Supermicroporous silica-based SiO2–Al2O3–NiO materials: Solid-state NMR, NMR relaxation and magnetic susceptibility

Microporous and Mesoporous Materials 118 (2009) 78–86

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Supermicroporous silica-based SiO2–Al2O3–NiO materials: Solid-state NMR, NMR relaxation and magnetic susceptibility Vladimir I. Bakhmutov, Boris G. Shpeizer, Andrey V. Prosvirin, Kim R. Dunbar, Abraham Clearfield * Contribution from the Department of Chemistry, Texas A&M University, College Station, TX 77842-3012, United States

a r t i c l e

i n f o

Article history: Received 26 March 2008 Received in revised form 6 August 2008 Accepted 9 August 2008 Available online 20 August 2008 Keywords: Porous oxide mixtures 1 H, 2H, 29Si, 27Al MAS NMR Relaxation time Nanoparticles Silica–alumina–NiO based materials

a b s t r a c t Microporous materials SiO2–Al2O3–NiO with a pore size of 8–20 Å, prepared by the sol–gel method at wide variation in Ni2+ concentrations, have been studied by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, magnetic susceptibility measurements and multinuclear NMR. It has been shown that the MAS NMR spectra and particularly NMR relaxation in amorphous paramagnetic solids can be successfully used for their characterizations. The 1H and 29Si spin-lattice NMR relaxation is always non-exponential and governed by direct dipolar interactions with paramagnetic centers while the spin-diffusion mechanism is negligible. The 29Si relaxation rates, obtained in the limits of the stretched exponential, reflect distribution of paramagnetic centers. It has been found that aluminum atoms are incorporated into the silica matrix of the materials while nickel centers are accumulated within pores. The nature of the nickel centers has been studied by magnetic susceptibility measurements supported by the XPS and X-ray experiments. These centers represent NiO and Ni0 (observed at high nickel loadings) aggregated into nanoparticles. The Ni0 nanoparticles are responsible for the room-temperature ferromagnetic behavior of the materials prepared with high nickel loadings. Ó 2008 Elsevier Inc. All rights reserved.

1. Introduction Ever since the discovery of mesoporous materials, such as the MCM-41 family [1,2] intense research into the synthesis and characterization of related porous materials has followed [3]. By using surfactants with moderately long alkyl chain lengths, pores in the range of 20 to greater than 100 Å diameters have been prepared. While these materials are semi-crystalline, the pores are well ordered. The well-known zeolites are more highly crystalline and commercially valuable, but are limited by pore sizes or pore entrances of about 10 Å diameter [4]. There is thus a gap in materials that are in the 10–20 or 25 Å range. We have utilized sol–gel techniques, with amines as templates, to prepare amorphous mixed oxides with pore sizes largely in between those of zeolites and the mesoporous materials [5]. Previous reports on our mixed silica–alumina–MO (M = Ni, Mn and Zn) families of materials with broad range values of MO have been published [6–8]. Because of the amorphous nature of these materials it is difficult to determine whether a single phase is present over this broad range of compositions. Potentially such materials may be active as catalysts [6] or sorbents but because of their poor crystallinity structural informa-

* Corresponding author. Tel.: +1 979 845 2936; fax: +1 979 845 4719. E-mail address: clearfi[email protected] (A. Clearfield). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.08.023

tion via X-ray diffraction methods is not forthcoming. Thus, a key structural question remains: are transition metal atoms uniformly incorporated into the silica/alumina matrix of the mixed oxide materials or are they situated within the cavities? In addition, organic templates, applied in the sol/gel synthesis, can reduce metal ions under high-temperature calcinations of the materials. Solid-state NMR is one of the powerful physical methods widely applied in materials science [9,10]. This method is particularly successful when materials are diamagnetic. Even for the strongly amorphous molecular systems, the various solid-state MAS NMR techniques can show important structural details that are often unavailable by other physical methods. In contrast, applications of solid-state NMR for paramagnetic solids are often non-trivial because of strong electron–nucleus coupling, intense dipole–dipole electron–nucleus interactions and also effects of the relaxation and BMS (bulk magnetic susceptibility) nature [7,11,12]. Big contact chemical shifts with large anisotropies [13–18] in combination with short relaxation times [19] lead to a situation where detection of NMR signals is difficult and their interpretation is problematic. Generally the nuclei, which are located closely to paramagnetic centers, for example, in their first coordination sphere, are ‘‘invisible” in the standard MAS NMR spectra while the nuclei, remote from these centers, show resonances accompanied by intense sidebands [11,17,20]. Additional difficulties are connected with the fact that short relaxation times and sideband effects can be observed even in systems where paramagnetic ions have been mechanically

79

V.I. Bakhmutov et al. / Microporous and Mesoporous Materials 118 (2009) 78–86

mixed with amorphous oxides [7]. Under these circumstances it seems to be very important to reveal what structural information can be extracted from the solid-state NMR data. In our previously published papers we reported the synthesis of two series of supermicroporous mixed oxides ZnO–Al2O3–SiO2 [8] and MnO–Al2O3–SiO2 [7], by the sol/gel method, in which tetraethylorthosilicate (TEOS) was the solvent, as well as the source of SiO2. Primary amines of different chain lengths were used as templates. In the case of the ZnO series the surface area increased and the pore size range became larger as the chain length of the amine increased for a fixed oxide composition. Although changes in the solid-state MAS NMR of the zinc-containing oxide mixture relative to a prepared baseline alumina–silica mixture indicated the presence of Zn–O–Al and Si–O–Zn bonding, the homogeneity of the mixture could not be determined. This was not the case for the manganese containing samples. Paramagnetic ions have a pronounced effect on magnetic nuclei depending upon the proximity of the two species. By comparing the SS MAS NMR spectra of a mixture of Mn(O2CCH3)2 and a silica–alumina baseline composition with spectra of the MnO– Al2O3–SiO2 series in which MnO was increased from 1 wt% to 20 wt%, information on the homogeneity was forthcoming [21]. MnO was found to be homogeneously dispersed in the silica–alumina matrix up to about 2.5 wt%. At larger amounts the excess Mn2+ migrated out of the mix to line the pore surfaces [7,21,22]. Previously we synthesized a series of NiO–SiO2–Al2O3 materials and utilized them in reduced form as catalysts [6]. In this paper, we focus on multinuclear NMR and on NMR relaxation and magnetic susceptibility measurements in order to establish the nature and distributions of paramagnetic centers in systems where Ni2+ loadings have been varied from 0 to 30 wt% in a silica–alumina matrix. 2. Experimental Materials 1–8 (Table 1) were obtained by a sol–gel method by using tetraethylorthosilicate (TEOS) and aluminum tri-sec-butoxide (ATSB) as sources of silicon and aluminum, respectively. Nickel(II) acetate tetrahydrate was used as the source of nickel and cyclohexylamine was added as an organic template and a base. The approximately constant content of carboxyl ions was controlled by addition of benzoic acid. All reagents were of ACS grade from Aldrich. The synthesis procedure was very similar to that reported earlier in details discussed for the zinc-containing materials [8]. Typically, TEOS is placed into a beaker and completely homogenized with ATSB. After the homogenization, the amine was added followed by Ni(OOCCH3)2  4H2O and the mixture stirred for 45– 50 min. Then, ddi H2O (distilled and deionized) was added to the beaker as required. The amount of water to be added was calculated by taking into account the four moles of water present per mole of nickel acetate. After the whole was aged for 20 h, the beaker was placed into an oven and the temperature was gradually

raised to 200 °C within 3–4 h. The gels were heated at this temperature for 24 h in air to dryness. The final products were calcined by overnight heating at 450 and 540 °C in air to remove the organic components [6]. The samples are marked through the text as 1(450)–8(450) and 1(540)–8(540), respectively. The 29Si, 1H, 2H, 27Al, 13C MAS NMR experiments were performed with a Bruker Avance-400 spectrometer equipped with standard 4 and 7 mm MAS, and wide-line probe heads. The 29Si, 27 Al Hahn-echo MAS NMR spectra were obtained with echo-delays corresponding to rotor periods. Phase corrections in these spectra were carried out at PHC0 = 0 and PHC1 = SWH  (echo delay)  360. The 27Al MAS NMR spectra were recorded with low-power pulses to excite selectively the central transitions. The 13C and 29Si MAS spectra were also collected with a common cross-polarization pulse sequence. The external standards used for the 27Al and 13C, 29 Si, 1H NMR experiments were [Al(OH2)6]3+ and TMS, respectively. A wide-line probe was used for the variable-temperature 2H NMR experiments performed with a quadruple-echo pulse sequence. The 29Si, 1H and 2H T1 relaxation measurements were preformed by the standard inversion–recovery (180°–s–90°) experiments with accurate pulse adjustments because a bad adjustment, itself, can lead to an exponential relaxation process [19]. The collected spectra were improved by manual phasing and baseline corrections. The relaxation inversion–recovery curves were treated using the appropriate nonlinear fitting routine. The T1 values are measured with errors 610% and well reproduced by independent experiments. X-ray diffraction patterns were collected with a Bruker D8 diffractometer using CuKa radiation (1.5418 Å) at 40 kV and 40 mA and a diffraction beam graphite monochromator. The measurements were recorded in steps of 0.02–0.04° with a count time of 1–2 s in the 2h range of 2–50–70°. Surface area measurements were performed on an Autosorb-6 (Quantachrome) unit with nitrogen absorption at liquid nitrogen temperature. Both, pure N2 as an absorbate and He as a carrier gas were utilized. Pre-calcined samples were out-gassed at 300 °C for 18 h. The resulting data were analyzed using the standard Autosorb-6 software supplied by Quantachrome Corp. Surface areas were calculated on the basis of the BET model. The more detailed information on the cumulative pore volume and surface area data, as well as pore size distribution characterization, were based on the NLDFT calculations carried out by using the advance Autosorb-1 Version 1.51 package supplied by Quantachrome Corp. The X-ray photoelectron spectroscopy experiments were carried out with a Kratos Axis Ultra Imaging X-ray photoelectron spectrometer equipped with a Mono(Al) anode and a multichannel detector. Charge referencing was measured against adventitious carbon (C 1s, 284.8 eV). Shirley-type background was subtracted from the signals. The recorded spectra were treated with Gauss– Lorentz curves to determine the binding energy of the different element core levels.

Table 1 The analytical data on nickel contents and NLDFT cumulative surface area and pore volume values in microporous silica-based materials 1–8 (aluminum contents are determined as 2–3 wt%) Compound

Ni (wt%)

NLDFT cumulative surface area (m2 g1)

NLDFT cumulative surface area due to micropores (m2 g1)

NLDFT cumulative pore volume (cm3 g1)

NLDFT cumulative pore volume due to micropores (cm3 g1)

1 2 3 4 5 6 7 8

0.0 1.4 3.0 4.8 13.4 19.7 23.0 29.3

708 788 709 735 611 497 562 522

609 626 663 670 550 449 388 173

0.238 0.298 0.216 0.240 0.204 0.169 0.239 0.359

0.177 0.192 0.188 0.201 0.166 0.139 0.122 0.059

Temperature of calcination, 540 °C.

V.I. Bakhmutov et al. / Microporous and Mesoporous Materials 118 (2009) 78–86

The transmission electron microscopy experiments were performed at the Texas A&M University Microscopy and Imaging Center with a JEOL JEM2010 at a working voltage of 200 kV, with a point resolution of 0.23 nm. All imaging magnifications were calibrated using standards of SiC lattice fringes [23] for high magnifications and commercial cross-line grating replica for low magnifications. The samples were dispersed in ethanol solution, a small drop of the solution was transferred onto the top surface of a 400-mesh carbon-film supported Cu grid which was previously glow discharged to achieve a better dispersion, and then the material was dried in air. The energy-dispersive spectroscopy (EDS) measurement was performed using an Oxford Instruments EDS detector with INCA energy platform. Magnetic measurements were performed on a Quantum Design SQUID magnetometer MPMS-XL in an applied field of 1000 Oe in the 2–300 K temperature range. The nickel contents were obtained via an instrumental neutron activation analysis performed at Texas A&M University Center for Chemical Characterization and Analysis. The absolute concentrations of silicon and aluminum were determined via ICP and duplicated by AA analysis for randomly selected samples at Anderson Analytical, Flynn, Texas, and varied within the range 35–40 wt% and 2–3 wt%, respectively. 3. Results and discussion 3.1. Pore structure Table 1 lists the nickel content of the mixed oxides along with the results of the BET N2 sorption–desorption measurements. EDS analysis performed on samples 2, 4 and 6 at five different positions on each sample yielded the same Ni content per measurement within experimental error attesting to the uniformity of the Ni distributions throughout the samples. Four of the eight isotherms are shown in Fig. 1 and all eight are type-I similar to the four shown. These curves are also similar to those obtained for the Al2O3–SiO2–ZnO materials [8] and those for the Al2O3–SiO2– MnO system. It is seen that the bulk of the nitrogen has been sorbed at the lowest pressure which is 103P/P0. This is an indication that the majority of the pores are in the micropore region. Pore sizes as determined by the MP method indicate average pore sizes in the 8–13 Å range. For the DFT method to access pores below about 15 Å in diameter, data collection at pressures in the 107– 103 range are required. However, the curves in Fig. 2 show that at a pore width of 20 Å, the limit of microporosity, the cumulative

0.35 0.30

Cumul. Pore Volume [cc/g]

80

0.25 0.20 0.15 0.10 0.05 0

10

20

30

40

50

60

70

80

Pore Width [Å] Fig. 2. Cumulative pore volumes versus pore width for the samples with varying nickel contents from top to bottom: 2(540), 1(540), 3(540) and 5(540).

pore volumes vary from 85% for sample 3–65% for sample 2 of the total pore value as micropores. It is evident that in each sample, irrespective of the metal oxide added to the SiO2–Al2O3 the majority of nitrogen uptake occurs within the pores below 20 Å, while the larger pores of 20–30 Å are responsible for the remaining nitrogen sorption. A more detailed characterization based on the data obtained at the P/P0 range from 107 to 103 will be reported in a separate publication that is in progress. 3.2. X-ray and XPS data The X-ray diffraction patterns of the materials are typical of amorphous alumina–silica systems where only diffuse scattering is observed. However, at nickel loadings of 13.4% and higher, the X-ray patterns show weak peaks at 2h = 37.13° (d = 2.42 Å) and 43.24° (d = 2.09 Å) that can be assigned to NiO [24]. Nickel oxide particles are also detected by the XPS experiments performed for the materials with nickel concentrations of 13.4% and higher. As seen from Fig. 3, the XPS pattern collected for material 5(450) exhibits a peak centered at 854 eV with a satellite at 860 eV [25]. It should be emphasized that the observed lines are very wide and the presence of more than one type of nickel species cannot be excluded. On the other hand, deconvolutions of such XPS patterns seem to be doubtful because two wide bands in Fig. 3 can be fit to two peaks at 854.2 and 859.8 eV, belonging to ‘‘free” NiO, or to three peaks at 854.3, 857.8 and 859.9 eV, as a combination

250

150

Intensity

Volume (cc/g)

200

100

50

0 0.00

0.25

0.50 Relative pressure (P/P0)

0.75

1.00 890

885

880

875

870

865

860

855

850

845

840

Binding Energy (eV) Fig. 1. Nitrogen sorption/desorption isotherms for the samples with varying nickel contents from top to bottom: 2(540), 1(540), 3(540) and 5(540).

Fig. 3. The XP Ni 2p3/2 spectrum of sample 5(540) calcined at 540 °C.

V.I. Bakhmutov et al. / Microporous and Mesoporous Materials 118 (2009) 78–86

81

of free NiO and NiO interacting with the SiO2 support [26], or even four peaks at 854.3, 855.9, 858.3 and 860.6 eV. Thus, because of the nature of the materials investigated, the X-ray and XPS data are only minimally instructive. 3.3. Solid-state NMR Samples 1–8 calcined at 450 and 540 °C were characterized by single-pulse 29Si, 27Al and 1H MAS NMR spectra, and by the Hahnecho 29Si MAS NMR spectra, recorded at different carrier frequencies and the T1 relaxation measurements performed for 1H, 2H and 29Si nuclei [22]. The same technique was successfully applied for characterization of the systems doped by Mn2+ ions [7]. The NMR data were collected for non-calcined and calcined diamagnetic samples 1 and 1(450). Their 29Si and 27Al MAS NMR spectra are typical of amorphous based systems [26]. According to the 29 Si CP and 29Si{1H} MAS NMR spectra of 1 and 1(450), site Q4(Si) remarkably dominates in the 29Si{1H} MAS NMR spectrum of 1(450) (110 ppm) while site Q3(Si) is observed as a weak and poorly-resolved shoulder at 100 ppm (Fig. 4). The 27Al MAS NMR spectrum of 1(450) shows an intense resonance at 53.5 ppm and a weak line at 0.7 ppm (Fig. 5). The relative intensity of this weak line increases in the 27Al CP MAS NMR spectrum of 1(450). Thus, the aluminum atoms are incorporated into the silica matrix as four-coordinated species [27] with the content of the extraframework aluminum being small. All the organic components, clearly observed in the 13C CP MAS NMR spectrum of 1 (sharp resonances at 169, 129, 60.3, 50.7, 33.3, 26.4 and 18.5 ppm), are not detected in the calcined material 1(450). The 1H MAS NMR spectrum of 1(450), exhibits a single resonance at 5.1 ppm, which can be attributed to water molecules located within pores [28]. As evidence for this assignment it was found that the intensity of this signal decreases for the sample packed into an NMR rotor immediately after drying at 65 °C for 1 h. Also, the sample, treated with D2O and then dried at 65 °C (sample 1-2H(450)) shows a narrow resonance in the static 2H NMR spectrum, recorded at 260 K with a quadruple-echo pulse sequence (Fig. 6). Thus quadrupolar interactions, typical for the solid state, are completely averaged due to fast isotropic or pseudo-isotropic motions of the 2H nuclei (for example, rapid tetrahedral jumps) [29]. Careful inspection of the 2H NMR spectrum did not reveal the pres-

Fig. 5. The 27Al MAS NMR spectra recorded at a spinning rate of 6 kHz (from the top the bottom: for sample 1(450) with a 27Al{1H} pulse sequence, for sample 1(450) with a CP H–Al pulse sequence and for sample 5(450) with a single-pulse sequence.

Fig. 6. The variable-temperature 2H NMR spectra recorded for 1-2H(450) (from the top the bottom: 260 K, 220 K, 200 K and 150 K.

ence of a rigid powder-shaped line probably due to a fast exchange between ‘‘free” water molecules and molecules located directly on the silica surface. In accord, the resonance line broadens to 2.2 and 5.5 kHz when the temperature decreases to 230 and 220 K, respectively. At 200 K the line transforms towards a rigid deuterium powder pattern which is resolved at 150 K showing quadrupole splitting of 148 kHz, typical of solid water (Fig. 6) [30]. It should be emphasized that such temperature evolutions are typical of small molecules located in pores of materials and having a high mobility [31]. The room-temperature 1H and 2H T1 NMR relaxation measurements performed for 1(450) and 1-2H(450) independently support the assignment. Generally spin-diffusion completely dominates in spin-lattice relaxation of protons in solids leading to an exponential recovery of the magnetization [28,31]. However, the 1H spinlattice relaxation in sample 1(450), spinning with a rate of 6 kHz, is non-exponential and described by a stretched exponential [32,33]:

I ¼ I0 ð1  2 expððs=T 1 Þb ÞÞ 1

Fig. 4. The 29Si MAS NMR spectra recorded at a spinning rate of 6 kHz (from the top to the bottom): for sample 1 with a 29Si{1H} pulse sequence, for sample 1(450) with a CP H–Si pulse sequence and for material 1(450) with a 29Si{1H} pulse sequence.

ð1Þ

with H T1 = 0.22 s and b = 0.71. The same treatment of the nonexponential 2H T1 relaxation in sample 1-2H(450) results in the 2H T1 time and the b values of 0.014 and 0.76 s, respectively. The non-exponential behavior is typical of molecules situated in pores due to the presence of T1 distributions [34], and the b parameters determined for the 1H and 2H relaxation curves are reasonably similar. Finally, the 1H, 2H T1 times of liquid water and water in pores

82

V.I. Bakhmutov et al. / Microporous and Mesoporous Materials 118 (2009) 78–86

differ strongly. For example, the 2H T1 time in the bulk water is as long as 0.42 s [34] versus 0.014 s measured in sample 1-2H(450). The 29Si spin-lattice relaxation in diamagnetic sample 1(450) is also non-exponential and well treated by a stretched exponential function: 29Si T1 time = 55 s and b = 0.82. It should be noted that a very similar b parameter (0.77) was determined for aryl-sulfonate silica gels [35] and discussed in terms of spin-diffusion when the latter is limited [36,37]. In accounting for the spin-diffusion coefficient of nuclei 29Si, which is equal to 1019 m2 s1 [37], spin-diffusion of 29Si nuclei could be actually effective at delay times s > 20 s in the inversion–recovery experiments. However, since the T1 measurements performed at spinning rates from 1.5 to 11 kHz led to the identical (within 10% errors) 29Si T1 values, the spin-diffusion mechanism in 1(450) can be ruled out [31]. Generally, the 29Si nuclei, for example, in silicate glasses, relax very slowly with T1 times reaching >30 min in the absence of paramagnetic impurities [37]. Therefore, the shorter non-exponential 29Si relaxation in sample 1(450) rather occurs by dipolar interactions of nuclei 29Si with non-controlled paramagnetic centers, for example, O2, situated close to the surface of the silica matrix [38]. The 29Si MAS NMR spectra of samples 2(450)–8(450) or 2(540)– 8(540) show the sideband patterns that are more pronounced at the higher Ni loadings and invisible in the spectrum of the nickel-free material (Fig. 7). The appearance of such intense sideband patterns has been earlier used as a criterion for invoking the incorporation of paramagnetic metal ions into the silica matrix of the materials [20,39]. According to another point of view, the appearance of the sideband patterns, themselves, even with shortened 29Si T1 relaxation times, shows the presence of paramagnetic centers only and does not prove the incorporation of paramagnetic metal ions [40]. The 29 Si MAS NMR experiments, performed for a sample, containing diamagnetic material 1(450) carefully mixed with Ni(CH3COO)2  4H2O in an 1/1 weight ratio (marked through the text as 1M), supported this idea. First, the 29Si MAS NMR spectrum of 1M with 12% (weight) of Ni2+, recorded at a spinning rate of 3 kHz (i.e. smaller than those in Fig. 7), shows the sideband pattern invisible in 1(450) at this rate. Second, the 29Si T1 time in 1M is remarkably shortened (0.29 s). It is obvious that these features cannot be explained by quite close contacts between 29Si and Ni2+ that are practically impossible in the mixture. On the other

Fig. 7. The 29Si MAS NMR spectra (from the top to the bottom) recorded for calcined samples 1(450), 2(450), 3(450), 4(450) (at 5 kHz) and 5(450), 6(450) and 8(450) (at 6 kHz).

hand, the sideband and 29Si relaxation effects are still smaller than those observed for 4(450) with a significantly smaller nickel content. As seen from the 29Si MAS NMR spectra in Fig. 7, the sideband intensities increase sequentially from 0.0 to 13.4 Ni2+%, reach a maximum in material 5(450), do not change from 5(450) to 6(450) and decrease again in 7(450) and 8(450). In spite of the above-mentioned complexity in the spectral behavior of paramagnetic systems, this tendency shows clearly that in the materials, obtained at high nickel loadings, the content of paramagnetic centers is becoming lower. This conclusion is valid even in the absence of their more accurate localization. Since the shape and the chemical shift of the 29Si isotropic resonance in Fig. 7 do not change, the detected nuclei, belonging to the silica lattice, are remote from paramagnetic centers. Closelylocated 29Si nuclei, located in a first coordination sphere, for example, in moieties Si–O–Ni, incorporated into the silica matrix or situated on its surface, could become ‘‘spectrally invisible” due to strong paramagnetic effects [40]. Usually such nuclei can be observed as very wide lines in the Hahn-echo MAS NMR spectra, collected at different carrier frequencies and then summarized [14,40]. However, such Hahn-echo 29Si MAS NMR experiments were unsuccessful and the spectra, collected in 1(450)–5(450) or 1(540)–5(540), did not reveal even a remarkable loss in intensities of the 29Si resonances on going from diamagnetic 1(450) to paramagnetic systems. However, it is obvious that small contents of such moieties cannot be completely excluded due to the wellknown problem of NMR sensitivity particularly in the cases of rare nuclei with broadened signals. The isotropic 27Al resonance in 2(450)–8(450) and 2(540)– 8(540) is also accompanied by intense sidebands and observed at 54 ppm (Fig. 5). The Hahn-echo 27Al MAS NMR experiments at different carrier frequencies gave the same result. Again, the detected Al atoms are in the silica framework and remote from paramagnetic centers. Finally, the 1H MAS NMR spectra of 2(450)–5(450) and 2(540)–5(540) exhibit sideband patterns with the isotropic resonances very close to that of the pore water in the diamagnetic sample 1(450). In good agreement with increasing the Ni loadings the sideband effects become more pronounced from 2(450) to 5(450) and the line-widths increase from 430 Hz in 1(450) to 990, 1670, 4280 and 20,000 Hz in 2(450), 3(450), 4(450) and 5(450), respectively. These strong 1H broadenings in combination with the relatively weak broadening effects observed for 29Si and 27 Al nuclei in Figs. 7 and 5, respectively, show accumulations of the paramagnetic Ni2+ centers within pores of the materials. Additional data supporting this idea were obtained by NMR relaxation experiments that revealed a very interesting effect. We have found that the 29Si T1 relaxation times are different in the freshly-prepared paramagnetic samples, calcined at different temperatures (450 and 540 °C): the T1 times are remarkably shorter in the materials, calcined at 540 °C. For example, the 29Si T1 time in freshly-prepared 4(450) and 4(540) was measured as 1.0 and 0.64 s, respectively. However, after one month, the values reduce and become identical: 0.24 and 0.23 s in 4(450) and 4(540), respectively. Then, after two and three months, the 29Si T1 values do not change in the 10% limit. This phenomenon seems to be connected with the movement of the nickel centers. We believe that the relatively mobile water in systems 2(450)–8(450) and 2(540)–8(540) can promote slow room-temperature migrations of the paramagnetic centers along pores. The nickel migrations cause more homogeneous redistributions through the volumes of the samples and increase effectively a number of close contacts NiSi. The 29Si and 1H T1 relaxation times measured in such calcined paramagnetic systems are presented in Tables 2 and 3. As these data indicate, the proton relaxation is not exponential and described by a stretched exponential with the b parameters between 0.65 and 0.7.

V.I. Bakhmutov et al. / Microporous and Mesoporous Materials 118 (2009) 78–86 Table 2 The 29Si MAS NMR T1 data collected for calcined materials 1(450)–8(450) and 1(540)– 8(540) treated with stretched exponential functions (the data are obtained as a spinning rate of 6 kHz) Sample number

wt% of Ni2+

T1 (s)

b

1 2 3 4 5 6 7 8

0 1.35 2.95 4.8 13.4 19.7 23.0 29.3

55 0.86 0.50 0.23 0.16 0.25 0.30 0.40

0.82 0.58 0.55 0.75 0.64 0.63 0.60 0.69

Table 3 The 1H T1 data obtained at a spinning rate of 6 kHz in the calcined materials 1–8 and treated by the stretched exponents System (wt% of Ni2+)

T1 (450 °C) (s)

b (450 °C)

1 2 3 4 5 8

0.22 0.0011 0.00051 0.00028 0.00014 0.00049 0.00051a

0.71 0.69 0.66 0.65 – 0.6 0.6

(0) (1.35) (2.95) (4.80) (13.4) (29.3) a

At 11 kHz.

Since the 1H inversion–recovery experiments, performed at spinning rates 6–12 kHz, resulted in the identical 1H T1 times (for example, 4.9  104–5.1  104 s in 8(450)), the 1H spin-diffusion mechanism is negligible [31] and the relaxation is controlled by dipolar interactions with paramagnetic centers [9,19,40]. The same conclusion can be drawn for the 29Si T1 relaxation. It is also nonexponential (Fig. 8), well treated by a stretched exponential function with the b values between 0.58 and 0.75 (Table 2) and is independent of spinning rates (it should be noted that in some cases the inversion–recovery curves can be treated in terms of a

Fig. 8. The 29Si inversion–recovery curve obtained at a spinning rate of 6 kHz for sample 4(450) treated with a stretched exponential (top) and bi-exponential function (bottom).

83

bi-exponential process, for example, material 4(450) in Fig. 8); however, since such treatments give generally larger errors, they are not discussed). Finally, the linear correlation between the 1H and 29Si T1 relaxation rates in Eq. (2)

R1 ð1 HÞ ðs1 Þ ¼ 1170R1 ð29 SiÞ ðs1 Þ  720

ð2Þ

illustrates clearly the same relaxation mechanism operating for both nuclei. This dipolar relaxation mechanism is expressed by

ð3Þ where Np is the number of paramagnetic centers, se is the electron relaxation time and the other constants are well known [41]. As follows from Tables 2 and 3, the 29Si and 1H T1 times shorten strongly even at a Ni2+ concentration of 1.35%. Generally such effects are attributed to homogeneous distributions of paramagnetic centers through the volume of the samples [42] due to their incorporation into the silica matrix. However, the 29Si isotropic resonances and the their sidebands observed in material 4(450) or 4(540) have shown the identical 29Si T1 times in contrast to the systems where paramagnetic manganese centers were actually incorporated into the silica lattice [21,22]. The remarkably shorter T1 relaxation times of sidebands with respect to isotropic resonances have been also reported for nuclei 31P in systems with paramagnetic centers incorporated into the matrix of the materials [43]. Thus, the relaxation data show that the nickel centers are homogeneously distributed within pores of our calcined systems in agreement with the TEM micrograph illustrated for sample 5(450) in Fig. 9. It should be emphasized that equal [29] Si T1 times are measured for the isotropic resonance and its sidebands in the mixture 1M where incorporation of Ni2+ is completely excluded. As the data in Tables 2 and 3 reveal, a R1(1H)/R1(29Si) ratio, obtained for the 1H and 29Si relaxation rates, changes insignificantly from one paramagnetic sample to another one and calculated as 900. At equal internuclear distances r(SiNi) and r(HNi), the dipolar mechanism must be more effective for 1H nuclei by a factor of 25 (see c2 in Eq. (3)). Thus, it is obvious that 1H nuclei are situated closer to paramagnetic centers than 29Si nuclei by a factor of 1.8. Again, the data correspond to accumulations of the paramagnetic centers within pores of the materials. The formalism in Eq. (3) requires decreasing the 29Si T1 times proportionally to [Ni]2. The pattern in Fig. 10 is more complex: the 29Si relaxation rate increases from 1(450) to 5(450), reaches a maximum and decreases at higher Ni contents. It should be noted that this pattern correlates with the sideband effects observed in

Fig. 9. The TEM micrograph of calcined material 5(450).

84

V.I. Bakhmutov et al. / Microporous and Mesoporous Materials 118 (2009) 78–86 Table 4 Magnetic properties of materials 2(450)–4(450) and 2(540)–4(540)

Fig. 10. The 29Si spin-lattice relaxation rate as a function of the nickel loadings in samples 1(450)–8(450) (d); point (j) corresponds to the relaxation rate in 1M.

the 29Si MAS NMR spectra (Fig. 7). It is also important that the 29Si T1 time in mixture 1M obtained by a stretched-exponent treatment at b = 0.64 is short (0.29 s) but the corresponding point in Fig. 10 completely drops out of the plotted sequence. As emphasized above, nuclei 29Si in materials 2(450)–8(450) relax via direct dipolar interactions with the paramagnetic centers (Eq. (3)). However, the linear dependence of the 29Si relaxation rate on the Ni concentration from 1(450) to 4(450) plotted on a logarithmic scale shows a slope of 0.6 instead of 2 expected theoretically (see Eq. (3)). This disagreement could be explained by aggregations of paramagnetic centers and reducing the electron relaxation time, se, due to exchange interactions [20]. The deviation of material 5(450) from the linearity in Fig. 10 is too large, however, and the further decrease in the relaxation rate rather corresponds to a partial transformation of the paramagnetic nickel centers into a non-paramagnetic state, for example, Ni0 [44]. Then, extrapolation of the linear section into the region of 5(450) provides an estimation of the content of the metal nickel in this material as 50%. As described below, even this rough estimation agrees well with magnetic measurements performed on powder samples 2(450)–5(450) and 2(540)–5(540) to reveal the nature of the paramagnetic centers.

System

Curie constant, C (emu K g1)

h (K)

g

2(450) 2(540) 3(450) 3(540) 4(450) 4(540)

0.254 0.263 0.45 0.495 0.96 0.84

0.8 1.3 2.2 3.2 +11.5 +9.7

2.33 2.37 2.1 2.2 2.4 2.25

magnetic interactions between magnetic centers. In accord with this assumption is the fact that the temperature dependences are well described by the Curie–Weiss law leading to a negative Weiss constant h of 2.2 K and 3.2 K for samples 3(450) and 3(540). The room-temperature vT values in samples 4(450) and 4(540) are calculated as 0.96 and 0.84 emu g1 K, respectively. In this case, however, the vT magnitude gradually increases to a broad maximum at 10 K and then it decreases again (Fig. 11). A treatment in terms of the Curie–Weiss law leads to positive Weiss constants h of +11.5 and +9.7 K for 4(450) and 4(540), respectively. Although bulk NiO is an antiferromagnet below TN = 525 K [47], this result suggests ferromagnetism due to the formation of NiO nanoparticles [48]. In fact, according to Néel [49], fine particles of an antiferromagnetic material should exhibit magnetic properties such as superparamagnetism and weak ferromagnetism, wherein the permanent magnetic moment is attributed to an uncompensated number of spins on two sublattices. The zero field cooling–field cooling (ZFC-FC) measurements performed in 4(450) and 4(540) at 10 Oe confirm this conclusion and show a 3D magnetic ordering at 6 K. This ordering is accompanied by a history dependence of the magnetization process in that the remnant magnetization does not follow the FC-curve and corresponds to glassy behavior [50]. The hysteresis observed at 1.8 K with coercivity at 750 Oe and 400 Oe for the materials 4(450) and 4(540) is depicted in Fig. 11 (inset) by the region ±7500 Oe. Indeed, NiO nanoparticles were found to exhibit large coercive fields at low temperature, due to surface anisotropy [51–53]. Finally, the frequency dependence of the AC magnetic susceptibility studied below the phase transition (Fig. 12) agrees with the presence of a degree of cluster-glass like behavior with the Mydosh parameter, / ¼ DT m =T g =D log x [54], being estimated as 0.1. Here DTm is the shift of the peak in v0 , log x is the logarithm of the applied frequency, and Tg is freezing temperature.

3.4. Magnetic measurements The magnetic behavior of the materials, calcined at 450 and 540 °C, are similar. Samples 2(450) and 2(540) exhibit roomtemperature vT values of 0.254 and 0.263 emu g1 K, respectively (Table 4). Taking into account the elemental analyses, the spin-state of the magnetic centers is estimated as S = 1, g = 2.33 and g = 2.37 typical of the magnetically-isolated Ni2+ ions (S = 1, g = 2.0–2.8) [45]. Below 10 K, the vT value decreases due to zero field splitting effects [46] and the temperature dependence of the magnetic susceptibility can be fit to the Curie–Weiss law giving negative Weiss constants, h, of 0.8 K and 1.3 K for the materials 2(450) and 2(540), respectively. Similar room-temperature magnetic properties are observed in 3(450) and 3(540) (Table 4). However, upon cooling, the vT values decrease indicating random antiferro-

Fig. 11. Temperature dependence of the vT product for sample 4(450). The solid and dashed lines correspond to the best fit to Curie–Weiss law. Inset: Hysteresis loop for 4(450) at 1.8 K.

85

V.I. Bakhmutov et al. / Microporous and Mesoporous Materials 118 (2009) 78–86

1 Hz

0.2

1.4

10 Hz

0.7

1 Hz 10 Hz

1.3

100 Hz

100 Hz 0.15

χ

χ

0.3 0.2

0.05

0.1

1

3

0.1

1 kHz

1.1

3

0.4

χ · 10 (emu/g)

0.5

1.2

3

1 kHz

· 10 (emu/g)

3

· 10 (emu/g)

0.6

χ · 10 (emu/g)

0.8

0.9

0.05

0.8

0.1

0.7

0

0 2

4

8

6

0.6

10

0 4

2

Temperature (K)

8

6

10

12

Temperature (K) 0

00

Fig. 12. Temperature dependence of the real v (full symbols) and imaginary v (open symbols) components of the AC susceptibility with oscillating field of 3 Oe at different frequencies for 4(450).

Fig. 14. Temperature dependence of the real v0 (full symbols) and imaginary v00 (open symbols) components of the AC susceptibility with oscillating field of 6 Oe at different frequencies for 5(450).

The variable-temperature magnetic measurements in samples 5(450) and 5(540) (corresponding to the top point of the relaxation dependence in Fig. 10) differ completely from those in materials with lower nickel concentrations. These data show a substantial temperature-independent component of the ferromagnetic nature usually observed for particles Ni0 [53]. The room-temperature field-dependent magnetization in this material reveals an unusually fast rise at low field without saturation even at 70 kOe (Fig. 13). Generally such behavior is attributed to the formation of ferromagnetic Ni0 nanoparticles [55] which have blocking temperature close to the ordering temperature of bulk nickel Tc = 630 K [56]. Nevertheless, the curves do not follow the simple Langevin expression valid for a superparamagnet [57]. Since the pattern M versus H has a nonzero slope even at higher H, the curves were fitted to the modified Langevin function [58–60]:

magnetization of 55 emu g1 [61,62] one can estimate the Ni2+ to Ni0 transformation as 30%. In order to understand better the above feature better, the lp magnitudes were determined from the equation lp/lB = MsqV, where q and V are the density and volume of the particle, respectively. Assuming for bulk nickel a spherical particle with q = 8.9 g cm3 and Ms = 55 emu g1 gives a diameter D of 8.1 nm. Materials 5(450) and 5(540), which exhibit similar magnetic behavior, also show a narrow hysteresis at 300 K with coercivity of approximately 100 Oe (Fig. 13, inset), again in accord with previously reported data for Ni nanoparticles [63]. The frequency dependence of the AC magnetic susceptibility reveals two peaks (Fig. 14) corresponding to a two phase system. The Mydosh parameter, /, was estimated as 0.14 in a full agreement with the superparamagnetic behavior. Finally materials 6(450)–8(450) with higher nickel concentrations exhibit mostly room-temperature ferromagnetic properties. As follows from the collective data, the magnetization and 29Si T1 relaxation experiments carried out for 5(450) are in good agreement, and nanoparticles with sizes similar to those calculated by the magnetic measurements can be found in the TEM micrograph (Fig. 9). In contrast, standard deconvolution procedures, performed for the wide XPS bands in Fig. 3, did not reveal the presence of peaks at 852.3 and 868.0 eV for nickel metal [26]. This result is a good illustration of the intrinsic problem of the XPS method which probes only near-surface atom concentrations. The Ni0 cluster sizes are ‘‘invisible” in most of the X-ray diffraction patterns of paramagnetic materials even with high nickel loadings. However, fortunately,

M ¼ Ms Lðlp H=kB TÞ þ va H

ð4Þ

where lp is the magnetic moment of a nanoparticle, kB is the Boltzmann constant, L(x) = coth x  1/x, and va is the high-field susceptibility. The plot of magnetization versus field at T = 300 K shown in Fig. 13 reveals superparamagnetism with lp = 14,000lB, and Ms = 1.4 emu g1. Using the analytic data and assuming a magnetic behavior of the particle close to that of bulk metal and a saturation

120

100

Intensity

80

60

40

20

0 0

Fig. 13. Room-temperature field-dependent magnetization curve for 5(450). The solid line corresponds to the best fit to Eq. (4). Inset: Hysteresis loop for 5(450) at 300 K.

10

20

30

40

50

2Θ Fig. 15. The X-ray pattern of material 5(450).

60

70

86

V.I. Bakhmutov et al. / Microporous and Mesoporous Materials 118 (2009) 78–86

these Ni0 clusters are well observed in the X-ray pattern recorded for material 5(450) (Fig. 15). Partial reduction of Ni2+ to Ni0, responsible for the magnetic properties of materials 5(450)–8(450) and 5(540)–8(540), is not surprising and is attributed to the presence of organic templates used in the synthetic procedures. It is interesting, however, that sample 8(450), heated in oxygen at 540 °C for 6 h, exhibits the same 29 Si NMR MAS spectrum and the same 29Si T1 time (0.33 s at b = 0.73). It is generally accepted that the metallic nickel is completely oxidized in an oxygen atmosphere at 430 °C [64,65]. In this connection we believe that the above stability is explained by the location of the nickel particles within a NiO shell. 4. Conclusions A series of materials SiO2–Al2O3–NiO with a predominant pore size in the range 8–20 Å have been synthesized by the sol–gel method where Ni2+ loadings have been varied between 0 and 30 wt%. The materials calcined at 450 and 540 °C were studied by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, variable-temperature magnetic susceptibility measurements and 29Si, 27Al, 1H, 13C solid-state MAS NMR spectra recorded with single-pulse and Hahn-echo pulse sequences. It has been shown that solid-state NMR and 1H and 29Si spin-lattice NMR relaxation in such amorphous paramagnetic solids can be successfully used for their characterizations. The 1H and 29 Si spin-lattice NMR relaxation is a non-exponential process and governed by direct dipolar interactions with paramagnetic centers while the spin-diffusion mechanism is negligible. The 29Si and 1H relaxation rates, obtained in the limits of the stretched exponential, nicely reflect the distribution of paramagnetic centers. It was found that aluminum atoms as four-coordinated species are incorporated into the silica matrix while nickel centers are accumulated within pores. The nature of the nickel centers was probed by variable-temperature magnetic susceptibility measurements supported by XPS and X-ray experiments. The Ni species are concluded to be NiO and Ni0 (observed at high nickel loadings) aggregated into nanoparticles. The Ni0 nanoparticles are responsible for the room-temperature ferromagnetic behavior of the materials prepared with high nickel loadings whereas the lowtemperature ferromagnetism, observed in the materials with lower nickel loadings, is caused by NiO nanoparticles. Acknowledgments The work reported here was supported by the National Science Foundation under Grant Nos. DMR-0332453 and CHE-0234931, for which grateful acknowledgment is made. K.R.D. is grateful to the Department of Energy and the Welch Foundation for financial support. The SQUID magnetometer was purchased with a grant from the National Science Foundation. We sincerely thank Dr. W.D. James and Mr. M. Raulerson for performing the neutron activation analysis on our samples. References [1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olsen, W.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [2] C.T.Kresge,M.E. Leonowicz,M.E.Roth,W.J. Vartuli, J.S. Beck, Nature359(1992) 710. [3] (a) A. Monnier, F. Schuth, Q. Huo, D. Kumar, D.I. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, I.M. Janicke, B.F. Chmelka, Science 261 (1993) 1299; (b) Q. Huo, P. Petroff, F. Schuth, G.D. Stucky, Nature 368 (1994) 317. [4] M.E. Davis, Acc. Chem. Res. 26 (1993) 111. [5] B.G. Speizer, A. Clearfield, J.M. Heising, Chem. Commun. (2005) 2396. [6] S.R. Kirumakki, B.G. Shpeizer, G.V. Sagar, K.V.R. Chary, A. Clearfield, J. Catal. 242 (2006) 319.

[7] V.I. Bakhmutov, B.G. Shpeizer, A. Clearfield, Magn. Reson. Chem. 44 (2006) 861. [8] B.G. Shpeizer, V.I. Bakhmutov, A. Clearfield, Micropor. Mesopor. Mater. 90 (2006) 81. [9] M.J. Duer, Solid-State NMR Spectroscopy: Principles and Applications, Blackwell Science, Oxford, 2002. [10] C. Bonhomme, C. Coelho, N. Baccile, C. Gervias, T. Avaiz, F. Babonneau, Acc. Chem. Res. 40 (2007) 738. [11] G. Mali, A. Ristic, V. Kaucic, J. Phys. Chem. B 109 (2005) 10711. [12] A. Kubo, T.P. Spaniol, T. Terao, J. Magn. Reson. 133 (1998) 330. [13] R. Rosas-Salas, J.M. Dominguez, F. Rachdi, C. Alvarez, Stud. Surf. Sci. Catal. 146 (2003) 131. [14] L. Canesson, A. Tuel, Chem. Commun. (1997) 241. [15] H. Hiese, F.H. Kohler, X. Xie, J. Magn. Reson. 150 (2001) 198. [16] A.N. Clayton, C.M. Dobson, C.P. Grey, J. Chem. Soc. Chem Commun. (1990) 72. [17] C.P. Grey, C.M. Dobson, A.K. Cheetham, R.J.B. Jakeman, J. Am. Chem. Soc. 111 (1989) 505. [18] A.M. Beale, G. Sankar, C.R.A. Catlow, P.A. Anderson, T.L. Grenn, Phys. Chem. Chem. Phys. 7 (2005) 1856. [19] V.I. Bakhmutov, Practical NMR Relaxation for Chemists, Wiley, Chichester, 2005, pp. 179–181. [20] D. Goldfarb, Zeolites 9 (1989) 509. [21] V.I. Bakhmutov, B.G. Shpeizer, A. Clearfield, Magn. Reson. Chem. 44 (2006) 985. [22] V.I. Bakhmutov, B.G. Shpeizer, A. Clearfield, Magn. Reson. Chem. 45 (2007) 118. [23] Z.P. Luo, Acta Mater. 54 (2006) 47. [24] T. Ahmad, K.V. Ramanujachary, S.E. Lofland, A.K. Gaguli, Solid State Sci. 8 (2006) 425. [25] G. Poncelet, M.A. Centeno, R. Molina, Appl. Catal. A Gen. 288 (2005) 232. [26] R. Mokaya, Micropor. Mesopor. Mater. 44–45 (2001) 119. [27] S. Ganapathy, K.U. Gore, R. Kumar, J.P. Amoureax, Solid State Nucl. Magn. Reson. 24 (2003) 184. [28] X. Xue, M. Kanzaki, Solid State Nucl. Magn. Reson. 31 (2007) 10. [29] R.J. Witterbort, M.G. Usha, D.J. Ruben, D.E. Wemmer, A. Pines, J. Am. Chem. Soc. 110 (1998) 5668. [30] A.J. Benesi, A.M.W. Grutzeck, B. O’Hare, J.W. Phair, J. Phys. Chem. B 108 (2004) 17783. [31] S. Hayashi, Solid State Magn. Reson. 3 (1994) 323. [32] G. Williams, D.C. Watts, Trans. Faraday Soc. 66 (1970) 80. [33] C.A. Angel, L.M. Torell, J. Chem. Phys. 78 (1983) 937. [34] R. Chen, P.E. Stallworth, S.G. Greenbaum, R.L. Kleinberg, J. Magn. Reson. A 110 (1994) 77. [35] M.H. Alaimo, J.E. Roberts, Solid State Nucl. Magn. Reson. 8 (1997) 241. [36] B. Maiti, B.R. McGarvey, J. Magn. Reson. 58 (1984) 37. [37] M.G. Motruza, R. Dupree, D. Holland, J. Non-Cryst. Solids 281 (2001) 108. [38] C.A. Fyfe, D.H. Brouwer, J. Am. Chem. Soc. 126 (2004) 1306. [39] C. Montes, M.E. Davis, B. Murray, M. Narayana, J. Phys. Chem. 94 (1990) 6425. [40] L. Canesson, Y. Boudeville, A. Tuel, J. Am. Chem. Soc. 119 (1997) 10754. [41] G. Mali, V. Kaucic, Solid State Nucl. Magn. Reson. 12 (1998) 243. [42] S.H. Chen, S.P. Sheu, K.K. Chao, J. Chem, Soc. Chem. Commun. (1992) 1504. [43] B. Suttler, R.E. Taylor, L.R. Hossner, D.W. Ming, Soil Sci. Soc. Am. J. 66 (2002) 455. [44] R.A. Shunn, R. Baker, R.J. Cole, J.D. Gilbert, D.P. Madden, Inorg. Synth. 15 (1975) 5. [45] E.A. Bourdreaux, L.N. Mulay, Theory and Application of Molecular Magnetism, Wiley, New York, 1976. p. 210. [46] O. Kahn, Molecular Magnetism, VCH Publisher, New York, 1993. p. 26. [47] C. Kittel, Introduction to Solid State Physics p. 448, seventh ed., Wiley and Sons, New York, 1996 (Chapter 15). [48] R.H. Kodama, A. Salah, A.E. Berkowitz, Phys. Rev. Lett. 79 (1997) 13934. [49] L. Neel, in: C. Dewitt, B. Dreyfus, P.D. Gennes (Eds.), Low Temperature Physics, Gordon and Beach, New York, 1962, p. 413. [50] R.L. Carlin, Magnetochemistry, Springer-Verlag, Berlin, Heidelberg, 1986. [51] L. Li, L. Chen, R. Qihe, G. Lia, Appl. Phys. Lett. 89 (2006) 134102. [52] E. Winkler, R.D. Zysler, M. Vasquez Mansilla, D. Fiorani, Phys. Rev. B 72 (2005) 132409. [53] Y. Ichiyanagi, N. Wakabayashi, J. Yamazaki, S. Yamada, Y. Kimishima, E. Komatsu, H. Tajima, Physica B 329–333 (2003) 862. [54] J.A. Mydosh, Spin Glasses, Taylor and Francis, London, 1993. [55] C. Parada, E. Moraan, Chem. Mater. 18 (2006) 2719. [56] H. Amekura, Y. Fudamoto, Y. Takeda, N. Kishimoto, Phys. Rev. B 71 (2005) 172404. [57] P. Dutta, A. Manivannan, M.S. Seehra, N. Shah, G.P. Huffman, Phys. Rev. B 70 (2004) 174428. [58] A. Punnoose, T. Phanthavady, M.S. Seehra, N. Shah, G.P. Huffman, Phys. Rev. B 69 (2004) 054425. [59] S.A. Makhlouf, F.T. Parker, A.E. Berkowitz, Phys. Rev. B 55 (R14) (1997) 717. [60] M.S. Seehra, P. Dutta, H. Shim, A. Manivannan, Solid State Commun. 129 (2004) 721. [61] C. Estournes, T. Lutz, J. Happich, T. Quaranta, P. Wissler, J.L. Guille, J. Magn. Magn. Mater. 173 (1997) 92. [62] Y. Wang, Q. Zhu, H. Zhang, J. Mater. Chem. 16 (2006) 1212. [63] J.H. Hwang, V.P. Dravid, M.H. Teng, J.J. Host, B.R. Elliott, D.L. Johnson, T.O. Mason, J. Mater. Res. 12 (1997) 1076. [64] C. Hoang-Van, Y. Kachaya, S.J. Teichner, Y. Arnaud, J.A. Dalmon, Appl. Catal. 46 (1989) 281. [65] S. Narayan, R.P. Unnikrishan, V. Vishwanathan, Appl. Catal. 129 (1995) 9.