Dependence of the properties of Ce-Zr-Y-La-M-O systems on synthetic conditions and on the nature of the transition metal M (Mn,Fe, Co)

The effects of synthetic conditions, component ratios, and the nature of the transition metal on the physicochemical and catalytic properties of Ce-Zr-Y-La-M-O (M = Mn, Fe, Co) systems are studied. The Ce-Zr-Y-La-M-O samples precipitated at ～23°C and calcined at 600°C are single-phase and are solid solutions with a fluorite structure, which persists upon calcination at 1150°C. The Ce-Zr-Y-La-Fe(Co)-O samples precipitated at 70°C and calcined at 1150°C consist of two solid solutions, one cubic, and the other tetragonal. The specific surface area (S _sp) of the samples precipitated at ～23°C and calcined at 600°C increases in the order Ce-Zr-Y-La-O < Ce-Zr-Y-La-Mn-O < Ce-Zr-Y-La-Co-O ≈ Ce-Zr-Y-La-Fe-O. The specific surface area of the samples precipitated at 70°C is independent of M and is ～110 m²/g. Calcination at 1150°C reduces S _sp approximately by two orders of magnitude. The TPR of the unpromoted systems in H₂ proceeds in two steps at 600–650 and 750–840°C. The introduction of M decreases the reduction temperatures and gives rise to a lower temperature peak between 150 and 300°C. The most effective promoter is cobalt. The fluorite-type catalysts containing no noble metal are active in NO reduction (X _NO ≈ 100%) at T _react = 400–450°C. The cobalt-containing catalysts are the most active in the oxidation of CO (X _max = 28%) and hydrocarbons (X _max = 4.3%).     

Real structure of metastable forms of aluminum oxide

Differences in the real structure of γ-Al₂O₃ samples obtained by the thermal decomposition of pseudoboehmite and boehmite prepared by the hydrothermal treatment of bayerite were found. The transformations of these structures during their conversion to δ-Al₂O₃ as the treatment temperature increased were studied. The rate of conversion of metastable alumina species into the stable α-Al₂O₃ phase significantly depends on the real structure of samples. The rate of this transformation is drastically retarded in the presence of extended defects in the oxides originated from boehmite, and the stability of metastable alumina species increased as the degree of surface dehydroxylation increased.     

Nonsteady-state self-interaction in a medium with striction nonlinearity

An analytic and numerical investigation is made of the self-focusing of a wave beam allowing for the inertia of the nonlinear response of the medium described by an acoustic type of equation. Some characteristics of the dynamics of self-interaction of the wave fields are analyzed in the paraxial optics approximation and the self-similar structures and space-time instability of a plane wave are considered. The stages of instability buildup, structure formation, and the establishment of a steady state are studied numerically.     

Gamma Spectrometry of Composite Models of Planetary Matter on the JINR Accelerator Proton Beam with Tagged Protons

Physics of Particles and Nuclei Letters - The results of the second stage of laboratory tests of the space gamma-spectrometer model on the phasotron installed at the JINR Laboratory of Nuclear...     

Simulation of Space Experiments for Nuclear Planetology: Measurement of Relative Intensities of Lines of Gamma Ray Emitted upon Thermal-Neutron Capture by Nuclei

The results obtained by experimentally studying gamma rays emitted by samples prepared as analogs of planetary matter and irradiated with thermal neutrons are presented. The intensities of spectral lines of gamma rays emitted by such samples differing in chemical composition are compared. These results will be used in processing data on gamma-ray spectra of the Moon and Mercury from measurements performed onboard spacecrafts with the aim of studying the composition of the surface of these celestial bodies.     

Mean Field Analysis of Personalized PageRank with Implications for Local Graph Clustering

We analyse a mean-field model of Personalized PageRank (PPR) on the Erd?s–Rényi (ER) random graph containing a denser planted ER subgraph. We investigate the regimes where the values of PPR concentrate around the mean-field value. We also study the optimization of the damping factor, the only parameter in PPR. Our theoretical results help to understand the applicability of PPR and its limitations for local graph clustering.     

Test facility for nuclear planetology instruments

An experimental facility for testing and calibrating nuclear planetology instruments has been constructed in partnership between the Space Research Institute (Moscow) and the Joint Institute for Nuclear Research. A model of Martian soil with a size of 3.82 × 3.21 m² and an overall mass of about 30 t is assembled from silicate glass. Glass is chosen in order to imitate absolutely dry soil close in composition to the Martian one. The heterogeneous model allows one to imitate the average elemental composition of Martian soil in the best possible way by adding layers of the necessary materials to it. Near-surface water ice is simulated by polyethylene layers buried at different depths within the glass model. A portable neutron generator is used as the neutron source for testing active neutron and gamma spectrometers. The facility is radiation-hazardous and is thus equipped with interlock and radiation monitoring systems in accordance with the effective regulations.     

Microwave synthesis of LaMO3 (M = Mn, Co, Fe) perovskites from crystalline hydrates of nitrates

The possibility of synthesizing perovskite-type LaMO₃ (M = Mn, Co, Fe) oxides by microwave irradiation of crystalline hydrates of nitrates was studied. Oxides with the perovskite structure form at the microwave irradiation stage; however, the resulting product is not singe-phase. Additional thermal treatment of the microwave synthesis product at 600 to 900°C for 5 h is needed for a single-phase oxide to be formed in the case of M = Mn. In the case of M = Co or Fe, the samples contain considerable amounts of the simple oxides La₂O₃ and Fe₂O₃ or Co₃O₄ along with the perovskite. The synthesized products were investigated in nitrous oxide decomposition and methane oxidation as model reactions. As compared to the samples obtained by other techniques, they have a larger specific surface area and are more active.