L. Vradman

Using Sonochemical Methods for the Preparation of Mesoporous Materials and for the Deposition of Catalysts into the Mesopores

Ultrasound radiation can be used to synthesize a variety of mesporous materials. The reaction time is considerably shorter than the conventional methods. Ultrasonic waves can be further used for the insertion of amorphous nanosized catalysts into the mesopores. A detailed study demonstrates that the nanoparticles are deposited as a monolayer on the inner mesopores walls without blocking them. When the ultrasonically prepared catalyst/mesoporous-subtrate composite is used in catalysis a high conversion into product is obtained.

Structure–Function Relations in Supported Ni–W Sulfide Hydrogenation Catalysts

Ni–W catalysts were prepared by impregnation of commercial γ-alumina and silica supports. The sulfidation, performed directly after drying at 100°C, yielded fully sulfided Ni–W species on both supports (SEM-EDAX, XPS, XRD). At optimal metals loading (∼50 wt% NiO + WO3, Ni/W = 2), the sulfided catalysts had similar texture (N2 adsorption) and displayed similar activity in dibenzothiophene hydrodesulfurization (DBT HDS), while the activity of the Ni–W/SiO2 catalyst in toluene hydrogenation (HYD) was six times higher than that of Ni–W/Al2O3. This is due to the more than two times higher WS2 slabs stacking number in Ni–W/SiO2 compared with Ni–W/Al2O3 (XRD, HR-TEM), yielding stronger adsorption of toluene (TPD).

Magnetic properties of nanocrystalline La1- xMnO3+δ manganites : Size effects

The magnetic properties of nanocrystalline manganites La1?xMnO3+? with particle size of 20 (LMO20), 25 (LMO25), and 30 nm (LMO30), prepared by the citrate method, have been investigated in the temperature range 5?320?K, magnetic field up to 90?kOe and under quasi-hydrostatic pressures up to 14.5?kbar. The studies involve sequential zero-field-cooled magnetization (M) measurements followed by magnetization measurements during cooling in the same magnetic field (H) and complementary measurements of ac susceptibility. Additional measurements of M versus H were carried out at ambient and applied pressures. All nanoparticles exhibit a paramagnetic to ferromagnetic transition (PFT) at a Curie temperature TC>200?K. It was found that the relative volume of the ferromagnetic phase increases for larger particle size and approaches a value of about 93% for LMO30. The real part of the ac susceptibility of sample LMO20 exhibits strong frequency dependence in a wide temperature range below TC, whereas for sample LMO30 only relatively weak frequency dependence was observed. The magnetization of sample LMO30 exhibits a PFT of second order; the type of transition could not be established for the smaller particles. It was found that an applied pressure enhances the TC of La1?xMnO3+? nanoparticles with a pressure coefficient of dTC/dP?1.9?K?kbar?1 for LMO20 and dTC/dP?1.4?K?kbar?1 for LMO25 and LMO30 samples. Peculiar magnetic memory effects observed for sample LMO20 are discussed.

Magnetotransport in granular LaMnO3+δ manganite with nano-sized particles

Transport and magnetic properties of compacted LaMnO3+? manganite nanoparticles of an average size of 18?nm have been investigated in the temperature range 5?300?K. The nanoparticles exhibit a paramagnetic-to-ferromagnetic (FM) transition at the Curie temperature TC ~ 246?K. However, the spontaneous magnetization disappears at a higher temperature of about 270?K. It was found that at low temperatures the FM core occupies about 50% of the particle volume. The temperature dependence of the resistivity shows a metal?insulator transition and a low-temperature upturn below the resistivity minimum at T ~ 50?K. The transport at low temperatures is controlled by the charging energy and spin-dependent tunnelling through grain boundaries. It has been found that the charging energy decreases monotonically with increasing magnetic field. The low temperature I?V characteristics are well described by an indirect tunnelling model while at higher temperatures both direct and resonant tunnelling dominates. The experimental features are discussed in the framework of a granular ferromagnet model.

High loading of short W(Mo)S2 slabs inside the nanotubes of SBA-15. Promotion with Ni(Co) and performance in hydrodesulfurization and hydrogenation.

Abstract Layered nanoslabs of a M0S2 and WS2 phases with a well-defined hexagonal crystalline structure were inserted into the nanotubular channels of SBA-15 at loadings up to 60 wt%. Sonication of a slurry containing SBA-15 in a W(Mo)(CO)6-sulfur-diphenylmethane solution yielded an amorphous W(Mo)S2 phase inside the mesopores that was transformed into hexagonal crystalline W(Mo)S2 nanoslabs by further sulfidation. The nanoslabs were distributed exclusively inside the mesopores in a uniform manner (HRTEM, local quantitative microanalysis), without blocking the pores (N2-sorption). The Ni(Co) promoters were introduced into the W(Mo)S2/SBA-15 composites by impregnation from an aqueous solution of nickel (cobalt) acetate. The activity (based on the volume of the catalyst loaded into reactor) of the optimized Ni-W-S/SBA-15 catalyst in hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of toluene was 1.4 and 7.3 times higher, respectively, than that of a sulfided commercial CO-MO/Al2O3. The HDS activity of Co-Mo- S/SBA-15 catalyst was 1.2 times higher than that of commercial catalyst. After promotion with Co, the directly introduced M0S2 slabs and M0S2 slabs prepared by sulfidation of Mo- oxide monolayer spread over SBA-15 displayed similar HDS performance.

Behaviour of NiO and Nio pohases at high loadings, in SBA-15 and SBA-16 mesoporous silica matrices

NiO nanocrystals were inserted into SBA-15 and SBA-16 mesoporous silica matrices by gelation on nickel hydroxide with propylene oxide inside the nanotubular channels followed by thermal dehydration. The NiO crystals size was higher inside the SBA-15 (5.5–7 nm) than inside SBA-16 (3–3.5 nm) increasing with NiO loading from 22 to 69 wt.% This reflects the lower mesopore diameter in SBA-16 compared with SBA-15. At all NiO loadings the nanocrystals were located inside the nanotubular channels of mesoporous silicas (XRD, SEM, HRTEM, EDS) without blocking them (N 2 -adsorption). The small NiO nanocrystals contributed significantly to the surface area of NiO/SBA composites especially at high loadings. After reduction with hydrogen large 16–20 nm Ni o particles were formed with liberating the nanotubular channels which remain intact. The possible mechanism of nickel phase movement after NiO reduction from the internal mesopores to the external surface of the mesoporous silica microcrystals is discussed.

Thermal decomposition-precipitation inside the nanoreactors. High loading of W-oxide nanoparticles into the nanotubes of SBA-15.

Abstract The solution thermal decomposition-precipitation (ThDP) of oxide precursor inside the nanotubes (nanoreactors) of mesoporous silica support under atmosphere saturated with solvent at the oxide precursor decomposition temperature was explored for loading the W- oxide nanoparticles into SBA-15. ThDP of W-Ethoxide solution in decalin yielded WO3/SBA-I5 composites with W-phase located exclusively inside the pores in form of nanocrystals strongly blocking the pores. ThDP of W(CO)6 yielded W-phase up to 32 wt% spread as an amorphous monolayer on the pore walls with minimal pore blocking.

Effect of nanoporous ZrO2 crystal size on the surface sulphur capacity and performance of sulfated zirconia as an acidic catalytic material

Three sulfated zirconia materials were prepared with crystal size of tetragonal ZrO 2 of 2.5–15 nm and sulphur content corresponding to the full surface sulphur capacity of this phase. They were synthesized by sulfation of amorphous Zr hydroxide precipitated in presence and absence of block-copolymer non-ionic surfactant and of tetragonal zirconia preformed inside mesopores of SBA-15 silica matrix. It was demonstrated that while the crystal size of tetragonal ZrO 2 phase and its surface are determine the maximal amount of sulfate ions and concentration of acid sites in the material the acidity strength and catalytic activity in three different reactions is a function of preparation history.

Effects of gaseous and liquid components on rate of deep desulfurization of heavy atmospheric gas oil

Abstract The effects of concentrations of sulfur, nitrogen, bi-(BA) and monoaromatics (MA) in heavy atmospheric gas oil(HAGO), H2S and ammonia in gas phase on HDS rate at deep desulfuriation stage (Sin 1110-60 ppm) were studied with Co-Mo-Al and Ni-W-Si catalysts using HAGO with FBP of 390°C and initial sulfur content of 1.24 wt.%. The complete elimination of hydrogen sulfide, ammonia, polyaromatics and partial elimination of monoaromatics prior to the deep desulfurization stage increases the overall rate of deep HDS by a factor of about six.

Liquid flow deposition of PbS films on GaAs(100)

Chemical bath deposition (CBD) is a simple and inexpensive technique for thin film deposition of a variety of semiconductors. Nevertheless, this technique has some drawbacks such as the change of reactant concentrations and spatial non-uniformity. Liquid flow deposition (LFD) is a variation of CBD with potentially improved performance. In the current research, LFD of PbS films was studied for the first time using a custom-made reactor. The reactor was made of Pyrex and has a tubular geometry, and the substrate was placed vertically on a hemi-cylindrical stage made of Teflon. The main objective of this research was to enhance our understanding of thin film deposition by comparing the results obtained in LFD and CBD methods. High resolution scanning electron microscopy and X-ray diffraction showed oriented films for both methods. A notable advantage of the LFD method is the possibility to deposit films of the same quality simultaneously on both sides of the substrate, which has a variety of practical implications. Kinetic studies showed that the maximal growth rate in the reactor was achieved when the residence time corresponded to half of the induction time, which indicates that film growth occurred by direct nucleation on the substrate. LFD can provide monocrystalline films with thickness exceeding 5 μm at room temperature, whereas such a film thickness is rarely obtained in CBD. Moreover, a growth time series in LFD demonstrated autocatalytic deposition and its effect on the resulting film. The growth rate in LFD increases with time and reaches a maximum of 80 nm min−1 after 30 min of deposition at 30 °C, after which it stays constant throughout the deposition, while in CBD the growth rate drops after 30 min of deposition.