O. V. Netskina

Effect of the nature of the active component and support on the activity of catalysts for the hydrolysis of sodium borohydride

The effect of the nature of an active component and a support on the rate of hydrolysis of aqueous sodium borohydride solutions was studied. It was found that the activity of supported catalysts, which were reduced in a reaction medium of sodium borohydride, decreased in the order Rh > Pt ≈ Ru ≫ Pd regardless of the nature of the support (γ-Al2O3, a Sibunit carbon material, or TiO2). The catalysts based on TiO2 exhibited the highest activity. As found by UV-vis diffuse reflectance spectroscopy, the composition and structure of the supported precursor of an active component depend on the nature of the support. It is likely that rhodium clusters with different reaction properties were formed on various supports under the action of a reaction medium.

Cobalt boride catalysts for hydrogen storage systems based on NH3BH3 and NaBH4

The catalytic activity of cobalt borides forming in situ under conditions of NH3BH3 and NaBH4 hydrolysis have been investigated. The reaction properties of the catalysts depend on the nature of the hydride. According to high-temperature X-ray diffraction, thermal analysis, high-resolution transmission electron microscopy, IR spectroscopy, and chemical analysis data, the nature of the hydride determines the particle size, chemical composition, and crystallization properties of the cobalt borides.

Activity of Rh/TiO2 catalysts in NaBH4 hydrolysis: The effect of the interaction between RhCl3 and the anatase surface during heat treatment

The reaction properties of Rh/TiO2 sodium tetrahydroborate hydrolysis catalysts reduced directly in the reaction medium depend on the temperature at which they were calcined. Raising the calcination temperature to 300°C enhances the activity of the Rh/TiO2 catalysts. Using diffuse reflectance electronic spectroscopy, photoacoustic IR spectroscopy, and chemical and thermal analyses, it is demonstrated that, as RhCl3 is supported on TiO2 (anatase), the active-component precursor interacts strongly with the support surface. The degree of this interaction increases as the calcination temperature is raised. TEM, EXAFS, and XANES data have demonstrated that the composition and structure of the rhodium complexes that form on the titanium dioxide surface during different heat treatments later determine the state of the supported rhodium particles forming in the sodium tetrahydroborate reaction medium.

Catalytic hydrodechlorination on palladium-containing catalysts

The catalytic liquid-phase hydrodechlorination of chlorobenzene on supported palladium-containing catalysts has been investigated. The following processes capable of deactivating the catalyst occur during the liquid-phase hydrodechlorination: the coarsening of supported metal particles, the washoff of the active component with the reaction medium, and potassium chloride deposition on the catalyst surface. The effects of the active component composition and of the preparation method on the hydrodechlorination activity and deactivation stability of the catalysts have been studied. The catalysts have been characterized by several physical methods.

Granulated rhodium catalysts of sodium borohydride hydrolysis for generators of high-purity hydrogen

Sodium borohydride hydrolysis in a flow reactor with turbulent mixing of reactants in a catalytic bed by the evolving hydrogen bubbles was studied. The stability of catalytic systems decreases in the order 1% Rh/Sibunit > 1% Rh/TiO2 > 1% Rh/γ-Al2O3. The decrease in the hydrogen generation rate is caused by the formation of a metaborate film on the catalyst surface, by the loss of the active component, and by disintegration of support granules and their removal with the flow of the spent liquid. High granule strength and macroporous structure of 1% Rh/Sibunit ensure stable generation of hydrogen.

Synthesis of Fine Vanadium-Carbide (VC0.88) Powder Using Carbon Nanofiber

The synthesis of fine vanadium-carbide (VC0.88) powder is considered. To produce the vanadium carbide, vanadium(III) oxide is reduced by means of carbon nanofiber in an induction furnace with an argon atmosphere. The carbon nanofiber is produced by the catalytic decomposition of light hydrocarbons. The specific surface of the carbon nanofiber is very high: ~150000 m2/kg, as against ~50000 m2/kg for soot. The impurity content in the carbon nanofiber is 1 wt %. By analysis of the phase diagram of the V–C system, the batch composition and the upper temperature limit in carbide formation may be determined such that vanadium carbide is formed as powder. Thermodynamic analysis yields the initial temperature at which the vanadium( III) oxide is reduced in a furnace with different CO pressures. The characteristics of the vanadium carbide are determined by the following methods: X-ray phase and elementary analysis; pycnometric analysis; scanning electron microscopy with local energy dispersion X-ray microanalysis (EDX); low-temperature nitrogen adsorption with subsequent determination of the specific surface by the BET method; sedimentation analysis; and synchronous thermogravimetric analysis and differential scanning calorimetry (TG/DSC). The material obtained with optimal reduction parameters consists of a single phase: vanadium carbide VC0.88. The powder particles are predominantly clumped together in aggregates. The mean size of the particles and aggregates is 9.2–9.4 μm, with a broad size distribution. The specific surface of the samples is 1800–2400 m2/kg. Oxidation of vanadium carbide begins at about 430°C and is practically over at 830°C. Optimal synthesis requires stoichiometric proportions of the reagents in the production of vanadium carbide VC0.88 at 1500–1600°C, with 20-min holding. In this process, carbon nanofiber effectively produces the carbide as the reduction product. The vanadium(III) oxide is reduced practically completely to VC0.88.

Synthesis of Highly Dispersed Zirconium Carbide

The reduction of zirconium oxide with nanofibrous carbon to obtain highly dispersed zirconium carbide was studied. The optimum reduction conditions were determined. The reaction products were identified using modern physicochemical methods (scanning electron microscopy, low-temperature nitrogen adsorption, sedimentation analysis, differential scanning calorimetry). The product obtained appeared to be single-phase zirconium carbide containing no more than 2 wt % impurities. The powder particles are aggregated (mean diameter 14.9–15.0 μm, specific surface area 1.5–1.7 m2 g–1). The oxidation of zirconium carbide starts at 480°С and is complete at 800°С.

SYNTHESIS OF FINELY DISPERSED VANADIUM CARBIDE (VC 0.88 ) USING NANOFIBROUS CARBON

The paper presents the experimental data on the synthesis of finely dispersed powder of vanadium carbide (VC 0.88  ). Vanadium carbide was prepared by the reduction of vanadium oxide  (III) with nanofibrous carbon (NFC) in the induction furnace under an argon atmosphere. NFC is a product of catalytic decomposition of light hydrocarbons. The main characteristic of a NFC is a high specific surface area (~150  000  m 2 /kg), which is significantly higher than that of soot (~50  000  m 2 /kg). The content of impurities in the NFC is at the level of 1  %  wt. Based on the analysis of the state diagram of the V – C system, the composition of the charge and the upper temperature limit of the carbide formation reaction for obtaining vanadium carbide in the powder state are determined. Based on the thermodynamic analysis, the temperature of the onset of the carbothermic reduction reaction of vanadium oxide (III) at various CO pressures was determined. The characteristics of vanadium carbide were studied using X-ray and elemental analyzes, pycnometric analysis, scanning electron microscopy using local energy dispersive X-ray microanalysis (EDX), low-temperature adsorption of nitrogen, followed by determination of the BET specific surface area, sedimentation analysis, synchronous thermogravimetry and differential scanning calorimetry (TG/DSC). The material obtained at optimal parameters is represented by a single phase  – vanadium carbide VC 0.88  . The powder particles were predominantly aggregated. The average size of the particles and the aggregates equaled 9.2  –  9.4  μm within a wide range of size distribution. The specific surface value of the obtained samples was 1800  –  2400  m 2 /g. Oxidation of vanadium carbide began from the temperature of ~430  °C and practically ends at ~830  °C. Optimum parameters of synthesis are the ratio of reagents according to stoichiometry to obtain carbide of composition VC 0.88 at a temperature of 1500  –  1600  °С and a holding time of 20  minutes. It is shown that for this process nanofibrous carbon is an effective reducing agent and that vanadium oxide  (III) is almost completely reduced to carbide VC 0.88

Developing Effective Cobalt Catalysts for Hydrogen-Generating Solid-State NaBH4 Composite

Hydrogen-generating solid-state NaBH4 composite are promising systems for storing and transporting hydrogen intended for use in low-temperature proton-exchange membrane fuel cells. Catalysts are introduced into the composites to ensure the generation of hydrogen at ambient temperatures. In this work, the effect of the synthesis conditions for cobalt catalyst on the gas generation rate is analyzed. It is found that the efficiency of hydrogen generation depends on the nature of the cobalt salt and pH of the aqueous solution of the salt in which the active component precursor is reduced under the action of sodium borohydride because these factors determine the composition, degree of dispersion, and magnetic behavior of the cobalt systems. It is found that the highest rate of gas generation—505 cm3/min per gram of the composite with a hydrogen content of 8.4 wt %—is observed for a sample reduced with sodium borohydride in a hydrochloric acid solution of cobalt chloride with a pH of 1.3. The results can be used to develop effective inexpensive cobalt catalysts for the production of hydrogen from pelletized solid-state NaBH4 composite.

Synthesis of Highly Dispersed Zirconium Diboride for Fabrication of Special-Purpose Ceramic

Reduction of zirconium dioxide with boron carbide and nanofibrous carbon in argon yielded a highly dispersed powder of zirconium diboride. Characteristics of zirconium diboride powders were examined by various analytical methods. The material obtained is represented by a single phase, zirconium diboride. Powder particles are for the most part aggregated. The average size of particles and aggregates is 10.9–12.9 μm with a wide size distribution. The specific surface area of the samples is 1.8–3.6 m2 g–1. The oxidation of zirconium diboride begins at a temperature of 640°C The optimal synthesis parameters were determined: ZrO2: B4C: C molar ratio of 2: 1: 3 (in accordance with stoichiometry), process temperature 1600–1700°C, synthesis duration 20 min.