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The simultaneous processes of diffusion, adsorption and chemical reaction, considering the transient nature of the concentration profiles in the porous catalyst particles as applied to the analysis of consecutive reactions  A → B → C , where reactant and products are subjected to diffusion limitations, are analyzed. The concentrations of the desired intermediate product  B , both the average in the catalytic particles and the observed in the fluid phase, initially increase as a function of time until reaching a maximum value and then decline due to the consumption in the secondary reaction. Due to the diffusion restrictions and the adsorption effect, the observed selectivities, calculated from the concentrations in the fluid phase, are always lower than the true selectivities, which also include the amounts accumulated in the particles. Besides depending on the rates of the primary and secondary reactions, the observed yield of product  B  also depends on the system adsorption capacity, i.e., the relationship between the capacities of the particles and the external fluid phase to accumulate the reactant species. For a given relationship between the intrinsic rates of the primary and secondary reactions, the higher the system adsorption capacity, the lower the observed yield of  B  as a function of conversion. The relationship between the observed yield of  B  and the observed conversion of  A , calculated considering the transient state of the concentration profiles in the particles, is coincident with that predicted by classical models, which assume the steady state in the particles, when the system adsorption capacity is extremely small.

Unsteady state diffusion-adsorption-reaction. Selectivity of consecutive reactions on porous catalyst particles