Finding a Mechanism Consistent with Experimental Observations

2 Finding a Mechanism Consistent with Experimental Observations 

We now propose a mechanism for the hydrodemethylation of toluene. We assume that toluene is adsorbed on the surface and then reacts with hydrogen in the gas phase to produce benzene adsorbed on the surface and methane in the gas phase. Benzene is then desorbed from the surface. Since approximately 75% of all heterogeneous reaction mechanisms are surface-reaction-limited rather than adsorption- or desorption-limited, we begin by assuming the reaction between adsorbed toluene and gaseous hydrogen to be reaction-rate-limited. Symbolically, this mechanism and associated rate laws for each elementary step are  

For smface-reaction-limited mechanisms, we use the adsorption rate Equation (10-72) to obtain 

Substituting Equations (10-75) and (10-76) into Equation (10-77) and rearranging, we obtain 

substitute for Cj.s and Cg-s and then substitute for Q. in Equation (10-73) to obtain the rate law for the case of surface-reaction control  

Rate law for Eley-Rideal surface-reaction-limited mcchanUm 

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