Journal 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 15261 1 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, Ce0.75Zr0.25O2, and Pd/Ce0.75Zr0.25O2 Keywords: disproportionation, carbon monoxide, ceria, zirconia, Raman spectroscopy Introduction automotive catalytic converters as an oxygen The disproportionation of CO to carbon buffer and the oxygen buffering capacity of and CO2 is favored thermodynamically at the catalyst is measured using the assumption temperatures less than 1000 K; however, the of stoichiometric oxidation of CO [21]. gas phase reaction kinetics are extremely The objective of this research was to slow [1]. Metals such as Ni [2-6], Fe [7, 8], Co provide unambiguous evidence whether or [4, 8, 9], Pt [10], Pd [11-13], Ru [14], and Rh not CeO2 catalyzes CO disproportionation. [15] readily catalyze the reaction. And it has Specifically, experiments were conducted also been reported that CO disproportionation with Ce0.75Zr0.25O2 and ZrO2, both as pure is facile on metal oxides such as MgO [16] materials and with deposited Pd metal, and and Fe3O4 [17]; albeit these metal oxides are the results were compared to the reported considered as less active catalysts than the Raman bands at 1582 and 1331 cm-1 observed metals. after CeO2 is exposed to CO [19,20]. Zirconia Based on FTIR investigations of the is not expected to catalyze the reaction and surface species formed upon exposure to was used as a reference material, yet, it has CO, Li et al. inferred CeO2 catalyzes CO been suggested that ceria is active for CO disproportionation [18]. A recent Raman disproportionation [18-20]. The Raman spectra study also provided results consistent with CO obtained for the two oxides were compared disproportionation catalyzed by CeO2 [19, 20]. to spectra obtained for the 1% Pd-supported The propensity of ceria-based materials to oxides for purposes of delineating between CO catalyze CO disproportionation is important disproportionation catalysis by the metal oxide to understand because CeO2 is used in and the metal [11-13]. * Corresponding author E-mail address: [email protected] 1 © Siberian Federal University. All rights reserved – 207 – Meghan E. Swanson, Vladimir V. Pushkarev… Carbon Monoxide Disproportionation over Ceria-Containing Materials Experimental Ten scans, each at a resolution of 4 cm-1, were Zirconia was precipitated from an aqueous collected for each spectrum in the 100-4000 -1 solution of ZrO(NO3)2 (Alfa, 99.9%) with aqueous cm range. The in-situ Raman measurements ammonia at pH 10. The precipitate was aged in were conducted at atmospheric pressure in a the supernatant liquid for 24 h before filtering, THMS 600 Raman cell from Linkam Scientific. washing, and drying at 373 K for 12 h; it was Approximately 100 mg of the catalyst was then calcined in air at 773 K for 12 h. The ceria- pressed into a 10 mm pellet, which was then zirconia (Ce0.75Zr0.25O2, 99.9%) was supplied by mounted in the in-situ cell. Rhodia, and it was calcined in air at 823 K for 12 The He (Air Products, >99.998%) and CO h. The 1% Pd supported catalysts were prepared (Praxair, >99.995%) were scrubbed of water and by impregnation of ZrO2 and Ce0.75Zr0.25O2 with oxygen impurities using zeolite and OxyTrap aqueous solutions of Pd(NH3)4(NO3)2 (Strem filters, both from Alltech. The CO was fitted with Chemicals, 99.9%). The impregnated samples an additional trap, Vista B γ-alumina heated to were dried at 373 K for 12 h and then calcined in 573 K, to remove metal carbonyl contaminants. air at 823 K for 12 h. The 10% O2 in He (Praxair, UHP) was used After calcination, the specific surface without further purification. The reaction gases area, pore volume distribution, and average (CO, He, and 10% O2/He) were mixed prior to the pore diameter of the materials were obtained Raman cell, and the desired flow rate of each gas 3 from N2 physisorption data acquired using was maintained within ± 1 cm /min using Brooks a Micromeritics (ASAP 2010) volumetric mass flow controllers (model 5850E). The total sorption analyzer. The catalysts were degassed gas flow rate was 100 cm3/min for all treatments by evacuation for 2 h at 673 K prior to the and experiments, and the heating or cooling rate measurements, and these characterization was always 10 K/min. results are shown in Table 1. The sample pretreatment consisted of heating The Raman spectra were obtained using the sample from room temperature to 673 K in a Renishaw System 2000 Raman spectrometer 10% CO/He, and maintaining these conditions equipped with a Leica DMLM microscope and a for 1 h. Subsequently, the gas flow was changed + 514.5-nm Ar ion laser as the excitation source. to 10% O2/He at 673 K for 1 h. Then, the sample The laser power at the source was 5-25 mW, was cooled to 623 K in 10% O2/He, a spectrum which caused minimal sample damage [22]. An was taken, and the cell was purged with He for Olympus x50 objective was used to focus the 15 min at 623 K. Next, the gas flow was switched unpolarized laser beam onto a <3 µm spot on the to 10% CO/He for 1 h and then a spectrum was sample, and to collect the backscattered light. recorded. Table 1. Catalyst textural properties Average Material Surface area (m2/g) Pore volume (cm3/g) 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% Pd/Ce0.75Zr0.25O2 114 0.288 9.0 – 208 – Meghan E. Swanson, Vladimir V. Pushkarev… Carbon Monoxide Disproportionation over Ceria-Containing Materials Results and Discussion hydroxyl band was detected; however, the band(s) The Raman spectrum of ZrO2 at 623 K in may be obscured by the strong fluorescence in the -1 10% O2/He exhibited one band in the hydroxyl region of 2000-4000 cm . -1 region at 3667 cm (Fig. 1a). Subsequent exposure With Ce0.75Zr0.25O2 at 623 K in 10% O2/He, of the sample to CO at 623 K for 1 h resulted in the two bands were observed (Fig. 2a). One band, appearance of four new bands: 1385, 1561, 2872, at 3654 cm-1, is in the hydroxyl region. The and 2979 cm-1 (Fig. 1b). The four band positions other band at 1206 cm-1 is consistent with the are consistent with an assignment of surface oxide lattice vibrations. Prior Raman studies of -1 formate species. The bands at 1561 cm and 1385 CexZr1-xO2 did not present spectra above 1000 -1 -1 cm are assigned to the C-H bending and O-C-O cm [27-30]. However, CeO2 exhibits a band at stretching vibrations [23-25], respectively. The 1166 cm -1 that is assigned to a combination of -1 2872 cm band is attributed to the formate the A1g, Eg, and F2g vibrational modes of the ceria -1 C-H stretch [23-25], and the 2979 cm band is lattice [31], and both the CeO2 and the CexZr1-xO2 assigned to a combination [23-25] of the 1561 and have a cubic fluorite crystal structure [32]. One -1 -1 1385 cm bands [26]. The spectrum of Pd/ZrO2 would expect that the 1166 cm lattice vibration -1 at 623K after 1 h exposure to CO contained only of CeO2 shifted to 1206 cm in Ce0.75Zr0.25O2 bands at 1582 cm-1 and 1331 cm-1 (Fig. 1d). No because the smaller ionic radius of the Zr ZrO2 3667 2979 (b) 2872 1385 1561 (a) Pd/ZrO2 Intensity (a. u.) (a. Intensity (d) 1582 1331 (c) 3667 4000 3200 2400 1600 800 Raman shift (cm-1) 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 Swanson, et al, Figure 1 Meghan E. Swanson, Vladimir V. Pushkarev… Carbon Monoxide Disproportionation over Ceria-Containing Materials Ce0.75Zr 0.25O2 1206 2115 1068 2847 3654 1582 2958 (b) (a) Pd/Ce0.75Zr0.25O2 1582 Intensity (a. u.) (a. Intensity 3654 2930 1206 (d) 2847 (c) 4000 3200 2400 1600 800 Raman shift (cm-1) Fig. 2. Top: In situ Raman spectra of Ce0.75Zr0.25O2 (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/Ce0.75Zr0.25O2 (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 has been assigned to carbonate species [36-40].