Zbigniew Kowalczyk

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.

Hydrodechlorination of CCl2F2 (CFC-12) over Pd-Au/C catalysts

Abstract A series of carbon-supported palladium-gold (Pd-Au) catalysts prepared by direct redox reaction method and characterized by various techniques were investigated in the reaction of dichlorodifluoromethane (CFC-12) with dihydrogen. The selectivity towards difluoromethane (desired reaction product) was increased upon introducing gold to palladium, from ∼72 to ∼86%, at 180°C. Such a selectivity enhancement was not observed in our previous studies when Pd-Au/C catalysts prepared by incipient wetness impregnation showed inadequate extent of Pd-Au alloying. Conditions of preparation of Pd-Au/C catalysts by the direct redox reaction method are found to affect the amount of deposited metals and the degree of Pd-Au mixing. The latter factor is essential in determining hydrodehalogenation behavior of the catalysts.

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.

Effect of potassium on the high pressure kinetics of ammonia synthesis over fused iron catalyst

Measurements were performed of reaction rate in the process of ammonia synthesis (T=370–470°C) on doubly promoted (DP) (Al2O3, CaO) and triply promoted (TP) (K2O, Al2O3, CaO) iron catalysts. The latter were obtained by impregnation of the reduced and subsequently passivated DP precursors with alcoholic solution of KOH. The studies were carried out under high total pressure (10 MPa) in a wide range of ammonia partial pressure in the gas phase: from 0.25 to about 7 bar. The results are shown to be authoritative for the so-called kinetic regime. The effect of the presence of K+ cations in the catalyst was the stronger, as the temperature of the reaction was the lower and, in particular, the ammonia pressure in the gas phase the higher. The obtained results are in good accordance with the results of Somorjais studies on activity of iron single crystal surfaces both clean and covered with (K+O) adlayer.

Potassium-Promoted Carbon-Based Iron Catalyst for Ammonia Synthesis. Effect of Fe Dispersion

Potassium-promoted iron catalysts supported on thermally modified, partly graphitized carbon were studied in the ammonia synthesis reaction. Iron nitrate was used as a precursor of the active phase and KOH or KNO3 were used as promoters. The kinetic studies of NH3 synthesis were carried out in a differential reactor under 63 bar and 90 bar pressure. Hydrogen chemisorption, X-ray diffraction and transmission electron microscopy experiments were performed to determine the dispersion of iron in the specimens. All the K+–Fe/C catalysts proved to be sensitive to ammonia, the NH3 partial pressure dependencies of their reaction rates being close to that of the commercial magnetite catalyst (KM I, H. Topsoe). The catalytic properties of the promoted Fe particles on carbon were shown to be dependent upon the iron dispersion, i.e. smaller particles exhibited higher turnover frequency in NH3 synthesis. It is suggested that either small Fe crystallites expose more highly active sites, e.g. C-7 (B-5) or the promotion of small crystallites by the alkali is more efficient.

Activity and thermoresistance of fused iron catalysts for ammonia synthesis

Abstract The activity and thermoresistance of five industrial catalysts were examined. Kinetic measurements were carried out in an isothermal, flow differential reactor under 10.0 MPa in the 400–520°C temperature range. In thermoresistance tests, overheating of the catalysts was carried out at 650°C for 24 h, or at 620°C for 24 to 192 h. The relationships obtained during both experimental tests were similar, with 24-h operating at 650°C giving a similar activity decrease as ageing for 192 h at 620°C. The chemical composition of the catalysts (XRF) was determined.

Influence of aluminium and potassium on activity and texture of fused iron Catalysts for ammonia synthesis

Abstract BET, scanning electron microscopy and X-ray diffraction were used to investigate the texture of fused iron Catalysts which had been previously reduced and passivated, and into which potassium hydroxide was introduced by impregnation or aluminium removed by extraction. Activity tests were carried out under 10.0 MPa in the 400–520 °C temperature range. Investigations were performed on the doubly promoted (Al, Ca) and industrial, triply promoted (K, Al, Ca) Catalysts. Introduction of potassium hydroxide into the doubly promoted Catalyst increases the process rate by about 15–20 times per unit surface area. Sintering and recrystallization are observed in the Catalyst, proving that interaction takes place during the process between the potassium hydroxide introduced and the alumina present on the iron surface. Partial removal of aluminium from the surface of the doubly and triply promoted Catalysts also leads to sintering and recrystallization under ammonia synthesis conditions. This shows that the presence of alumina on the surface, and not within the iron, is responsible for textural promotion. Loss of Catalysts activity is proportional to reduction in total surface area.