KINETIC AND TRANSIENT KINETIC INVESTIGATIONS OF THE SYNTHESIS OF OXYGENATES FROM CARBON-MONOXIDE HYDROGEN MIXTURES ON SUPPORTED RHODIUM CATALYSTS

A detailed kinetic investigation has been undertaken on the CO/H2 reaction on promoted rhodium catalysts at high pressures. It has been found that hydrocarbon production is inhibited by increasing the carbon monoxide pressure and enhanced by increasing the hydrogen pressure. Ethanol production is generally negative order with respect to carbon monoxide and positive order with respect to hydrogen. Both acetaldehyde and acetic acid formation are positive order with respect to carbon monoxide and hydrogen pressures. The results are interpreted in terms of a model in which the availability of hydrogen is a critical factor in determining the activity and selectivity of the various catalysts. Promotion of rhodium by reducible oxides causes a change in the orders of reaction, the net effect being that the reactions become much less sensitive to hydrogen pressure. It is concluded that such oxide promoters enhance the availability of hydrogen by increasing the effective surface hydrogen pressure on the catalyst adjacent to active sites. Transient kinetic experiments have been performed in which the reaction mixture has been switched on and off and products analysed at short intervals of time. The results are in agreement with the conclusions of the kinetic experiments concerning the orders of reaction and the sensitivity to hydrogen pressure. No evidence has been found for long delays in the production of ethanol when the catalyst is exposed to the CO/H2 reaction mixture. Possible reasons for differences observed in this work and in other published work are considered. The overall conclusion is that the formation of ethanol is rapid, but that the detection of ethanol may be delayed by adsorption effects in the catalyst bed.