Hydrogen addition effectively reduces the fuel consumption of spark ignition engines. We propose a new on-board reformer that produces hydrogen at high concentrations and enables multi-mode operations. For the proposed reformer, we employ a catalytic fuel decomposition reaction via a commercial NiO-CaAl2O4 catalyst. We explore the physical and chemical aspects of the reforming process using a fixed bed micro-reactor operating at temperatures of 550-700 degrees C. During reduction, methane is decomposed to form hydrogen and carbon. Carbon formation is critical to hydrogen production, and free space for carbon growth is essential at low temperatures (<= 600 degrees C). We define a new accumulated conversion ratio that quantitatively measures highly transient catalytic decomposition. The free space of the coated monolith clearly aided low-temperature decomposition with negligible pressure drop. The coated substrate is therefore suitable for on-board applications considering that our reformer concept also utilizes the catalytic fuel decomposition reaction. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.