Kinetics and percolation: coke in heterogeneous catalysts

Abstract

In the conventional lattice percolation models, bonds or sites are open at random, whereas in reality there is often interplay of percolation and the kinetics under consideration. An interesting and practically important example is hydrocarbon conversion occurring in a reactor containing mm-sized porous pellets with catalytic nanoparticles deposited at walls of the nanopores. Such catalytic heterogeneous reactions are often accompanied by coke formation deactivating catalytic nanoparticles and blocking pores for reactant diffusion, so that one needs to remove coke from time to time e.g. via reaction with oxygen. Herein, I first present generic coarse-grained Monte Carlo simulations of coke formation in a single pellet with the emphasis on the reaction regime influenced by reactant diffusion in pores. Then, the obtained coke distributions are used for similar simulations of coke removal. This combination of the models has allowed me to illustrate qualitatively new spatio-temporal regimes of the processes under consideration. For example, the removal of coke can be slow in the beginning, due to blocking of oxygen diffusion near the external pellet-gas interface and preventing its penetration to the central part of a pellet, and then fast when the pathways for diffusion to the center become to be open.