Using the ammonia decomposition as an example, the kinetics of a heterogeneous reaction are discussed. On a continuously non‐uniform surface, no single reaction step is the limiting one; on the optimum sites two reaction steps are equally slow. The expression for over‐all velocity is developed, and solved exactly for the case of an exponential distribution of activation energies. It appears that if the distribution is broad, its exact shape is unimportant, but that a factorr,connected with the amount of adsorption energy available for catalytic work, is important. It is shown that for a broad distribution, the pressure dependence of the over‐all rate is the same as the pressure dependence on the optimum sites. Because the position of the optimum depends on pressure, one cannot assume that the surface is ``effectively uniform.'' The simplest of ammonia kinetics is discussed, but it appears that on a non‐uniform surface, much more extensive experiments than have been available are necessary to elucidate properly the kinetics of a reaction.
展开▼
