CC BY
15Journal of Siberian Federal University. Chemistry 3 (2010 3) 207-215
УДК 546
Carbon Monoxide Disproportionation over Ceria-Containing Materials
Meghan E. Swanson, Vladimir V. Pushkarev, Vladimir I. Kovalchuk* and Julie L. d'Itri
Department of Chemical Engineering, University of Pittsburgh,
Pittsburgh, Pennsylvania 152611
Received 6.09.2010, received in revised form 13.09.2010, accepted 20.09.2010
The disproportionation of CO catalyzed by ceria is demonstrated in this Raman spectroscopic investigation of the interaction of CO with ZrO2, Pd/ZrO2, Ce0.75Zr025O2, and Pd/Ce0.75Zr025O2
Keywords: disproportionation, carbon monoxide, ceria, zirconia, Raman spectroscopy
Introduction
The disproportionation of CO to carbon and CO2 is favored thermodynamically at temperatures less than 1000 K; however, the gas phase reaction kinetics are extremely slow [1]. Metals such as Ni [2-6], Fe [7, 8], Co [4, 8, 9], Pt [10], Pd [11-13], Ru [14], and Rh [15] readily catalyze the reaction. And it has also been reported that CO disproportionation is facile on metal oxides such as MgO [16] and Fe3O4 [17]; albeit these metal oxides are considered as less active catalysts than the metals.
Based on FTIR investigations of the surface species formed upon exposure to CO, Li et al. inferred CeO2 catalyzes CO disproportionation [18]. A recent Raman study also provided results consistent with CO disproportionation catalyzed by CeO2 [19, 20]. The propensity of ceria-based materials to catalyze CO disproportionation is important to understand because CeO2 is used in
automotive catalytic converters as an oxygen buffer and the oxygen buffering capacity of the catalyst is measured using the assumption of stoichiometric oxidation of CO [21].
The objective of this research was to provide unambiguous evidence whether or not CeO2 catalyzes CO disproportionation. Specifically, experiments were conducted with Ce0.75Zr0 25O2 and ZrO2, both as pure materials and with deposited Pd metal, and the results were compared to the reported Raman bands at 1582 and 1331 cm-1 observed after CeO2 is exposed to CO [19,20]. Zirconia is not expected to catalyze the reaction and was used as a reference material, yet, it has been suggested that ceria is active for CO disproportionation [18-20]. The Raman spectra obtained for the two oxides were compared to spectra obtained for the 1% Pd-supported oxides for purposes of delineating between CO disproportionation catalysis by the metal oxide and the metal [11-13].
* Corresponding author E-mail address: [email protected]
1 © Siberian Federal University. All rights reserved
Experimental
Zirconia was precipitated from an aqueous solution of ZrO(NO3)2 (Alfa, 99.9%) with aqueous ammonia at pH 10. The precipitate was aged in the supernatant liquid for 24 h before filtering, washing, and drying at 373 K for 12 h; it was then calcined in air at 773 K for 12 h. The ceria-zirconia (Ce075Zr025O2, 99.9%) was supplied by Rhodia, and it was calcined in air at 823 K for 12 h. The 1% Pd supported catalysts were prepared
by impregnation of ZrO2 and Ce0.75Zr0.25O2 with
aqueous solutions of Pd(NH3)4(NO3)2 (Strem Chemicals, 99.9%). The impregnated samples were dried at 373 K for 12 h and then calcined in air at 823 K for 12 h.
After calcination, the specific surface area, pore volume distribution, and average pore diameter of the materials were obtained from N2 physisorption data acquired using a Micromeritics (ASAP 2010) volumetric sorption analyzer. The catalysts were degassed by evacuation for 2 h at 673 K prior to the measurements, and these characterization results are shown in Table 1.
The Raman spectra were obtained using a Renishaw System 2000 Raman spectrometer equipped with a Leica DMLM microscope and a 514.5-nm Ar+ ion laser as the excitation source. The laser power at the source was 5-25 mW, which caused minimal sample damage [22]. An Olympus x50 objective was used to focus the unpolarized laser beam onto a <3 ^m spot on the sample, and to collect the backscattered light.
Ten scans, each at a resolution of 4 cm-1, were collected for each spectrum in the 100-4000 cm-1 range. The in-situ Raman measurements were conducted at atmospheric pressure in a THMS 600 Raman cell from Linkam Scientific. Approximately 100 mg of the catalyst was pressed into a 10 mm pellet, which was then mounted in the in-situ cell.
The He (Air Products, >99.998%) and CO (Praxair, >99.995%) were scrubbed of water and oxygen impurities using zeolite and OxyTrap filters, both from Alltech. The CO was fitted with an additional trap, Vista B y-alumina heated to 573 K, to remove metal carbonyl contaminants. The 10% O2 in He (Praxair, UHP) was used without further purification. The reaction gases (CO, He, and 10% O2/He) were mixed prior to the Raman cell, and the desired flow rate of each gas was maintained within ± 1 cm3/min using Brooks mass flow controllers (model 5850E). The total gas flow rate was 100 cm3/min for all treatments and experiments, and the heating or cooling rate was always 10 K/min.
The sample pretreatment consisted of heating the sample from room temperature to 673 K in 10% CO/He, and maintaining these conditions for 1 h. Subsequently, the gas flow was changed to 10% O2/He at 673 K for 1 h. Then, the sample was cooled to 623 K in 10% O2/He, a spectrum was taken, and the cell was purged with He for 15 min at 623 K. Next, the gas flow was switched to 10% CO/He for 1 h and then a spectrum was recorded.
Table 1. Catalyst textural properties
Material Surface area (m2/g) Pore volume (cm3/g) Average Pore diameter (nm)
ZrO2 51 0.117 7.5
Ce0.75Zr0.25O2 106 0.287 9.3
1% Pd/ZrO2 49 0.103 8.0
1% PdACe0.75Zr0.25O2 114 0.288 9.0
Results and Discussion
The Raman spectrum of ZrO2 at 623 K in 10% O2/He exhibited one band in the hydroxyl region at 3667 cm-1 (Fig. 1a). Subsequent exposure of the sample to CO at 623 K for 1 h resulted in the appearance of four new bands: 1385, 1561, 2872, and 2979 cm-1 (Fig. 1b). The four band positions are consistent with an assignment of surface formate species. The bands at 1561 cm-1 and 1385 cm-1 are assigned to the C-H bending and O-C-O stretching vibrations [23-25], respectively. The 2872 cm-1 band is attributed to the formate C-H stretch [23-25], and the 2979 cm-1 band is assigned to a combination [23-25] of the 1561 and 1385 cm-1 bands [26]. The spectrum of Pd/ZrO2 at 623K after 1 h exposure to CO contained only bands at 1582 cm-1 and 1331 cm-1 (Fig. 1d). No
hydroxyl band was detected; however, the band(s) may be obscured by the strong fluorescence in the region of 2000-4000 cm-1.
With Ce0.75Zr0.25O2 at 623 K in 10% 02/He, two bands were observed (Fig. 2a). One band, at 3654 cm-1, is in the hydroxyl region. The other band at 1206 cm-1 is consistent with the oxide lattice vibrations. Prior Raman studies of CexZr1-xO2 did not present spectra above 1000 cm-1 [27-30]. However, CeO2 exhibits a band at 1166 cm-1 that is assigned to a combination of the Aig, Eg, and F2g vibrational modes of the ceria lattice [31], and both the CeO2 and the CexZr1-xO2 have a cubic fluorite crystal structure [32]. One would expect that the 1166 cm-1 lattice vibration of CeO2 shifted to 1206 cm-1 in Ce0.75Zr0.25O2 because the smaller ionic radius of the Zr
Fig. 1. Top: In situ Raman spectra of ZrO2 (a) after pretreatment in 10% O2/He at 673 K for 1 h and (b) after subsequent exposure to 10% CO/He at 623 K for 1 h. Bottom: In situ Raman spectra of 1% Pd/ZrO2 (c) after pretreatment in 10% O2/He at 673 K for 1 h and (d) after subsequent exposure to 10% CO/He at 623 K for 1 h. All spectra were recorded at 623 K
Fig. 2. Top: In situ Raman spectra of Ce0J5Zr0.25O2 (a) after pretreatment in 10% O2/He at 673 K for 1 h and (b) after subsequent exposure to 10% CO/He at 623 K for 1 h. Bottom: In situ Raman spectra of 1% Pd/Ce0J5Zr0.25O2 (c) after pretreatment in 10% O2/He at 673 K for 1 h and (d) after subsequent exposure to 10% CO/He at 623 K for 1 h. All spectra were recorded at 623 K
cations, compared to the Ce cations, results in a contraction of the lattice [32-33]. Such a lattice contraction manifests itself in the Raman spectra as a shift of the lattice vibrational bands to higher wavenumbers [34-35].
Exposure of the Ce0.75Zr0.25O2 to CO resulted in the appearance of four new bands: 2847, 2115, 1582, and 1068 cm-1 (Fig. 2b). The presence of a low intensity band at 2958 cm-1 cannot be ruled out despite the poor signal to noise ratio. As well, a low intensity band at 1331 cm-1 cannot be ruled out because it may be obscured by the broad band at 1206 cm-1. In accordance with the band assignment for ZrO2, the 2958 and 2847 cm-1 bands observed with Ce0.75Zr0.25O2 are attributed to formate species [26]. The band at 1068 cm-1
has been assigned to carbonate species [36-40]. Several earlier investigations concluded the 2115 cm-1 band is characteristic of Ce3+ species. In one study the band was assigned to the CO stretching mode of CO-Ce3+ [41], and in two other studies it was assigned to the symmetry-forbidden 2F5/2 ^ 2F7/2 electronic transition of Ce3+ [37,39].
The bands observed with Pd/Ce075Zr025O2 at 623 K in 10% O2/He were similar to those obtained with Ce075Zr025O2 under the same condition: a hydroxyl vibration at 3654 cm-1 and a lattice vibration at 1206 cm-1 (Fig. 2c). After exposure of Pd/Ce075Zr0 25O2 to CO at 623 K for 1 h, a distinct band appeared at 1582 cm-1, with a shoulder peak in the vicinity of 1331 cm-1 (Fig. 2d). As well, formate bands at 2930 and 2847
cm-1 were observed, which in not surprising for zirconia-containing materials; such species have been reported to be stable at high temperatures [42-43].
The most exciting result of this investigation is the presence of a band at 1582 cm-1 after exposure of Ce0.75Zr025O2 to CO at 623 K. The band position is inconsistent with formate species on metal free ZrO2 (Fig. 1b), and the band cannot be attributed to a formate species on ceria. In addition, only traces of formates have been detected on ceria exposed to CO after oxidation; however, appreciable amounts of formates have been reported for ceria pre-reduced with H2 prior to CO exposure [44-45]. Rather, cerium carbonates and/or carboxylates form after pre-oxidation and subsequent exposure to CO [38,39,45]. As well, the band cannot be assigned to zirconium carbonates/carboxylates, as they are thermally unstable at 623 K [42-43]. However, a Raman band at 1582 cm-1 has been reported for CeO2 exposed to CO after pre-oxidation with O2 [19-20]. Moreover, this investigation reports the presence of a Raman band at 1582 cm-1 for both Pd/Ce0.75Zr025O2 and Pd/ZrO2 exposed to CO after pre-oxidation (Fig. 1, 2).
The bands at 1582 and 1331 cm-1 observed with Pd/ZrO2, Ce 075Zr025O2, and Pd/Ce 075Zr025O2 after exposure to CO (Figures 1,2) are in the same position as the bands tentatively assigned to carbonaceous deposits on CeO2 [19-20]
upon exposure to CO after pre-oxidation [4654]. Graphite and other sp2-hybridized carbons exhibit strong Raman bands in these positions -namely, there is a Raman active E2g carboncarbon stretching mode at 1582 cm-1 [46-52], and a Raman active breathing mode of a sp2-hybridized carbon with A1g symmetry in the vicinity of 1331 cm-1 [48-51,53,54]. The A1g breathing mode is forbidden in large perfect crystals, but becomes Raman active in small crystallites [48-51,53,54].
Conclusion
It has been demonstrated that CO disproportionates on CeO2 and ceria-based mixed oxides at conditions of industrial significance. With Pd/ZrO2 the bands at 1582 and 1331 cm-1 are the result of carbon formed via the CO disproportionation on Pd; these bands are not observed with ZrO2 under the same conditions. Bands in the same positions are observed with Ce075Zr025O2 after exposure to CO at 623 K - clear evidence that these metal oxides catalyze carbon formation via a CO disproportionation route.
Acknowledgements
Support from the Petroleum Research Fund (grant No PRF 39801 AC5S) of the American Chemical Society and National Science Foundation (CTS 0086638) is gratefully acknowledged.
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Диспропорционирование моноксида углерода на катализаторах, содержащих оксид церия
М.Е. Свансон, В.В. Пушкарёв, В.И. Ковальчук, Дж.Л. д'Итри
Департамент химической технологии, Питсбургский университет
Питсбург, Пенсильвания 15261
Методом спектроскопии комбинационного рассеяния исследовано взаимодействие СО с ZrO2, Pd/ZrO2, Ce0.75Zr0.25O2 и Pd/Ce0.75Zr0.25O2 катализаторами дожигания выхлопных газов. Показана возможность диспропорционирования моноксида углерода в условиях измерения буферной ёмкости каталитических материалов по кислороду.
Ключевые слова: реакция Будуара, моноксид углерода, оксид церия, оксид циркония, Рамановская спектроскопия.
