J. Phys. Chem. 1986, 90, 5883-5887
5883
Characterization of [Li'0-1 Centers in Lithium-Doped MgO Catalystst Ji-Xiang Wang and Jack H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: April 2, 1986)
A previous study of Li-promoted MgO catalysts, which are effective in the partial oxidation of methane, led to the conclusion that [Li'O-] centers are important in the formation of CH3' radicals. Because of their role in this catalytic process, the formation and thermal stability of these centers have been studied in more detail. The [Li+O-] centers are present in the Li/MgO powders at temperatures as low as 673 K. They have a positive heat of formation (AH= 3 kcal mol-'); thus, the concentration of the centers increases with increasing temperatures. Upon quenching the powder in liquid O2to 77 K, the [Li+O-] centers are frozen in and can be. detected by EPR spectroscopy. In addition to the [Li'O-] centers, the paramagnetic 02-and 03-ions also are present. The latter species is formed by the reaction of 0-with O2in the solid or on the surface, depending upon the treatment of the samples. The [Li'O-] centers also may be formed by irradiation of the Li/MgO powders at 77 K with 254-nm light. Both methods of preparation (quenching and irradiation) result in [Li'O-] centers which are thermally unstable at temperatures above 200 K. This indicates that the centers are formed at isolated substitutional Li' ions. By contrast, previous work by Abraham and co-workers has demonstrated that when Li-doped single crystals are quenched from higher temperatures the resulting [Li'O-] centers are stable up to 750 K. This remarkably greater stability has been attributed to the presence of substitutional Li' in clusters termed "microgalaxies".
Recent work on lithium-promoted magnesium oxide has demonstrated that this is an effective catalyst for the partial oxidation of The sites responsible for the activation of methane are believed to be [Li'O-] centers, which is consistent with the role of 0- in hydrogen atom abstraction from alkane^.^^^ During catalysis the surface-generated methyl radicals escape into the gas phase where they subsequently may be identified by using a matrix isolation techniqueS or they may combine to form ethane. Over 7 wt % Li-promoted MgO at 993 K in a conventional flow reactor, ethane and ethylene were obtained with a 50% combined selectivity a t 38% conversion of CH4.2 Since lithium and magnesium do not have variable oxidation states, this represents a new type of oxidation catalyst on which thermally generated 0- ions activate methane. The [Li'O-] center has been extensively studied in MgO and other group IIA (2) oxides by Abraham and co-workers.611 These centers were formed in single crystals of the oxides, either by high-energy irradiation or by quenching crystals which had been heated to high temperatures (-1500 K) in oxygen or air. The different modes of generation resulted in centers which were identical, except for remarkably different thermal stabilities. The centers formed by y-irradiation were unstable at >200 K, whereas the centers formed by quenching were stable up to 750 K. A model proposed by Abraham and co-workers, which adequately accounts for most of these unusual results, will be described in a subsequent section. The [Li+O-] center in MgO is characterized by gll = 2.0049, g, = 2.0545, All = +0.031 G, and A , = -2.40 G , as determined by EPR and ENDOR spectroscopies." These g values are consistent with those expected for an 0- ion (V-type center) having a unique symmetry axis. The four-line hyperfine structure reflects the weak interaction of the trapped hole with the 'Li nucleus ( I = 3/2). The center also gives rise to an optical absorption band at 1.83 eV (678 nm).lo Were the [Li+O-] centers only formed thermally at T > 1400 K, they would have little significance as sites for the activation of methane in the Li-promoted MgO catalysts; however, as previously demonstrated, the centers may be formed on powder samples at temperatures as low as 773 Unlike the thermally generated centers in the single crystals, the quenched [Li'O-] centers in powders are unstable at temperatures of 200 K. The In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the pblock elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the numbering: e.g., I11 3 and 13.) f
purpose of this paper is to explore further the generation and thermal stability of [Li'O-] centers in MgO powders relative to single crystals. In addition, the roles of molecular oxygen will be described in more detail.
Experimental Section The samples used in this study were prepared from high-purity MgO (Aldrich, 99.999%) and Li2C03 (Fisher, >99.0%). The standard preparation involved adding Li2C03to deionized water and stirring for 30 min. After adding MgO to the solution, the slurry was boiled for 1 h. The resulting material, in the form of a thick paste, was dried overnight in air at 413 K. Unless otherwise noted, the samples were heated to an elevated temperature under vacuum and subsequently quenched in a controlled atmosphere, using the apparatus depicted in Figure 1. The powder sample first was pressed into a thin wafer, then broken into 2-mm chips, and finally loaded into the holder (c). The thermally treated material consisted mainly of Li2C03and MgO phases.5 For quenching into liquid oxygen, 600 Torr of O2was added at 298 K and the bottom of the apparatus was cooled in liquid nitrogen (b), which, of course, liquified part of the O2in the vessel. After the sample was heated in the residual O2vapor for 1 h at the desired temperature, the ground-glass joint (d) was rotated and the sample fell into the liquid oxygen. Note that sufficient volume was provided to accommodate the oxygen which rapidly entered the gas phase. The remaining liquid was boiled off by removing the liquid nitrogen bath, stopcock f was closed, and the bottom of the apparatus was immersed in the liquid nitrogen bath immediately. The lower section of the apparatus was disconnected at g, and the sample was transferred rapidly to the EPR tube a, (1) Ito, T.; Lunsford, J. H. Nature (London) 1985, 314, 721. (2) Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J. Am. Chem. SOC. 1985, 107, 5062. (3) Aika, K.; Lunsford, J. H. J . Phys. Chem. 1977, 81, 1393. (4) Bohme, D. K.; Fehsenfeld, F. C. Can. J. Chem. 1969, 47, 2717. (5) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. J . Am. Chem. SOC.1985, 107, 58.
( 6 ) Olson, D. N.; Orera, V. M.; Chen, Y.; Abraham, M. M. Phys. Reu. B. 1980, 21, 1258. (7) Boldu, J. L.; Abraham, M. M.; Chen, Y. Phys. Reu. E 1979, 19, 4421. (8) Lacy, J. B.; Abraham, M. M.; Boldu, J. L.; Chen, Y.; Narayan, J.; Tohver, H. T. Phys. Rev. B 1978, 18, 4136. (9) Chen, Y.; Tohver, H. T.; Narayan, J.; Abraham, M. M. Phys. Reu. B 1977. 16. 5535.
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0022-3654/86/2090-5883$01.50/0
0 1986 American Chemical Society
5884 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
Wang and Lunsford
02
05
2070 [Li+O-lL 2.079 1 2.054
‘m
/!)
2.016 2.432
\A
cr3+
vacuum c
2.d02
MI?+
Figure 1. Quenching apparatus: (a) EPR tube, (b) liquid N2, (c) sample holder, (d) rotating joint, (e) thermocouple well, ( f ) stopcock, (g) ball joint, and (h) manometer.
which also was immersed in a liquid nitrogen bath. The transfer of the sample typically required about 1 or 2 s. After the transfer any residual O2 was removed by evacuation. As a variation of this procedure, a modification of the apparatus was employed in which a 4-mm EPR tube was attached directly to the bottom of the apparatus of Figure 1 . Thus, the brief warming of the sample during the transfer was avoided. It was found, however, that results were essentially the same, and the smaller EPR tube was favored because it was easier to transfer to the spectrometer. Spectra were recorded by using a Varian E-6S EPR spectrometer. The g values were determined relative to a phosphorus-doped silicon standard with g = 1.9987 or to the intrinsic Cr3+ impurity line with g = 1.9797. All spectra were obtained with the sample at 77 K. Spin concentrations were obtained by double integration of the unknown and the standard. The error in absolute concentration is estimated to be i 2 5 % , but for spectra having identical shape the relative error is estimated to be i 3 % since one may use pzaks heights for comparison purposes. Irradiation experiments were carried out by using a low-pressure mercury vapor lamp having an intensity of ca. 60 pW/cm* at 254 nm. The samples, which were contained in the EPR tube, were irradiated at 77 K.
Results Quenching us. Irradiation. In Figure 2 we compare the effects of slowly cooling a sample from 923 K, quenching from 923 K into liquid oxygen, or irradiating. All three samples contained ca. 7 wt % Li. The sample giving rise to spectrum a was heated in 192 Torr of O2 a t 923 K for 1 h and slowly cooled to 298 K. In order to obtain spectrum c, the same sample was irradiated in 15 Torr of O2 for 30 min. Spectrum b was obtained on a sample which was heated in 200 Torr of O2at 923 K for 1 h and quenched in liquid oxygen. Spectrum a of Figure 2 exhibits weak lines due to Mn4+lo and the polycrystalline spectrum of Fe3+ in Mg0.12 The narrow line at g = 2.0005 is due to a center in the fused-quartz Dewar. Upon irradiation the perpendicular component of the [Li+O-] center appears at g , = 2.054 (spectrum c). This spectrum, which resulted from irradiation of the sample for 0.5 h in 15 Torr of 02, corresponds to 9 X 10l6 [Li+O-] centers/g, whereas irradiation for 1 h in 120 Torr of O2resulted in 6 X 10’’ [Li+O-] centers/g. A much weaker line is present at g = 2.032 which is the region (12) Lunsford, J. H. J. Chem. Phys. 1965, 42, 2617.
Figure 2. EPR spectra of 7 wt % Li/MgO after heating in 192 Torr of O2 at 923 K for 1 h: (a) sample cooled slowly to 298 K, (b) sample quenched in liquid O2 at 77 K, and ( c ) sample (a) irradiated in 15 Torr of O2 for 30 min. g.=2.054
Figure 3. EPR spectrum of 7 wt % Li/MgO after being evacuated at 923 K for 1 h, heated in oxygen at 923 K for 1 h and in vacuo at 573 K for 1 h. and then irradiated in vacuo at 77 K for 1 h.
where the spectrum of “Li pairs” has been observed in Li-doped MgO single crystals.’JO The hyperfine lines, which are resolved in the spectra of the single crystals, are not resolved in this spectrum. The remaining dominant feature is the spectrum of the ozonide ion, O8-10 A) so that neither spin exchange nor dipolar broadening occurs. The substitutional Li+ ions which form the microgalaxies are derived from LizO precipitates which are present in the Li-doped MgO. The migration of oxygen to achieve reaction 3 is surprising in the single crystals as the diffusion coefficient of oxygen ions at 13o(t-1600 K is too small to account for the process. To explain this phenomenon Abraham and co-workers invoke Schottky defects, whereby Mg2+ions migrate to the surface and combine with oxygen to form new MgO monolayers. The present results on Li-doped MgO powders differ from the single-crystal results in three significant ways: (1) In the powder samples the [Li+O-] centers were apparent after quenching from much lower temperatures; (2) other paramagnetic oxygen species were detected; and (3) as mentioned previously, the [Li+O-] centers were much less stable. The differences in the temperature of formation and thermal stability may, in part, result from the inability to quench the large single crystals at a sufficiently rapid rate. In fact, most of the quenching studies carried out by the Oak Ridge group were only to 298 K, although in earlier work quenching was carried out by cooling the crystals to 77 K.l0 If quenching to
