J. Phys. Chem. 1989, 93, 6581-6582 functions, and convenient tables from ref 19 were used to evaluate the resultant rovibrational symmetry of each particular state (now including all the vibrational quantum numbers as well as J and K ) . In figure 3, results are presented for N,(E) with J = 5 (panel a ) and with J = 20 (panel b). Clearly, there are differences depending on the J value: at low values of J, the A2 and B, states occur more often than the A, and B2. However for J = 20, the numbers of states for the species are almost the same. This fact can be seen more clearly in Figure 4, where the ratio of N A I ( E ) / N A 2 ( E is ) depicted as a function of energy for the two values of J. For J = 5 this ratio is close to 0.833, and for J = 20 it is close to 1.05. These ratios are predicted by Quack? who concluded that for J > 0 the ratio W(E,J,A+)/W(E,J,A-) is equal to [ ( J l)//(-I)’ (his notation; in our notation, A, = A+ and A2 = A-). The present results show that convergence as a function of energy to the limiting values is much faster than for the J = 0 case, probably due to the much higher density of states, which makes the classical approximation valid even at low energies. Similar results for C H 2 0 at J = 5 give densities of states that are almost equal for the four symmetry species.
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4. Conclusions In this work we showed that, by making a simple modification of a semiclassical Monte Carlo method, symmetry-specificdensities of states can be calculated for nonseparable degrees of freedom, with relatively small additional computational effort. This is related to the fact that, in the process of evaluation of the density of states according to the method of ref 1, all the quantum numbers are specified in each trial, making the symmetry evaluation relatively simple. The technique is general and can be applied to any molecule. These calculations show that is possible determine symmetry-specific densities of states for more realistic systems (including intermode and rovibrational coupling terms) and that the simple assumption of a regular representation may be adequate at high densities of states, but significant departures can occur when the state densities are sparse.
Acknowledgment. This work was funded, in part, by the US. Department of Energy, Office of Basic Energy Sciences, and B.M.T. thanks CONICET of Argentina for a postdoctoral fellowship.
The Structure of Sulfate Species on Magnesium Oxide M. Bensitel, M. Waqif, 0. Saw, and J. C. Lavalley* URA CNRS “Catalyse et Spectrochimie”, ISMRa, Universiti de Caen, 14032 Caen Cedex, France (Received: June 1 , 1989)
The nature of the sulfate species formed by SO2oxidation on two MgO samples with very different surface areas has been studied by use of infrared spectroscopy. On the low area sample, bulklike sulfate (SO,”) was the major product whereas both bulk and surface sulfates were formed on the high area sample. Exchange using H2I80showed that the surface sulfate had a terminal trigonal O=S=O group, a structure which is different from that previously found for sulfated A1203,Ti02, and Zr02. The unique formation of bulk sulfates on MgO among these four oxides can partially explain why this material is used as an SO, transfer catalyst.
Introduction
Experimental Section
The structure of sulfate species formed on the surface of metal oxides by oxidation of H2S or SO2 or by impregnation with (NH4)2S04solutions has been recently discussed. Tanabe et al.’J proposed a chelating organic sulfate structure with two covalent SO double bonds for the species adsorbed on the surface of FeZO3, Ti02, or Zr02. However, using I60 l80 exchange experiments with H2I80,we propose a structure with only one SO double bond for the SO, species at the surface of A120s, Ti02,3 or Zr02.4 Among the numerous basic solids used as SO, adsorbents in the fluid catalytic cracking units, some are based on MgO derivative~.~-’ Accordingly we have studied the sulfation of magnesium oxide using infrared spectroscopy. Two samples having different surface areas were used in order to differentiate between surface and bulk sulfates. Very sharp bands arise from surface sulfates after evacuation at high temperature, and isotopic exchange has allowed us to determine the structure of this surface species.
MgO was obtained either by thermal decomposition (820 K) of a commercial hydroxide Mg(OH)2 from Merck or by burning a magnesium ribbon in air (MgO smoke). Their respective BET surface areas after heat treatment at 820 K were 290 and 10 m2 g-l. Sulfation has been performed on the samples (pressed into disks of about 10 mg cm-2) activated at 723 K by heating at 723 K, in the IR cell, different amounts of SO2 in a large excess of oxygen during 14 h. Spectra were recorded at room temperature with a Nicolet MX-1 Fourier spectrometer, allowing the subtraction of the absorbance of the oxide (difference spectra).
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(1) Yamaguchi, T.; Jin, T.; Tanabe, K. J . Phys. Chem. 1986, 90, 3148. (2) Yamaguchi, T.; Jin, Y.; Ishida, T.; Tanabe, K. Mater. Chem. Phys. 1987, 17, 3. ( 3 ) Saur, 0.; Bensitel, M.; Saad, A. B. M.; Lavalley, J. C.; Tripp, C. P.; Morrow, B. A. J. Catal. 1986, 99, 104. (4) Bensitel, M.; Saur, 0.;Lavalley, J. C.; Morrow, B. A. Mater. Chem. Phys. 1988, 19, 147. ( 5 ) Van Houte, G. Ed. Off. Int. Librarie, Bruxelles 1973. ( 6 ) Carpentier, P., Theses, Paris VI, 1987. (7) Bertolacini, R. G.; Hirschberg, E.H. M.; Modica, F. J. US 4381991 Patent, Standard Oil.
0022-3654/89/2093-658 1$01.50/0
Results
Figure 1 shows the spectrum of the species obtained by heating MgO smoke with 200 pmol g-’ SO2and ca. 2 mmol of oxygen at 723 K followed by evacuation at 723 K. Well-defined bands occur at 1260, 1170, 1100, 1025, and 1015 cm-I. They are not sensitive to the addition of water vapor at room temperature, and their intensity is practically not modified by heating under vacuum up to 923 K. At higher temperature all peaks decrease in intensity in unison and disappear at 1023 K. The same experiment performed on the ex-hydroxide sample leads to a very different spectrum (Figure 2a). After evacuation at 723 K, rather broad bands appear at 1385, 1220, 1175, 1045, and 975 cm-I. They shift toward higher wavenumbers when the sample is evacuated at higher temperature and become quite sharp at 923 K (Figure 2b). These modifications can be related to the hydroxylation state of the surface since, after addition of H 2 0 a t room temperature to the sample heat treated at 923 K, evac0 1989 American Chemical Society
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The Journal of Physical Chemistry, Vol. 93, No. 18, 1989
Letters TABLE I: Isotopic Effects on the Wavenumbers of the Bands Due to the Trigonal O=S=O Croups; Comparison between Results Relative to Superficial MgO Sulfate Species and to Sulfate Monoesters" Va(SO2)
isotope
9 ' 6 0
S+1e0
$:I
1500
p
VS(S02)
MgO
ref 11
MgO
ref 11
1422
1414
1231
1201
1410 1390
1401 1392
1221 1210
1 I83 1172
1370
1378
1191
1157
S+M0
,
,
,
1300
1'100'
$00
WRVENUMBERS Figure 1. IR spectrum of species formed by oxidation at 723 K of 200 pmol g-I SO2 in a large excess of O2 on MgO smoke.
of SO2introduced. The wavenumbers of the IR bands due to the species formed on the MgO smoke or appearing on the exhydroxide sample using a large amount of SO2 are very similar to those reported in literature for anhydrous MgSOd8with the SO-: anion in a C , symmetry site. Such species can be considered as bulklike or subsurface species in agreement with their insensitivity to the adsorption of water. However, their low-temperature stability compared to that of pure MgS02*10is an unexpected result which could be related to the small amount of sulfate formed and/or its dispersion in the oxide. The bands 1422 and 1231 cm-I due to the surface SO -: species observed (Figure 2b) shift and split when I60 l80exchange occurs. The result obtained (Figure 2c,d and Table I) can be compared to the recent work by Lowe et al.," who studied the absolute conformation of the organic sulfate monoesters. The infrared spectra of such compounds show two bands at 1414 and 1201 cm-' assigned to the antisymmetric and symmetric vibrations of the group
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0 0
S
O
The presence in the spectrum, for partial oxygen-18 exchange, of more than two bands in the 1420-1370- and 1230-1200-cm-' ranges is in agreement with such an interpretation and is due to the SI6O2,S160180,and SI8O2groups. Moreover the splitting of the bands due to the S 1 6 0 1 8 0group is in favor of two S=O nonequivalent bonds. Taking into account the results of Lowe et al." and of our L--rwexperimental work, we propose a structure of the type 1400
1200
1000
800
WAVENUMBERS
Figure 2. IR spectra of species formed by oxidation at 723 K of 200 pmol g-I SO2 in a large excess of O2 on MgO ex-hydroxide (i) after final evacuation at 723 (a) and 923 K (b); (ii) after the first (c) and the third (d) exchange with H 2 ' * 0 at 723 K, followed by evacuation at 923 K.
uation at 723 K leads to a spectrum analogous to Figure 2a. Oxidation of a larger amount of SO2 (for instance 400 pmol g-I) on the MgO smoke slightly increases the absorbance of the bands observed in Figure 1 whereas on the MgO ex-hydroxide, new bands develop in addition to those previously noted in Figure 2a, in particular at 1175 cm-l. This wavenumber is very close to that of the intense band observed on the MgO smoke. To study the structure of the species which are present after oxidation of 200 pnol g-' SO2 on the MgO ex-hydroxide sample, exchange experiments with H2I80have been performed. After each exchange at 723 K, the sample was evacuated at 923 K before recording the spectrum. The spectra are reported in Figure 2c,d. All bands are shifted or split and their wavenumbers are reported in Table I. The slight decrease of their intensity is due to a partial decomposition arising above 873 K as shown by vacuum microbalance experiments. Nevertheless retroexchange with H2% leads back to bands similar to those observed in the original spectrum showing that the bands obtained by isotopic exchange are not due to decomposition products.
Discussion Results clearly show that two kinds of sulfate species are formed, depending on the surface area of the MgO sample and the amount
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-0NS*o for the sulfate species on the MgO surface and we assign the multiple bands observed in the spectra of the partially exchanged species to two nonequivalent S=O bonds (Table I). In our previous studies relative to sulfation of A1203,Ti02, or Zr02?s4 the spectra of surface sulfate species were quite different: only one v(S=O) band was observed, it was situated near 1380 cm-', and it gave rise to only one additional band near 1340 cm-I I8O exchange whatever the degree of exchange. upon I6O Evidently the structure of surface sulfate species depends on the nature of the metal oxide. We also previously found that the amount of sulfate species formed on alumina by oxidation of SO2 is limited to about 2.2 Kmol m-2 because only surface species are formed.I2 There is no corresponding limit in the case of MgO due to the formation of bulklike or subsurface species. This could partly explain the use of this oxide in the fluid catalytic cracking units as SO, transfer catalyst.
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(8) Hezel, A.; Ross, R. D.Spectrochim. Acta 1966, 22A, 1949. (9) Lowell, P.S.; Schwitzgebel, K.; Parsons, T. B.;Sladek, K. J. I d . Eng. Chem. Process Des. Deo. 1971, 10, 384. (10) Mu, J.; Pelmutter, D. Ind. Eng. Chem. Process Des. Deu. 1981, 20, 640. ( 1 1 ) Lowe, G.;Paratt, M. J. J . Chem. SOC.,Chem. Commun. 1985, 1073
and 1075. (12) Saussey, H.; Vallet, A,; Lavalley, J. C. Muter. Chem. Phys. 1983, 9, 457.
