3346.     TEBBOTH, J. A.  Decomposition of Carbon Monoxide by Nickel Catalysts.  Reaction Mechanism Between 250° and 450°.  Jour. Soc. Chem. Ind., vol. 67, 1948, pp. 62-66.

        Principal factors involved in the catalytic formation of CH₄ from water gas have been described recently.  A major difficulty encountered was the progressive deposition of C on the Ni-ThO₂-kieselguhr catalyst used.  An attempt is made here to ascertain the mechanism of the formation of C and how to minimize it.  It was found that unsupported and unpromoted Ni catalysts, derived from precipitated NiCO₃, showed little activity for the decomposition of CO at all temperatures between 250° and 450°.  The presence of ThO₂ and kieselguhr in a Ni catalyst prepared from the precipitate NiCO₃ caused a marked increase in the rate of decomposition of CO at the same temperatures.  Elementary C formation occurred on Ni catalysts at al temperatures from 250°-450°, being least at 250° and greatest at 325°-350°.  The Ni of such promoted and supported Ni catalysts was converted almost quantitatively to Ni₃C at 250°.  Above this temperature, carbide formation was incomplete.  Decomposition of Ni₃C in the presence of ThO₂ and kieselguhr occurred above 250° and was rapid at 300°.  The proportion of C combined as carbide decreased with increasing temperature, but some persisted at 450°.  When using promoted and supported catalysts for CH₄ synthesis from CO and H₂, the conditions prevailing at the inlet end of the catalyst bed were very favorable for elementary C formation.  The results were accounted for by 2 reactions:  3 Ni+2 CO=Ni₃C+CO₃ and Ni₃C=C+3 Ni.  The latter reaction may have occurred in 2 stages with the formation of a lower carbide Ni₆C, but evidence here was incomplete.

        ----------.  See abs. 304.