D.W. McKee

Mechanisms of the alkali metal catalysed gasification of carbon

Abstract The catalytic effects of alkali metal salts in the gasification of carbonaceous materials by oxygen, steam and carbon dioxide are described. The most effective catalysts are generally the carbonates, oxides and hydroxides; other active salts tend to convert to these species under gasification conditions. Current theories of the mechanism of this type of catalysis are reviewed. Thermodynamic considerations, the results of thermal analysis and the magnitude of kinetic isotope effects suggest that cyclic sequences of elementary reactions are responsible for the catalytic phenomena.

Metal oxides as catalysts for the oxidation of graphite

Abstract The oxides of a number of transition metals have been studied as catalysts for the oxidation of graphite by dry oxygen. Thermogravimetric analysis was used to measure ignition temperatures of doped polycrystalline graphite and changes in the microtopography of natural graphite crystals during the catalytic oxidation were investigated by optical microscopy. In many cases information on the chemical nature of the catalytic entities could be obtained by oxidizing mixtures of metal acetates and polycrystalline graphite in the thermobalance. Only those oxides which could be reduced by graphite to a lower oxide or to the metallic state appeared to function as active catalysts. The localized action of the catalyst particles gave rise to catalytic channeling and/or the promotion of etch pit formation on the graphite basal plane, depending on the nature of the catalyst.

The catalytic behavior of alkali metal carbonates and oxides in graphite oxidation reactions

Abstract The reactions that occur in different gaseous environments between graphite and alkali metal carbonates and oxides have been studied by simultaneous thermogravimetry-differential thermal analysis and hot-stage microscopy. The catalytic effects of these salts during gasification of graphite in oxygen and carbon dioxide are interpreted on the basis of distinct oxidation-reduction cycles, involving the intermediate formation of peroxide in the former case and alkali metal in the latter. The thermodynamic feasibility of the proposed cycles has been discussed in detail.

Oxidation behavior and protection of carbon/carbon composites

Abstract The oxidation behavior of C/C composite sheet materials in air has been studied over a wide range of temperature. Gasification was detectable at around 500°C and above about 900°C, under the flow conditions used in the experiments, the overall rates of gasification were controlled by gas phase diffusion. The presence of catalysts reduced the temperature for the onset of gasification but had no effect on the kinetics in the diffusion-controlled region. Borate-based coatings containing refractory particulates and silicon carbide coatings sealed with borates have been found capable of protecting C/C composites against air oxidation for extended periods to temperatures of at least 1200°C.

Effect of metallic impurities on the gasification of graphite in water vapor and hydrogen

Abstract The catalytic effects of metallic impurities on the reactivity of graphite towards water vapor and hydrogen in the temperature range 25° to 1100°C have been investigated as a function of the oxidation state of the impurity. Iron, cobalt and nickel are active catalysts for the former reaction between 600° and 1000°C when the metal is kept in the reduced state by means of added hydrogen. Motion of the metallic particles on the basal plane surface of the graphite during the reaction leads to the formation of channels which with the smallest catalyst particles are oriented mostly in the 〈1120〉 directions. Vanadium and molybdenum are weak catalysts under these conditions, whereas copper, zinc, cadmium, silver, chromium, manganese and lead are inactive. When hydrogen is absent so that the metal remains in the oxidized state, the catalytic activity of all these impurities is low or negligible. The reaction of graphite with dry hydrogen occurs less readily but is again strongly catalyzed by metallic iron, cobalt and nickel. Manganese, chromium, molybdenum and vanadium show a slight catalytic effect in the hydrogenation reaction at temperatures around 1000°C, whereas copper, zinc, cadmium, silver and lead are inactive.

Borate treatment of carbon fibers and carbon/carbon composites for improved oxidation resistance

Carbon fibers and carbon/carbon composites have been treated with borate additives and then cured at 500–600°C to produce a continuous film of boron oxide on all exposed surfaces.This treatment has been found to be highly effective in retarding oxidation of the carbonaceous substrate for extended periods in flowing air at temperatures up to 1000°C. At higher temperatures, and in the presence of water vapor, borate species were appreciably volatile and the oxidation protection provided by the coatings was less effective.

The inhibition of graphite oxidation by phosphorus additives

Abstract Impregnation of graphite with organic phosphate and phosphite esters has been found to result in increased resistance to air oxidation at elevated temperatures. Thermal decomposition of the phosphorus compounds at 200–600°C leaves a hydrophilic residue strongly adsorbed on the graphite surface at active sites where oxidation normally occurs.

The effects of boron additives on the oxidation behavior of carbons

Abstract Impregnation of graphite with aqueous solutions of boric oxide or with organo-borates has been found to result in marked reduction in the rate of oxidation of the graphite in dry or moist air between 600 and 1000°C. At these temperatures, the additives probably form a glassy oxide residue which blocks active sites on the graphite surface.

Gasification of graphite in carbon dioxide and water vapor—the catalytic effects of alkali metal salts

Abstract The catalytic effects of a series of alkali metal salts in promoting the gasification of a graphite powder by carbon dioxide and water vapor have been studied by thermogravimetry between 700 and 1100°C. Lithium salts, specifically the carbonate and hydroxide, were the most active catalysts for both reactions. Cyclic processes which may account for the observed catalytic effects were evaluated from the standpoint of thermodynamic feasibility.

The catalyzed reaction of graphite with water vapor

Abstract The effects of Li, Na and K carbonates in catalyzing the reaction between graphite and water vapor have been studied in the temperature range 700–1100°C, using simultaneous thermogravimetry-differential thermal analysis. The catalyzed reaction, which is rapidly accelerated at temperatures in the vicinity of the melting point of the alkali metal carbonate, occurs most readily in the presence of the lithium salt. Thermodynamic analysis of the possible reactions suggests that the catalytic mechanism involves an oxidation-reduction cycle with the intermediate formation of the hydroxide of the alkali metal.