Dynamic Behavior of Carbonate Species on Metal Oxide Surface

Hideto Tsuji,*,† Akie Okamura-Yoshida,‡ Tetsuya Shishido,§ and Hideshi Hattori. Center of Advanced Research for Energy Technology (CARET), Hokkai...
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Langmuir 2003, 19, 8793-8800

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Dynamic Behavior of Carbonate Species on Metal Oxide Surface: Oxygen Scrambling between Adsorbed Carbon Dioxide and Oxide Surface Hideto Tsuji,*,† Akie Okamura-Yoshida,‡ Tetsuya Shishido,§ and Hideshi Hattori Center of Advanced Research for Energy Technology (CARET), Hokkaido University, Sapporo 060-8628, Japan Received February 17, 2003. In Final Form: June 23, 2003 The oxygen exchange between surface carbonate species and oxide surfaces was investigated by the temperature-programmed desorption (TPD) of C18O2 adsorbed on metal oxides, MgO, CaO, and ZrO2 at room temperature. Although desorption plots were quite different for these metal oxides, extensive oxygen exchange was commonly observed. The fraction of C16O2 in the total desorbed CO2 increased monotonically with desorption temperature, and each desorption peak in the plots for the total CO2 could not be characterized by the isotopic distribution. The difference in proportion of incorporated lattice oxygen in the total CO2 at each desorption temperature was small between the low and high concentrations of C18O2. Among these metal oxides, ZrO2 showed the highest fraction of C16O2 in the total CO2 desorbed at 400 °C when C18O2 was adsorbed on these metal oxide with same surface concentration. The value of [C16O18O]2/{[C16O2][C18O2]} was close to 4 over almost all the temperature region of desorption for all these metal oxides, indicating that the three oxygen atoms composing carbonate were entirely scrambled before desorption as CO2. The amount of fixed monodentate carbonate with no exchange of oxygen was quite small. The manner of surface lattice oxygen incorporation into carbon dioxide is discussed on the basis of the mechanism proposed in the previous study. Successive adsorption experiments using 13CO2 with MgO demonstrated that the carbonate species switches from the weak basic sites to the strong basic sites during the heating process. It was also suggested that the dynamic behavior of carbonate species varies the coordinative environment and the adsorption strength of basic sites, which results in their further migration and desorption as CO2.

Introduction The interaction of carbon dioxide with basic sites on metal oxide surfaces is involved in important reactions catalyzed by metal oxides. For instance, oxidative coupling of methane,1 CO2-reforming of methane,2 the water gas shift reaction,3 ketonization of carboxylic acid,4 and carboxylation of epoxide5 are given as the catalytic reactions in which carbon dioxide is coproduced or activated on a metal oxide. Strong adsorption of carbon dioxide on a basic metal oxide such as the alkaline earth oxides, MgO and CaO, also results in poisoning their surface sites relevant to the adsorption of molecules and the initiation of catalytic reactions, although such poisoning happens to prevent undesirable side reactions from proceeding.1,2 Thus, carbon dioxide is one of the most suitable acidic molecules to prove and characterize the basicity of a surface.6 Many studies were reported for the * Corresponding author. E-mail: [email protected]. † Present address: Mitsubishi Chemical Corporation, Science & Technology Research Center, Komoshida-cho 1000, Aoba-ku, Yokohama 227-8502, Japan. Tel: +81-45-963-4385. Fax: +81-45-9633184. ‡ Present address: International Test & Engineering Services Co., Ltd., Ichimiyake 800, Yasu-cho, Shiga 520-2392, Japan. § Present address: Department of Chemistry, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan. (1) Lunsford, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 970. (2) (a) Yamazaki, O.; Nozaki, T.; Omata, K.; Fujimoto, K. Chem. Lett. 1992, 1953. (b) Guerrero-Ruiz, A.; Rodrı´guez-Ramos, I.; Sepu´lvedaEscribano, A. J. Chem. Soc., Chem. Commun. 1993, 487. (c) RostrupNielsen, J. R.; Bak Hansen, J.-H. J. Catal. 1993, 144, 38. (3) Shido, T.; Asakura, K.; Iwasawa, Y. J. Catal. 1990, 122, 55. (4) Sugiyama, S.; Sato, K.; Yamasaki, S.; Kawashiro, K.; Hayashi, H. Catal. Lett. 1992, 14, 127. (5) Yano, T.; Matsui, H.; Koike, T.; Ishiguro, H.; Fujihara, H.; Yoshihara, M.; Maeshima, T. Chem. Commun. 1997, 1129. (6) Hattori, H. Chem. Rev. 1995, 95, 527.

basicity of metal oxide surfaces using carbon dioxide with different methods such as IR spectroscopy, temperatureprogrammed desorption (TPD), microcalorimetry, and surface titration employing pulsing techniques.7-10 Above room temperature, most of the carbon dioxide is chemically adsorbed on the surface defect sites of alkaline earth oxides and forms surface carbonate species (CO3). Fukuda and Tanabe reported using IR measurements at room temperature that mono- and bidentate carbonates are formed for the adsorption of carbon dioxide on a MgO surface.7b Pacchioni demonstrated by means of the ab initio cluster model SCF and correlated calculations that the carbonates form at low-coordinated defect sites via a nonactivated process.11 The stability of the chemisorbed species depends on the efficiencies of the charge donation from lattice oxygen.12 Idriss and Seebauer pointed out that the carbonate formation does not merely reflect the titration of surface Lewis base sites but is also sensitive to the ability of the surface to share or donate oxygen atoms to the adsorbates.10b In addition, the contribution of surface cations should be taken into account especially for the formation of bidentate carbonate.7c (7) (a) Gregg, S. J.; Ramsay, J. D. J. Chem Soc. A 1970, 2784. (b) Fukuda, Y.; Tanabe, K. Bull. Chem. Soc. Jpn. 1973, 46, 1616. (c) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89. (d) Hertl, W. Langmuir 1989, 5, 96. (e) Pokrovski, K.; Jung, K. T.; Bell, A. T. Langmuir 2001, 17, 4297. (8) (a) Zhang, G.; Hattori, H.; Tanabe, K. Appl. Catal. 1988, 36, 189. (b) Tsuji, H.; Yagi, F.; Hattori, H. Chem. Lett. 1991, 1881. (c) Yamaguchi, T.; Morita, T.; Salama, T. M.; Tanabe, K. Catal. Lett. 1990, 4, 1. (9) Auroux, A.; Gervasini, A. J. Phys. Chem. 1990, 94, 6371. (10) (a) Idriss, H.; Seebauer, E. G. J. Mol. Catal. 2000, 152, 201. (b) Idriss, H.; Seebauer, E. G. Catal. Lett. 2000, 66, 139. (11) Pacchioni, G. Surf. Sci. 1993, 281, 207. (12) Barteau, M. A.; Idriss, H. Adv. Catal. 2001, 45, 261.

10.1021/la0342666 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/11/2003

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Scheme 1. Proposed Processes for the Mechanism of Migration of Surface Bidentate Carbonate in Ref 14

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corresponding to these migration process by means of ab initio calculations with various cluster models including coordinatively unsaturated oxygen sites.17 In the present work, the dynamic behavior of carbonate species including the oxygen exchange between adsorbed carbon dioxide and the oxide surface was investigated for MgO, CaO, and ZrO2 by means of the TPD of adsorbed 18 O-labeled carbon dioxide (C18O2) and 13C-labeled carbon dioxide (13CO2). Further evidence of the carbonate species migrating over the surface was obtained even for ZrO2 which does not possess the strong basic sites on the surface. We wish to discuss the mechanism of incorporation of surface lattice oxygen into carbon dioxide and the effect of the carbonate migration on the coordinative environment of the basic sites. Experimental Section

For a long time, chemisorption of carbon dioxide on metal oxide surfaces has been discussed on the premise that surface carbonates stay at the basic sites. It has been conceivable that one carbon dioxide molecule interacts with one basic site on the surface. One of the reasons is that carbon dioxide is a stable molecule with a CdO bond energy of 191 kcal mol-1. A second reason is that the adsorption is generally so strong that a high temperature above 700 °C is usually required for complete removal of CO2 to expose the low-coordinated defect sites. However, Yanagisawa’s group and our group found by means of mass spectrometry with 18O-labeled CO2 that the oxygen exchange between carbon dioxide and the surface of MgO, one of the representative basic metal oxides, occurs even at room temperature.13,14 It was also revealed that the dominant species on the MgO surface is a bidentate carbonate that has a structure with exchangeable oxygen. Moreover, significant desorption of C16O2 was observed in the TPD of adsorbed C18O2. This meant that the surface carbonate underwent multiple oxygen exchange without leaving the surface. We confirmed the occurrence of similar multiple oxygen exchange in the TPD of adsorbed C18O2 for other basic metal oxides such as La2O3.15 In the preceding paper on this issue, we proposed the processes in which bidentate carbonate migrates without leaving the MgO surface and explained the mechanism for multiple oxygen exchange between carbon dioxide and the oxide surface (Scheme 1).14 In process I, carbon dioxide rolls over the surface in such a way that the free oxygen atom in the bidentate carbonate approaches an adjacent Mg atom on the surface. Three C-O bonds do not break during the migration in this process. In process II, the adjacent O atom on the surface approaches a carbon atom. One of the C-O bonds breaks when the C atom forms a bond with the adjacent O atom. In addition to processes I and II, a further process, III, was possible, although this process is essentially the same as the mechanism proposed for the oxygen exchange between carbon dioxide and an oxide surface at high temperature as described earlier.16 IR spectra suggested that these migration processes occur during heating of the sample in the TPD run. Yanagisawa and co-worker also reported the plausible surface sites (13) (a) Yanagisawa, Y.; Shimotama, Y.; Ito, A. J. Chem. Soc., Chem. Commun. 1993, 610. (b) Shishido, T.; Tsuji, H.; Gao, Y.; Hattori, H.; Kita, H. React. Kinet. Catal. Lett. 1993, 51, 75. (14) Tsuji, H.; Shihido, T.; Okamura, A.; Gao, Y.; Hattori, H.; Kita, H. J. Chem. Soc., Faraday Trans. 1994, 90, 803. (15) Tsuji, H.; Okamura-Yoshida, A.; Hattori, H. unpublished results. (16) (a) Peri, J. B. J. Phys. Chem. 1975, 79, 1582. (b) Gensse, C.; Anderson, T. F.; Fripiat, J. J. J. Phys. Chem. 1980, 84, 3562.

The MgO sample was prepared from commercially available MgO (Merck). The MgO powder (Merck) was soaked in distilled water and once hydrated at ambient temperature for 24 h. After evaporation of the water, the resulting magnesium hydroxide was dried at 100 °C for 24 h and used as a precursor of the MgO sample. Commercially available Ca(OH)2 (Kanto Chemicals) was used for the CaO sample. Calcium hydroxide was also treated in distilled water in the same way as MgO. The ZrO2 sample was prepared by hydrolysis of ZrO(NO3)2 by NH3 aqueous solution according to the literature.8c The precipitate was washed with distilled water, followed by drying at 110 °C, and calcination at 600 °C for 5 h. X-ray diffraction analysis revealed that the crystal structure of the resultant sample was monoclinic ZrO2. Carbon dioxide without an isotope-labeled atom was obtained by purification of dry ice through freeze-thaw cycles under vacuum. The 18O-labeled carbon dioxide and the 13C-labeled carbon dioxide were supplied by Icon, and the isotopic purity was 99% for C18O2 and 95% for 13CO2. All operations were carried out using a high-vacuum system made of glass. The Mg(OH)2 or Ca(OH)2 samples were placed in an adsorption vessel made of quartz and heated at 700 °C in a vacuum for 3 h (ca. 10-3 Pa). Thus, the oxide surfaces were directly prepared by thermal decomposition of hydroxides in a vacuum prior to the TPD measurements. The BET surface areas of the oxides prepared in the same way as the TPD measurements were 166 and 59 m2/g for the MgO and CaO, respectively. For the ZrO2, the vessel containing the oxide sample was evacuated at 650 °C for 1 h. The BET surface area was 26 m2/g for the ZrO2 pretreated in the same way. For the TPD measurements, a known amount of C18O2 measured using a volumetric vessel and a manometer was introduced into the adsorption vessel after cooling the sample to room temperature. The residual pressure was negligible. Practically all of the carbon dioxide introduced into the adsorption vessel was adsorbed on the sample. The TPD was run from room temperature to 1000 °C at a heating rate of 10 °C/min in a vacuum with a turbo molecular pump and an ion pump. The small amounts of desorbed gases introduced into the chamber were analyzed by mass spectrometry, using an Anelva AQA100R quadrupole mass spectrometer. A definite amount of argon was continuously introduced into the system during the TPD run, and peak intensity of each mass in the spectrum was normalized to the argon peak intensity. The plots of mass intensity relative to that of argon allowed us to quantify desorption amount empirically for every TPD measurement.

Results Figure 1 shows TPD plots for desorption of each type of isotopically labeled CO2 from the MgO sample prepared at 700 °C. The concentration of the adsorbed C18O2 was 410 µmol/g, which corresponds to one molecule of carbon dioxide per 67 Å2. This concentration is close to that of the (17) Yanagisawa, Y.; Takaoka, K.; Yamabe, S. J. Chem. Soc., Faraday Trans. 1994, 90, 2561. (18) Pacchioni, G.; Ricart, J. M.; Illas, F. J. Am. Chem. Soc. 1994, 116, 10152.

Dynamic Behavior of Carbonate Species

Figure 1. TPD plots for C18O2 adsorbed on MgO. Preparation temperature, 700 °C; admitted C18O2 concentration, 410 µmol/ g; adsorption temperature, room temperature; heating rate, 10 °C/min. C18O2 (m/e ) 48), dash-dotted line; C16O18O (m/e ) 46), dotted line; C16O2 (m/e ) 44), solid line; total CO2, gray line. Mass peak intensity is normalized to that of Ar (m/e ) 40).

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Figure 3. Isotopic distribution of CO2 in TPD plots for C18O2 adsorbed on MgO. Original data and line types for each isotopelabeled CO2 are the same as those used for Figure 1.

Figure 4. Isotopic distribution of CO2 in TPD plots for C18O2 adsorbed on MgO. Original data and line types for each isotopelabeled CO2 are the same as those used for Figure 2. Figure 2. TPD plots for C18O2 adsorbed on MgO. Preparation temperature, 700 °C; admitted C18O2 concentration, 210 µmol/ g; adsorption temperature, room temperature; heating rate, 10 °C/min. C18O2 (m/e ) 48), dash-dotted line; C16O18O (m/e ) 46), dotted line; C16O2 (m/e ) 44), solid line; total CO2, gray line. Mass peak intensity is normalized to that of Ar (m/e ) 40).

CO2 remaining on the MgO after exposure to 20 Torr CO2, followed by evacuation at room temperature. In TPD measurements without admission of CO2, no significant desorption peaks were observed. There are three regions in the plot for the total CO2 in terms of desoption temperature. Carbon dioxide desorbed in the range of high temperature was composed mostly of C16O2, and the desorption of C18O2 was scarcely observed. Appearance of C16O18O just above room temperature indicates that the oxygen exchange occurs at room temperature. The incorporation of surface lattice oxygen into CO2 was investigated for the adsorption of smaller amounts of C18O2 on the MgO sample. The TPD plots for desorption of each type of isotopically labeled CO2 are shown in Figure 2 for which concentration of adsorbed C18O2 was 210 µmol/ g, corresponding to one molecule per 134 Å2. For the low CO2 concentration, carbon dioxide began to desorb at a slightly higher temperature in the TPD compared to that for the high CO2 concentration. The peak appearing just above room temperature in Figure 1 disappeared, and two peaks are observed in Figure 2. To compare the degree of lattice oxygen incorporation between the low and high concentrations of CO2, the isotopic distributions in desorbed CO2 were analyzed for the TPD plots shown in Figures 1 and 2. The fractions of

C18O2 (m/e ) 48), C16O18O (m/e ) 46), and C16O2 (m/e ) 44) for each TPD plot in Figures 1 and 2 are shown in Figures 3 and 4, respectively. More 16O atoms were incorporated into CO2 desorbed at higher temperature for both concentrations. The fraction of C18O2 began to decrease from the beginning of TPD and decreased straightforwardly. However, the difference in proportion of incorporated lattice oxygen in the total CO2 at each desorption temperature was small between the low and high concentrations of C18O2. The TPD for desorption of CO2 and the oxygen exchange between adsorbed CO2 and the oxide surface were investigated for other alkaline earth oxides. Figure 5 shows the TPD plots for desorption of CO2 remaining on CaO prepared at 700 °C after exposure to 20 Torr C16O2 followed by evacuation at room temperature. For the CaO sample, relatively sharp peaks were observed, and their magnitudes were more intense than those for the MgO sample, indicating that the adsorbed species are more homogeneous for the CaO sample than for the MgO sample in terms of adsorption strength. The low-temperature component was completely missing even for the sample evacuated at room temperature after exposure to excess CO2. The desorption temperature was higher for CaO than for MgO, which coincides with the order of basic strength, CaO > MgO, observed in catalytic activities for many basecatalyzed reactions and predicted in the theoretical study.6,18 Similar experiments using 18O-labeled CO2 were performed for the CaO sample. The TPD plots for desorption of each type of isotopically labeled carbon dioxide from

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Figure 5. TPD plots for CO2 remaining on CaO after exposure to 20 Torr C16O2, followed by evacuation at room temperature. Preparation temperature, 700 °C; adsorption temperature, room temperature; heating rate, 10 °C/min. C16O2 (m/e ) 44), solid line; total CO2, gray line. Mass peak intensity is normalized to that of Ar (m/e ) 40).

Figure 6. TPD plots for C18O2 adsorbed on CaO. Preparation temperature, 700 °C; admitted C18O2 concentration, 438 µmol/ g; adsorption temperature, room temperature; heating rate, 10 °C/min. C18O2 (m/e ) 48), dash-dotted line; C16O18O (m/e ) 46), dotted line; C16O2 (m/e ) 44), solid line; total CO2, gray line. Mass peak intensity is normalized to that of Ar (m/e ) 40).

the CaO sample prepared at 700 °C are shown in Figure 6. The concentration of the adsorbed C18O2 was 438 µmol/ g, which corresponds to one molecule of carbon dioxide per 23 Å2. This concentration was about one-fifth of that of the CO2 remaining in the CaO after exposure to 20 Torr CO2 followed by evacuation at room temperature. Whereas the plot for the total CO2 can be divided into two regions in terms of the desorption temperature, the detection of C18O2 was small even for the peak at lower temperature around 450 °C, and C16O2 was dominant for both peaks. The fractions of C18O2 (m/e ) 48), C16O18O (m/e ) 46), and C16O2 (m/e ) 44) analyzed for the TPD plots in Figure 6 are shown in Figure 7. It is also clear for the CaO sample that more 16O atoms were incorporated into CO2 desorbed at higher temperature. The fraction of C16O2 increased monotonically from the beginning of desorption, and it reached ca. 0.8 at the end of the TPD. Figure 8 shows the TPD plots for desorption of each type of isotopically labeled carbon dioxide from the ZrO2 sample pretreated at 650 °C. The concentration of the adsorbed C18O2 was 66 µmol/g, which corresponds to one molecule of carbon dioxide per 66 Å2. This concentration is close to that of the CO2 remaining on the ZrO2 after exposure to 20 Torr CO2 followed by evacuation at room temperature. A broad desorption peak appeared in the plot for the total CO2, and the high-temperature compo-

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Figure 7. Isotopic distribution of CO2 in TPD plots for C18O2 adsorbed on CaO. Original data and line types for each isotopelabeled CO2 are the same as those used for Figure 6.

Figure 8. TPD plots for C18O2 adsorbed on ZrO2. Pretreatment temperature, 650 °C; admitted C18O2 concentration, 66 µmol/g; adsorption temperature, room temperature; heating rate, 10 °C/min. C18O2 (m/e ) 48), dash-dotted line; C16O18O (m/e ) 46), dotted line; C16O2 (m/e ) 44), solid line; total CO2, gray line. Mass peak intensity is normalized to that of Ar (m/e ) 40).

Figure 9. Isotopic distribution of CO2 in TPD plots for C18O2 adsorbed on ZrO2. Original data and line types for each isotopelabeled CO2 are the same as those used for Figure 8.

nent which was observed in the TPD plots for MgO and CaO was missing in the TPD plot for ZrO2. This shows that the number of strongly basic sites for ZrO2 is small as compared to those for MgO and CaO. However, the situation of ZrO2 for the oxygen exchange was similar to those of MgO and CaO. As shown in Figure 9, extensive oxygen exchange between carbon dioxide and the oxide surface was observed for the ZrO2 sample. With a temperature increase, the fraction of C16O2 increased to 400 °C and decreased above 400 °C. The fraction of ca. 0.8

Dynamic Behavior of Carbonate Species

Figure 10. TPD plots for 13CO2 adsorbed on MgO once evacuated at 200 °C after adsorbing 12CO2. Preparation temperature, 700 °C; adsorption temperature, room temperature; heating rate, 10 °C/min. First admitted 12CO2 concentration, 310 µmol/g; second admitted 13CO2 concentration, 60 µmol/ g. 13CO2 (m/e ) 45), dotted line; 12CO2 (m/e ) 44), solid line; total CO2, gray line. TPD plot for 12CO2 (m/e ) 44) without second admission of 13CO2, dotted gray line. Mass peak intensity is normalized to that of Ar (m/e ) 40).

reached at 400 °C was substantially higher than those for MgO and CaO at 400 °C after taking into account of the concentration for the admitted CO2. Next, we examined for the MgO sample prepared at 700 °C by means of successive adsorption using 13C-labeled CO2 whether the carbonate species move to other sites with different basic strength. The successive adsorption procedures were performed as described bellow. At first, nonlabeled CO2 was adsorbed at room temperature. The concentration of the first adsorbed CO2 was 310 µmol/g. After the first run in which the sample adsorbing nonlabeled CO2 was evacuated up to 200 °C at a heating rate of 10 °C/min, the sample was immediately cooled to room temperature in a vacuum, and then 59 µmol/g of 13 CO2 was supplementarily adsorbed. The sample was further evacuated at room temperature for a few minutes prior to the second run. Figure 10 shows the TPD plots for desorption of each type of isotopically labeled carbon dioxide in the second run. When the supplemental admission of 13CO2 was absent, the desorption of 12CO2 was not detected in the region up to 200 °C (dotted gray line in Figure 10). This means that CO2 adsorbed on the strong basic sites does not move to the weak basic sites even if the weak basic sites are exposed by the evacuation at 200 °C. However, a considerable amount of 12CO2 adsorbed previously was detected below 200 °C when the small amount of 13CO2 was adsorbed at room temperature prior to the second run. The isotopic distribution for the second run is shown in Figure 11. The fraction of 13CO2 decreased with a temperature increase and then reached a plateau at 250 °C, and the fraction of 13CO2 incorporated into the high-temperature component was constantly ca. 0.13 to the end of the TPD. On the other hand, the situation of the successive adsorption of nonlabeled CO2 and 13CO2 without the intermediate evacuation at elevated temperature was different in isotopic distribution during the TPD from that with the intermediate evacuation up to 200 °C. Figures 12 and 13 present the TPD plots for desorption and fraction of each type of isotopically labeled CO2 after the adsorption procedure in which 303 µmol/g of nonlabeled CO2 was first adsorbed, and then 71 µmol/g of 13CO2 was secondarily adsorbed at room temperature. After each adsorption, the sample was evacuated at room temperature for a few minutes. As shown in Figure 13, the proportion of

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Figure 11. Isotopic distribution of CO2 in TPD plots for 13CO2 adsorbed on MgO once evacuated at 200 °C after adsorbing 12 CO2. Original data and line types for each isotope-labeled CO2 are the same as those used for Figure 10.

Figure 12. TPD plots for 12CO2 and 13CO2 adsorbed on MgO successively at room temperature. Preparation temperature, 700 °C; heating rate, 10 °C/min. First admitted 12CO2 concentration, 303 µmol/g; second admitted 13CO2 concentration, 71 µmol/g. 13CO2 (m/e ) 45), dotted line; 12CO2 (m/e ) 44), solid line; total CO2, gray line. Mass peak intensity is normalized to that of Ar (m/e ) 40).

Figure 13. Isotopic distribution of CO2 in TPD plots for 12CO2 and 13CO2 adsorbed on MgO successively at room temperature. Original data and line types for each isotope-labeled CO2 are the same as those used for Figure 12.

secondarily adsorbed 13CO2 was almost constant throughout the TPD in contrast to the results for the successive adsorption procedure in which the MgO sample was once evacuated at 200 °C after the first adsorption of CO2. Discussion Type of Carbonate Species on Metal Oxide Surfaces. As mentioned in the preceding work on this issue,

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Figure 14. Structures of surface monodentate carbonate, bidentate carbonates I and II, bicarbonate, formed by adsorption of CO2 on well-degassed oxide surface.

the monodentate carbonate should contribute to the desorption of C18O2 for the TPD of adsorbed C18O2 because of its structure with no exchangeable oxygen (see Figure 14). There was no region in which C18O2 was dominant in the TPD plots for MgO, CaO, and ZrO2. The present results indicate that the fraction of CO2 fixed on the surface as monodentate carbonate during the heating process is quite small for all the metal oxides examined. A multiple oxygen exchange between adsorbed carbon dioxide and surface was observed for ZrO2 with a different structure from MgO and CaO (rock salt). There is a qualitative similarity of the TPD plots for desorption of each type of isotopically labeled CO2 shown in Figure 8 to those reported by Bachiller-Baeza et al. for a commercial available ZrO2.19 Tret’yakov et al. inferred by means of IR spectroscopy that bidentates I and II (see Figure 14) formed upon adsorption of CO2 onto ZrO2.20 They pointed out the different stability of the two bidentate carbonates; bidentate I was removed by evacuation at 150 °C while bidentate II was removed by evacuation at 300 °C. However, neither divided peak suggesting the existence of two distinguishable carbonates nor the threshold of isotopic distribution was observed in the TPD plot below 400 °C. If bidentate carbonates I and II exist on the ZrO2 with different stability, there is no difference in the manner of oxygen exchange with surface lattice oxygen between them. A decrease in the fraction of C16O2 above 400 °C was observed in the TPD plot for ZrO2. This increase in the component involving 18O above 400 °C strongly suggests the existence of a minor but distinguishable species remaining on the ZrO2 surface above 400 °C. According to the IR spectra for CO2 adsorption on ZrO2 reported by Pokrovski et al., the species that decompose finally as the temperature is raised are monodentate species.7e Bicarbonate species formed with increasing surface coverage were reported to be less stable than carbonate species on ZrO2. Incorporation of Surface Lattice Oxygen into Carbon Dioxide. As mentioned in the Introduction, we proposed the existence of three processes during the TPD to explain the multiple oxygen exchange between adsorbed carbon dioxide and the surface of MgO. In this study, it was shown that the same processes are involved in the interaction of carbon dioxide with CaO and ZrO2 before desorption. In particular, the desorption of carbon dioxide in which double 16O was incorporated strongly supports the existence of process II, and the incorporation of 16O exceeding 33% of total oxygen for the desorbed CO2 should be attributed to process II.21 The increase in the fraction of 16O with desorption temperature indicates that process II occurs during heating of the sample in the TPD run. It is also suggested that the rate of process II is faster at higher temperature. Carbon dioxide desorbed at higher (19) Bachiller-Baeza, B.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Langmuir 1998, 14, 3556 (20) Tret’yakov, N. E.; Pozdnyakov, D. V.; Oranskaya, O. M.; Filimonov, V. N. Russ. J. Phys. Chem. 1970, 44, 596.

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temperature would pass through process II more frequently before desorption. As illustrated in Scheme 1, process II involves the nucleophilic attack of surface lattice oxygen on the carbon atom of bidentate carbonate, and the ease of oxygen incorporation in process II seems to be related to the nucleophilicity of surface lattice oxygen. Barteau and Idriss mentioned, in their recent review, the nucleophilicity of surface lattice oxygen similar to that involved in process II.12 They pointed out that surface lattice oxygen plays a role as a nucleophile in such a type of basecatalyzed reaction as the Tishchenko reaction in which the carbonyl carbon of aldehyde is activated by surface lattice oxygen of a basic metal oxide. In the present study, it was suggested that the nucleophilicity of surface lattice oxygen is sufficient to substitute an oxygen atom of carbon dioxide which is one of the most inert molecules. The carbon atom of the carbonate species accepts the nuclophilic attack of lattice oxygen with leaving a new oxygen vacancy on the surface. Simultaneously, the oxygen atom originally composing carbonate is left on another oxygen vacancy. An oxygen vacancy appears and disappears around the carbonate species and this process should take place repeatedly in the TPD run. In other words, the behavior of carbonate species varies the coordinative environment of the surface consecutively. Metal oxide surface is variable to carbon dioxide through oxygen exchange, although these oxides are not regarded as the kind of redox oxide such as MoO3 and V2O5 operating by the Mars-van Krevelen mechanism. A similar effect of the carbonate migration on the coordinative environment is adapted to process I. In process I, the migration of carbonate would fill in an oxygen vacancy and generate another via transient tridentate-like species similar to that proposed for zeolite A with narrow pores.22 The surface carbonates no longer stay at one site. The site from which CO2 molecules are desorbed should be different from those on which CO2 molecules are adsorbed. The microscopic coodinative environment of the surface exposed after desorption of CO2 should be different from that before adsorption of CO2. The fraction of C16O2 at 400 °C was higher for the ZrO2 sample than for the MgO and CaO samples. This suggests that the surface lattice oxygen of ZrO2 is easily incorporated into CO2; that is, part of the surface metal-oxygen lattice bond breaks more easily for ZrO2 than for MgO and CaO. The metal-oxygen bond strength E(M-O) calculated on the basis of the heat of formation and the coordination number would be a most adequate property to discuss such reactivity of surface lattice oxygen for these nonredox metal oxides.12,23 However, the easy incorporation of surface lattice oxygen of ZrO2 cannot be explained by this bulk property because the E(M-O) of ZrO2 is much higher than those of MgO and CaO.12 Although the factor dominating the oxygen exchange between adsorbed carbon dioxide and the oxide surface is not clear at present, this type of reactivity of lattice oxygen and oxygen vacancy would be an important surface property relating to the catalysis over the oxide surfaces. Oxygen Scrambling via Carbonate Migration. To elucidate the mechanism of oxygen exchange and the (21) Without process II, the carbonate species would always contain two 18O atoms during repetition of processes I and III. The repetition of processes I and III would result in the exchange of one oxygen atom but not the exchange of more than one oxygen atom of carbon dioxide; that is, the desorption of C16O2 would not be generated. (22) Takaishi, T.; Endoh, A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 411. (23) Rethwisch, D. G.; Dumesic, J. A. Langmuir 1986, 2, 73.

Dynamic Behavior of Carbonate Species

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Figure 15. Variation in [C16O18O]2/{[C16O2][C18O2]} with the temperature increase in TPD plots for C18O2 adsorbed on MgO. 410 µmol/g of admitted C18O2 concentration, solid line; 210 µmol/g of admitted C18O2 concentration, dotted line. Original data are the same as those used for Figure 1 and Figure 2.

Figure 16. Variation in [C16O18O]2/{[C16O2][C18O2]} with the temperature increase in TPD plots for C18O2 adsorbed on CaO. Admitted C18O2 concentration, 438 µmol/g. Original data are same as those used for Figure 6.

dynamic behavior of carbonate species, the isotopic distributions in desorbed CO2 were analyzed again. Chemical equations for the processes illustrated in Scheme 1 are expressed by

C18O18O + 16Osc ) C16O18O + 18Osc

(1)

C16O 18O + 16Osc ) C16O16O + 18Osc

(2)

for processes I and III and by

C18O18O + 16Osc + 16Ol ) C16O18O + 16Osc + 18Ol

(3)

C16O18O + 16Osc + 16Ol ) C16O16O + 16Osc + 18Ol

(4)

for process II, where Osc represents lattice oxygen composing bidentate carbonate (basic site) and Ol nucleophilic lattice oxygen (another basic site). These equations are reduced to the following equation.

C16O2 + C18O2 ) 2C16O18O

(5)

Therefore, the value of [C16O18O]2/{[C16O2][C18O2]} should be equal to 4 when the system is at equilibrium.24 The isotopic distributions represented by the value of [C16O18O]2/{[C16O2][C18O2]} were plotted against the desorption temperature in Figures 15, 16, and 17 for MgO, CaO, and ZrO2, respectively. Surprisingly, the value of [C16O18O]2/{[C16O2][C18O2]} demonstrating the degree of oxygen scrambling was close to 4 over almost all the temperature region of desorption for these samples. This means that there is no distinction between the original 18 O and the incorporated 16O; that is, the three oxygen atoms composing carbonate are entirely scrambled by repetition of the processes before desorption. The two oxygen atoms of the desorbed CO2 are also picked at (24) The value of 4 is also drawn from the probability of choosing two oxygen atoms at random out of an unlimited amount of isotopic oxygen mixture. When the ratio of 16O to total oxygen is x, the probability of choosing the combination of 16O and 18O is expressed in terms of 2x(1 - x), and the probability of choosing two 16O and the probability of choosing two 18O are expressed in terms of x2 and (1 - x)2, respectively. Thus, for the relationship among the numbers of oxygen pairs, we have the equation: the squared number of 16O-18O pairs divided by the number of 16O-16O pairs and by the number of 18O-18O pairs equals 4; that is, (2x(1 - x))2/(x2 (1 - x)2) ) 4. This might be more suitable for considering the present case than the equilibrium constant for the reaction, C16O2 + C18O2 ) 2C16O18O, on the basis of the statistical mechanics using a partition function.

Figure 17. Variation in [C16O18O]2/{[C16O2][C18O2]} with the temperature increase in TPD plots for C18O2 adsorbed on ZrO2. Admitted C18O2 concentration, 66 µmol/g. Original data are same as those used for Figure 8.

random out of the three oxygen atoms of carbonate. A possible mechanism could be described as follows for explaining the compatibility of the constant value close to 4 for [C16O18O]2/{[C16O2][C18O2]} with the increase in the 16O fraction as desorption temperature increase. The rates of processes I and III are fast compared to that of process II, and the carbonate oxygen becomes indistinguishable rapidly after the incorporation of surface lattice oxygen by process II. Carbonate Migration to Sites with Different Basic Strength. The successive adsorption experiments using 13 CO2 for the MgO sample revealed another aspect of the dynamic behavior of carbonate species on the surface. Krylov et al. reported the results of successive adsorption experiments using radioisotopic 14CO2 in their study of MgO.25 They observed that a 14CO2 concentration was proportional to the total CO2 remaining on the surface after desorption at each temperature whenever 14CO2 was adsorbed first or secondarily at 50 °C. They assumed that the surface of MgO is homogeneous with respect to the activation energy for CO2 adsorption. Auroux and Gervasini also pointed out this homogeneous CO2 adsorption behavior in their microcalorimetric study.9 They reported a plateau for the differential heats of adsorption for CO2 on MgO and CaO. In the present study, we observed similar homogeneity of CO2 adsorption in the TPD for the (25) Krylov, O. V.; Markova, Z. A.; Tret’yakov, I. I.; Fokina, E. A. Kinet. Katal. 1965, 107.

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sample on which CO2 was successively adsorbed at room temperature (see Figures 12 and 13). This indicates that the adsorption priority is not given to the strong basic sites at least when CO2 is admitted at room temperature. Carbon dioxide would be adsorbed at random regardless of the strength of the basic sites in the initial stages and would then migrate to the stronger basic sites during the heating process. The evaluation of basic sites on the surface by means of the TPD for desorption of CO2 must be different from that by means of the differential heats of adsorption for CO2. In the TPD run for MgO after the adsorption of CO2 followed by the evacuation up to 200 °C, desorption below 200 °C was scarcely observed without supplemental admission of CO2 (dotted gray line in Figure 10). This means that CO2 once migrated to the strong basic sites did not move to the weak basic sites. Therefore, the supplementarily admitted 13CO2 should be adsorbed on the weak basic sites exposed after the first run up to 200 °C. It was, however, observed that the previously adsorbed 12 CO2, which are anticipated to move to the strong basic sites, are incorporated into the low-temperature component in the TPD plots after the supplemental adsorption of 13CO2 (see Figure 11). Namely, the adsorption strength of a part of basic sites was changed by the supplemental adsorption of CO2. After taking into account the migration of carbonate and its effect on the coordinative environment of basic sites, the decrease in basic strength of the sites adsorbing the former CO2 is ascribed to the dynamic behavior of the carbonate species formed in the later adsorption. The coordinative environment of basic sites forming carbonate should be varied with desorption and crossing over of other carbonates because these accompany the appearance and disappearance of oxygen vacancies. This would change the adsorption strength and would cause successive desorption as CO2 and migration of the carbonate to other sites. The basicity of each site and its adsorption strength for CO2 oscillate during the heating process in the TPD run. This heterogeneity might be one reason for the broad desorption peak appearing generally in the TPD plots for adsorbed CO2 on basic metal oxide surfaces. It is not simple to assign desorbed CO2 molecules to each basic site in terms of adsorption strength.

Tsuji et al.

Conclusions We demonstrated the dynamic behavior of carbonate species formed in the adsorption of CO2 on the surface of basic metal oxides by means of the TPD using isotopic labeled CO2 and analysis of the isotopic distribution for desorbed CO2. The results obtained in this study are summarized as follows: 1. In addition to the exchange among oxygen atoms composing bidentate carbonate, the nucleophilic incorporation of surface lattice oxygen into CO2 participates as the temperature increase. 2. The bidentate carbonate migrates over the surface, and the three oxygen atoms composing carbonate are entirely scrambled before desorption as CO2. 3. As for isotopic distribution, the value of [C16O18O]2/ {[C16O2][C18O2]} is close to 4 over almost all the temperature region of desorption for the metal oxides investigated in the present study. Carbonate species are indistinguishable in terms of the manner of oxygen exchange. The direct desorption from the fixed monodentate carbonate is quite small for MgO, CaO, and ZrO2. 4. The successive adsorption of CO2 on MgO revealed that carbon dioxide is adsorbed at random regardless of the strength of the basic sites at room temperature and then migrates to the stronger basic sites during the heating process. The assignment of desorbed CO2 molecules to each basic sites in terms of adsorption strength is not simple because the dynamic behavior of the carbonate species diversify the coordinative environment and the adsorption strength of basic sites. The correlation between the surface basicity and nucleophilic incorporation of surface lattice oxygen into carbon dioxide is now under investigation. Full description of this complicated behavior of carbonate species would be very difficult because the coordinative environment of the surface must vary successively. However, further data should contribute to our understanding of the nature of metal oxide surfaces. Supporting Information Available: Figures showing the TPD plots for SrO and CaO with low concentration of adsorbed C18O2 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA0342666