THE CO/H2 REACTION ON RH/MGO STUDIED BY TRANSIENT ISOTOPIC METHODS

Various transient isotopic methods have been used to study the CO/H2 reaction over 2.5 wt% Rh/MgO catalyst at temperatures in the range 493-573 K. The steady-state tracing method permits the determination of the surface coverage of CO and that of the very active carbon C-alpha. The results confirm that the steps for methane formation pass through a large reservoir of surface CO and through a small reservoir of active carbon (theta-c-alpha < 0.01). Rate control is largely determined by the rate of CO dissociation on the metal surface. The results also suggest that formate on the support may be an active intermediate to make CO2. Two pools of surface CO, one of alpha-CO* that readily exchanges with gaseous CO, and another one of beta-CO* that does not exchange with gaseous CO are present on Rh and participate in the methanation reaction. The amount of beta-CO* is found to increase with time on stream at 573 K. Some alpha-CO* and beta-CO* interact with -OH groups of the support during a temperature-programmed reaction in He to give CO2 and H-2, probably via a formate intermediate.