J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 803-807 Oxygen Exchange between Magnesium Oxide Surface and Carbon Dioxide Hideto Tsuji, Tetsuya Shishido, Akie Okamura, Yunzhi Gao, Hideshi Hattori' and Hideaki Kita Division of Materials Science Graduate School of Environmental Earth Science Hokkaido University, Sapporo 060,Japan The isotopic distribution of carbon dioxide desorbed from an MgO surface containing adsorbed '80-labelled carbon dioxide (C'802)has been measured by temperature-programmed desorption (TPD), in order to study acid-base pair sites on the surface. Three desorption peaks, differing in both isotopic distribution and tem- perature of desorption, were observed. The desorption peak in the temperature range 300-400 K (region I) is due to C'60180and Cl8O, in a ratio of nearly 1 :1.The desorption peak in the range 400-600 K (region II) is composed of Cl6O,, C' 60'80and Cl8O,,among which C"0, was most dominant. The third peak appeared in the temperature range 600-1000 K (region Ill). This peak consists mostly of C1602. For most of the adsorbed species, the surface Mg2+ contributes to CO, adsorption. It is concluded that the adsorbed CO, undergoes multiple oxygen exchange with the surface while migrating over it. Based on IR measurements of the adsorbed CO, , it is suggested that migration takes place during heating of the sample to 473 K in the TPD run. Carbon dioxide is frequently used as a probe molecule for investigating the basic properties of metal oxide surfaces in different methods such as IR and TPD.In the TPD of adsorbed CO, ,the concentration of the basic sites is reflected in the peak areas of the TPD plot, and the strength of the basic sites in the temperature at which the CO, desorption peak appears.lW3 In the IR study of adsorbed CO,, carbon-ate species of different types such as unidentate carbonates, bidentate carbonates, carbonate ions and hydrogen-carbonates are formed, depending on the adsorption condi- tions and the surface ~tructure.~ On a well degassed MgO at an elevated temperature (pretreatment temperature) in a vacuum. The surface areas were 166 and 90 m2 g-' for the MgO samples prepared at 973 and 1273 K, respectively. The '80-labelled carbon dioxide was supplied by Icon and its iso- topic purity was 99%.For the TPD experiments, the Mg(OH), sample (100 mg) was placed in an adsorption vessel and pretreated at 973 K or 1273 K in a vacuum for 3 h (ca. 10-3 Pa). After cooling to room temperature, a known amount of C1'0, was intro- duced into the vessel. The residual pressure was negligible. Practically all of the CO, introduced into the vessel was surface, unidentate and bidentate carbonates are f~rrned.~.~ adsorbed on the MgO. The TPD was run from room tem- Fukuda and Tanabe6 reported that at a high coverage of adsorbed CO, , unidentate carbonates are predominant, while at a low coverage, bidentate carbonates become pre- dominant. Recently, Yanagisawa et a/.' have reported that oxygen exchange between adsorbed CO, and the MgO surface takes place to a considerable extent.They observed a TPD desorp- tion peak consisting mainly of Cl6O, and Cl60l8O after C"0, adsorption on MgO, and suggested that the adsorbed C'80, interacts with the peroxy ion [('60s)22-] on a defect in the MgO surface. We have observed essentially the same phenomena in the TPD study of adsorbed Cl80,. The occurrence of oxygen exchange between CO, and the oxide surface indicates that CO, interacts not only with the basic sites of 02-,but also with the acidic sites of surface metal cations. Therefore, elucidation of the exchange mecha- nisms is expected to reveal the surface acid-base properties. The isotope distribution reflects the variety of adsorption sites and acid-base properties on the surface.In the present paper, we report a detailed TPD study of the MgO surface interacting with C1802, as well as an IR study of the adsorbed CO, . Experimental The MgO sample was prepared from commercially available MgO (Merck). Impurity levels in the MgO were at most 0.2 wt.% for Na and 0.035 wt.% for the other cations. The MgO powder (Merck) was soaked in distilled water and hydrated at ambient temperature for 24 h. After evaporation of water, the resulting magnesium hydroxide was dried at 373 K for 24 h, and used as a precursor of the MgO sample. The MgO sample was obtained by thermal decomposition of Mg(OH), perature to 1073 K at a heating rate of 10 K min-'. The desorbed gases were analysed by mass specrometry, using an Anelva AQAl00R quadrupole mass spectrometer. A small quantity of argon was continuously introduced into the system, and each peak in the mass spectrum was normalized to the argon peak intensity. For the IR experiments, an Mg(OH), disk was placed in an in situ IR cell, and pretreated similarly.A known amount of CO, was introduced into the cell. The sample with adsorbed CO, was heated in a vacuum, increasing the temperature by 100 K increments up to 973 K. After evacuating the sample at each step for 30 min, the sample was cooled to room tem- perature and an IR spectrum was recorded on a Jasco FTIR 5300 spectrometer. Results and Discussion TPD plots for desorption of each type of isotopically labelled carbon dioxide from the MgO sample pretreated at 973 K are shown in Fig.1. The concentration of the adsorbed carbon dioxide was 410 pmol g- which is equal to one mol- ecule of carbon dioxide per 67 A'. This concentration is close to that of the CO, remaining on the MgO after exposure to 20 Torr CO,, followed by evacuation at room temperature. In the case of TPD measurements without admission of CO, ,no significant desorption peaks were observed. The plot for the total CO, can be divided into three regions in terms of both desorption temperature and isotopic distribution. The first region (region I) ranges from room temperature to 400 K, the second one (region 11) from 400 to 600 K, and the third (region 111)above 600 K. In region I, the desorbed CO, comprised C1802 and C'60'80 in a ratio of close to 1 : 1.The detection of I I 1, I1 I 200 400 600 800 1000 1200 desorption temperature/K Fig. 1 TPD plots for C"0, adsorbed on MgO pretreated at 973 K. (-) Total CO,, (--) Ci802, (---) C'60180, (---) Cl60,. CO, concentration, 410 pmol g- '. (Peak heights are relative to that of Ar.). C160180just above room temperature indicates that the oxygen exchange occurs at room temperature. In region 11, C'60180 was dominant, though considerable fractions of Cl6O, and Cl80, were included. In region 111, the desorbed CO, was composed mostly of C1602, a small fraction of C160180being included. The desorption of C1802 was scarcely observed. The appearance of three regions in the TPD plot indicates the existence of three kinds of adsorption site, differing in adsorption strength for CO, .Since the isotopic distribution for each region was different, a mixing of the CO, adsorbed on different kinds of site does not take place upon adsorption at room temperature. Note that the classification of the surface adsorption sites becomes much clearer by the use of isotopic CO, and measurement of the isotopic distribution. On the well degassed MgO surface, two types of CO, adsorption were reported on the basis of the IR st~dy.~,~ One type of adsorbed species was unidentate carbonate and the other bidentate carbonate. In the form of unidentate carbon- ate, the adsorbed species is bound to the surface through a bond between the C in CO, and the surface 0 as shown in Fig.2(a). A single adsorption-desorption will not involve oxygen exchange between CO, and the MgO surface. In the form of bidentate carbonate, the adsorbed species is bound to the surface through two bonds: one between the C in CO, and the surface 0, and the other between an 0 in CO, and Mg as shown in Fig. 2(b). From the bidentate formed upon adsorption of C1*02,C180, will be desorbed if bonds I1 and I11 break, and C'60'80 will be desorbed if bonds I and IV break. This mechanism was proposed by several workers8-l * for the oxygen-exchange reaction between CO, and oxide surfaces. The desorbed CO, will be a J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1:1 mixture of C'60'80 and C"0, upon TPD if the l8O bound to Mg is equivalent to the l6O originating from the MgO surface.In region I, the desorbed CO, was a nearly 1: 1 mixture of ClaO, and C'60'80.This suggests that a major part of the adsorbed species desorbed in region I is in the form of biden- tate carbonate, and that the two C=O bonds [bonds I and I1 in Fig. 2(b)] are equivalent. In regions I1 and 111, the major isotopic form of CO, desorbed was Cl60,. If C"0, were adsorbed on one pair of Mg-0 sites in the form of bidentate carbonate, C1602 should not have been included in the desorbed CO,. There-fore, it is suggested that for the desorption of CO, in regions I1 and 111, processes other than simple adsorption-desorption of CO, on one pair of MgO sites are involved. There is no region in which Cl8O, is dominant over C160'80 and Cl6O,.Thus, the fraction of CO, adsorbed as unidentate carbonate is quite small. This indicates that for most adsorbed species bond formation between Mg2+ and the oxygen of CO, is involved in adsorption. On a thoroughly degassed MgO surface, coordinatively unsaturated Mg2+ cations are exposed,' '*12 and exhibit Lewis-acidic nature.13 Molecular orbital calculations have also pointed out that the coordinatively unsaturated Mg2 + cations can be regarded as Lewis-acid sites and that the acidity of the Mg2+ becomes stronger as the coordination number of 02-to Mg2+ decrease^.'^ In fact, amm~nia,'~*'~ hydrogen' 7-20 and hydrocarbons' 1-23 undergo heterolytic dissociation upon adsorption on the Mg2+-02- acid-base pair sites of MgO pretreated at a high temperature.We believe that CO, is also adsorbed on Mg2+-02- pair sites in the form of bidentate carbonate with an acid-base bifunc- tional interaction, though CO, does not seem to undergo heterolytic dissociation. It is certain that bond formation between the Mg2+ acid sites and the oxygen of CO, plays a role in oxygen exchange with the MgO surface during the adsorption-desorption cycle. Fig. 3 shows the TPD plot for each type of isotopically labelled CO, after C"0, was adsorbed on the MgO sample pretreated at 1273 K. The concentration of adsorbed CO, was 410 pmol g-'. Compared with the TPD plot in Fig. 1, the desorbed amounts increase in regions I and 11, and decrease in region 111. It is obvious that the number of surface sites which adsorb CO, strongly has decreased.The changes are considered to be due to the decrease in surface area of the MgO. The isotopic distribution and temperature range of each region, however, remain unchanged. In the case 200 400 600 800 1000 1200 desorption temperature/K Fig. 2 Structures of (a) surface unidentate and (b)surface bidentate Fig. 3 TPD plots for C'*O, adsorbed on MgO pretreated at 1273 carbonate, formed by adsorption of CO, on well degassed MgO K. All details as Fig. 1. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 of the sample pretreated at 973 K, small amounts of OH groups remain on the MgO surface, and the OH groups are completely eliminated by pretreatment at 1273 K.24*2sThere-fore, it is suggested that the I60 atoms incorporated into the desorbed CO, originate from lattice 0,-, and not from the surface OH groups.Lower-coordinated ion-pair sites, expressed as MgEc-O&, become exposed on the MgO surface by increasing the pretreatment temperature,' and the catalytic properties change with pretreatment temperature.26 In par- ticular, Mgfg -0:; ion-pair sites for which unsaturation of the coordination is most extensive are considered to be revealed at pretreatment temperatures above 973 K. The reactivity of the surface ions may increase as the coordination number becomes lower because the protrusion of wavefunc- tions is greatly enhanced at edges and corners on the surface of partially ionic crystals.27 However, the CO, desorption temperature was the same for the MgO samples pretreated at 1273 and 973 K.This indicates that the pretreatment above 973 K does not enhance the adsorption strength of MgO towards CO, . The molecular orbital calculation studyI4 pointed out that the basic strength of the 0,-ions is enhanced in the following manner; fewer Mg2+ cations are coordinated to the central 0,-ion in the basic sites and more 0,-ions are coordinated to the Mg2+ cations adjacent to the central 0,-ion. Our observation for the adsorption strength of CO, also suggests that the basicity of the 0,-ion is not strongest for MgSb-O&. We wish to correlate each desorption region of CO, in the TPD plot with the surface structure. The results obtained in the present study reveal the rearrangement of the surface Mg-0 bonds by interaction with CO,.To correlate the desorption peak in the TPD plot of adsorbed CO, with the surface structure, such a rearrange- ment of the surface Mg-0 should be taken into account. To investigate the CO, desorption in regions I1 and I11 in more detail, smaller amounts of C1802were adsorbed on the MgO sample pretreated at 973 K, and subjected to TPD measurements. TPD plots for each isotopic CO, are shown in Fig. 4 and 5. The concentrations of adsorbed CO, were 210 pmol g-', corresponding to one molecule per 134 A2 for Fig. 4, and 41 pmol g-', corresponding to one molecule per 670 A2 for Fig. 5. In Fig. 4, two peaks are observed. They correspond to the two peaks in regions I1 and I11 in Fig. 1.By reduction of the adsorbed amount of CO, to about a half, the peak in region I was removed. The main isotopic CO, species were C160180 81 I 0;6t.-0, 24 n$ 1 ILP ~ ~~~~~~~ 200 400 600 800 1000 1200 desorption temperature/K Fig. 4 TPD plots for C1802 adsorbed on MgO pretreated at 973 K. As Fig. 1 except CO, concentration,210 pmol g-I. 805 1.2 1.a N 0 0.8 E .P 0.6 E A3 0.4 a QJ .--0.2 2 0 200 400 600 800 1000 1200 desorption temperature/K Fig. 5 TPD plots for C'802 adsorbed on MgO pretreated at 973 K. As Fig. 1, except CO, concentration, 41 pmol g-'. for region 11, and Cl6O, for region 111.The fraction of C"0, was small for region 11, and not appreciable for region 111.Extensive 0 exchange between adsorbed CO, and the surface 0 should take place. The exchange is more extensive for the CO, desorbed in region 111. Extensive 0 exchange was more clearly demonstrated by further reduction of the amount of adsorbed C1802.In Fig. 5 where one C1802 molecule per 670 A2 was adsorbed, only one peak appeared in the TPD plot of the total CO,. This peak coincides with the peak in region I11 (Fig. 1). The coin- cidence of the peak in Fig. 5 with the region I11 peak in Fig. 1, and the disappearance of the peaks in regions I and I1 indicate that CO, is selectively desorbed from the strong adsorption sites when a small amount of CO, was admitted. The desorbed CO, consisted of 90% Cl6O, and 10% C160180,with no appreciable amount of C1802.Ito" has reported that multiple oxygen exchange between CO, and the lattice oxygen of MgO takes place by repetitive adsorption-desorption of CO, molecules. However, multiple oxygen exchange was observed in the presence of gas-phase CO, (> 1 kPa) at a reaction temperature of around 1123 K. The reaction conditions of his study were greatly different from those of the present study. In the conditions for Fig. 5, the partial pressure of CO, after admission of one molecule per 670 A' was negligible, which is evidenced by the absence of a desorption peak below 500 K. Therefore, it is excluded that the multiple 0 exchange between CO, and the surface 0 occurs through repetitive adsorption-desorption of CO, molecules.As the mechanism for the multiple oxygen exchange between CO, and the surface oxygen atoms, Yanagisawa et d7 proposed that the adsorbed C180, interact with the surface peroxy ions [( l60s);-] to form tentative intermediate rolling carbonate ions. Takaishi and Endoh,' also proposed the rolling three-fold coordinated carbonate as a mechanism for the oxygen exchange between CO, and the framework oxygen of zeolite A. Both of these mechanisms would result in the desorbed CO, containing at least 17% CI8O, even if four C-0 bonds are formed (with equivalent bonds in the tentative intermediates), and therefore, cannot explain the absence of C"0, and the extensive desorption of CI6O, in region 111, as observed in the present study.To explain the absence of C1802, the adsorbed species must encounter several surface oxygen atoms before desorption. We propose that extensive oxygen exchange results from the adsorbed CO, migrating over the surface without leaving the surface. During the migration over the surface, the adsorbed CO, forms bidentate carbonate and undergoes 0 exchange with the surface 0 atoms. The proposed processes are shown in Fig. 6. There are at least two ways, processes (I) and (11), for the adsorbed car- bonate species to migrate over the surface. In process I, carbon dioxide rolls over the surface in such a way that the free oxygen atom in the bidentate carbonate approaches the adjacent Mg atom on the surface. Three C-0 bonds do not break during the migration in this process.In process (11), the carbon atom approaches the adjacent 0 atom on the surface. One of the C-0 bonds breaks when the C atom forms a bond to the adjacent 0 atom. In process (I), the carbonate species always contains two l80atoms while migrating over the surface. Therefore, repe- tition of process (I) will result in the exchange of one oxygen atom, but not the exchange of two oxygen atoms in the desorbed CO,. The repetition of process (11) will also result in the exchange of one oxygen atom in the desorbed CO,. The free oxygen atom (0')is always away from the surface and not exchanged with the surface oxygen atom. For evolu- tion of Cl60, both processes (I) and (11) should be involved, assuming that replacement of the surface Mg-0 bond by the bond between 0 in CO, and surface Mg occurs upon desorption.The extensive formation of I60-rich carbonate species must be caused by repetition of processes (I) and (11). The occurrence of processes (I)or (11)seems to depend on the surface structure of MgO. A difference in the probabilities of processes I and I1 results in a difference in the isotopic dis- tribution in the desorbed CO, . In addition to processes (I) and (11), a further process, (111), should be considered. This process is essentially the same as the mechanism proposed for the oxygen exchange between bidentate carbonate and oxide surface as described earlier.*-'* The carbonate species could migrate over the surface over a long distance by a combination of process (111) with processes (I) and (11) without leaving the surface if process (111) exists.The Mg-0 sites from which CO, molecules are desorbed in regions I1 and I11 should be different from those on which CO, molecules are adsorbed. In other words, the CO, mol-ecule is adsorbed on a certain site on the MgO surface and desorbed from a different sites. On the CO, migration path, the surface Mg-0 bonds break and reform as CO, migrates over. 05 0"cP process (II) O$+7 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 It is not certain at this moment whether the migration takes place at room temperature or during heating in the TPD run. IR spectra of adsorbed Cl6O, were measured after heating the C' 60,adsorbed sample at different temperatures for 30 min in a vacuum, and are shown in Fig.7. The spectra, with subtraction of the spectrum of MgO pretreated at 973 K (background), are shown in the range from 800 to 2000 cm-'. The concentration of the adsorbed CO, was one molecule per 670 A', and is the same as the TPD run shown in Fig. 5. No peaks assigned to CO, weakly adsorbed onto cationic sites were observed in all spectra. It is confirmed that the weakly adsorbed linear species of CO, is not involved in the processes of the migration. At room temperature adsorption, two bidentate carbonates were observed. The peaks appeared at 1668, 1320, 1005 and 849 cm-' and those at 1630, 1277, 955 and 833 cm-' are assigned to bidentate carbonates.6 The broad peaks at 1517 and 1398 are assigned to unidentate carbonate.6 These peaks were not changed by evacuation at room temperature.By elevating the temperature to 373 K [Fig. 7(b)], the spectrum was changed. The peaks at 1630, 1277, 955 and 833 cm-' which are assigned to one bidentate carbonate disappeared and those at 1666, 1323, 1007 and 850 cm-' increased in intensity. As the temperature was raised, the spectrum grad- ually changed to give sharper peaks and the peak positions were slightly shifted. At 573 K [Fig. 7(d)], the peaks assigned to unidentate carbonate converted to bidentate carbonate and four peaks assigned to bidentate carbonate were posi- tioned at 1655, 1331, 1036 and 856 cm-'. These peaks increased in intensity up to 573 K and decreased greatly on 10.1 (f) IIII IIII I09 o~c-p'o process (III) 08-Cfl Fig.6 Proposed processes for the mechanism of migration of surface bidentate carbonate J. CHEM. SOC. FARADAY TRANS., 1994. VOL. 90 evacuation at 673 K for 30 min. The changes in the shdpe and position of the peaks were most extensive when the tem- perature was raised from room temperature to 473 K. Above 473 K. the change in the spectrum was small. It is, therefore. suggested that the bidentate carbonate formed on room tem- perature adsorption of CO, migrates over the surface as the temperature is raised in the TPD run. The migration occurs mostly in the temperature range from room temperature to 473 K. Conclusions The results are summarized as follows: (1) Oxygen exchange between adsorbed CO, and the MgO surface was found on TPD analysis by using C'*O, as adsorbate.(2)The oxygen exchange occurs at room temperature. 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