The production of energy from fossil fuels poses significant environmental concerns for today's energy systems, driving the need for carbon-free energy carriers. Hydrogen is widely regarded as a carbon-free energy carrier, and ammonia has emerged as a practical alternative for its storage, transport, and on-site production through catalytic decomposition. This work focuses on the development of a computational model of a laboratory-scale fixed-bed reactor operated at Technische Universität Darmstadt for ammonia decomposition catalyst testing. While the experimental reactor is primarily intended for kinetic investigations, the modeling study aims to provide insight into the coupled transport and reaction phenomena governing reactor behavior and to establish a framework that can be extended toward scalable reactor configurations.