Jan Sentek

An alkali-promoted ruthenium catalyst for the synthesis of ammonia, supported on thermally modified active carbon

Ruthenium catalysts deposited on active carbon and pre-calcined at 1900°C in He atmosphere were studied. The precursors of the active phase were RuCl3·0.5H2O and Ru3(CO)12. Potassium and cesium (hydroxides or nitrates) were used as promoters. The catalysts were characterized by TEM, WAXS and H2 chemisorption. Activity measurements in NH3 synthesis were performed in a flow integral reactor under a pressure of 10 MPa at 470, 430, and 400°C (H2:N2=3:1). It has been found that the alkali-promoted catalyst deposited on the thermally modified carbon is much more active than that based on untreated carbon. The form of the Ru precursor used in the manufacture of the K-promoted catalyst had no effect on its dispersion or activity. The promoting effect of potassium was comparable to that of cesium. A monotonous increase in the activity of alkali-promoted catalyst vs. Ru content (5--15%) was observed, accompanied by a decrease in dispersion.

Thermally modified active carbon as a support for catalysts for NH3 synthesis

The effect of thermal treatment of a typical active carbon at 1300 °C and 1900 °C on its structural properties was investigated. It has been found that only a high-temperature heating (1900 °C) produced substantial changes in the structure of the carbon used: a disappearance of a considerable part of open micropores and formation of a turbostratic structure. The prepared materials were used as a support in catalysts for NH3 synthesis. The precursor of the active phase was Fe(NO3)3 · 9H2O. The iron nitrate deposited on the raw amorphous carbon decomposes to finely dispersed, hardly reducible iron oxides on heating to 470 °C in a H2 + N2 mixture. Even if reduced to more than 33%, iron is inactive in NH3 synthesis (400–470 °C, p = 10MPa). When deposited on the turbostratic, low-surface area carbon, iron forms, however, well developed crystallites (~60 nm), and is active in ammonia synthesis. Potassium activates strongly the surface of Fe, and, in the case of the amorphous support, it stimulates the reduction of iron oxides.

Effect of K, Cs and Ba on the kinetics of NH3 synthesis over carbon-based ruthenium catalysts

The kinetics of NH3 synthesis over carbon-based ruthenium catalysts promoted with barium or alkali was studied. Both the ammonia partial pressure dependencies of the reaction rates (T = 400°C, p = 63 bar, H2 : N2 = 3 : 1) and the pressure variations of the activity (T = 370°C, p= 4–63 bar, H2 : NN2 = 3 : 1) were found to be different for Ba and for the alkali (K, Cs). Ba–Ru/C proved to be more sensitive to the NH3 content and to the total pressure. The rate of synthesis over the alkali-promoted catalysts is, in turn, much stronger influenced by the ruthenium dispersion. TOFs of NH3 synthesis for the promoted samples at 370°C and 4 bar (Ba 0.085 1/s, Cs 0.05 1/s, K 0.035 1/s) are significantly higher than that for the Ru(0001) basal plane (0.0085 1/s results from the literature data at 370°C, 2 bar). The most active Ru/C samples (Ba or Cs) exceed significantly the fused iron catalyst, especially at high conversions.

Studies on kinetics of ammonia synthesis over ruthenium catalyst supported on active carbon

Abstract The rate of ammonia synthesis was measured for ruthenium and iron catalysts under high (100 bar) and under atmospheric pressures. The effect of ammonia concentration in the gas phase on reaction rate was measured at 400, 430, and 470°C for p = 100bar at 370 and 400°C for p = 1bar . It was found that ruthenium is less sensitive than iron to the increase of ammonia pressure in the gas phase. It is also much less sensitive to changes in the total pressure (for p H 2 :p N 2 =const. ). At relatively high conversion degrees the ruthenium catalyst used was several times more active than the fused iron catalyst at 100 bar and 25–40 times more active under atmospheric pressure.

Decomposition of ammonia over potassium promoted ruthenium catalyst supported on carbon

The rate of NH3 decomposition has been measured over the K-Ru/C catalyst, the studies being accompanied by the N2-TPD experiment. The catalyst proved to be extraordinarily active in ammonia decomposition. The TOF values over K-Ru/C were almost 103 times higher than those for the triply promoted magnetite, both referred to the nitrogen chemisorption. The rate of nitrogen desorption was found to be significantly lower than the rate of NH3 decomposition under the comparable conditions. It is suggested that the adsorbate–adsorbate interaction is responsible for the higher rate of the latter. The modelling based on the Langmuir–Hinshelwood kinetics shows the coverage of Nad in high pressure NH3 synthesis to be very small.

Methane Conversion into C2 Hydrocarbons and Carbon Black in Dielectric-barrier and Gliding Discharges

Abstract The methane conversion into C2 hydrocarbons, carbon black, and hydrogen was studied under plasma conditions at atmospheric pressure using dielectric barrier discharges (DBD) and gliding discharges. Ethane was the main product of the methane conversion under the filamentary DBD action in the mixtures CH4+Ar, thus it may be concluded that under the conditions of DBD microdischarges, the methane transformation can be terminated after one of the first steps of the chain reaction when ethane is produced. The methane conversion rate increased in the presence of dielectric granular packing (quartz glass and silica gel). Using the gliding discharges, two processes were examined: the methane conversion into unsaturated hydrocarbons C2 (mainly acetylene) and into carbon black. For these processes, it was found that the methane concentration in the starting gas mixtures (CH4+H2 and CH4+Ar) is one of the main parameters influencing the conversion rate and the unit energy consumption. The equilibrium in the CH4+H2 mixtures at the pressures of 1 and 10 bar was examined and the temperature ranges of the solid carbon stability were determined.

Wear and Nanomechanical Studies of Silicon Oxide and Silicon Nitride Thin Films for MEMS Applications

Thin films produced by PE-CVD techniques are often used in microelectronic and MEMS applications. New kind of discharges, glow discharges so called Atmospheric Pressure Glow (APG) can be otained when a dielectric barrier is used. The process of discharges stabilized by a dielectric barrier having non-uniform character (similar to the “silent” discharges) was used to produce thin films of silicon oxide and silicon nitride applicable in MEMS technology. The substrate temperature (silicon or metal) was usually 150-250°C. Such film deposited using polycondensation of tetraethoxysilane (TEOS) and hexamethylodisilazane (HMDS) for silicon oxide and silicon nitride respectively were tested using a special microtribometer for wear studies and Hysitron Inc.Triboscope TM which has the capability to carry out indentations at very low loads and to make load-displacement measurements with subnanometer indentation depth sensitivity. The wear and nanomechanical behaviors of the films were found to be very sensitive to the thickness (films up to 300 nm thick have been investigated) and the material composition.