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Magnetic properties of faujasites
Abstract
This chapter gives data and a brief introduction about the magnetic properties of faujasites.
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Temperature-dependent EPR data of potassium-electro-sodalite (PES), K8[Al6Si6O24](e-)2, are consistent with the occurrence of an antiferromagnetic phase transition at 71±2 K. PES is a Mott insulator which contains an unpaired electron in every sodalite cage. The same transition in sodium-electro-sodalite occurs at a considerably lower temperature (42 K), indicating that the exchange interaction among localized electrons is stronger in PES. PES is obtained by the inclusion of one potassium atom in every cage of potassium sodalite. The 27Al MAS NMR resonance of PES is shifted downfield in respect to diamagnetic potassium-sodalite, K6[Al6Si6O24]. The NMR shift is due to unpaired electrons and is caused by hyperfine Fermi contact interaction.
This chapter discusses the influence of calcium ions on the properties of nickel faujasite catalysts for the hydrogenation of carbon monoxide. In the experiment described in the chapter, high initial catalytic activity (NiCaX 1.2, NiCaX 5, NiX (17), NiX (57)) might have been due to a relatively large portion of Ni ions in positions easily accessible for reduction. For samples exchanged in the sequence calcium followed by nickel, the Ca ions seemed to hinder the Ni ions from reaching SI sites. This effect was balanced by either a high Ni content (NiCaX 1.2) or a simultaneous exchange (NiCaX 5). The samples with the lowest initial catalytic activity (NiCaX 3.1, NiCaX 1.1) exhibited the highest relative increase of activity, raising the temperature from 250° C to 300° C. This effect might have been due to especially pronounced additional reduction of Ni ions in other than SI sites. The samples also had a strong tendency to sinter, according to the activity decline at 350° C and the X-ray analysis data. The highest final activity and stability at 350° C was observed for samples NiCaX 5, NiX (17), and NiX (57)—that is, for catalysts, which also had the highest initial activity. This means that the initial reduction condition determines the quality of the catalyst.
In a porous crystal of zeolite low-silica X (LSX), β-cages and supercages (cavities) are arrayed in a diamond structure, respectively. We loaded potassium metal into zeolite LSX which has a chemical formula of Na7.3K4.7Al12Si12O48 per β-cage (or supercage), and generated Na-K alloy clusters in β-cages and/or supercages. We have investigated the magnetic properties, the optical ones and the electrical resistivity at various values of K-loading density n per β-cage (or supercage) up to n = 9.7. Localized magnetic moments are observed at 8.2 < n < 9.7. Almost simultaneously, nearly pure ferromagnetism is observed at 8.4 < n < 9.7. The highest Curie temperature is ≈12 K at n ≈ 9. Optical reflection spectra for 8 < n have a new band at 2.8 eV which is assigned to the optical excitation of s-electrons of clusters generated at β-cages. The origin of the magnetic moments is assigned to these β-cage clusters because of the coincidence between the growths of the 2.8 eV band and localized magnetic moments. The origin of magnetic ordering is explained by a ferromagnetic interaction between β-cage clusters. All samples are found to be insulating from the temperature dependence of electrical resistivity. A direct magnetic interaction between β-cage clusters is not expected because of the high electronic barrier between them. A ferromagnetic superexchange coupling between β-cage clusters is newly proposed via the sp3-like closed-shell clusters at supercages. A thermal hysteresis is observed in the electrical resistivity at intermediate temperatures, and the origin is assigned to low-density carriers generated in the Na-K eutectic alloy structures in nanospace.
Nickel (Ni) nanoparticles were synthesized in micropores of zeolite by the adsorption and decomposition of a sublimated Ni organometallic compound, Ni(C5H5)(2), to invent metallic catalysts with nanosize, which are smaller than 5 nm and keep the nanosize at high temperature. In the decomposition process, Ni species were partially decomposed by ultraviolet light irradiation and fixed in zeolite pores prior to thermal reduction under H-2 flow. Note that the Ni nanoparticles showed an excellent thermal stability, because they kept the high dispersion with diameters smaller than 5 nm even after heating at 400 degrees C. On the other hand, the Ni particles supported on zeolite by a conventional method, which is an incipient wetness impregnation process, became larger than 10 nm after heating at the same temperature. The synthesized Ni nanoparticles acted as a metallic catalyst because they showed higher selectivity for H-2 generation than C2H4 generation during ethanol steam reforming reaction. Copyright
The temperature dependence of the DC magnetization of a series of NiNaY zeolite samples has been measured at 2 T from 1.5 K to 140 K. The results show typical paramagnetic behavior for all the samples. The obtained magnetic moment of the paramagnetic unit implies a ferromagnetic coupling between the nearest neighboring Ni2+ ions in a non-reduced sample. For the Ni0 cluster in a supercage of the reduced samples, the number of atoms in the cluster is close to the magic numbers 18 and 20 for alkali metal clusters. No magnetic coupling between the Ni2+ pairs and between the Ni0 clusters was found.
27Al NMR property of rubidium-loaded potassium-type sodalite, which has antiferromagnetic transition temperature TN of 80K, is reported. Similar to the case of potassium-loaded case, monotonous narrow spectrum is seen above TN and broadened one is observed below TN. Separately shifted component, which is seen in the case of sodium-loaded case, is not observed as the potassium-loaded case. When two Gaussian functions are used for fitting, widths of each Gaussian component fairly scale to each other. The ratio between two Gaussian widths takes close value to the case of potassium loading below 65K and shows difference above 65K. This difference comes from the raising of TN by replacement of K to Rb.
At least five different paramagnetic RhII species have been detected by e.s.r. following activation by O2 at 200–475 °C of ion-exchanged RhNa-Y prepared either from [Rh(NH3)5Cl]Cl2(sample I) or from RhCl3· 3H2O (sample II). The conditions of formation of these species, their stability and their possible locations have been studied. Almost no paramagnetic species were detected in samples activated in O2 at temperatures higher than 500 °C, indicating a total oxidation of rhodium. In samples activated in O2 at temperatures > 200 °C and then evacuated at increasing temperatures, all RhII species disappeared progressively, while a new species E(Y) with g∥= 2.068 and g ⊥= 2.674 developed. The same species E(Y) was also generated in sample II following evacuation at temperatures > 200 °C without O2 activation. It is believed that species E(Y) is composed of isolated RhO atoms stabilized in the secondary channel of Y zeolite. Possible reduction mechanisms are discussed. A striking similarity between the overall chemistry of Rh in Na-X as revealed by e.s.r. and that of Rh in Na-Y was found.
Na43+ and K43+ clusters arrayed in aluminosilicate sodalite are known to show antiferromagnetism below respective Néel temperatures TN=48 and ~=70 K. We newly prepared K-Rb alloy (K3Rb)3+ clusters in addition to Na43+ and K43+ ones, and performed muon-spin relaxation (muSR) measurements. We find a large amplitude of precession signal in zero-field muSR spectra, indicating that antiferromagnetism takes place in the major volume of the samples. The TN is estimated from the temperature dependence of the precession signal to be ~=80 K for (K3Rb)3+. The internal field at the muon stopping site is estimated from the precession frequency to be ~=92, ~=142, and ~=155 Oe for Na43+, K43+, and (K3Rb)3+, respectively. Therefore, TN and the internal field are found to increase in the increasing order of atomic weight. These results are explained by the increase in the size of the s-electron wave function of clusters and the decrease in their potential depth.
We fabricated zeolite samples containing manganese ions (Mn-zeolite) and measured DC-magnetization with a flux meter based on the SQUID. Between 4 K and 300 K, the DC-susceptibility of Mn-zeolite obeys the Curie-Weiss law. The value of the Curie constant is 4.0×10−3 emu K/g and the Weiss temperature is −0.05 K. Mn-zeolite sample contains 6.22×1020 manganese ions ( Mn2+ ) per gram. The manganese ions in the zeolite sample are bound to the frame of zeolite. Then these ions can be considered as localized spins and behave as almost free magnetic dipoles in the lattice. The mean distance among ions is estimated to be 10 Angstrom. The main contribution of the magnetic interactions between ions may be dipole-dipole interaction. Mn-zeolite, therefore, is expected to keep paramagnetic properties in the temperature range even below 1 K. But the Weiss temperature is too high comparing with the value expected from the dipole-dipole interaction.
Electron microscopy and magnetic measurements of the reduced gadolinium zeolites show formation of gadolinium particles with extremely narrow particle size distribution and diameters less than 1.3 nm. These particles are paramagnetic even at 5 K and 1 T.
Silver clusters have been prepared in zeoliteY by ion exchange and subsequent reduction. The intensity of the strong conduction electron spin resonance atg=1.960±0.002 follows a Curie-Weiss law, with F=-80 K.
The long-range magnetic ordering of Na43+ clusters with spin S=1/2 on a body-centered-cubic lattice in sodium electrosodalite has been studied in zero applied magnetic field by positive muons as local magnetic probes. An experimental determination of the temperature dependence of the magnetic order parameter of an s-electron antiferromagnet is presented. The order parameter is measured via the local magnetic field at the muon site (Bloc). The temperature dependence Bloc(T) exhibits critical behavior near the Néel temperature TN=(50.3±0.2)K. The critical exponent β=(0.36±0.1) is close to the predicted value for a three-dimensional Heisenberg system. The precession signal is ascribed to muons in a diamagnetic state. A fraction of muons not contributing to the precession signal below TN is ascribed partly to muons in domains which are not magnetically ordered and partly to muons forming a bound state with an unpaired electron.
The dc magnetization of Y-type zeolite containing Fe ions and that after reduction treatment were measured in the temperature range of 1.5-300 K. The samples show paramagnetic behavior. The obtained value for the magnetic moment of the paramagnetic units strongly implies the existence of ferromagnetic coupling between the Fe ion at site I in hexagonal prism and the one at the nearest site I' in β cage.
A range of NiNa-Y and NiK-Y zeolites was prepared by ion exchange. The location of Ni2+ cations within the zeolite framework was monitored after various stages of thermal treatment and the reduction process of the transition metal ion in a flowing hydrogen atmosphere was investigated and correlated with such factors as the hydrogen flow rate, reduction time, reduction temperature, sample precalcination, and NH3 pretreatment. Cation location was probed by means of Cs+ back exchange and confirmed by the observation of the IR spectra of CO adsorbed on the activated zeolites. Iodometric titrations and Na+ back-exchange techniques were used to measure the levels of Ni2+ reduction. The data generated reveal how the interrelated Ni2+ ion location and the nature of the charge-balancing alkali metal co-cation serve to influence the level of Ni2+ reduction.
Na concentration (x) dependence of ferrimagnetic properties is investigated for Na–K alloy clusters incorporated in low-silica X (LSX) zeolite. In the LSX zeolite, β-cages of inner diameter ≈7 Å are arranged in a diamond structure, and supercages of inner diameter ≈13 Å are formed among them. The used LSX zeolite contains xNa+ and (12−x)K+ cations per β-cage or supercage. Guest nK atoms are loaded into the zeolite, namely the loading density is given by n per β-cage. The samples at x=4 have been reported to show Néel’s N-type ferrimagnetism in the specific region of n. This ferrimagnetism is explained by the model of antiferromagnetic coupling between two non-equivalent magnetic sublattices of clusters, the ones in β-cages and the others in supercages. In the present study, the value of x is changed from 4 to 0. Ferrimagnetic properties are found to show strong x-dependence. A systematic increase in loading densities of ferrimagnetic region is clearly observed with decreasing x. A remarkable change in temperature dependence of spontaneous magnetization is observed depending on x. Na+ cations are known to be mainly distributed in β-cages. Hence, the decrease in Na concentration is proposed to change the electronic potential depth for clusters in β-cage, which leads to important differences in the interaction between electrons localized in β-cages and those in supercages.
Magnetic properties of manganese ion spin systems in zeolite-Y have been measured. These ions act as almost free ions in the zeolite. Low frequency ac-susceptibility can be described with the Curie-Weiss law down to 50 mK. The susceptibility has a maximum around 30 mK showing a magnetic anomaly. Characteristic relaxation times deduced from frequency dependence of the maxima are studied. They diverge towardsT
c = 0 K according to the following formula, = 0exp{(b/T)+1} where 0 = 3 x 10–10 sec, b = 0.28K, and = 0.4, respectively. It is expected from ESR measurements that the dominant interaction between the manganese ions is dipole-dipole like. This divergence is interpreted as that of a dipolar spin glass.
Potassium clusters are stabilized in the cages of low silica X zeolite, where the cages with the inside diameter of ∼13Å are arrayed in a diamond structure. The average number of potassium atoms per cage, n, is changed from ∼6.3 to the saturated value of ∼8.8 by controlling the guest potassium loading density. The temperature dependence of magnetization shows the typical behavior of the N-type ferrimagnetism at n∼6.8 and n∼8.3. The zero field μSR spectra show significant increase in the relaxation rate below the Curie temperature, 5K, in the sample with n∼6.8. The time dependence of asymmetry shows almost full amplitude of decrease, indicating that the internal field below the magnetic phase transition arises in the almost full volume of the sample. The longitudinal field μSR spectra at 1.7K show the typical decoupling behavior of a static internal field. A model of ferrimagnetism is proposed based on the structure of the zeolite, where a magnetic sublattice of itinerant electron ferromagnetism is assumed.
The g shift of the conduction-electron spin resonance of Na-Cs alloy clusters is zeolite NaY has been measured and is found to follow the simple Elliot relation. In this model, the g shift depends upon the ratio of the spin-orbit coupling and energy-band separation. The assumption of a linear variation of both these parameters across the alloy series gives rise to a nonlinear variation of the g shift, as is found experimentally. The spin susceptibility changes from a Curie to a Curie-Weiss law with increasing Cs content, contrary to the predictions of the quantum size effect for noninteracting particles.