J. Phys. Chem. 1992, 96, 9035-9038 (36) Stevens, G. C.; Edmonds, T. J. Catal. 1975, 37, 544. (37) Linee, J. R.; Stewart, T. B.; Fleischauer, P. D.; Yarmoff, J. A.; Taleb-Ibrahimi, A. J. Vac. Sci. Technol. 1989, A7, 2469. (38) Grünert, W.; Stakheev, A. Y.; Feldhaus, R.; Anders, K.; Shpiro, E. S.; Minachev, K. M. J. Phys. Chem. 1991, 95, 1323.
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(39) Haber, J.; Marczewski, W.; Stock, J.; Ungier, L. Ber. Bunsen-Ges. Phys. Chem. 1975, 79, 970. (40) Nag, N. K. J. Catal. 1985, 92, 432. (41) Broclawik, E.; Haber, J. J. Catal. 1981, 72, 379. (42) McCormick, R. L.; Schrader, G. L. J. Catal. 1988, 113, 529.
FTIR Spectroscopic Study of C02 Adsorption/Desorptlon on MgO/CaO Catalysts Rosemarie Philippi and Kaoru Fujimoto*
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Department of Synthetic Chemistry, Faculty of Engineering, The University Tokyo 113, Japan (Received: September 30, 1991)
of Tokyo,
Hongo, Bunkyo-ku,
The nature of adsorption states of C02 on mixed Mg/Ca oxide catalysts was investigated by FTIR spectroscopy. At room temperature, the unidentate is predominantly formed besides bicarbonate. On CaO-rich catalysts additional structures were detected and attributed to bridging carbonate and linearly adsorbed C02. Desorption behavior of these species is discussed in dependence on temperature. Surface basicity was quantitatively measured by the wavenumber difference of symmetric and asymmetric carbonate absorption bands and related to catalytic activity for oxidative coupling of methane.
Introduction
TABLE I: Surface Areas and Adsorbed Amounts of C02 in
It
has been well-known that magnesium oxide, especially when doped with lithium ions, is an excellent catalyst for the oxidative
Dependence on Catalyst Composition
specific surface area
coupling of methane (e.g., ref 1). Also, mixed oxides of magnesium and calcium are effective catalysts for this reaction.2 It was found that both the yield and selectivity for C2 hydrocarbons were highest at 85 mol % MgO (Figure 1). Surface basicity was suggested to be essential for this effect. Temperature-programmed desorption (TPD) curves of previously adsorbed C02 and volumetric C02 adsorption measurements confirm this idea.2 This study presents additional FTIR spectroscopic investigations on the adsorption of C02 on mixed MgO/CaO catalysts. C02 was chosen as probe molecule because it interacts mainly with basic sites and is therefore suitable for characterizing basic properties of the oxide surfaces (e.g., refs 3-5). Basicity is one of the key elements for methane oxidative coupling. By FTIR spectroscopy it should be possible to clarify: (a) Which kind of adsorption states are formed on the catalysts? (b) Does the catalyst composition influence the desorption behavior of the adsorbed species? (c) Are there any correlations of IR data with surface basicity or catalytic activity?
(m2/g)
MgO 95% MgO/CaO 85% MgO/CaO 75% MgO/CaO 50% MgO/CaO 25% MgO/CaO
17.5
CaO
20.2 26.5 39.7 62.2 49.0 30.7
of C02
(µ
/m2) 4.31 5.32 4.93
6.93
9.99
signment of the bands was done by comparison with a wide range of literature data reported for metal oxides3"13 and metal carbonato complexes.14
Catalytic activities were determined by using a conventional fixed bed flow-through reactor which was operated under atmospheric pressure. The typical reaction conditions were as follows: temperature 1003-1103 K, W/F = 0.02-1 g cat. h mol"1, = 13-35 kPa, P0l = 1.4-5.7 kPa. PCh4 Specific surface areas were determined by BET method and C02 uptake was measured volumetrically.
Experimental Section Catalysts were prepared by coprecipitating the hydroxides from aqueous solution of the corresponding nitrates with sodium hydroxide. Details are described elsewhere.2 Pure MgO and pure CaO, and their mixed oxides (mostly 95 mol % MgO/CaO, 85 mol % MgO/CaO, 75 mol % MgO/CaO, 50 mol % MgO/CaO, and 25 mol % MgO/CaO) were used. For FTIR spectroscopic measurements a Perkin-Elmer 1600 Series FTIR instrument was used. The oxides were pressed into thin self-supporting wafers and calcined in the heating zone of a home-made IR cell at 950 6C for 45 min in flowing helium. After cooling to room temperature the samples were moved into the optical path and the spectra were recorded in helium stream by applying transmission absorption technique in the range 3000-900 cm'1. The samples were exposed to a C02 stream for
Results 1. Physical Properties of MgO, CaO, and Their Mixtures. Data of specific surface areas of the samples and adsorbed amount of C02 are listed in Table I. Specific surface area is highest for the 50% MgO/CaO sample, whereas the adsorbed amount of C02
per m2 increases with increasing CaO content. The latter indicates a rise in the total number of weak and strong basic sites. None of these properties correlate with catalytic activity directly. 2. IR Spectroscopic Results. 2.1. Adsorption of C02 at Room Temperature, (a) Pure Oxides. Calcined MgO shows one single band at 981 cm-1; calcined CaO shows an increase in absorbance at 1000 cm'1 (Figure 2), due to crystal lattice vibration.5,6 After C02 exposure on MgO strong bands appear at 1526 and 1419 cm"1 with a shoulder around 1658 cm"1. A weaker band at 1075 cm'1 and a very weak one at 1223 cm"1 are observed. No bands are found at wavenumbers higher than 1800 cm"1. The bands at 1526, 1419, and 1075 cm'1 are assigned to surface
min. Desorption experiments were carried out by heating the disks on which C02 was previously adsorbed at a fixed temperature for 10 min in helium stream. After cooling down to room temperature the corresponding spectrum was recorded and the procedure was repeated with continuously increasing desorption temperatures. Peak positions were obtained using the cursor functions of the instrument after expanding the section of the spectrum. As10
unidentate carbonate (antisymmetric stretching vibration of C^COj, symmetric stretching vibration of 0,C0] and stretching vibration of OnC, respectively3,5"7) (Figure 3). The shoulder band at 1658 cm"1 is most probably attributed to the antisymmetric stretching mode of OrCOn of a surface bicarbonate3,6,7 (Figure 3) which is formed by interaction of C02 with surface hydroxyl. Bicarbonate produces a symmetric , stretching band3,4,6 in
*On leave from the Central Institute of Physical Chemistry, Berlin, Germany.
0022-3654/92/2096-9035S03.00/0
composition
adsorbed amount
©
1992 American Chemical Society
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The Journal
of Physical Chemistry,
Vol. 96, No. 22, 1992
Philipp and Fujimoto
TABLE II: Wavenumbers of IR Bands (in cm'1) of Adsorbed C02 at Room Temperature and Their Assignment CaO-rich CaO MgO MgO-rich assignment 1075 1419 1526 1658 1223
1067-1072 1415-1418
1071
1069
1360-1550
1360-1560
1506 1639
1550 1630 1226 1781
1560 1630 1213 1776
around 2500
around 2554 and 2900
1220-1217
unidentate, OggC stretching unidentate, 0]CO] sym stretching + bicarbonate, OgCOn sym stretching unidentate, OgCOg asym stretching bicarbonate, OgCOgg asym stretching bicarbonate, COH bending “bridging” carbonate, CO„ stretching linearly adsorbed C02
,0.
V--Z
,o,
“I!
G
,o
?“
M unidentate carbonate
X
C
X
o,
/1
bidenlate carbonate
OH
11° 11II
c
X
1
I|
M
bicarbonate
MgO mol% in CaO
Figure 1. Catalytic activity of mixed MgO/CaO oxides for the oxidative coupling of methane at 750 °C, atmospheric pressure, CH4:02:N2 = 13:1.4:85.6, W/F = 1 g-cat. h mol'1.
carbonate ion
1° M
C
X i1 M
"bridging" carbonate
Figure 3. Surface carbonates, formed by adsorption of C02 on metal 5'7 oxide
2000
1800
1600
1400
1200
1000
Figure 4. FTIR spectra for 85% MgO/CaO and 50% MgO/CaO at temperature after calcination (—) and subsequent exposure to C02
room 2800
2400
1400 2000 1800 wavenumber/cm-’
1000
Figure 2. FTIR spectra for MgO and CaO at room temperature after calcination (—) and subsequent exposure to C02
the range 1410-1480 cm'1. In the present case this band is superimposed by the broad symmetric stretching mode of the unidentate complex. The weak band at 1223 cm'1 is attributed to the COH bending vibration of bicarbonate.3,4·6 Exposure Of CaO to C02at room temperature causes a broad structureless band between 1560 and 1360 cm"1 with a shoulder at 1630 cm"1 (Figure 2). A sharp band of lower intensity is found at 1069 an'1 ami still weaker bands appear at 1213 and 1776 cm"1, around 2554 cm"1, and around 2900 cm'1. The most probable assignments are the following: The main band between 1560 and 1360 cm'1 is formed by the symmetric and asymmetric OgCOg stretching band of unidentate carbonate and the symmetric OiCOn stretching band of bicarbonate. The 1069-cm"1 band is attributed to the OnC stretching vibration of unidentate and the bands at 1630 and 1213 cm"1 correspond to the asymmetric OiCOn stretching and the COH bending mode of bicarbonate. The band at 1776 cm'1 is tentatively assigned to the COh stretching mode of a “bridging” carbonate4,7,8 (Figure 3). The symmetric OiCOi stretching mode7 (1020-970 cm'1) is
(-----)
superimposed by metal oxide lattice vibrations. The bridging carbonate is reported to be formed on different Al203-containing surfaces4,8,9 but not yet reported for CaO.5,10 The bands around 2554 and 2900 cm"1 are most probably caused by linearly adsorbed C02, where the molecules interact via one oxygen atom with metal cations. Such species are well-known on A12034 and mixed Al203/Mg0 surfaces.3,11 The bands observed in these cases, however, have somewhat higher wavenumbers. The data show that the unidentate complex seems to be the dominant adsorption state on MgO and CaO. Bicarbonate is formed in minor amounts. On CaO bridging carbonate and linearly adsorbed C02 were found additionally. Assignments of all bands are summarized in Table II. (b) Mixed Oxides. Adsorption of C02 on MgO-rich oxides (MgO content >85%) gives spectra very similar to the corresponding curves for paire MgO. Unidentate is the main adsorption state, ami bicarbonate is present in minor amounts. Representative curves are shown for the 85% MgO/CaO catalyst in Figure 4. The band intensities are higher than on pure MgO, indicating a higher C02 uptake and an increase in the number of basic sites. The wavenumbers of all bands are shifted to slightly lower values.
The Journal
C02 Adsorption/Desorption on MgO/CaO
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1600
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1000
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It was already reported11·12 that the increase in temperature and dehydration weakens the basicity of surface oxygen on A1203 and MgAl204. This leads to transformation of unidentate into species where an additional bond between one oxygen atom of C02 and metal cation stabilizes the surface structure11,12 (bidentate or bridging carbonate, see Figure 3). Also in the present case a decreased basicity of surface oxygen may explain the observed features. After heating to 600 °C the broad band around 1470 cm'1 is drastically decreased and all bands at higher wavenumbers disappear. These facts correspond to a large desorption peak at 600 °C in the temperature-programmed desorption (TPD) curve2 of adsorbed C02. The remaining bands decrease continuously with further increase in temperature. The bands at 1542 and 1428 cm'1 are assigned to unidentate carbonate, whereas the band at 1304 cm"1 and the small shoulder band around 1622 cm'1 may be caused by traces of bidentate carbonate (COn stretching and asymmetric OjCO, stretching mode) for which the splitting of the asymmetric CO stretching mode of carbonate ion is reported5'7·10 to be ca. 300 cm"1. The coexistence of uni- and bidentate carbonate at higher temperatures and lower coverages was already found by Tanabe et al.5. It may be explained, as mentioned above, by a partial transformation of unidentate into bridging structures because of lowering the basic strength of surface oxygen. Discussion
Figure 5. FTIR spectra for 85% MgO/CaO and 50% MgO/CaO at room temperature after subsequent heating the sample, where C02 was previously adsorbed, at the temperatures, indicated on the curves (in eC).
Adsorption behavior of C02 on CaO-rich samples (MgO content
