A catalytic site is introduced into a two-dimensional Lorentz gas system consisting of three disks arranged in an equilateral triangle to model reactive dynamics. This system is studied at a microscopic level using an N-cylinder description where the exact dynamics is replaced by a symbolic dynamics which is a generating partition. The Kolmogorov-Sinai entropy and its finite and colored varieties are discussed. These are then related to the colored escape rate, a macroscopic property. Lastly, escape is eliminated by extending the three disk system to an infinite lattice, and the color correlation function is studied. For large catalytic regions the Poisson process rate law expression breaks down.