Above 38o0-42oo, the reaction, also leads to elementary carbon formation.
Schmidt,8 however, in an X-ray examination, failed to find any evidence for the existence of the higher carbide Ni,C,.
In considering whether Bahr and Bahr's mechanism can be applied to interpret the results obtained with the precipitated catalysts used in the present work, j t is clear that the reaction rates are appreciably modified by the promoter and support. Thus reaction (I) is always accompanied by reaction or rcactions leading to elementary carbon formation, i.e., by the reaction 2CO=CO,+C, whatever the mechanism of the latter reaction may be; and has only been found, to be faster than this concurrent reaction at 250". In addition, reaction (3) also occurs below 400' and is rapid above 300~. Above 350°, the rate of reaction diminishes with increasing temperature, and this decrease appears to run parallel with the decrease in the percentage of combined carbon in the catalyst. The results obtained in the present work are therefore best accounted for by the two simple reactions 3Ni + 2CO = Ni,C + CO, and Ni,C = C f 3Ni.
The latter reaction may however occur in two stages, with the formation of a lower carbide Ni,C, but evidence of this is as yet incomplete.
Summary of results .
I. Unsupported and unpromoted nickel catalysts, dcrived from precipitated nickel carbonate, showed little activity for the decomposition of carbon monoxide at all temperatures between 250' and 450".
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The presence of thoria and kieselguhr in a nickel catalyst prepared from the precipitated nickel carbonate caused a marked increase in the rate of decomposition of carbon monoxide at 3. Elementary carbon formation occurred on nickel catalysts at all temperatures from 250" to 45O0, being least at 250" and greatest at 3~5~-350".
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The nickel of such promoted and supported nickel catalysts was converted almost quantitatively to Ni,C at 250'. Above this temperature, carbide formation was incomplete.
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Decomposition of nickel carbide, in the presence of thoria and kieselguhr, occurred above 250' and was rapid at 300' .
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The proportion of carbon combined as carbide decreased with increasing temperature, but some persisted at 450". 7. When using promoted and supported catalysts for methane synthesis from carbon monoxide and hydrogen, the conditions prevailing at the inlet end ofthe catalyst bed were very favourable for elementary carbon formation. 25Oo-45O0.