The effective frequency-dependent rate coefficient for conversion of particles diffusing in a disordered system of static spherical catalysts is calculated for low density of catalysts by the method of multiple scattering theory. The rate coefficient follows from a Foldy-Lax type formula. The catalysts are assumed uniform, but different energies, conversion rates, and diffusion coefficients are allowed inside and outside the catalysts. The frequency-dependent rate coefficient is characterized by its steady-state value and by a spectrum of relaxation rates. The coefficient displays a wide variety of behavior as a function of the parameters of the model.