Reaction efficiency on the surface of a porous catalyst

The influence of geometrical factors on the efficiency of diffusion-control led reactive processes that take place on the surface of a porous catalyst particle is studied using the theory of finite Markov processes. The reacti on efficiency is monitored by calculating the mean walklength [n] of a rand omly diffusing atom/molecule before it undergoes an irreversible reaction a t a specific site (reaction center) on the surface. The three cases (geomet ries) considered are as follows. First, we assume that the surface is free of defects and model the system as a Cartesian shell (Euler characteristic, Omega = 2) of integral dimension d = 2 and uniform site valency v(i) = 4. Then, we consider processes in which the diffusing reactant confronts areal defects (excluded regions on the surface); in this case, both cl and Omega remain unchanged, but there is a constriction of the reaction space, and t he site valencies v(i) are no longer uniform. Finally, the case of a cataly st with an internal pore structure is studied by modeling the system as a f ractal solid, viz. the Menger sponge with fractal dimension cl = 2.73. The sensitivity of the reaction efficiency to the dimensionality of the reactio n spare (integer vs fractal), to the local symmetry at the reaction center las defined by the site valency v(i)), and to the size of the catalyst part icle las specified by the number N of lattice sites defining the system) is quantified by comparing the numerically exact values of [n] calculated in each case.