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Noise-like magnetic resonance absorption in zeolites
6 March 1998
Chemical Physics Letters 284 Ž1998. 435–439
Noise-like magnetic resonance absorption in zeolites V.F. Yudanov a , O.N. Martyanov a , Yu.N. Molin
b
a
b
BoreskoÕ Institute of Catalysis, NoÕosibirsk 630090, Russia Institute of Chemical Kinetics and Combustion, NoÕosibirsk 630090, Russia Received 6 November 1997
Abstract Unusual X-band magnetic resonance spectra have been observed over the 77–500 K temperature range in polycrystalline specimens of zeolites treated to thermal oxidation. Narrow individual lines form a noise-like spectrum over the whole magnetic field scan range Žup to 8000 G.. However, the pattern differs from the real noise by strict reproducibility. A simple model is proposed to account for the phenomenon as the overlapping resonance lines of a large number of randomly oriented ferromagnetic microcrystal impurities. q 1998 Elsevier Science B.V.
1. Introduction An investigation of the properties of natural and synthetic zeolites, belonging to aluminosilicates with a geometrically rigid set of pores and channels, is largely stimulated by their unique properties, like high catalytic and adsorption activity w1,2x. In many cases the chemical processes in zeolites proceed via the formation of radical cation intermediates w3–6x. Thus, it is of great interest to study the structure and properties of the zeolite surface active centers capable of accepting electrons from the adsorbed molecules. A recent interest in such research is aroused also due to the application of zeolites as a matrix for stabilization of active paramagnetic species Žsee, e.g. Ref. w7x and references therein.. The catalytic and adsorption activity of zeolites changes essentially after the high-temperature Ž700– 1000 K. treatment in an oxygen atmosphere w6x. No noticeable amount of paramagnetic species is detected in such samples by ESR, except for the Fe 3q ions that are usually present as impurities or are deliberately introduced upon the synthesis w8x. The
g-values of the corresponding ESR lines are 4.3, ; 2.3 and 2. The signal with g s 4.3 supposedly belongs to the tetrahedral Fe 3q in the zeolite lattice. Two other signals can be assigned to the Fe 3q ions located in the channels as well as in the oxide phase of the different dispersion on the outer crystal surface w8,9x. The present Letter reports on the discovery and preliminary research of the unusual magnetic resonance signals in zeolites treated to high-temperature oxidation.
2. Experimental The specimens of zeolite ZSM-5 prepared by hydrothermal synthesis have been studied. A molar ratio, M s SiO 2rAl 2 O 3 and the content of Fe 2 O 3 were determined using chemical analysis. For samples A and B, the ratio M is 32 and 91, while the content of Fe 2 O 3 is 0.043 and 0.004 weight percent, respectively. The specific size of individual micro-
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 4 4 9 - 8
V.F. YudanoÕ et al.r Chemical Physics Letters 284 (1998) 435–439
436
crystalline particles is 3–5 mm. When preparing the specimens, the ion-exchange was performed in aqueous solution of NH 4 Cl and then the zeolites were kept in an O 2 atmosphere at 850 K for 1–2 h. An X-band ESR spectrometer ŽBruker ESR-300. was used to record the spectra. The samples were placed at the center of a rectangular TE 102 cavity of the spectrometer, the magnetic component of microwave field being perpendicular to the steady external magnetic field. The temperature varied over the range of 77–500 K.
3. Results All the specimens studied exhibit the familiar intense broad ESR signals caused by the absorption of paramagnetic Fe 3q ions. Against a broad spectrum background, however, one can observe quite weak sharp absorption lines in the unusually large range of magnetic field, i.e. from the values close to zero to the maximum ones Ž; 8000 G. attainable by the spectrometer ŽFig. 1.. With a single magnetic field sweep, these lines are virtually lost into noise and their reliable recording requires a digital accumulation of about 10 2 scans. For convenience, the spectra below are given as the signals resulting from the subtraction of a broad background component. As follows from Fig. 2Ža., the spectra consist of the multitude of lines, that are rather narrow for a solid, with a width of the order of a few gauss split by about 5–10 G. Superficially, these spectra have a noise-like character but a reliable criteria for their reality is the strict reproducibility for a given sample
Fig. 1. The ESR spectrum of zeolite ZSM-5 Žsample A..
Fig. 2. Ža. The noise-like component of the magnetic resonance spectrum for the zeolite ZSM-5 Žsample A. obtained by subtraction of the broad background component. The registration conditions: modulation frequency – 100 kHz, modulation amplitude – 1 G, MW power – 100 mW, the time of magnetic field sweep – 40 s; Žb, c. a sequence of spectra recorded under Žb. modulation frequency of 100 kHz and Žc. of 50 kHz.
in the independent registration series. A comparison between spectra Žb. and Žc. ŽFig. 2. shows that the position and shape of the lines are reproducible with a high accuracy and, hence, the lines observed are not random noise. We have changed registration conditions, namely, the modulation frequency and amplitude Žwithin the individual linewidth., the microwave power, the integration time constant and the field sweep rate. All these variations leave the spectrum unchanged and allow the absorption pattern to be recorded, invariant over indefinite periods. The spectra reproducibility is observed, however, only with a fixed position of specimen in the spectrometer cavity. A minute rotation of specimens causes noticeable changes in the spectrum. Fig. 3 shows that, a 0.5 degree rotation results in virtually
V.F. YudanoÕ et al.r Chemical Physics Letters 284 (1998) 435–439
Fig. 3. The spectrum of zeolite ZSM-5 Žsample A.. Žb. the specimen is rotated by 0.5 deg relative to Ža.. Rotation was performed in a magnetic field of 3000 G. The registration conditions are similar to those in Fig. 2.
uncorrelated spectral patterns. Similarly, if the specimen is remounted in the spectrometer cavity, a general view of the spectrum is preserved while one can observe variations in the position and shapes of the individual lines in any spectral section. In the significantly inhomogeneous external magnetic field the noise-like spectra fail to show any line broadening typical of the ordinary ESR spectra. The spectra obtained in the inhomogeneous magnetic field Žthe H0 gradient ; 100 Grcm. look similar to those detected in the homogeneous field. If the specimen mounted in the spectrometer cavity is UV-irradiated upon registration, its noise-like spectrum changes totally with a slight increase in the intensity. At room temperature the spectrum recovers all its details upon completion of irradiation. At 77 K, the relaxation occurs rather slowly and the recovery kinetics is easily registered in spite of the long spectrum accumulation time Ž3–5 min.. The long-wave length boundary of the UV-irradiation effect is 350–400 nm. The line intensity falls smoothly Žbut no more than 5–7 times. in the 77–450 K range. If the temperature is increased further, the spectrum intensity falls much quicker and the spectrum accumulation time, necessary to get a reasonable signal to noise ratio, increases. We have failed to detect the noiselike spectra at temperatures above 500 K. The noise-like spectra reveal a high sensitivity to the interaction between a specimen and gas phase molecules. A change in the O 2 or N2 pressure from 1 to 10 3 Torr over the specimen causes total modifi-
437
cation of a fine spectrum structure. This phenomenon, reversible with respect to pressure, is identical for O 2 and N2 but is missing for Ar atoms which are adsorbed much more weakly under the experimental conditions Žroom temperature.. The line broadening effect was not observed for the paramagnetic O 2 molecules. It should be noted that the noise-like magnetic resonance absorption is not unique for the ZSM-5 zeolites. The other investigated zeolites ŽZSM-12, mordenite. show similar spectra. By now we have also observed such spectra at 300 K for oxidized silver supported on TiO 2 as well as for Bi 2 O 3 and TiO 2 . Compounds giving rise to such spectra, are chemically stable. Thus the specimens preserve their spectra for more than a year when kept out of doors at room temperature. However the annealing of specimens at about 600 K in an H 2 atmosphere leads to the disappearance of the spectra.
4. Discussion The studied noise-like spectra and their characteristics are highly unusual for ESR spectroscopy. The most curious feature is the variation in the spectral fine structure with a small change in specimen position with respect to the external magnetic field. Since we used a polycrystalline zeolite specimen Žfine powder., any preferred orientation of particles in the sample seems to be improbable. This possibility could also be eliminated because of the lack of the broadening effect of the inhomogeneous external magnetic field. The spectra observed, can be attributed to a set of the absorption lines of the randomly oriented separate microcrystals. Their number is inadequate to form a smooth contour of the absorption lines usually observed in the ESR spectra of randomly oriented paramagnetic centers. This may be realized in powder if the lines of microcrystals are narrow and strong and the anisotropic interaction is large enough. To illustrate this possibility, let us consider the results of the numerical simulation of the ESR anisotropic spectra for a finite number N, of the randomly oriented paramagnetic particles ŽFig. 4.. The computations were performed for g 1 s 4.0, g 2
438
V.F. YudanoÕ et al.r Chemical Physics Letters 284 (1998) 435–439
Fig. 4. The simulated anizotropic ESR spectra occurring after addition of the N Lorentzian lines of paramagnetic particles randomly oriented relative to the external magnetic field H0 . D H is the line halfwidth.
s 2.1 and g 3 s 0.9 Žwhich roughly fits the resonance field range of the spectra observed. with a Lorentzian individual line. We have used a modified ESR-1 program w10x to simulate the anisotropicaly broadened ESR spectra. Fig. 4Ža. shows the computation result, involving 10 4 particles with an individual linewidth D H s 50 G. As the number of individual lines is insufficient to provide a full smooth averaging, the resulting spectrum demonstrates ‘‘noise’’ whose intensity is comparable to that of the real spectral lines. Nonetheless, the sufficiently strong lines complying with the main g 1 and g 2 values of g-tensor are clearly distinguishable in the resulting spectrum. Only a weaker line, corresponding to g 3 s 0.9, is almost lost under the ‘‘noise’’. The same computation with an individual linewidth D H s 1 G
Žwhich is closer to the experimental values. yields the spectrum consisting of the noise-type set of lines ŽFig. 4Žb.., among which no components can be isolated that are associated with the main g-tensor values of the absorbing particle. A hundred fold increase in the summed up orientations does not result in these components. A relatively smooth spectrum can be obtained only with a subsequent significant increase of the summarized spectra number. As seen from Fig. 4Žc., with N s 6.4 = 10 7, the components corresponding to the g-tensor main values are most noticeable. The above simulations support the possibility of noise-like spectra formation due to incomplete averaging of the lines belonging to the randomly oriented monocrystalline particles. At present, only preliminary assumptions as to the nature of these particles can be put forward. The existence of the noise-like spectrum in magnetic fields as high as 8000 G as well as the narrow individual linewidth are difficult to explain using the known ESR parameters of Fe 3q paramagnetic centers. The spectra of the isolated Fe 3q ions in different coordination environments are predominantly centered around g-values of about 2 to 4 and the lines are spread not wider than up to 5000 G w11x. Their individual linewidths are not less than 20–30 G for single crystals w12x. One can, however, suggest that the studied zeolite specimens contain the traces of the ferromagnetic phase produced by iron admixtures under thermooxygen treatment. The ferromagnetic resonance ŽFMR. absorption line position can be basically specified by the shape of ferromagnetic material particles w13x. In the case of thin films, for example, the external resonance magnetic field H0 highly depends on the relative orientation of the film in the magnetic field. The extreme H0 values are defined by the equations
v0 g
(
s H0 P Ž H0 q 4 p Ms .
Ž 1.
for H0 parallel to the film plane and
v0 g
s H0 y 4 p Ms
Ž 2.
for H0 perpendicular to the plane. Here, Ms is the saturation magnetization of the material given, v 0 is the working frequency and g is the magnetogyric
V.F. YudanoÕ et al.r Chemical Physics Letters 284 (1998) 435–439
ratio of free electrons. According to Eqs. Ž1. and Ž2., the resonance field range extends from H H ; 10 2 G to H 5 ; 10 4 G for Ms ; 10 3 G, typical of the particular ferromagnetic particles. In single crystals, the FMR absorption linewidth may vary in wide limits. The most narrow lines with D H ; 0.2 G were observed in the crystalline specimens of yttriferrous garnets, Y3 Fe 5 O 12 w14x. Thus, FMR may provide both a wide spectrum and a small width of the individual lines. Therefore, the conditions under which the noise-like spectra are formed can be satisfied for FMR of the randomly oriented particles with a strong anisotropy of the effective g-value. An important prerequisite for the formation of the noise-like spectrum is also a sufficient line intensity of the isolated microcrystal. With a linewidth of 1 G, the threshold concentration detectable by means of an ordinary ESR spectrometer, is about 10 10 spins per sample and provisionally by three orders of magnitude lower for the FMR signals. Since according to simulation, the well-formed noise-like spectrum is formed with a number of single crystals N ; 10 4 , we conclude that in the case of FMR, the observed spectra may arise with spin concentration being not less than 10 11 per sample. This concentration is a few orders of magnitude lower than that of iron in the purest zeolite sample studied. Note that the noise-like spectrum intensity is found to increase negligibly with the overall iron content in the zeolite specimen as could be expected. The experiments with sample B in which the iron content was an order of magnitude less than that in sample A, show that the noise-like spectrum line intensity was virtually the same. It is assumed then that only a small portion of iron, present in the samples, forms the ferromagnetic phase which provides the noise-like spectrum. We suggest that this ferromagnetic phase appears in the form of either strongly anisotropic single crystals or thin films isles on the zeolite crystal surface under thermal oxidation. The effect of UV-illumination and gas adsorption on the structure of the noise-like spectra can be related to reversible
439
changes in either magnetization or the orientation of ferromagnetic particles. We believe that a further study of the nature and properties of the noise-like spectra will allow us to develop a useful method for investigating small ferromagnetic particles in disordered systems. Acknowledgements We are grateful to K.I. Zamaraev and V.N. Parmon for the interesting discussions and support. We also thank Dr. V.N. Romannikov for the kindly supplied specimens of zeolites and Dr. A.A. Shubin for his help in the calculations. The work was performed using the equipment of the Center on ESR spectroscopy ŽInstitute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences. with a financial support of RFBR ŽGrant 96-03-34020.. References w1x J.A. Rabo, Zeolite Chem. Catal. ACS 171 Ž1976. 80. w2x P.A. Jacobs, Zeolite Chemistry and Catalysis, Elsevier, New York, 1991. w3x P.L. Corio, S. Shih, J. Phys. Chem. 75 Ž1971. 3475. w4x F. Chen, X.J. Guo, J. Chem. Soc. Chem. Commun., 1989, p. 1682. w5x F.R. Chen, J.J. Fripiat, J. Phys. Chem. 97 Ž1993. 5796. w6x A.V. Kucherov, A.A. Slinkin, D.A. Kondratyev, J. Mol. Catal. 37 Ž1986. 107. w7x D.W. Werst, E.E. Tarakovsky, E.A. Piocos, A.D. Trifunac, J. Phys. Chem. 98 Ž1994. 10249. w8x R. Szostak, T.L. Thomas, J. Catal. 100 Ž1986. 555. w9x L.E. Iton, R.B. Beal, T.D. Hodul, J. Mol. Catal. 21 Ž1983. 151. w10x A.A. Shubin, G.M. Zhidomirov, J. Struct. Chem. 30 Ž1989. 67. w11x A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon Press, Oxford, 1970. w12x R.S. de Biasi, M.L.N. Grillo, J. Alloys and Compounds 189 Ž1992. 201. w13x C. Kittel, Phys. Rev. 73 Ž1948. 155. w14x E.G. Spencer, R.C. Le Craw, A.M. Clogsten, Phys. Rev. Lett. 3 Ž1959. 32.
Chemical Physics Letters 284 Ž1998. 435–439
Noise-like magnetic resonance absorption in zeolites V.F. Yudanov a , O.N. Martyanov a , Yu.N. Molin
b
a
b
BoreskoÕ Institute of Catalysis, NoÕosibirsk 630090, Russia Institute of Chemical Kinetics and Combustion, NoÕosibirsk 630090, Russia Received 6 November 1997
Abstract Unusual X-band magnetic resonance spectra have been observed over the 77–500 K temperature range in polycrystalline specimens of zeolites treated to thermal oxidation. Narrow individual lines form a noise-like spectrum over the whole magnetic field scan range Žup to 8000 G.. However, the pattern differs from the real noise by strict reproducibility. A simple model is proposed to account for the phenomenon as the overlapping resonance lines of a large number of randomly oriented ferromagnetic microcrystal impurities. q 1998 Elsevier Science B.V.
1. Introduction An investigation of the properties of natural and synthetic zeolites, belonging to aluminosilicates with a geometrically rigid set of pores and channels, is largely stimulated by their unique properties, like high catalytic and adsorption activity w1,2x. In many cases the chemical processes in zeolites proceed via the formation of radical cation intermediates w3–6x. Thus, it is of great interest to study the structure and properties of the zeolite surface active centers capable of accepting electrons from the adsorbed molecules. A recent interest in such research is aroused also due to the application of zeolites as a matrix for stabilization of active paramagnetic species Žsee, e.g. Ref. w7x and references therein.. The catalytic and adsorption activity of zeolites changes essentially after the high-temperature Ž700– 1000 K. treatment in an oxygen atmosphere w6x. No noticeable amount of paramagnetic species is detected in such samples by ESR, except for the Fe 3q ions that are usually present as impurities or are deliberately introduced upon the synthesis w8x. The
g-values of the corresponding ESR lines are 4.3, ; 2.3 and 2. The signal with g s 4.3 supposedly belongs to the tetrahedral Fe 3q in the zeolite lattice. Two other signals can be assigned to the Fe 3q ions located in the channels as well as in the oxide phase of the different dispersion on the outer crystal surface w8,9x. The present Letter reports on the discovery and preliminary research of the unusual magnetic resonance signals in zeolites treated to high-temperature oxidation.
2. Experimental The specimens of zeolite ZSM-5 prepared by hydrothermal synthesis have been studied. A molar ratio, M s SiO 2rAl 2 O 3 and the content of Fe 2 O 3 were determined using chemical analysis. For samples A and B, the ratio M is 32 and 91, while the content of Fe 2 O 3 is 0.043 and 0.004 weight percent, respectively. The specific size of individual micro-
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 4 4 9 - 8
V.F. YudanoÕ et al.r Chemical Physics Letters 284 (1998) 435–439
436
crystalline particles is 3–5 mm. When preparing the specimens, the ion-exchange was performed in aqueous solution of NH 4 Cl and then the zeolites were kept in an O 2 atmosphere at 850 K for 1–2 h. An X-band ESR spectrometer ŽBruker ESR-300. was used to record the spectra. The samples were placed at the center of a rectangular TE 102 cavity of the spectrometer, the magnetic component of microwave field being perpendicular to the steady external magnetic field. The temperature varied over the range of 77–500 K.
3. Results All the specimens studied exhibit the familiar intense broad ESR signals caused by the absorption of paramagnetic Fe 3q ions. Against a broad spectrum background, however, one can observe quite weak sharp absorption lines in the unusually large range of magnetic field, i.e. from the values close to zero to the maximum ones Ž; 8000 G. attainable by the spectrometer ŽFig. 1.. With a single magnetic field sweep, these lines are virtually lost into noise and their reliable recording requires a digital accumulation of about 10 2 scans. For convenience, the spectra below are given as the signals resulting from the subtraction of a broad background component. As follows from Fig. 2Ža., the spectra consist of the multitude of lines, that are rather narrow for a solid, with a width of the order of a few gauss split by about 5–10 G. Superficially, these spectra have a noise-like character but a reliable criteria for their reality is the strict reproducibility for a given sample
Fig. 1. The ESR spectrum of zeolite ZSM-5 Žsample A..
Fig. 2. Ža. The noise-like component of the magnetic resonance spectrum for the zeolite ZSM-5 Žsample A. obtained by subtraction of the broad background component. The registration conditions: modulation frequency – 100 kHz, modulation amplitude – 1 G, MW power – 100 mW, the time of magnetic field sweep – 40 s; Žb, c. a sequence of spectra recorded under Žb. modulation frequency of 100 kHz and Žc. of 50 kHz.
in the independent registration series. A comparison between spectra Žb. and Žc. ŽFig. 2. shows that the position and shape of the lines are reproducible with a high accuracy and, hence, the lines observed are not random noise. We have changed registration conditions, namely, the modulation frequency and amplitude Žwithin the individual linewidth., the microwave power, the integration time constant and the field sweep rate. All these variations leave the spectrum unchanged and allow the absorption pattern to be recorded, invariant over indefinite periods. The spectra reproducibility is observed, however, only with a fixed position of specimen in the spectrometer cavity. A minute rotation of specimens causes noticeable changes in the spectrum. Fig. 3 shows that, a 0.5 degree rotation results in virtually
V.F. YudanoÕ et al.r Chemical Physics Letters 284 (1998) 435–439
Fig. 3. The spectrum of zeolite ZSM-5 Žsample A.. Žb. the specimen is rotated by 0.5 deg relative to Ža.. Rotation was performed in a magnetic field of 3000 G. The registration conditions are similar to those in Fig. 2.
uncorrelated spectral patterns. Similarly, if the specimen is remounted in the spectrometer cavity, a general view of the spectrum is preserved while one can observe variations in the position and shapes of the individual lines in any spectral section. In the significantly inhomogeneous external magnetic field the noise-like spectra fail to show any line broadening typical of the ordinary ESR spectra. The spectra obtained in the inhomogeneous magnetic field Žthe H0 gradient ; 100 Grcm. look similar to those detected in the homogeneous field. If the specimen mounted in the spectrometer cavity is UV-irradiated upon registration, its noise-like spectrum changes totally with a slight increase in the intensity. At room temperature the spectrum recovers all its details upon completion of irradiation. At 77 K, the relaxation occurs rather slowly and the recovery kinetics is easily registered in spite of the long spectrum accumulation time Ž3–5 min.. The long-wave length boundary of the UV-irradiation effect is 350–400 nm. The line intensity falls smoothly Žbut no more than 5–7 times. in the 77–450 K range. If the temperature is increased further, the spectrum intensity falls much quicker and the spectrum accumulation time, necessary to get a reasonable signal to noise ratio, increases. We have failed to detect the noiselike spectra at temperatures above 500 K. The noise-like spectra reveal a high sensitivity to the interaction between a specimen and gas phase molecules. A change in the O 2 or N2 pressure from 1 to 10 3 Torr over the specimen causes total modifi-
437
cation of a fine spectrum structure. This phenomenon, reversible with respect to pressure, is identical for O 2 and N2 but is missing for Ar atoms which are adsorbed much more weakly under the experimental conditions Žroom temperature.. The line broadening effect was not observed for the paramagnetic O 2 molecules. It should be noted that the noise-like magnetic resonance absorption is not unique for the ZSM-5 zeolites. The other investigated zeolites ŽZSM-12, mordenite. show similar spectra. By now we have also observed such spectra at 300 K for oxidized silver supported on TiO 2 as well as for Bi 2 O 3 and TiO 2 . Compounds giving rise to such spectra, are chemically stable. Thus the specimens preserve their spectra for more than a year when kept out of doors at room temperature. However the annealing of specimens at about 600 K in an H 2 atmosphere leads to the disappearance of the spectra.
4. Discussion The studied noise-like spectra and their characteristics are highly unusual for ESR spectroscopy. The most curious feature is the variation in the spectral fine structure with a small change in specimen position with respect to the external magnetic field. Since we used a polycrystalline zeolite specimen Žfine powder., any preferred orientation of particles in the sample seems to be improbable. This possibility could also be eliminated because of the lack of the broadening effect of the inhomogeneous external magnetic field. The spectra observed, can be attributed to a set of the absorption lines of the randomly oriented separate microcrystals. Their number is inadequate to form a smooth contour of the absorption lines usually observed in the ESR spectra of randomly oriented paramagnetic centers. This may be realized in powder if the lines of microcrystals are narrow and strong and the anisotropic interaction is large enough. To illustrate this possibility, let us consider the results of the numerical simulation of the ESR anisotropic spectra for a finite number N, of the randomly oriented paramagnetic particles ŽFig. 4.. The computations were performed for g 1 s 4.0, g 2
438
V.F. YudanoÕ et al.r Chemical Physics Letters 284 (1998) 435–439
Fig. 4. The simulated anizotropic ESR spectra occurring after addition of the N Lorentzian lines of paramagnetic particles randomly oriented relative to the external magnetic field H0 . D H is the line halfwidth.
s 2.1 and g 3 s 0.9 Žwhich roughly fits the resonance field range of the spectra observed. with a Lorentzian individual line. We have used a modified ESR-1 program w10x to simulate the anisotropicaly broadened ESR spectra. Fig. 4Ža. shows the computation result, involving 10 4 particles with an individual linewidth D H s 50 G. As the number of individual lines is insufficient to provide a full smooth averaging, the resulting spectrum demonstrates ‘‘noise’’ whose intensity is comparable to that of the real spectral lines. Nonetheless, the sufficiently strong lines complying with the main g 1 and g 2 values of g-tensor are clearly distinguishable in the resulting spectrum. Only a weaker line, corresponding to g 3 s 0.9, is almost lost under the ‘‘noise’’. The same computation with an individual linewidth D H s 1 G
Žwhich is closer to the experimental values. yields the spectrum consisting of the noise-type set of lines ŽFig. 4Žb.., among which no components can be isolated that are associated with the main g-tensor values of the absorbing particle. A hundred fold increase in the summed up orientations does not result in these components. A relatively smooth spectrum can be obtained only with a subsequent significant increase of the summarized spectra number. As seen from Fig. 4Žc., with N s 6.4 = 10 7, the components corresponding to the g-tensor main values are most noticeable. The above simulations support the possibility of noise-like spectra formation due to incomplete averaging of the lines belonging to the randomly oriented monocrystalline particles. At present, only preliminary assumptions as to the nature of these particles can be put forward. The existence of the noise-like spectrum in magnetic fields as high as 8000 G as well as the narrow individual linewidth are difficult to explain using the known ESR parameters of Fe 3q paramagnetic centers. The spectra of the isolated Fe 3q ions in different coordination environments are predominantly centered around g-values of about 2 to 4 and the lines are spread not wider than up to 5000 G w11x. Their individual linewidths are not less than 20–30 G for single crystals w12x. One can, however, suggest that the studied zeolite specimens contain the traces of the ferromagnetic phase produced by iron admixtures under thermooxygen treatment. The ferromagnetic resonance ŽFMR. absorption line position can be basically specified by the shape of ferromagnetic material particles w13x. In the case of thin films, for example, the external resonance magnetic field H0 highly depends on the relative orientation of the film in the magnetic field. The extreme H0 values are defined by the equations
v0 g
(
s H0 P Ž H0 q 4 p Ms .
Ž 1.
for H0 parallel to the film plane and
v0 g
s H0 y 4 p Ms
Ž 2.
for H0 perpendicular to the plane. Here, Ms is the saturation magnetization of the material given, v 0 is the working frequency and g is the magnetogyric
V.F. YudanoÕ et al.r Chemical Physics Letters 284 (1998) 435–439
ratio of free electrons. According to Eqs. Ž1. and Ž2., the resonance field range extends from H H ; 10 2 G to H 5 ; 10 4 G for Ms ; 10 3 G, typical of the particular ferromagnetic particles. In single crystals, the FMR absorption linewidth may vary in wide limits. The most narrow lines with D H ; 0.2 G were observed in the crystalline specimens of yttriferrous garnets, Y3 Fe 5 O 12 w14x. Thus, FMR may provide both a wide spectrum and a small width of the individual lines. Therefore, the conditions under which the noise-like spectra are formed can be satisfied for FMR of the randomly oriented particles with a strong anisotropy of the effective g-value. An important prerequisite for the formation of the noise-like spectrum is also a sufficient line intensity of the isolated microcrystal. With a linewidth of 1 G, the threshold concentration detectable by means of an ordinary ESR spectrometer, is about 10 10 spins per sample and provisionally by three orders of magnitude lower for the FMR signals. Since according to simulation, the well-formed noise-like spectrum is formed with a number of single crystals N ; 10 4 , we conclude that in the case of FMR, the observed spectra may arise with spin concentration being not less than 10 11 per sample. This concentration is a few orders of magnitude lower than that of iron in the purest zeolite sample studied. Note that the noise-like spectrum intensity is found to increase negligibly with the overall iron content in the zeolite specimen as could be expected. The experiments with sample B in which the iron content was an order of magnitude less than that in sample A, show that the noise-like spectrum line intensity was virtually the same. It is assumed then that only a small portion of iron, present in the samples, forms the ferromagnetic phase which provides the noise-like spectrum. We suggest that this ferromagnetic phase appears in the form of either strongly anisotropic single crystals or thin films isles on the zeolite crystal surface under thermal oxidation. The effect of UV-illumination and gas adsorption on the structure of the noise-like spectra can be related to reversible
439
changes in either magnetization or the orientation of ferromagnetic particles. We believe that a further study of the nature and properties of the noise-like spectra will allow us to develop a useful method for investigating small ferromagnetic particles in disordered systems. Acknowledgements We are grateful to K.I. Zamaraev and V.N. Parmon for the interesting discussions and support. We also thank Dr. V.N. Romannikov for the kindly supplied specimens of zeolites and Dr. A.A. Shubin for his help in the calculations. The work was performed using the equipment of the Center on ESR spectroscopy ŽInstitute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences. with a financial support of RFBR ŽGrant 96-03-34020.. References w1x J.A. Rabo, Zeolite Chem. Catal. ACS 171 Ž1976. 80. w2x P.A. Jacobs, Zeolite Chemistry and Catalysis, Elsevier, New York, 1991. w3x P.L. Corio, S. Shih, J. Phys. Chem. 75 Ž1971. 3475. w4x F. Chen, X.J. Guo, J. Chem. Soc. Chem. Commun., 1989, p. 1682. w5x F.R. Chen, J.J. Fripiat, J. Phys. Chem. 97 Ž1993. 5796. w6x A.V. Kucherov, A.A. Slinkin, D.A. Kondratyev, J. Mol. Catal. 37 Ž1986. 107. w7x D.W. Werst, E.E. Tarakovsky, E.A. Piocos, A.D. Trifunac, J. Phys. Chem. 98 Ž1994. 10249. w8x R. Szostak, T.L. Thomas, J. Catal. 100 Ž1986. 555. w9x L.E. Iton, R.B. Beal, T.D. Hodul, J. Mol. Catal. 21 Ž1983. 151. w10x A.A. Shubin, G.M. Zhidomirov, J. Struct. Chem. 30 Ž1989. 67. w11x A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon Press, Oxford, 1970. w12x R.S. de Biasi, M.L.N. Grillo, J. Alloys and Compounds 189 Ž1992. 201. w13x C. Kittel, Phys. Rev. 73 Ž1948. 155. w14x E.G. Spencer, R.C. Le Craw, A.M. Clogsten, Phys. Rev. Lett. 3 Ž1959. 32.