NOTES Electron Paramagnetic Resonance Evidence for the Presence of Aluminum at Adsorption Sites on Decationated Zeolites
by Katherine M. Wang and Jack H. Lunsford Department of Chemistry, Texas A & M University, College Station, Texas 77843 (Received October 10, 1968)
The concept that trigonal aluminum functions as a Lewis acid site on alumina, silica-alumina, and zeolite surfaces has been proposed to explain a variety of catalytic results, yet, there is very little direct experimental evidence that aluminum is available at the surface for such reactions. A study of magnetic interactions between an adsorbed paramagnetic species and an aluminum nucleus is one technique which will demonstrate whether aluminum is part of an adsorption site. A recent investigation of K O adsorbed on decationated Y zeolites has shown that aluminum hyperfine structure was present in the electron paramagnetic resonance (epr) spectrum of the nitric oxide molecule.' The spectrum, however, was not well resolved because of the large number of closely spaced hyperfine lines. The superoxide ion, 02-,leads to a simpler spectrum than that of nitric oxide since l60has no nilclear magnetic moment in contrast to I 4 S where I = 1. In addition to detecting magnetic interactions, epr spectra of the superoxide ion and the nitric oxide molecule have been used to probe crystal field interactions on materials of catalytic interest.2-s This technique, which is extended in the present investigation, is based upon the principle that a low-symmetry crystal field will split the degenerate 2pn* orbitals of the paramagnetic molecule. The splitting of energy levels is rather strongly exhibited in the y tensor of the epr spectrum. Again, because of increased resolution, the superoxide ion is a better probe than the nitric oxide molecule, particularly where the principal values of the y tensor are nearly equal. Positive identification of the polycrystalline spectrum assigned to the 02- species has recently been made by Tench and Holroydl8 mho used 170isotope labeling to demonstrate that the spectrum on RlgO was indeed that of a diatomic ion. Iianzig and coworkers7 have also extended their work on 02in a single crystal of KCl.
Experimental Section The NH4Y zeolite was prepared by exchanging the sodium form of a Linde type-Y zeolite with ammonium ions from an NH4NOs solution. Approximately 90% of the sodium ions were exchanged. Chips of the dried KH4Y zeolite were placbd in a Vycor sample tube with a quartz side arm of 20-em length and 4-mm inner diam-
eter. The samples were degassed under vacuum Torr) to 600" with a heating interval of 2 hr/100". During the degassing process, the side arm was heated to complete the degassing procedure. After a total of 12 hr degassing, the samples were heated in 1 atm of Hz a t 600" for 1 hr and then evacuated. Upon cooling to room temperature, the chips were tapped into the side arm and the sample tube was filled with oxygen at pressures ranging from 1 to 400 Torr. The 0 2 was distilled onto the sample from the liquid state. The side arm was sealed with a torch, pulled off, and spectra of the samples were obtained prior to irradiation. The tube was then y irradiated at a dose rate of 413 R/min for 24 or 48 hr. After irradiation the samples were tapped into one end of the quartz tube and the other end was heated with a flame for 5 min t o remove color centers in the quartz. The samples were cooled in liquid nitrogen for at least 15 min before the epr spectrum was obtained a t 77°K. Zeolite samples after irradiation were sometimes transferred in a nitrogen atmosphere to another tube fitted with a stopcock so that the gas-phase O2 could be removed by evacuation or added back t o the sample. In one case the decationated zeolite was irradiated under vacuum without any O2present. A Varian spectrometer (Nodel 4502) with a TEm mode cavity was employed. A phosphorus-doped silicon standard was used to determine the spin concentration and y values from the 02-spectrum. The experimental spectra were analyzed by comparing the curves with theoretical spectra for polycrystalline material which are reported in the l i t e r a t ~ r e . ~ , ~
Results The spectrum shown in Figure l a was observed at 77°K when a decationated Y zeolite was irradiated in the presence of oxygen. After a sample had been transferred under nitrogen and evacuated, the spectrum was essentially the same as that observed in Figure la. The spectrum shown in Figure l b was observed at 77°K when the irradiated sample had been heated at temperatures ranging from 150 to 250" for 30 min to 24 hr. At temperatures above 300" the spectrum began uniformly to disappear. The spectrum in Figure l b can be resolved into three sets of six hyperfine lines. Each set of lines represents the hyperfine interaction along one of the three principal axes (5,y, and x ) which are defined by the orienta(1) J. H. Lunsford, J . Phys. Chem., 72, 4163 (1968). (2) J. H. Lunsford, ibid., 72, 2141 (1968). (3) J. H. Lunsford, J . Chem. Phys., 46, 4347 (1967). (4) J. H. Lunsford and J. P. Jayne, ibid., 44, 1487 (1966). (6) P. H. Kasai, ibid., 43, 3322 (1965). (6) A. J. Tench and P. Holroyd, Chem. Commun., 471 (1968). (7) H. R. Zeller and W. Kanzig, Helv. Phys. Acta, 40, 873 (1967). (8) M. C. R. Symons, Advan. Phys. Org. Chem., 1, 283 (1963). (9) P. W. Atkins and M. C. R. Symons, "The Structure of Inorganic Radicals," Elsevier Publishing Co., New York, N. Y., 1967, pp 268-272.
Volume 73, Number 6 June 1969
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ple a t room temperature. When 02 was not present in the sample tube, the spectrum of the y-irradiated decationated zeolite exhibited only a weak, single peak with a g value of 2.0017.
Discussion It appears that the three sets of six hyperfine lines
Igyy 3 2.003
Figure 1. (a) The spectrum of the 0 2 - ion on a decationated zeolite after y irradiation. (b) The spectrum after heating the same sample to temperatures from 150 to 250". Both spectra were recorded at 77°K.
tion of the molecule with respect to the external magnetic field. Here, x is chosen along the internuclear axis while x and y are along the pir functions. For each set the six lines are symmetrically disposed about the magnetic field which corresponds to that principal g value. Since g,, N g,,, two sets of hyperfine lines partially overlap, but the six lines for the x direction are well defined. The 02-spectrum is therefore characterized by gzz = 2.009 f 0.001, g,, = 2.002 A 0.001, and g,, = 2.038 A 0.001, with /u,,l = 4.7 k 0.5 G, lauu]= 5.7 0.5 G, and ( a z E=/ 6.5 k 0.5 G. A spin concentration of 10ls spins/g was obtained after the sample had been irradiated for 24 hr. Upon doubling the irradiation time the spin concentration almost doubled. The oxygen pressure during irradiation, however, had little effect on the spin concentration over the pressures from 1 to 400 Torr. Exposure of the sample after irradiation to 700 Torr of 0 2 appreciably reduced the amplitude of the spectrum because of spinspin interactions between the chemisorbed oxygen and physically adsorbed oxygen. The spectrum shown in Figure l b could be reproduced by evacuating the samThe Journal of Physical Chemistry
occur because of the interaction between an 27Al nucleus ( I = 5/2) and the unpaired electron of 02-. The isotropic nature of the hyperfine splitting (azzN uuv N azz,assuming the same sign) as well as its magnitude indicates that the splitting is mainly the result of a small aluminum s-orbital contribution to the wave function of the unpaired electron. Magnetic dipole interaction would have led to a smaller, anisotropic sp1itting.l The simple six-line sets confirm that the unpaired electron is interacting with only one 27Al nucleus in contrast to F-type centers where the unpaired electron often interacts with several neighboring atoms. These results furnish strong evidence that an aluminum atom is involved in the adsorption site for the 02-ion, yet, they do not require that the aluminum be adjacent to the superoxide ion. The hyperfine lines could not be seen clearly without heating the y-irradiated sample to 150" for a t least 30 miri because of line-broadening effects. The broadening may be due to random magnetic dipole interactions with other strongly adsorbed paramagnetic species which desorb upon heating. It is important to note that the principal g values did not change as the resolution increased. This means that the ion did not move from one site to a different one upon heating. The maximum concentration of superoxide ions formed was about a factor of 10 less than the number of paramagnetic NO species detected on the same material. The number of 02-spins still appeared to be a linear function of the radiation dosage over the range studied, which indicates that sites were still available and only required the necessary electron excitation energy for the formation of 02-. The g values of the 02-molecule ion are related to the crystal field splitting (A) of the 2pn* energy levels according to the following equations given by Kanzig and Cohen'o g,
g e ( G )
+ 2[X2/(X24- A2)]'/'Z
(IO) W. Kanzig and AM.H. Cohen, Phys. Rev. Letters, 3, 509 (1959).
NOTES where X is the spin-orbit coupling constant, and 1 is a correction to the angular momentum about x caused by the crystal field. For those cases where the g values have been very accurately determined the value of 1 has been shown to be quite close to unity. Assuming I = 1.00” and using the value of X = 0.014 eVJ4A can be calculated as 0.78 eV. This is the largest value of A yet observed,ll and it may be compared with the value of A = 0.60 eV for NO on the same zeolite material.’ It is interesting that the crystal field interaction with the weakly adsorbed NO molecule is essentially the same as the interaction with the more tightly bound 02- ion. When O2 was absent from the sample, the spectrum of the yirradiated decationated Y zeolite appeared to be a single weak line, whereas Stamires and Turkevich12 found a six-line spectrum which was ascribed to the interaction of trapped electrons with 27Alnuclei (X2 center). The difference in results can be attributed to the radiation dosages in the two experiments. As Stamires and Turkevich showed, the X2center required an induction dosage of lo6 R while the dosage for the sample without O2was only 0.6 X lo6 R.
Acknowledgment. This work was supported by National Science Foundation Grant GP-8319. (11) I n previous work4 values of A / & A/E, and 1 have been calculated explicitly by using the three principal g values and eq 1-3. A small error in g, and g,, however, will result in a sizable error in A/A, AIE, and 1. For the polycrystalline spectra of 02- on many surfaces, it appears that a better value of A/A can be obtained by taking 1 equal to unity and calculating A/A directly from eq 1. For this reason a better value of A for 02- on ZnO is probably 0.57 eV rather than 0.82 eV.2’4 (12) D. N. Stamires and J. Turkevich, J. Am. Chem. SOC.,86, 757 (1964).
On the Nature of the Products of y Radiolysis of Aerated Aqueous Solutions of Benzene
by T. K. K. Srinivasan, I. Balakrishnan, and M. P. Reddy Communication No. 1891 from the National Chemical Laboratory, Poona-8, India (Recehed November 4, 1068)
I n the y-radiolysis of aerated aqueous solutions of benzene, products other than phenol have been variously described as mucondialdehyde,l a compound of unknown identity which behaves like a phenol,2 and an acidU3 Some conclusions regarding this problem are submitted below. 1. The freshly irradiated solution (neutral a t the start) shows strong absorption maxima at 268 and 345 mp. Three ether extractions reduce exclusively the maximum a t 268 mp and the resulting aqueous layer has a spectrum which is virtually unaffected by subsequent
207 1 ether extraction and shows the p H dependence so meticulously described earlier.2 Bicarbonate washings of the ether layer show hardly any uv absorption and do not yield any appreciable amount of precipitate of hydrazone with dinitrophenylhydrazine reagent (DNPH). After subsequent alkali washing the ether layer shows only uv absorption characteristic of remnants of benzene. After acidification the alkali washings show the uv spectrum of phenol, with the 268-mp peak corresponding closely to the diminution in this peak of the original irradiated solution as a result of ether extraction. The solution couples with p-nitrobenzenediazonium chloride (PNDC) and the dye has an ir spectrum identical with that of the dye obtained with a solution of phenol. 2. The aqueous layer after ether extraction, with two strong nearly equal uv peaks at 268 and 345 mp a t neutral pH, upon acidification to p H