1739
J . Phys. Chem. 1993,97, 1739-1742
A Novel Li Ionic Cluster at the Surface of MgO Damien Murphy, Elio Giamello,. and Adriano Zecchina Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universitb di Torino, Via Pietro Giuria 9, 10125 Torino, Italy Received: November 17, 1992; I n Final Form: January 15, 1993
A novel paramagnetic ionic cluster containing three lithium ions (likely Li32+)has been obtained upon treatment of the dehydrated MgO surface with vapors of lithium. The corresponding EPR spectrum is characterized by g = 2.0011 and a hyperfine splitting of 8.26 G (1 G = 0.1 mT).
The ability of zeolites to stabilize small charged metal clusters has been known for many years, starting from the pioneering work of Kasai et a1.l4 on the formation of Na43+46 centers in Y zeolite. Since that time, many different kindsofcationicclusters stabilized in X, Y,and A zeolites have been widely reported including Na43+,46 Na54+,7Na65+,2*7 b3+,4 K32+,8-10Agsn+,II Ag,P*gAg+,I and Ag32+.l In additionto the variety of cationicspecies produced, a similar variety of generation procedures have also developed, including exposure of the zeolite to vapors of the metal,l” metal azide decompositi~n,’~ 7-irradiation of the alkali metal loaded zeolite,l3 and treatment of the sample with solutions of the alkali metal in liquid ammonia.15 However, the utilization of other support materials for cluster stabilization, particularly nonzeolitic materials, has been less investigated. In the present contribution we report on the formation and stabilization of a paramagnetic cationic species of lithium on the surface of thermally activated MgO obtained by exposure of the sample to lithium vapors. This is, to the best of our knowledge,the first observation of such a cluster on the surface of a nonzeolitic ionic oxide. Other paramagnetic species, such as surface point defectsI6(F centers) and small metallic particles, are also formed when Li vapors interact with the oxide support, as confirmed by EPR spectroscopy. High surface area MgO was prepared by thermal decomposition of the hydroxide at 523 K under vacuum. The oxide was then activated by thermovacuum treatment at 1073 K for 1 h. After the oxide was cooled to room temperature, a small piece of lithium metal was brought into contact with the sample. Details on the experimental procedure for Li vapor production and sample preparation (hereafter indicated as Li/MgO) are given in ref 16. EPR spectra were recorded at X-band frequencies on a Varian E-109 spectrometer equipped with a dual cavity and coupled to a CS-EPR data station from Stelar (Italy) for spectra recording and elaboration. Varian pitch (g = 2.0028) was used for gvalue calibration. Upon contact with lithium vapors the microcrystallineMgO attains an intense blue color and simultaneously exhibits an EPR spectrum which is shown, recorded at three different microwave powers, in Figure 1. The spectra in Figure 1 are due (as it will be shown in the following) to the superimposition of the signals of three different paramagnetic entities (whose spin Hamiltonian parametersare listed in Table I), which are (a) surface F,+ centers, (b) small metallic particles, and (c) Li32+cationic species. Each entity is best observed at one particular microwave power. The features of the three distinct paramagnetic centers are briefly described in the following. (a) Surface F,+ Centers. Surface F,+ centers on MgO are usually thought of as an electron trapped in an oxygen vacancy at the cubic planar face of the oxide (rock-salt type structure) which can be described as a cage constituted by ( 5 ) positive ions in a square-pyramidal arrangement. As previously reported by
V
Figure 1. Room temperature EPR spectra of Li/MgO recorded at powers of (a) 0.1 mW, (b) 10 mW, and (c) 50 mW.
TABLE I: Spin Hamiltonian Parameters for tbe Three Paramagnetic Species Observed 011 LJ/MgO F,+ center Li metal particle Li?+
81= 2.0014 gi =2.0003 2.0013 2.001 1
~0.95 nl.1
4.5
1
8.26
some of us,16 these centers can be generated by exposure of the thoroughly outgassed MgO powder to vapors of low ionization energy metals, such as magnesium and lithium. The EPR signal due to the center can be easily recognized on the basis of the g values, of the axial nature of the spectrum (which is typical of the monoelectronic point defects on the surface of the alkaline earth ~ x i d e s ) , ’ of ~ J the ~ narrow line width, and of the ease of saturation. Due to this latter property, the center is best observed at low microwave power levels (Figure la). Thesaturationcurve of the F,+ centers on Li/MgO (measured on the basis of the height of peak A in Figure 1) is reported in Figure 2a as line A.
0022-3654/93/2097-1739$04.00/0 0 1993 AmZrican Chemical Society
Letters
1740 The Journal of Physical Chemistry, Vol. 97, No. 9, 1993
Peak Height
Peak Height
1.2- 1
1.2
I
lb
0.6
0.4
0.2
0 0
2
4
6
8
1
0
1
2
1
4
2
0
4
P ”‘I mW1” -3 Line A
6
8
1
0
1
2
1
4
P “‘I mw’”
+- F centre on MelMgO
+Line B
Peak Height 1.2
C
0
2
4
8
6
10
12
$12, mw1/2
* Peak C
-8 Peak
D
4- Peak E
Figure 2. Peak height (in arbitray units) versus the square of the microwave power (P1/2/mW112)for the three paramagnetic species observed on the Li/MgO system. The peak heights as representative of each paramagnetic species were chosen from Figure 1 as follows: line A, Fst centers (a); line B, Li metal particles (b); peaks C-E,Li charged cluster (c). The saturation trend of a “pure” F,+ center on Mg-treated MgO sample (Mg/MgO) is shown in (a) for comparison. No accurate data can be obtained for line A after 10 mW power levels due to interference from Li metal particles.
Despite a littleand unavoidable interferencewith theother signals present in the spectrum, the curve is similar to that observed for “pure” F,+ centers on MgO which are generated by neutron or y-irradiation17 or, alternatively, by exposure to Mg vapors.I”’* (b) Small Lithium Particles. For microwave powers higher than 20-30 mW a symmetric structureless line (labeled B in Figure 1) dominates the spectrum. As seen in Figure 2b, no saturation over 50 mW was observed for this line. In addition, the gvalue of the signal (g = 2.0013) and the line width (AH/G = 4.5 f 1 G)did not change drastically between 300 and 77 K. These results areconsistentwith conductionelectron paramagnetic resonance of very small metallic particles.sJ9 Xu and Kevan9-20have recently reported on the formation of alkali metal particles in alkali metal cation exchanged X zeolites. For the lithium metal particles a g value of 2.0034 and AH/G = 3.0 f 0.2 G were reported, indicating the presence of particles with diameters smaller than 10 Both the g values and the line width observed in the present case deviate slightly from the values previously reported for alkali metal species in This deviation is likely due to the larger particle size distribution on the MgO surface (resulting in the lowering of the gvalue and
an increase in the line width) because of the lack of any threedimensional cage structure which could specifically limit the size of the particle formed (as in the X (c) L i P Centers. At moderate powers (5-10 mW, Figure 1b), where the interference of both the F,+ centers and the metal particles is minimal, a 10-linemultiplet can be easily distinguished. The multiplet (moreclearly seen in Figure 3a,b which reports the first- and second-derivative spectra of Figure lb) has an approximate intensity ratio of 1:3:6: 10:12:12:10:6:3:1, which correctly fits the binominal distribution for an unpaired electron interacting with three quilvalent nuclei with a nuclear spin (I) = 3 / 2 and a hyperfine separation of 8.26 G. Since lithium is the only nucleus present having I = 3/2 (’Li: I = 3/2, 92.58%), we can confidently assume that the electron responsible for the 10line multiplet is interacting with three equivalent lithium nuclei. The spectrum shape closely resembles that recently obtained by reaction of potassium vapor (via decomposition of the metal azide) with dehydrated X, Y, and A zeolites as reported by Anderson et a1.* and Kevan et al.”J and independently assigned to K32+clusters. Previously reported EPR spectra on the same Li/MgO system by some of usI6 exhibited, in comparison to the
Letters
Figure3. (a) Second-derivativeand (b) third-derivativespectraof Figure lb.
present data, a weaker and less resolved hyperfine structure. This was assigned to the interaction of the unpaired electron trapped in an anionic vacancy (F,+ centers) with one or more Li+ ions resulting from the metalvapor ionization. These ions were thought to constitute the "walls" of those particular positive cages formed with the ions deriving from the vaporized metal (and indicated as "reconstructedsites"). The improved resolution of the spectra reported here allows us to discard this previous hypothesis for the following reasons: (i) The observed lithium hyperfine coupling constant is indeed in the range of those hyperfinevalues reported for bulk F+centers which areelectrons trapped by six equivalentcations in octahedral symmetry in lithium halides (e.g., 13.95 G in LiF and 6.82 G in LiCl).2' However, an ideal surface F,+ center, emerging at a planar surface of Mg0,I6 does not have the same octahedral environment of a bulk F center, but a square-pyramidal C4" symmetry surrounded by five cations, four of which areequivalent (equatorial cations). Tench and Nelson22 conclusively demonstrated that the paramagnetic electron of the surface F,+ centers on MgO interacts most strongly with the axial cation (having about 8% Mg 2s character) and only to a minor extent with the four equatorial cations (Le., less than 1%). In other words the unpaired electron of the surface F,+ center is mostly localized at the bottom of the potential well or anionic vacancy. Since the Li+ cations of the 'reconstructed sites" can be located exclusively at the equatorial positions, the expected hyperfine constant for such a center should be about 0.7-1 G, Le., by far lower than that observed in the present case (8.26 G). (ii) When we consider the hyperfine interaction between the point defects (F,+)and neighboring Li+ cations, we notice that the probability of the presence of quartets and septets with the related intensity patterns (related to an F,+ center interacting with one surface Li+ ion or two equivalent Li+ ions, respectively) should be higher than the probability of the presence of a 10-line multiplet (related to the less probable case of three equivalent Li+ cations interacting with the F,+ center). This is clearly not the case in the present experiment. (iii) As shown in Figure 2c, the 10-line multiplet has a significantly different saturation behavior compared to the F,+ centers (Figure 2a) and so cannot be associated to a hyperfine interactionof an F,+ center. In fact, thesaturation trenddisplayed by the multiplet resemblesthe general behavior recently described by Kevan and Xu9for K32+and Na6S+cationic clusters in zeolites.
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1741
A second possible candidate to account for the observed multiplet is a neutral Li3 cluster. However, the existence of a neutral species on the highly ionic surface of MgO seems rather unlikely. The ionization of a metal atom is actually endothermic in the gas phase, but the process becomes favorable and spontaneousonthesurfaceof an ionic solid. The surface ionization is facilitated in fact by the overall gain in the Madelung energy following the stabilization of the released electrons and resulting cations on the surface.16 Furthermore, Lis molecules have been prepared by cryogenic matrix isolation techniques in solid argon at 4 K (g = 2.0026) and in a hydrocarbon matrix at 77 K (adamantane, g = 2.OOl).23 The EPR spectrum obtained in both cases was typical of a system with three equivalent Li nuclei, but with hyperfine splittings of 32.2 G (in Ar matrix) and 33.1 G (in the hydrocarbon matrix), i.e., about 4 times higher than those found in our experiments, The hypothesis that a neutral Li3 molecule is responsible for the 10-line spectrum must therefore be ruled out. The only remaining candidate for the assignment of the trimeric Li species observed on the Li/MgO system is consequently the charged metal cluster List+. This assignment is in agreement with both the 10-line structure of the multiplet and its saturation behavior (Figure 2c) which is the same as that recently observed for the potassium analogue, K32+.9 This proposed assignment however does not easily account for the magnitude of the observed hyperfine coupling constant. The percentage of atomic character in the trimeric Li species (Le., the ratio between the experimental hyperfine constant and that of an isolated 7Liatom which is 143.36 G)25is in fact only 17.3%over the three Li+ cations. This should in principle be due either to a substantial s-p hybridization contribution, similar to what is observed for Li324and Li7,25or, alternatively, to extensive electron delocalization, similar to what is observed for K32+in X and A zeolites? In the former hypothesis,if onecalculates the anisotropic hyperfine term due to an electron in a Li 2p orbital (based on the fine structure splitting of the Li atom at the 2Pstate), one obtains A,, = +2 G and Al = -1 G. Thus, if the unpaired electron density unaccounted for by the Li 2s orbitals is distributed evenly between the three Li 2p orbitals (hence 28% each), the expected hyperfine anisotropy would be less than 1 G and so not detectable in the present case with Ai, = 8.26 G. On the basis of the present data, we cannot therefore rule out the possibility of s-p hybridization and consequentanisotropy in thecluster. However, the generally accepted model of the small cationic clusters is as excess electron traps with substantial delocalization of the electron into the space between the ions,*so that the low atomic character in Lij2+may indeed be due to extensive electron delocalization. We therefore tentatively suggest that the charged trimeric cluster, stabilized at the surface of MgO, is characterized by a poor interaction between the unpaired electron and the three Li+nuclei. This behavior is not surprising when consideration is given to the percentage of atomic or s character of other charged alkali metal clusters in various matrices. (For Kg3+ and K32+ a percentage atomic character of 80%and 47% has been reported, whereas for Na6J+ and Na43+ the percentage of atomic character is only 45% and 40%) As it can be noticed the percentage atomic character seems to decrease with decreasing atomic number of the alkali atom (as well as with decreasing number of cations in the cluster). Unfortunately,to our knowledge no data for Li32+ in zeolitic or other matrix are available for comparison. An alternative possibility is that theobserved spectrum belongs to a paramagnetic entity which also incorporates one Mg2+ion, i.e., (Li3Mg)4+. Following this hypothesis the Mg ion in the system should exert an electrostatic interaction toward the unpaired electron higher than that of a Li+ ion. Moreover, the magnesium ion (24Mg having Z = 0 and a natural abundance of 89.87%) would not in practice influence the hyperfine structure
'
1742 The Journal of Physical Chemistry, Vol. 97, No. 9, 1993
and so would account for the relatively low Li atomic character of the unpaired electron. In addition, the equivalence of the three Li+ ions suggests a highly symmetric arrangement of the four cations, for instance in tetrahedral geometry. In conclusion, the 10-line hyperfine multiplet, observed upon interaction of the Li metal vapors with the surface of MgO, can be assigned to an ionic cluster involving three lithium ions. The cluster is similar to other charged metal clusters reported in the literature, in the sense that the unpaired electron is scarcely bound to the surrounding ions, and so the species can be alternatively described as an ionic cluster or a free electron solvated by metal ions. Whether the stoichiometryof the species is better described by Li3*+(analogous to Ka2+observed in zeolites) or by (Li3Mg)4+ (Le., an electron in a tetrahedral array of cations analogous to the tetrahedral K43+ and Na43+clusters observed in zeolites) is not possible todecideon the basisofthe present data. It is however worth mentioning that in the present case the Li ionic cluster is stabilized on the surface of an ionic solid without the agency of the threedimensional cavity structure typical of zeolites: this latter property of zeolites appears to stronglyfavor the formation of this kind of species.
Refereacer rad Notes (1) Kasai, P. H. J . Chem. Phys. 1%5,43,3322. (2) Rabo, J. A.; Angell, C. J.; Kasai, P. H.; Schomaker, V. Discuss. Furuduy Soc. 1966,41,328. (3) Kasai, P. H.; Bishop, Jr., R. J. J . Phys. Chem. 1973, 77,2308. (4) Edwards, P.P.; Harrison, M. R.; Klinowski, J.; Ramdas, S.;Thomas, J. H.; Johnson, D. C. J . Chem. Soc., Chem. Commun. 1984, 982.
Letters ( 5 ) Anderson, M. R.; Edwards, P. P.; Klinowski, J.; Thomas, J. M.; Johnson, D. C.; Page, C. J. J . Solid Stute Chem. 1984, 508, 165. (6) Breuer, R. E. H.; de Boer, E.;Geismar, G.Zeolites 1989, 9, 337. (7) Anderson, P. A.; Edwards, P. P. J. Chem. Soc., Chem. Commun. 1991, 915. ( 8 ) Anderson, P. A.; Singer, R. J.; Edwards, P. P. J . Chem. Soc.,Chem. Commun. 1991,914. (9) Xu, B.; Kevan, L. J . Chem. Soc., Furuduy Trans I 1991. 87 (17) 3843. (10) Xu, B.; Kevan, L. J. Chem. Soc., Furuduy Truns I 1991, 87 (19) 3157. (11) Michalik, J.; Kevan. L. J. Am. Chem. Soc. 1986, 108, 4247. (12) Morton, J. R.; Preston, K. F.; Sayari, A.; Tse, T. S . J . Phys. Chem. 1987, 91, 2177. (13) Van der Pol, A.; Reijersc, E.J.; de Boer, E.; Wasowicz, T.;Michalik, J. Mol. Phys. 1992, 75, 37. (14) Martens, L. R.; Grobet, P. J.; Jacobs, P. A. Nature 1985,315,568. (15) Bordiga, S.;Ferrero, A.; Giamello, E.;Spoto, G.; Ztcchina, A. Cutul. Lett. 1991, 8, 375. (16) Giamello, E.;Ferrero, A,; Coluccia, S.;Zecchina, A. J . Phys. Chem. 1991, 95, 9385. (17) Nelson, R. L; Tench, A. J.; Harmswworth. B. J. Truns Furuduy Soc. 1967, 63, 1427. (1 8) Giamello, E.; Murphy, D. Unpublished data. (19) Blazey, K. W.; Muller, K. A.; Blatter, F.; Schunacker. E . Europhys Lett. 1987, 4, 857. (20) Xu. B.; Kevan, L. J . Phys. Chem. 1992, 96, 2642. (21) Holton, W. C.; Blum, H. Phys. Reo. 1962, 125, 89. (22) Tench, A. J.; Nelson, R. L. J. ColloidInterfoce Sci. 1968,26, 364. (23) Howard, J. A,; Sutcliffe, R.; Mile, B. Chem. Phys. Lett. 1984, 114, 84. (24) Weltner, Jr., W.; Van Zee. R. J. In Physics und Chemistry of Smull Clusters; Jena, P., Rao, B. K., Khanna, S.N. Eds.; Plenum Press: New York, 1987; pp 353-367. (25) Garland, D. A.; Lindsay, D. M. J. Chem. Phys. 1984,80,4761.
