THE DECOMPOSITION OF CARBON MONOXIDE BY NICKEL CATALYSTS. THE REACTION MECHANISM BETWEEN 250"
AND 450" By J. A. TEBBOTH T h e catalytic formation of methane from water gas has recently been studied with a view to obtaining information to render the process capable of commercial exploitation. The principal factors involved in the catalysis have been described by Booth, Wilkins, JoUey and Tebboth,' and orhcr workers.? A major difficulty cncountcred was the progressive deposition of carbon on the nickel-thoriakieselguhr catalyst used. T h e work described in this paper was a portion of that carried out in order to ascertain the mechanism of the formation of this carbon, so that measures could be taken to minimize it.
The thermal conditions prevailing in the catalyst bed during methane synthesis are far from uniform. Above the threshold temperature of the catalyst (the temperature at which the reaction becomes thermally self-supporting) passage of carbon monoxidehydrogen mixtures at space-velocities of ~ooo-~ooo vol./vOl. catalystlhr. resulted in immediate methane synthesis with a rise in temperature in the catalyst bed. With suitable design of reaction vessel a temperature of 350' could be maintained without external heat in the catalyst at the exit of the vessel. At this temperature virtually complete conversion of carbon monoxide was obtained. Typical figures for the measured temperature in the catalyst bed in laboratory-scale apparatus are given in Table I. The tubes used were of $ in. internal bore and each contained 8 C.C. of catalyst. A simple glass tube waused but the steel tube was surrounded by an outer jacket of mercury.
This very uneven temperature distribution is important in assessing the probability of different reactions by which carbon may beformed. Possible reactions and their equilibrium constants (Kp) are : Kp at Gooo K. Kp at 800" K. COI-i CH, = IC+ 2H.O 5'573 3'233 (1) CHa=C+zH? 9.009 lo-> 0.196 (2) CO+ HE= C +H20 1.694 10' 19.91