.4 eV, which is in good agreement with recently published data for natural minerals (2.6 eV).5,11 Looking at the spectrum of the highspin complex of Cr2O3 (Figure 2a), the main contribution of the WL at ∼533.0 eV can be assigned to a superposition of the spin-up eg orbital and the spin-down t2g orbital with a total number of five electrons and separated by ∼0.4 eV.5 The second peak (shoulder) at ∼2.3 eV higher photon energy relates to the remaining two spin-down eg electrons. This superposition is thus a consequence of the equivalent values of exchange and ligand-field splitting. Since, the highest occupied orbital of the high-spin complex of CrO2 (Figure 2b) is the partly filled spin-up t2g, there are four levels which are candidates for absorption and, therefore, are contributing to the shape of the WL. The broader asymmetric feature at ∼533.5 eV can be attributed to the superposition of the spin-up eg orbital (two electrons), the spin-down t2g orbital (three electrons), and the spin-down eg orbital (two electrons) with a total number of seven electrons separated by 0.3 and 1.5 eV as indicated in Figure 2b. The second peak (shoulder) preceding the broader feature is due to the O(1s) f O(2p)Cr(3d) (spin-up t2g) transition and can only be observed in the spectrum of CrO2 (Figure 2b) and appeared also in the spectrum of titania-supported CrO2 (Figure 3b) as a shoulder at ∼530.0 eV (indicated by an arrow). The shape of the WL of the chromium oxide O K-edge spectra strongly depends on the oxidation state of the chromium atoms and, therefore, can be used as a fingerprint of different environments in the supported chromium oxides. Thus, the shape of the difference spectra of the titania supported chromium oxides ([CrxOy on TiO2] - y[TiO2]; y ) 0.89 for Cr2O3/TiO2 and y ) 0.85 for CrO2/ TiO2) (Figure 3) indicates that the chromium oxide compound in Figure 3a can be assigned to the Cr2O3 due

Chromium Oxide Particles on Titanium Dioxide

to the similarity to the spectrum of Cr2O3 (see Figure 2c). The chromium oxide phase in Figure 3b is obviously different from the one in Figure 3a suggesting the different environment in this CrOx/TiO2 film due to the observation of the bulk CrO2 related 530-eV feature, which is clearly visible in the difference spectrum. The other features in the difference spectrum do not agree with the O K-edge spectrum of bulk CrO2 suggesting that the dispersed CrO2 particles interact with the titania particles, which has also been suggested in a very recent study of the magnetic properties of this system.7 Additionally, the presence of Cr2O3 or other compounds in this supported system can be excluded since the O K-edge difference spectrum (Figure 2b) does not consist of the superposition of both the O K-edge spectrum of CrO2 and Cr2O3 or other compounds like K2Cr2O7 (see Figure 3c) or CrOOH (not shown). We have found no indication for the presence of chromate(VI) and/or chromium(III) oxide hydroxide contaminations in the chromium oxide samples under investigation by comparing the oxygen K-edge absorption spectra of pure potassium dichromate (K2Cr2O7 see Figure

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3c) and CrOOH (not shown) with the chromium oxide data. These O K-edge XANES looked very different when compared to the chromium oxide O K-edge spectra: the O K-edge spectrum of the “d0” compound K2Cr2O7 as shown in Figure 3c exhibits a strong sharp WL at 532.0 eV followed by a small feature at ∼542 eV at the threshold while, on the other hand, the O K-edge spectrum of the “d3” compound CrOOH reveals a small WL at ∼533 eV. In summary we conclude with the suggestion that the shape of the White line at threshold of the oxygen K-edge XANES spectrum can be used as a fingerprint of the different environments in the titania-supported chromium oxide films. Acknowledgment. The authors thank Professor Dr. R. Schlo¨gl, Fritz-Haber-Institute, for helpful discussion and financial support. Dr. H. Schneider and Mrs. U. Klengler are acknowledged for preparative work and for the electron micrographs, respectively. LA960261F