N-doped carbon nanofibers with various nitrogen contents and chemistries have been grown on cordierite monoliths by varying the synthesis conditions. Ru has been impregnated on the N-doped CNFs, on undoped CNFs, H2O2-treated CNFs, and alumina coated monoliths. These catalysts have been characterized (XPS, TPR, STEM, CO chemisorption) after several preparation stages and they have been tested in ammonia decomposition. It has been found that nitrogen doping contributes to stabilize Ru to a small particle size and in a reduced state. The strong interaction of the precursor with the nitrogen groups enables the preparation of small Ru nanoparticles, uniformly distributed throughout all the CNFs coating the monolith. Catalysts supported on N-doped CNFs have exhibited NH3 decomposition activities higher than catalyst on N-free CNFs. It has been evidenced that the catalyst with the highest amount of nitrogen facilitates keeping Ru in reduced state upon air exposure. This enhanced reducibility correlates with substantially higher TOF in ammonia decomposition than for the other catalysts supported on CNFs with various functionalizations.