We report on the effect of interparticle magnetic interactions in an ensemble of superparamagnetic magnetite particles with an average size of ~8.4nm dispersed in the diamagnetic matrix on the ...blocking of this ensemble in external magnetic field. The two limit cases are investigated: the case of strongly interacting particles, when the value of magnetic dipole-dipole interaction between particles is comparable with the energy of other interactions in the ensemble (the interparticle distance is similar to the nanoparticle diameter) and the case of almost noninteracting particles distant from each other by about ten particle diameters. We demonstrate that the experimental dependence of the blocking temperature on external field is described well within the model 1, in which the density of particles in a nonmagnetic medium is taken into account and the correlation value depends on external magnetic field. The model for describing the magnetic properties of a disperse nanoparticle ensemble is proposed, which makes corrections related to the particle size and mean dipole-dipole interaction energy for the anisotropy constant. The surface magnetic anisotropy of Fe3O4 particles and parameters of the interparticle coupling are estimated.
•The interparticle interactions affect superparamagnetic behavior of nanoparticles;•Effective magnetic anisotropy constant depends on interparticle interaction energy;•Modified random anisotropy model was used for 3-D dispersed nanoparticles ensemble.
•The M(H) magnetization curves of NiO nanoparticles (NPs) measured in pulsed fields of up to 250 kOe have been studied.•A model of NiO NP obtained from analysis of M(H) data have been ...proposed.•Surface and size effects as well as the origin and the magnitude of uncompensated magnetic moment have been revealed.
-The analysis of the M(H) magnetization curves of antiferromagnetic nanoparticles yields information about magnetic subsystems formed in these objects, which are characterized by a large fraction of surface atoms. However, in the conventionally investigated experimental magnetic field range of up to 60–90 kOe, this analysis often faces the ambiguity of distinguishing the Langevin function-simulated contribution of uncompensated magnetic moments μun of particles against the background of a linear-in-field dependence (the antiferromagnetic susceptibility and other contributions). Here, this problem has been solved using a pulsed technique, which makes it possible to significantly broaden the range of external fields in which the μun contribution approaches the saturation. Nanoparticles of a typical NiO antiferromagnet with an average size of ~ 4.5 nm have been investigated. Based on the thorough examination of the M(H) magnetization curves measured in pulsed fields of up to 250 kOe, a model of the magnetic state of NiO nanoparticles of such a small size has been proposed. The average moment is ~130 μB (μB is the Bohr magneton) per particle, which corresponds to 60–70 decompensated spins of nickel atoms localized, according to the Néel hypothesis (μun~ 3/2), both on the surface and in the bulk of a particle. A part of the surface spins unrelated to the antiferromagnetic core form another subsystem, which behaves as free paramagnetic atoms. Along with the antiferromagnetic core, an additional linear-in-field contribution has been detected, which is apparently related to superantiferromagnetism, i.e., the size effect inherent to small antiferromagnetic particles.
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•Micron-sized Al2O3 particles are formed on Raney Ni during transfer hydrogenation•Air oxygen promotes micron-sized Al2O3 formation on catalyst surface•O2 inhibits transfer ...hydrogenation of benzene ring catalyzed by Raney Ni•O2 influence on Ni0 surface facilitates isomerization of menthols and borneols•Removing alumina species with aqueous alkali reactivates the catalyst
Transfer hydrogenation (TH) of six substrates catalyzed by Raney® nickel and conducted at 150 °C for 5 h under air or argon was studied. Significant decrease of conversion was observed under air as compared with TH under argon: 57% vs. 98% for menthone and 39% vs. 54% for camphor, respectively. Regardless of different reaction atmosphere used, aluminum segregated on the catalyst surface to form micron-sized alumina crystallites to modify the adsorption capacity, to inhibit the catalyst activity and to change the products selectivity of TH. Moreover, the changes of the catalyst structure under air entailed the change of reaction rates of both hydrodeoxygenation and dearomatization of acylbenzenes (ketones), as well as benzylic alcohols. Thus, manipulating by reaction atmosphere of TH catalyzed by Raney® nickel can be considered as an efficient tool to govern a chemo-, regio- and stereo-selectivity of products.
The transformation of petroleum asphaltenes in supercritical water was studied. The experiments were performed in autoclave at temperature 380
°C and pressure 226
atm with stirring for 3
h, medium ...density was about 0.33
g/cm
3. The reaction resulted in the formation of gas products, about 4.3%, and an insoluble residue (coke) with about 48.6% yield. The remaining products were separated into fractions by consecutive dissolution in hexane (30.0%), benzene (10.6%), and chloroform (5.7%). The properties of the obtained products were studied with FT-IR spectrometry and
1H NMR spectroscopy. The method of simulated distillation was used to demonstrate that the fractional composition of the hexane-soluble part of the products is close to the fractional composition of a mixture of the diesel fraction and vacuum gas oil of the corresponding oil in 1:1 ratio. The obtained data support the conclusion that asphaltene cracking proceeds in SCW, with most probable main processes being dealkylation of substituents in the aromatic fragments of molecules and aromatization. This leads to formation of gaseous products and hexane-soluble fraction consisting of lighter aliphatic and aromatic compounds, as well as carbonized solid residue.
•A sample of NiO nanoparticles with an average size of 8 nm was synthesized.•The sample exhibits the superparamagnetic behavior with a blocking temperature of 185 K.•The magnetization curves in ...fields up to 250 kOe at temperatures of 80–300 K was measured.•The FM, AFM contributions and paramagnetism of the surface spins was extract.
It is well-known that the fraction of surface atoms and the number of defects in an antiferromagnetic particle increase with a decrease in the particle size to tens of nanometers, which qualitatively changes the properties of the particle. Specifically, in antiferromagnetic nanoparticles, spins in the ferromagnetically ordered planes can partially decompensate; as a result, an antiferromagnetic particle acquires a magnetic moment. As a rule, uncompensated chemical bonds of the surface atoms significantly weaken the exchange coupling with the antiferromagnetic particle core, which can lead to the formation of an additional magnetic subsystem paramagnetic at high temperatures and spin-glass-like in the low-temperature region. The existence of several magnetic subsystems makes it difficult to interpret the magnetic properties of antiferromagnetic nanoparticles. It is shown by the example of NiO nanoparticles with an average size of 8 nm that the correct determination of the contributions of the magnetic subsystems forming in antiferromagnetic nanoparticles requires magnetic measurements in much stronger external magnetic fields than those commonly used in standard experiments (up to 60–90 kOe). An analysis of the magnetization curves obtained in pulsed magnetic fields up to 250 kOe allows one to establish the contributions of the uncompensated particle magnetic moment μun, paramagnetic subsystem, and antiferromagnetic particle core. The μun value obtained for the investigated NiO particles is consistent with the Néel model, in which μun ∼ N1/2 (N is the number of magnetically active atoms in a particle), and thereby points out the existence of defects on the surface and in the bulk of a particle. It is demonstrated that the anomalous behavior of the high-field susceptibility dM/dH of antiferromagnetic NiO nanoparticles, which was observed by many authors, is caused by the existence of a paramagnetic subsystem, rather than by the superantiferromagnetism effect.
An influence of various organic compounds on aggregation of asphaltenes separated from Tartar oil was studied by small-angle X-ray scattering and scanning tunneling microscopy methods. The compounds ...such as cyclohexane, methanol, formic acid, heptane, tetrabutylammonium fluoride, acetonitrile, decane, hexanol-1, furfural, and octene-1, added to the solution of asphaltenes in toluene, were shown to produce a considerable aggregation of asphaltene particles of ∼10–13 nm size and of an oblong ellipsoid shape along the longer axis. Decalin, dimethylformamide, dimethylsulfoxide, nitrobenzene, aniline, cyclohexanol, monoethanolamine, triethanolamine, 1-methylnaphtalene, and dichloroethane cause asphaltene aggregate sizes to decrease essentially. Acetone, glycerin, ethyl acetate, dioxane, benzyl alcohol, oleic acid, limonene, α-pinene, cycloheptatrien, and nitromethane were shown to influence weakly on aggregation/disaggregation processes of asphaltenes. Conceivable reasons of the observed effects are discussed from general physico-chemical properties of the compounds used.
A novel method for the synthesis of catalytically active composites based on gold clusters incorporated into an alumina matrix was proposed. The method combines sol–gel and supercritical fluid ...technologies. The composites were synthesized by precipitating of gold clusters and alumina sol mixture in the supercritical antisolvent (SAS) carbon dioxide medium. Gold clusters were synthesized in the colloidal solution of alumina sol in situ (one-pot synthesis) via controlled reduction of Au
III
using PPh
3
as a stabilizer for the first time. It was shown that this method allows us to stabilize gold clusters with sizes less than 2 nm in the alumina matrix with a developed specific surface area (
S
BET
= 441 m
2
/g) and a narrow pore size distribution. The obtained composite does not contain stabilizer, which may poison the catalyst. The developed approach makes possible to synthesize catalytically active composites with a certain amount of gold cluster and oxide matrix of different nature.
Highlights
The gold based cluster composites were obtained via supercritical antisolvent precipitation.
The composites contain gold clusters with sizes less than 2 nm.
The composites have a developed specific surface and narrow pore size distribution.
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•Spin-flip satellite line intensity measures total dipolar interactions with nuclei.•Trityl radicals have stronger interaction with solvent 1H than internal 1H.•Natural-abundance 13C ...hyperfine features overlap 1H spin-flip satellite lines.•ENDOR confirms dipolar coupling strength measured from spin-flip satellites.
The electron nuclear dipolar interactions responsible for some dynamic nuclear polarization (DNP) mechanisms also are responsible for the presence formally in CW EPR spectra of forbidden satellite lines in which both the electron spin and a nuclear spin flip. Such lines arising from 1H nuclei are easily resolved in CW EPR measurements of trityl radicals, a popular family of DNP reagents. The satellite lines overlap some of the hyperfine features from 13C in natural abundance in the trityl radical, but their intensity can be easily determined by simple simulations of the EPR spectra using the hyperfine parameters of the trityl radical. Isotopic substitution of 2H for 1H among the hydrogens of the trityl radical and/or the solvent allows the dipolar interactions from the 1H on the trityl radical and from the solvent to be determined. The intensity of the dipolar interactions, integrated over all the 1H in the system, is characterized by the traditional parameter called reff. For the so-called Finland trityl in methanol, the reff values indicate that collectively the 1H in the unlabeled solvent have a stronger integrated dipolar interaction with the unpaired electron spin of the Finland trityl than do the 1H in the radical and consequently will be a more important DNP route. Although reff has the dimensions of distance, it does not correspond to any simple physical dimension in the trityl radical because the details of the unpaired electron spin distribution and the hydrogen distribution are important in the case of trityls.
It has been shown for the first time that the high-temperature treatment of heavy oil with a high content of asphaltenes at 350–450 °C without a catalyst or H
2
leads to the formation of new ...vanadium(IV) porphyrin complexes detected by ESR spectroscopy. In anisotropic spectra of treated oil samples recorded at 130 K, together with the signals of the porphyrin complex of the vanadyl ion, VO
2+
, another component with distinct
g
and
A
spin Hamiltonian parameters was observed. A comparative analysis of the resonance parameters of the revealed complexes allowed us to unambiguously identify them as the thiovanadyl (VS
2+
) porphyrins formed during the thermal treatment. Thus, the formation of thiovanadyl porphyrins from vanadyl porphyrins in asphaltene-containing heavy oils at high-temperature processing has been experimentally proven for the first time.
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•TH of mono-, di- and trinuclear aromatic compounds was studied at 150 °C and 82 °C•2-PrOH/Raney® nickel combination demonstrates a high reactivity•Naphthalene and biphenyl are ...partially reduced at 82 °C with high selectivity•Trinuclear anthracene and phenanthrene at 150 °C are hydrogenated into octahydro-derivatives
Catalytic hydrogenation of mono-, di- and trinuclear aromatic compounds has been studied under hydrogen transfer conditions at 150 °C and 82 °C in 2-PrOH as a hydrogen donor and with Raney® nickel as a catalyst. In contrast to conjugated or condensed aromatic rings, isolated ones demonstrated low reactivity in transfer hydrogenation (TH) that can be used to increase the hydrogenation selectivity of the reaction. So, naphthalene and biphenyl are partially hydrogenated into tetralin and cyclohexylbenzene, respectively, with excellent conversion (≥ 96 %) and selectivity (≥ 98 %) for 5–6 h at 82 °C. Increasing the reaction temperature to 150 °C results expectedly in the hydrogenation of second aromatic ring, which occurs slowly enough. Only 8 % of decaline and 42 % of dicyclohexyl, correspondingly, were obtained after 5 h at 150 °C. At the same time, TH of trinuclear anthracene and phenanthrene at 150 °C resulted in the formation of deeper hydrogenated octahydro-anthracenes and -phenanthrenes, respectively.
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