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Mechanism of formation of ammonia synthesis catalysts
280
NEWS SOLID ELECTROLYTES: AND SOLUTIONS
PROBLEMS
L.C. De Jonghe [Am. Ceram. Sot. Bull., 65 (8) (1986) 1158-11601 briefly reviews four classes of degradation occurring in sodium B”-alumina solid electrolytes: darkening of the electrolyte; crack propagation; internal sodium deposition; and corrosion and microcracking of the electrolyte in contact with the polysulfide electrode.
in a catalytic reactor mounted in a gas reaction cell of an electron microscope. Their main reactions may be summarized as follows. The iron nucleates epitaxially on the magnetite and forms crystals of 50 nm in size. This reduction occurs via a cellular reaction which produces a fine structure of crystalline iron and pores. These pores develop preferentially for stacking faults in the magnetite associated with high calcium concentration.
PHOTO REACTIVITY AND CHEMICAL ANALYSIS OF MANGANESE DIOXIDES
THERMAL
SPECTROSCOPY
Some of the manganese dioxides used in syntheses, photo-electrochemistry, electrocatalysis and in various types of generators do not correspond to the MnO, formula. They include Mn4’ Mn3 + ions in their lattices and Mn4’ vacancies. J. Brenet [Bull. Sot. Chim. Fr., 1 (1987) 9-151 reviews and discusses the different analytical methods used to determine their exact composition.
This non-destructive and very sensitive method of characterization of solids is reviewed by Nabil Amer (Lawrence Berkeley Labs.) in Vol. 69 of the Materials Research Society Symposia Proceedings Series. The method makes it possible to detect a variation of temperature of 10-60C and to observe time dependent processes in the samples. Furthermore, as Nahil Amer points out, it is also possible to measure very small (10e3 A) bulking on materials.
MECHANISM OF FORMATION OF AMMONIA SYNTHESIS CATALYSTS
INTERACTION MATTER WITH
The formation of ammonia synthesis catalysts by reduction of magnetite by hydrogen at 450 o C has been studied by electron microscopy (TEM, SEM), X-ray energy dispersive microanalysis and electron energy loss analysis (EDAX, EELS) by G.M. Pemmck, H.M. Flower and S.P.S. Andrews [J. Catal., 103 (1987) l-191. Different models, such as the “core and shell” or the “crackling core” model, have been proposed to explain the activation reaction. In addition, the role of the structural or chemical promoters is not fully established. The authors attempt to explain this catalyst by duplicating the conditions of reduction
The catalytic activity of potassium salts (e.g. K&O, and KOH) in steam gasification of coal has been known for some time. The catalysts undergo secondary reactions with the coal mineral matter, especially with the clay minerals present. Experiments were carried out by Formella et al. [Fuel, 65 (1986) 14701 to investigate the influence of mineral matter on the reactivity of chars derived from bitummous coal during porassium-catalysed steam gasification. Chars with different ash contents were impregnated with different amounts of catalyst (K,CO,) and gasified at 970 K and 4 MPa in steam. Further experiments were per-
Reactivity
of Soliak, News Brief, Vol. 3, No. 3, June 1987
OF COAL MINERAL POTASSIUM
NEWS SOLID ELECTROLYTES: AND SOLUTIONS
PROBLEMS
L.C. De Jonghe [Am. Ceram. Sot. Bull., 65 (8) (1986) 1158-11601 briefly reviews four classes of degradation occurring in sodium B”-alumina solid electrolytes: darkening of the electrolyte; crack propagation; internal sodium deposition; and corrosion and microcracking of the electrolyte in contact with the polysulfide electrode.
in a catalytic reactor mounted in a gas reaction cell of an electron microscope. Their main reactions may be summarized as follows. The iron nucleates epitaxially on the magnetite and forms crystals of 50 nm in size. This reduction occurs via a cellular reaction which produces a fine structure of crystalline iron and pores. These pores develop preferentially for stacking faults in the magnetite associated with high calcium concentration.
PHOTO REACTIVITY AND CHEMICAL ANALYSIS OF MANGANESE DIOXIDES
THERMAL
SPECTROSCOPY
Some of the manganese dioxides used in syntheses, photo-electrochemistry, electrocatalysis and in various types of generators do not correspond to the MnO, formula. They include Mn4’ Mn3 + ions in their lattices and Mn4’ vacancies. J. Brenet [Bull. Sot. Chim. Fr., 1 (1987) 9-151 reviews and discusses the different analytical methods used to determine their exact composition.
This non-destructive and very sensitive method of characterization of solids is reviewed by Nabil Amer (Lawrence Berkeley Labs.) in Vol. 69 of the Materials Research Society Symposia Proceedings Series. The method makes it possible to detect a variation of temperature of 10-60C and to observe time dependent processes in the samples. Furthermore, as Nahil Amer points out, it is also possible to measure very small (10e3 A) bulking on materials.
MECHANISM OF FORMATION OF AMMONIA SYNTHESIS CATALYSTS
INTERACTION MATTER WITH
The formation of ammonia synthesis catalysts by reduction of magnetite by hydrogen at 450 o C has been studied by electron microscopy (TEM, SEM), X-ray energy dispersive microanalysis and electron energy loss analysis (EDAX, EELS) by G.M. Pemmck, H.M. Flower and S.P.S. Andrews [J. Catal., 103 (1987) l-191. Different models, such as the “core and shell” or the “crackling core” model, have been proposed to explain the activation reaction. In addition, the role of the structural or chemical promoters is not fully established. The authors attempt to explain this catalyst by duplicating the conditions of reduction
The catalytic activity of potassium salts (e.g. K&O, and KOH) in steam gasification of coal has been known for some time. The catalysts undergo secondary reactions with the coal mineral matter, especially with the clay minerals present. Experiments were carried out by Formella et al. [Fuel, 65 (1986) 14701 to investigate the influence of mineral matter on the reactivity of chars derived from bitummous coal during porassium-catalysed steam gasification. Chars with different ash contents were impregnated with different amounts of catalyst (K,CO,) and gasified at 970 K and 4 MPa in steam. Further experiments were per-
Reactivity
of Soliak, News Brief, Vol. 3, No. 3, June 1987
OF COAL MINERAL POTASSIUM