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Investigation of CO and CO2 Adsorption on Tetragonal and Monoclinic Zirconia Konstantin Pokrovski, Kyeong Taek Jung, and Alexis T. Bell* Chemical Sciences Division, Lawrence Berkeley National Laboratory, and Department of Chemical Engineering, University of California, Berkeley, California 94720-1462 Received December 8, 2000. In Final Form: March 9, 2001 The adsorption of CO and CO2 on tetragonal and monoclinic ZrO2 was investigated using infrared spectroscopy and temperature-programmed desorption spectroscopy. For this study, samples were prepared of tetragonal ZrO2 (t-ZrO2) with surface areas of 20 and 187 m2/g and of monoclinic ZrO2 (m-ZrO2) with surface areas of 19 and 110 m2/g. The CO2 adsorption capacity of m-ZrO2 is more than an order of magnitude higher than that of t-ZrO2. The principal species observed on m-ZrO2 are HCO3- and m- and b-CO32-, whereas the principal species observed on t-ZrO2 are p- and b-CO3-. The higher CO2 adsorption capacity of m-ZrO2 is attributed to the higher concentration and basicity of the hydroxyl groups on this polymorph, as well as the stronger Lewis acidity of Zr4+ cations and the stronger Lewis basicity of O2- anions. Depending on the adsorption temperature, the CO adsorption capacity of m-ZrO2 is 5- to 10-fold higher than that of t-ZrO2. Below 450 K, CO is adsorbed as HCO3- and CO32- species on both polymorphs of ZrO2. At higher temperatures, only CO32- and HCOO species are observed. The CO adsorption capacity of m-ZrO2 is higher than that of t-ZrO2 because of the greater strength of the adsorption centers on this phase.
Introduction The surface of zirconia contains a variety of catalytically active sites. These include Bro¨nsted acidic and basic hydroxyl groups and coordinatively unsaturated Lewis acidic-base Zr4+O2- pairs. Recent investigations have shown that all four types of sites can contribute to the progress of catalyzed reactions involving CO and CO2, such as the synthesis of branched hydrocarbons from CO/ H2, the synthesis of methanol from CO/H2 and CO2/H2,1-4 and the synthesis of dimethyl carbonate from methanol and CO2.5 These studies have motivated efforts aimed at understanding the interactions of CO and CO2 with ZrO2 and the effect that the bulk phase of zirconia might have on these interactions. By use of CO as the probe molecule, it has been established that Lewis acidic sites, while more abundant on monoclinic ZrO2, are not as strong as those on tetragonal ZrO2.6-14 The adsorption of CO results in the appearance of molecularly adsorbed CO, and formate species are formed at elevated temperatures via the reaction of CO with hydroxyl groups on the surface of ZrO2. Adsorption of CO2 on monoclinic ZrO2 produces bicarbonate and monodentate and bidentate carbonates, * Corresponding author. E-mail:
[email protected]. (1) Amenomiya, Y. Appl. Catal. 1987, 30, 57. (2) Denise, B.; Sneedon, R. P. A. Appl. Catal. 1986, 28, 235. (3) Fisher, I.; Bell, A. T. J. Catal. 1997, 172, 222. (4) Fisher, I.; Bell, A. T. J. Catal. 1998, 178, 153. (5) (a) Tomishige, K.; Sakaihori, T.; Ikeda, Y.; Fujimoto, K. Catal. Lett. 1999, 58, 225. (b) Tomishige, K.; Ikeda, Y.; Sakaihori, T.; Fujimoto, K. J. Catal. 2000, 192, 355. (6) Cerrato, G.; Bordiga, S.; Barbera, S.; Morterra, C. Surf. Sci. 1997, 50, 50. (7) He, M.-Y.; Ekerdt, J. G. J. Catal. 1984, 87, 381. (8) Hertl, W. Langmuir 1989, 5, 96. (9) Morterra, C.; Giamello, E.; Orio, L.; Volante, M. J. Phys. Chem. 1990, 94, 3111. (10) Guglielminotti, E. Langmuir 1990, 6, 1455. (11) Bolis, V.; Morterra, C.; Volante, M.; Orio, L.; Fubini, B. Langmuir 1990, 6, 695. (12) Morterra, C.; Bolis, V.; Fubini, B.; Orio, L.; Williams, T. B. Surf. Sci. 1991, 251/252, 540. (13) Bolis, V.; Morterra, C.; Fubini, B.; Ugliengo, P.; Garrone, E. Langmuir 1993, 9, 1521. (14) Bolis, V.; Cerrato, G.; Magnacca, G.; Morterra, C. Thermochim. Acta 1998, 312, 63.
whereas bidentate and polydentate carbonates are formed on tetragonal ZrO2.8,15-19 Monoclinic ZrO2 is found to form stronger bonds with CO2 than tetragonal ZrO2.19 The present work was undertaken with the aim of establishing the role of bulk phase zirconia on the adsorption capacity and strength for the adsorption of CO and CO2. For this purpose, samples of tetragonal and monoclinic ZrO2 were prepared with comparable surface areas. The interactions of CO and CO2 with tetragonal and monoclinic zirconia were investigated by both infrared spectroscopy and temperature-programmed spectroscopy. The effects of zirconia surface area, as well as the bulk phase, were also examined. Experimental Section The preparation of tetragonal and monoclinic zirconia used in this work has been described previously.20 Tetragonal zirconia was prepared by dropwise addition of a 30 wt % ammonium hydroxide solution to a 0.5 M solution of zirconyl chloride (ZrOCl2‚ 8H2O, Aldrich) maintained at a pH of 10. The precipitated material was heated in its mother liquor at 373 K and 1 atm for 240 h while maintaining the pH at 10. The final product was recovered by vacuum filtration. It was then redispersed in deionized water in order to remove residual chlorine and then filtered. Fifty such washings were carried out with a total of 10 L. After each washing, the filtrate was checked for Cl- by addition of a few drops of AgNO3 solution. The washed product was airdried at 373 K in a vacuum oven for 24 h and then calcined in a tube furnace in pure O2. The calcination temperature was raised from 298 K to the desired level at 10 K/min and then held at the final temperature for 5 h. A final calcination temperature of 973 K was used to obtain a sample with a Brunauer-EmmettTeller (BET) surface area of 187 m2/g, and a temperature of 1323 K was used to obtain a sample with a BET surface area of 20 m2/g. The presence of tetragonal zirconia as the only phase was (15) Nakano, Y.; Iizuka, T.; Hattori, H.; Tanabe, K. J. Catal. 1978, 57, 1. (16) Xu, B. Q.; Yamaguchi, T.; Tanabe, K. Chem. Lett. 1988, 1663. (17) Kondo, J.; Abe, H.; Sakata, Y.; Maruya, K.; Domen, K.; Onishi, T. J. Chem. Soc., Faraday Trans. 1 1989, 84, 511. (18) Morterra, C.; Orio, L. Mater. Chem. Phys. 1990, 24, 247. (19) Bachiller-Baeza, B.; Rodriquez-Ramos, I.; Guerrero-Ruiz, A. Langmuir 1998, 14, 3556. (20) Jung, K. T.; Bell, A. T. J. Mol. Catal. 2000, 163, 27.
10.1021/la001723z CCC: $20.00 © 2001 American Chemical Society Published on Web 06/02/2001
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Langmuir, Vol. 17, No. 14, 2001
Pokrovski et al.
Figure 1. Infrared spectra of CO2 adsorbed at 298 K on t-ZrO2(187) taken as a function of temperature during progressive heating in flowing He. confirmed for both samples by X-ray diffraction and Raman spectroscopy. The two samples of tetragonal zirconia are identified as t-ZrO2(20) and t-ZrO2(187). Monoclinic zirconia was prepared by boiling a 0.5 M solution of zirconyl chloride (ZrOCl2‚8H2O, Aldrich) under reflux at 373 K and 1 atm for 240 h, while maintaining the pH at 1.5. The precipitated material was washed, dried, and calcined in a manner identical to that used to produce tetragonal ZrO2. A calcination temperature of 573 K was used to obtain a material with a BET surface area of 110 m2/g, and a temperature of 973 K was used to obtain a material with a BET surface area of 19 m2/g. X-ray diffraction spectra of the calcined material confirmed that only monoclinic ZrO2 was present in the calcined material in both cases. The two samples of monoclinic zirconia are identified as m-ZrO2(19) and m-ZrO2(110). Transmission infrared spectra of adsorbed CO2 and CO were recorded using a specially designed in situ cell that could be heated to 523 K. For these experiments, 60 mg of zirconia was pressed into a wafer and then placed in the cell. Prior to the adsorption of CO or CO2, the zirconia was calcined in O2 for 2 h. CO or CO2 was adsorbed from a He stream containing 4% of the adsorbate for a total time of 20 min. Spectra were recorded with a Nicolet FTIR spectrometer by coadding 64 spectra at a resolution of 4 cm-1. Temperature-programmed desorption (TPD) of adsorbed CO or CO2 was carried out using a quartz microreactor connected to a flow manifold and a mass spectrometer. Each experiment was initiated by pretreating the zirconia sample in pure O2 flowing at 100 cm3/min for 2 h at 773 K. The sample was then flushed in He and cooled to the desired adsorption temperature. CO or CO2 was adsorbed from a He stream containing 4% of the adsorbate for a total time of 20 min. The sample was then cooled to room temperature in the presence of the adsorbate and flushed with He for 30 min. Temperature-programmed desorption was carried out in flowing He (100 cm3/min) with a heating rate of 25 K/min.
Results and Discussion CO2 Adsorption and Desorption. Figure 1 shows the infrared spectra taken following the adsorption of CO2 on t-ZrO2(187) at 298 K and during subsequent temperatureprogrammed desorption. Attempts were made to obtain similar spectra for t-ZrO2(20), but the adsorption capacity of this sample was found to be too low to obtain usable results. At 298 K, two sets of bands are observed in Figure 1. The peaks at 1620 and 1225 cm-1 are assigned to bidentate bicarbonate species, b-HCO3-, whereas the peaks at 1450 and 1430 cm-1 are assigned to polydentate carbonate species, p-CO32-. 7,8,15-18 During temperatureprogrammed desorption, new peaks appear at 1595, 1375, 1355, and 1315 cm-1. The peaks at 1595 and 1315 cm-1 can be assigned to bidentate carbonate species, b-CO32-, and the peaks at 1375 and 1355 cm-1 can be assigned to monodentate carbonate species, m-CO32-.7,8,15-18 With
Figure 2. Temperature-programmed desorption spectra of CO2 adsorbed at various temperatures on (a) t-ZrO2(20) and (b) t-ZrO2(187). Heating rate ) 25 K/min; He flow rate ) 100 cm3/ min.
increasing temperature, the intensity of the bands for b-HCO3- and p-CO32- decreases monotonically, whereas those for m-CO32- and b-CO32- first increase and then decrease. The variations in band intensity with temperature seen in Figure 1 suggest that b-HCO3- decomposes at relatively low temperatures (
