Ammonia decomposition for the production of hydrogen in a newly fabricated, aluminum framework post microreactor is modeled. A detailed microkinetic model describing the chemistry of ammonia decomposition on Ru is developed. A PFR model is used as a low hierarchical tool for developing a reduced rate expression using a computer-aided methodology. This reduced chemistry model is used in computational fluid dynamics simulations of microreactors, and good agreement with experimental data is observed. It is found that the overall conversion in the post microreactor better approximates that of a PFR than that of a CSTR. It is shown that the posts play an important role in enhancing the transverse mass transfer and providing a high catalyst surface area and that interesting flow patterns and back-diffusion occur at lower flow rates. Finally, parametric studies are performed, and a tradeoff between pressure drop and conversion is observed under certain conditions.