Diffusion coupling kinetics in multisite catalysis: a microkinetic framework

Multicomponent (or multisite) catalysts are gaining increasing importance and popularity in heterogeneous catalysis and yet the cooperation mechanism among them remains obscure and intriguing. In this work, a vital and characteristic phenomenon in the two-component catalytic systems, the diffusion of surface intermediates, and the induced activity enhancement by coupling with the surface reactions is systematically investigated with both rigorous theoretical deduction and microkinetic simulations. By virtue of the proposed pseudo-fluctuation method, a general analytical framework is established with the two-step reaction model and the influence and origin of intermediate diffusion are elaborated. We quantify that the difference of the chemical potential of surface intermediates on different sites leads to the diffusion and determines the diffusion flowing from the strong adsorption site to the weak adsorption site. More importantly, we demonstrate that an enhancement in the overall reaction rate can be achieved by integrating the advantages of different components. Specifically, the optimum promotion effect would appear when the adsorption abilities of the two-component catalysts for the surface intermediate are around the chemical potential of the reactant and product, respectively. From the aspect of practical preparation methods, the coupling kinetics of the mechanically mixed catalysts and the alloy-typed catalysts are theoretically compared and a preliminary strategy for choosing the coupled components is proposed. We believe that this work provides a fundamental insight into the understanding and manipulation of the intermediate diffusion process.