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L. E. Iton and J.
The Journal of Physical Chemistty, Vol. 82, No. 2, 1978
Turkevich
Trapping and Stabilization of Hydrogen Atoms in Intracrystalline Voids. Defected Calcium Fluorides and Y Zeolite Surfaced Lennox E. Iton” and John Purkevich Department of Chemistry, Princeton University, Princeton, New Jersey 08540 and Solid State Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received July 7, 1977) Publication costs assisted by Argonne National Laboratory and the U.S.Energy Research and Development Administration
Using EPR spectroscopy, it has been established that Ha atoms are absorbed from the gas phase when CaFz powder is exposed to H2 gas in which a microwave discharge is sustained, being trapped in sites that provide unusual thermal stability. The disposition of the trapped atoms is determined by the occluded water content of the CaF2. For ultrapure CaF2,atoms are trapped in interstitial sites (HP) having A. = 1463 MHz; for increasing water content, two types of trapped He atoms are discriminated, with preferential trapping in void sites (external to the regular fluorite lattice) that are associated with the H20 impurity. Characterization of these “extra-lattice” H- (and D.) atoms is presented, and their EPR parameters (material dependent; g 2.0022, A. = 1413 MHz) and behavior are discussed in detail. Failure to effect Hs-D. atom exchange with D, gas suggests that atoms m atoms are trapped exclusively in “extra-lattice” sites when the are not stabilized on the CaF, surface. H water-containing CaFz is y irradiated at 77 or 298 K indicating that the scission product atoms do not escape from the precursor void region into the regular lattice. It is concluded that the thermal stability of the “extra-lattice” atoms, like that of the interstitial qtoms, is determined ultimately by the high activation energy for diffusion of the H. atom through the CaF, lattice. For comparison, results obtained from H. atoms trapped in y-irradiated rare earth ion-exchanged Y zeolites are presented and discussed also; these “surface” trapped atoms do not exhibit great thermal stability. Distinctions in the H. atom formation mechanisms between the fluorides and the zeolites were deduced from the accompanying paramagnetic species formed. The intracavity electric fields in the Y zeolites have been estimated from the H. atom hfsc contractions, and are found to be very high, =l VIA.
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I. Introduction The history of the study of H- atoms trapped in solids, and retaining their atomic integrity, began with the EPR observations on y-irradiated acid ices by Livingston et a1.l These authors also reported the trapping of D.atoms in porous glass from irradiation of D20 adsorbed in the pores,2 although it was much later that Kazanskii and c o - w o r k e r ~demonstrated ~~~ the actual trapping of H. atoms in “surface” sites (on y-irradiated silica gel). For these and other systems in which H- atoms are trapped in “surface” sites, e.g., Y zeolite^,^-^ thermal stability is quite modest, the atoms being annealed a t -100-150 K in all cases. There would be considerable interest if a model system could be found in which He atoms trapped at surface sites exhibited marked thermal stability, stable to room temperature, e.g. This stems, not only from the consequent e atom ability to perform and study at will primary H reactions on the surface (that such reactions are important in hydrogenation processes using metal catalysts is widely accepted8), but also, more arcanely, from the energy storage and recovery purview. It has been known for some time that H. atoms trapped in bulk CaFz (single crystals) show excellent stability characteristic^,^-^^ so we sought, using very high surface area polycrystalline (“gel”) CaF2, to explore this model system with particular emphasis on the surface aspect. The results of this EPR investigation are presented here, along with some results on H. atoms trapped in rare earth ion-exchanged Y zeolites where “surface” trapping is assured. The experiments exploit two very different methods for the generation of trapped H. atoms, viz., (a) in situ generation in the solid via the action of high-energy ionizing Based on work performed under the auspices of the U.S. Energy Research and Development Administration. Contribution C3029-18. *Address correspondence to this author at the Solid State Science Division, Argonne National Laboratory.
radiation; and (b) capture of atoms by the solid from the gas phase following the action of a microwave discharge on the parent gas. There have not been any previous publications on stabilized H. atoms trapped in any solid by capture from the gas phase; to our knowledge, the only report of such a method having previously been exploited is in the original paper of Hall and Sch~macher.~ Therein is mentioned a private communication by Handler, who was able to “introduce or activate” H. atoms in CaFz by sustaining a gaseous discharge in tubes containing CaF2 powder; but an independent, fuller reporting of this experiment was apparently never made. Study of the trapping, and stabilization as neutral atoms, of (gas phase) H- atoms becomes of additional general interest when it is appreciated that findings in this domain may profitably be exploited or provide useful insights in the studies of surface modification by proton implantation techniques12 and of the hydrogen spillover phen0men0n.l~ Both of these processes could ultimately be used to prepare materials of enhanced catalytic activity, and we will remark briefly on one implication of our results for spillover studies.
11. Experimental Section 11.1. Materials and Pretreatment Procedures. The material designated “gel CaF,” is a special preparation obtained from the Exxon Co. It has a surface area of 125 m2/g and gives a fluorine analysis of 49.8% (vs. a theoretical value of 48.7% for stoichiometric, pure CaF2). (These data were provided by the supplier.) Reagent CaF2 is a commerical powder, BA Reagent grade; this material had a small impurity of Mn2+ ions which caused spectroscopic interference in EPR, so its use was quickly discontinued. Ultrapure CaFz is a commercial powder of ultrapure grade obtained from Alfa Inorganics. The material designated “precipitated CaF,” was specially prepared so as to obtain CaFz with occluded DzO.
0022-3654/78/2082-0200$0 1.0010 0 1978 American Chemical Society
Stabilization of Hydrogen Atoms in Intracrystalline Voids
To that end, CaF2 was precipitated by a simple double decomposition reaction from a concentrated solution enriched in D20 by the following procedure: A 3.6 M solution was prepared by dissolving 8 g of CaClz [Fisher, ACS Reagent grade] in 20 mL of DzO. [Volk, 99.8 atom %, redistilled before use (under N2 atmosphere).] To this was added, in a plastic tube, a small molar excess (-6 g) of 48% H F in H 2 0 [B&A reagent grade]. The highly dispersed precipitate was centrifuged out and washed by redispersal in D 2 0three times. [The supernate remained quite acidic (pH 1-2) after the final washing.] In general, this gelatinous precipitate proved to be rather awkward to manipulate due to its pastelike consistency and the washing operation was not conveniently managed. Rapid drying of the precipitate was achieved by smearing the gelatinous paste thinly in a crucible and heating it very briefly (10-15 min) in an oven at -513 K. A flaky solid could then be easily collected and powdered. Elemental analyses14 of this material yielded: Ca, 50.5% and F, 48.2%. The Na-Y zeolite (Union Carbide, Ultrapure grade) was obtained from Dr. J. Rabo. Samples of this material were exchanged with the various trivalent rare earth ions. Conventional ion-exchange procedures were performed at room temperature, solution and solid phases being kept in contact for a few hours. The degree of exchange was fairly high; elemental analyses of representative samples gave typical rare earth contents of -6% by weight. CaF2 samples in EPR tubes were usually first treated by heating at -393 K under high vacuum (10-5-10-6 Torr) for several hours to ensure thorough removal of air and surface water. Experiments to determine the effect of varying the pretreatment temperature on the y-irradiation H. atom yield were carried out over the range 393-723 K. The Y zeolite samples were typically sealed in vacuo after degassing a t room temperature and 10-3-10-4 Torr pressure for 1h; this treatment yields samples that are still substantially hydrated. Dehydrated samples were prepared by calcining the materials in vacuo at 823 K for -20 h. The effect of hydrogen gas treatment on the H. atom production in CaF2 was examined by heating samples for several hours in 500 Torr of Hz following the thermal pretreatment described above. The effect of the temperature of the hydrogen treatment was investigated between 393 and 723 K. A series of experiments designed to elucidate the role of surface water in this CaFz system was executed as follows: A sample tube ensemble, comprised of a treatment tube and four EPR sample tubes on side arms, was constructed of Pyrex. Enough powder for four samples was introduced into the treatment tube and subjected to thermal pretreatment at high temperature (573-723 K) in the normal way. A portion of the powder would then be removed and sealed off in one of the EPR tubes. The remainder of the sample was treated a t high temperature with H2 gas, and a second sample sealed off. Then, D 2 0 (degassed thoroughly by the freeze-pump-thaw method) was introduced by trap-to-trap distillation into the rest of the powder, until the powder was quite wet. After standing for 30 min a t room temperature, the excess DzO was distilled off, and a third sample removed. Finally, the first step would be repeated and the fourth sample obtained. An experiment designed to test for the presence of exchangeable surface-trapped H.atoms in the CaF, material was arranged by exposing a sample of y-irradiated gel CaF2 to D2 gas. The EPR spectrum of the sample at 77 K was recorded before and after D2 intromission, the
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The Journal of Physical Chemistry, Vol. 82, No. 2, 1978 201
Dz gas having been introduced while the evacuated sample was maintained a t 77 K. Thereafter, the sample was observed repeatedly in a series of spectra recorded after the sample had been in contact with the gas for 1day at 77 K, 17 h at 195 K, 3 days at 298 K, 17 h at 323 K, 30 h at 348 K, and 30 h at 358 K. All spectra were recorded a t 77 K for the purpose of making comparisons. 11.2. Production of Hydrogen Atoms. A 6oCosource was used for the y-irradiation experiments. Untreated or appropriately pretreated samples, contained in sealed EPR sample tubes, were exposed to the radiation at either 296 or 77 K; the zeolites were irradiated always a t 77 K. The radiation exposure ranged between 3.25 and 6.5 Mrd; the yields of trapped H. atoms were not markedly different at these two dosage extremes. Before spectroscopic examination, the sample tubes were always carefully flame annealed to remove paramagnetic defects. Two kinds of systems were used in the experiments on capture of gas phase H. atoms, viz., (a) static system; (b) continuous flow system. The microwave discharge was produced in an adjustable cavity by a Microtherm generator (Raytheon Mfg. Co.), Model CMD-10, operating at 2.45 GHz, having a peak power output capability of 125 W. The microwave power, either forward to the load or reflected to the transmitter, could be measured wit,h an attached Micro-Match power meter (Bendix Corp.) of appropriate range and operational frequency (120 W, 2.45 GHz, Model 725.3). The accessories (cavity, power meter, transmission cable, electrodeless discharge lamp) were obtained from the Opthos Instrument Co. For the static system experiments, the samples, in 4-mm 0.d. quartz EPR tubes equipped with high vacuum stopcocks, were always pretreated by pumping under high vacuum (10-5-10-6 Torr) at elevated temperatures (473-573 K) for several hours. After cooling the samples, hydrogen gas (Liquid Carbonic, >99.99% pure, and further purified by passage through a Deoxo catalytic hydrogen purifier (Engelhard Industries, available from Matheson Gas Products) and dried by passage through a cold trap containing Linde Type 5A (1/8 in.)molecular sieves and activated alumina (Fisher, 8-14 Mesh)) was admitted to the vacuum system which was then flushed three times and repumped to a hard vacuum. The sample tubes were then filled with hydrogen to a pressure of 5-6 Torr at room temperature. The gas pressure chosen was the empirical maximum consistent with the sustaining of an adequate discharge in the sample tube. The fitted stopcock allowed the hydrogen in the sample tube to be replenished at will when the discharge could no longer be satisfactorily sustained. A Varian liquid nitrogen insert dewar was used to support the sample in the microwave cavity during the discharge operation. This allowed for some measure of temperature control at the sample which tends to heat up enormously under the influence of the hot plasma formed in the discharge of the gas. Low temperature trapping was performed with liquid nitrogen (bp 77 K) in the insert dewar, while for “room” temperature trapping pentane (bp 309 K) was used. The discharge cavity itself and the exterior of the insert dewar were cooled by a stream of cold nitrogen gas, cooled by passage through a Pyrex U-tube immersed in a dewar flask of liquid nitrogen. In order that EPR spectra could be examined at &-band and directly compared with those observed at X-band, identity in the treatment of X- and Q-band samples was guaranteed by using a dual tube arrangement. All the powder was contained in the X-band tube throughout the treatment, and only at the end was a portion withdrawn
202
L. E. Iton and J. Turkevich
The Journal of Physical Chemistry, Vol. 82, No. 2, 1978
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and sealed off in the Q-band tube. The details of the flow system are given e1~ewhere.l~ The experiment consisted of sustaining the discharge in a stream of H2 gas flowing continuously over the powder. The system pressure was maintained at -1 Torr, and the sample temperature could be controlled as in the static system experiments. 11.3. Spectroscopic Measurements. EPR spectra were obtained using the following Varian spectrometers: the E-12 at X-band (9.3 GHz) with either a rectangular TElo2 mode cavity or a TElO4mode dual cavity, and the V4561 a t Q-band (34 GHz) with a cylindrical TEOl1mode cavity. All spectra were recorded with 100-kHz modulation. Temperatures from 77 K to room temperature (295 K) were accessible with conventional insert dewar and variable temperature (E257) accessories. Effective g values were measured by comparison with standard pitch (g = 2.0028) or DPPH (g = 2.0037) signals. The use of the dual cavity in X-band experiments enhanced the accuracy of these measurements. In addition, nonlinearity of the magnetic field sweep was corrected for by making absolute field measurements with a Nuclear Magnets Corp. precision gaussmeter with ancillary power amplifier (Boonton Radio Co., Type 230 A) and frequency counter (Hewlett-Packard, 5245L). Comparable precision could not be obtained in Q-band measurements, so no reliance was placed on Q-band data for accurate g value reporting. Microwave power saturation studies were routinely performed at X-band, at both 295 and 77 K, with the applied power in the range 0.01-200 mW. The low power extremes of this range were facilitated by a low power bridge accessory. At 100-kHz field modulation, this corresponds to a rapid passage condition. 111. Results 111.1. Calcium Fluorides. (A) Under y Irradiation. The initial experiments were performed on a sample of the gel CaF2 that had been pretreated with H2 gas at 303 K. y irradiation of this sample at 77 K yielded H. atoms stable a t room temperature, as shown in the EPR spectrum of Figure l a . We will use the general designation Ht. for trapped He atoms. Attempts to enhance the yield by substantially elevating the hydrogen pretreatment temperature (to 1173 K) were abortive; no H- atoms were
trapped upon irradiation. It was then established that the hydrogen gas pretreatment was unnecessary by demonstrating that H. atoms were stabilized in the untreated gel (y irradiated at 298 K in a sample tube containing air). This showed that the H. atom precursors were present as an intrinsic impurity in the material, probably as occluded H 2 0 in the partially hydrated gel. H,. formation was shown to be totally suppressed by high temperature pretreatment; vacuum treating at 723 K caused the gel to become colored (no sizeable concentration of paramagnetic centres are formed, however) and no H. atoms were observed when the material was subsequently irradiated. (It had previously been established that pretreatment at a temperature 523 < T 5 673 K was sufficient to prevent subsequent Ht- formation in reagent CaF2.) In order to discriminate between surface and bulk occluded H20 as possible precursors of the H,., a series of experiments was undertaken on gel that had previously been deactivated. No D,. was formed from samples that had been reexposed to D20, thereby confirming the inefficacy of surface water as precursor to the trapped atoms; moreover, no H. atoms were formed in deactivated gel that was subsequently subjected to high temperature treatment with H2 gas, thereby demonstrating that suitable H. atom precursors (e.g., substitutional hydride ions) could not be introduced into the material by this simple technique. These results strongly indicate that one is indeed dealing with occluded H20 as precursor. The H. atoms formed by y irradiation of the gel CaFz are not, as will be verified later, trapped in interstitial sites; but they show the unusual thermal stability associated with such defects (H:) in CaF2.11 The results of an incremental annealing experiment are graphically presented in Figure 2; it shows that most of the H. atoms stable at 303 K are stable up to 373 K, but that between 373 and 398 K they begin to anneal. The annealing process is slow, however, and even after 40 h at 423 K, a significant quantity of H,. survive. Complete annealing of the Ht- was ef€ected at 473 K. The kinetic stability of the atoms at room temperature is to be inferred from the fact that most of the H,. atom signal intensity remains after 2 years standing. Two experiments bear upon the fate of the Ht. upon annealing, and their accessibility to the surface. An attempt to effect the exchange of Ht- and D. by annealing a sample of the y-irradiated gel in a D2 gas atmosphere at a series of temperatures up to 358 K was unsuccessful. It is to be noted that this temperature exceeds that at which the least stable of the Ht. are annealed. Vanderspurt16 had effected such an isotopic exchange on porous vycor glass at a temperature at which the Ht*were unstable, so success in such an experiment is indicative that atoms can either be trapped on, or survive migration
Stabilization of Hydrogen Atoms in Intracrystalline Voids
The Journal of Physical Chemistry, Vol. 82, No. 2, 1978 203
TABLE I: Thermogravimetric Analyses of Calcium Fluoride Materialsa (A)
,
Percentage weight loss at
I
Material
413 K
613 K
Ultrapure CaF, Gel CaF, Precipitated CaF,
0 2.52 0.42
0 3.26 0.58
a Performed by the Analytical Services Department of the Baron Consulting Co. 100 G H
TABLE 11: Emission Spectroscopy Analysis of Trace Impurities in Calcium Fluoride Materials Percentage impurity Material I
I -H
Figure 3. Room temperature EPR spectra showing (a) H. atoms in uitrapure CaF, exposed to discharged H2 gas; (b) H. atoms in gel CaF, exposed to discharged H2 gas.
to, the surface. In a complementary experiment, it was established that reirradiation of a sample that had been annealed to 573 K (a temperature well above that for which complete annealing of the Ht. EPR signal is effected, and by which most of the weight loss in the sample dehydration has been achieved) resulted in a restoration of the Ht* signal with intensity comparable to that following the original irradiation. This suggests that the annealed atoms do not escape from the solid a t this temperature, being retained in a diamagnetic form that is again susceptible to ionization or decomposition by high energy radiation; and/or that the saturation yield of Ht. from the precursor impurity is independent of the irradiation history of the sample. Ultrapure CaF2 yielded no trapped H. atoms upon y irradiation; however, the CaF2 material prepared by precipitation from an aqueous solution enriched with deuterium yielded both H. and D. atoms stable at room temperature (Figure l b ) , confirming that there was no specificity with regard to the material’s origin, providing that it was not subjected to rigorously dehydrating conditions. The trapped atoms, Ht. and Dt., in this material also exhibited long term room temperature stability, their spectra being still readily observable after 2 years storage a t 300 K. The Ht. and Dt. in the precipitated CaF2 material were also noninterstitial. A sample pretreated in vacuo at 423 K still yielded trapped atoms; but, as in the case of the gel, pretreatment of the sample at 723 K was sufficient to deactivate the material and preclude the formation and trapping of atoms. Unlike the gel, this material was not colored by deactivating pretreatment, but instead became colored only after the subsequent y-irradiation exposure. Again, as with the gel material, treatment of a deactivated sample with D20 failed to induce any Dt. formation upon irradiation, thereby eliminating surface water from consideration as a suitable precursor. (B) Under Microwave Discharge in H2 Gas. The appearance of trapped H. atoms in ultrapure CaF2,which is inactive to y irradiation, first established that the atoms were indeed being trapped from the gas phase in the microwave discharge experiment, rather than merely being “activated” from a precursor already present in the solid. Moreover, from the form of the resulting spectrum, Figure 3a, which displays extensive superhyperfine structure and a very large isotropic hyperfine splitting, the trapping is readily identified as being in interstitial sites (H:); this
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Ultrapure CaF,a 0.01 0.005 0.02 0.25 0.01 Gel CaF, Precipitated CaF, 0.02 0.1 0.02 0.02 a The impurity listing provided by the suppliers of this material (Alfa Inorganics) mentions the following metals (the concentrations are stated in ppm): Ni Yrr
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precipitated CaFz that had been exposed to discharged H2 gas, since the details of the shf structure of the H: could not be well resolved in the weak signals under consideration. Figure 9 illustrates the suppression of the HP resonances relative to the type B resonances in precipitated CaFz upon cooling. The data on the multiple site trapping in each of the three CaFz materials are summarized in Table VI. 111.3. y-Irradiated Y Zeolites. (A) H. A t o m Signal Characteristics. In Table VI1 is presented a summary of the EPR parameters for Ha atoms in Y zeolite host materials for selected cations under a variety of pretreatment conditions. In some cases, trapped H.atoms (H,.) were observed even in samples that had been strongly dehydrated by calcination at 823 K, e.g., the case of a sample containing Sm3+ ions is cited; the signal intensity was considerably reduced by such pretreatment. Indeed, the intensity reduction was significant even upon mild dehydration effected by prolonged (- l h) evacuation at room temperature. The principal EPR parameters prove to be somewhat insensitive to the nature of the cation, be it alkali metal or rare earth (RE), diamagnetic or paramagnetic; altogether, these parameters show only modest variation throughout the ensemble of materials and samples investigated. Both g values and hfsc interaction constants characteristically show small negative shifts with respect to the free atom values. Comparison of data for untreated and mildly dehydrated samples of Na-Y and of Na/Eu-Y reveal a tendency for the hfsc splitting to undergo a small increase upon partial dehydration. Comparison of data for mildly and strongly dehydrated samples of Na/Sm-Y
o-MI=-1/2 COMPONENT
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indicate a tendency for the a value to be slightly reduced again upon calcination. The signal line width is the characteristic that proves to be most sensitive to sample pretreatment and to the natures of the cations. As the data on Na/Eu-Y samples show, the line width found in an untreated sample is essentially unchanged upon low temperature degassing (77 K) but is profoundly narrowed upon mild dehydration. The data for mildly and strongly dehydrated Na/Sm-Y samples show that calcination produces further sharpening of the signals. Signal line widths were also typically broader when the zeolite contained paramagnetic cations, whether the materials were pretreated or not. This is apparent from a comparison of data for untreated samples of Na-Y and Na/Eu-Y, and from a comparison of data on mildly dehydrated Na-Y and Na/La-Y samples with data on mildly dehydrated Na/Eu-Y and Na/Sm-Y samples. Ht. signal line shapes were also dependent upon sample pretreatments, and were in general asymmetric. A detailed consideration of these is given e1~ewhere.l~ The microwave power saturation behavior of the Ht. in untreated Na-Y and Na/Eu-Y samples is represented graphically in Figure 10; the presence of the paramagnetic ions has a very pronounced effect on both signal amplitude and line width variations, although the latter are not shown. It must be noted that the Na/Eu-Y sample contains both Eu3+(diamagnetic) and Eu2+(paramagnetic) ions; the ion exchange was performed with Eu3+ions, but partial reduction occurred by electron trapping under the
200
L. E. Iton and J. Turkevich
The Journal of Physical Chemistry, Vol. 82, No. 2, 1978
1
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0.5
1
100 G
+-+
TIME (secs)
Figure 12. Decay kinetics for H,. signal disappearance at 120 K in y irradiated (a) Na-Y; (b) Na/Eu-Y.
structure with the Ht. signals. This is rationalized in terms of the considerably greater ease of dehydration of the Na-Y sample containing, as it does, only monovalent cations; the pretreatment by room temperature evacuation ( Torr pressure) for 1h only mildly dehydrates those Figure 11. H. atom satellite structure occurring in hydrated Y zeolites zeolite samples exchanged with trivalent RE ions, but containing only nonparamagnetic cations: (a) untreated Na-Y; (b) untreated Na/Eu-Y (no satellites). results in extensive dehydration of the Na-Y sample. This issue will be considered at greater length in section IV. It influence of the ionizing radiation. (Concrete evidence for is to be noted that Ht. signal line widths in the mildly this is found in the EPR spectra of the ions t h e m s e l v e ~ . ~ ~ ) dehydrated samples containing RE ions are generally Whereas the amplitude saturation curve of the Na-Y comparable to, or narrower than those in, the untreated sample exhibits near homogeneous character, that for the Na-Y sample; it cannot be contended, therefore, that Na/Eu-Y sample reveals the presence of a considerable strong satellites are being masked by broad signals. inhomogeneous broadening contribution as judged from Satellite structure was not observed in any sample that had its much larger width. The asymmetric signal line shapes been strongly dehydrated by calcining in the pretreatment. preclude the use of the use of the standard proceduresz2 (B) H. Atom Stability and Annealing Kinetics. The for calculation of relaxation times via the progressive presence of the RE cations did not significantly alter the saturation technique. Qualitatively, however, it is clear thermal stability of the H. atoms trapped in the Y zeolite that the relaxation time is much shortened by the presence lattice. A comparison was made of some annealing of the paramagnetic ions since a much larger microwave properties of the H,. in untreated Na-Y and Na/Eu-Y field (by a factor of -7) is required to saturate the Ht. samples. The Ht. signal decayed rapidly for both samples signal in the Na/Eu-Y sample. at 120 K, a temperature slightly above that (110 K) at A more dramatically visible consequence of the introwhich complete annealing of the Ht. in a H-Y zeolite has been r e p ~ r t e d .The ~ kinetics of the decay are second-order duction of paramagnetic ions into the system is the effect with respect to H,. concentration for the Na-Y sample, but on forbidden transitions involving protons in the vicinity are complex (neither first nor second order in the Ht. of the trapped H. atom. In Figure 11are compared spectra concentration) for the Na/Eu-Y sample (Figure 12). of untreated Na-Y and Na/Eu-Y samples, recorded at Second-order kinetic behavior is as that found by identical microwave power conditions (1 mW); a pair of Abou-Kais et al.7,z5for the disappearance of the Ht. in an strong satellite lines accompanying each H. atom signal H-Y zeolite, and suggests Ht. decay by recombination of component is clearly visible in the case of the Na-Y H. atoms with each other, or with another species present sample, but they are conspicuously absent in the case of in equal concentration. (It should be noted that H-Y the Na/Eu-Y sample. These satellites represent forbidden zeolite samples are necessarily calcined in pretreatment.) transitions (Am, = f l ;Am, = f l )involving simultaneous Complex kinetics, on the other hand, suggests the situation spin flips of the electron and a neighboring proton in the treated by Sprague and Schulte-Frohlinde;26this implies trapping environs, since the satellite spacings correspond an initial distribution of the Ht. among a variety of shallow to the lH NMR frequencies at the respective fields.20 No traps with competition between reaction and retrapping resolvable satellite structure is discerned in spectra of the upon annealing. Na/Eu-Y sample even at maximum applied microwave 111.4. Other Paramagnetic Centers. Other signals power (0.2 W), whereas the satellite intensities in the case (“central signals”) besides those due to trapped H atoms of the Na-Y sample are strong enough that they still appear in the spectra of the various CaFz and Y zeolite discernible, albeit weakly, at the lowest microwave power materials exposed to y irradiation or to microwave disused (10 pW). charged Hz gas. Attention here will be limited to giving That this satellite suppression is a genuine and direct them consideration only in so far as they might give some effect of the paramagnetic ion presence, and not some insight into the nature of the H atom production mechother cause such as burial in the wings of too broad signals, anisms. is confirmed by the observation of strong satellites in a OH radicals are identified in the spectra of the y-irmildly dehydrated Na/La-Y sample, La3+ being a diaradiated hydrated Y zeolites, as seen in Figure 11from the magnetic ion with no 4f electrons. Satellite structure was -45-G splitting of the principal doublet in the central not observed in any of the other similarly pretreated signal region; these constitute the hyperfine components samples containing paramagnetic ions when examined of the g, and gy components of the OH signal. The much under similar instrumental conditions. Indeed, even Na-Y weaker and broader g, component, often overlooked and samples pretreated in the same way yielded no satellite I
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The Journal of Physical Chemisrry, Vol. 82, No. 2, 1978 209
Stabilization of Hydrogen Atoms in Intracrystalline Voids
(€9
bJ
li
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1
CI
CI
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Figure 13. Structures of the central signals in gel CaF, at 77 K: (a) sample y irradiated at 77 K (not annealed); (b) sample y irradiated at 298 K.
misassigned in the earliest observations of OH signals in ice, is apparent at much lower field in both spectra. The signal is characterized by g, 2.006, gii 2.092; A, -45 G, A, -30 G, A, undetermined (small). Even in the best case, that of the unexchanged Na-Y zeolite (Figure I l a ) , the x and y components are not fully resolved, and an axial approximation has been used in characterization of the g tensor, although it may be fully orthorhombic. (A referee’s suggested estimate of g - g, N 0.004 from our spectra is comparable with pubfished results on single crystals of ice, where g, - g, = 0.006, as determined from ENDOR rnea~urements.~~) The hfsc tensor components and the (averaged) g, value are similar to those of OH in irradiated ice,27,28 but the g, (gli) component is somewhat larger and is more like the value found for OH in irradiated CaS04.2H20.27The parameters are markedly different, however, from those presented in a recent characterization of OH in y-irradiated rehydrated H-Y zeolite,%there given as gl = 2.0028, g, = 2.0050, g3 = 2.010; A1 = -26 G, Az = -47 G, A3 = -53 G; Aiso = -42 G. We point out, based on both theoretical calculations and the currently accepted experimental data on OH in the ~ a p o r ,in~ ice,27*28 ~ ’ ~ ~and in hydrated salts,27that (a) g, is always 22.05, (b) A,
