Coupled reduced mechanisms were developed for the hetero-/homogeneous combustion of fuel-lean and fuel-rich H-2/CO/O-2/N-2 mixtures in a Pt-coated planar channel, using a method based on the Directed Relation Graph (DRG) and for a wide range of operating conditions for which detailed measurements are available. It is demonstrated that catalytic and gas-phase reaction mechanisms can be reduced together for all the fuel-lean cases. On the other hand, when joint reduction is performed for low-pressure fuel-rich cases (using a strict threshold value to capture the relatively weak, yet still important coupling between the catalytic and gas-phase reaction pathways) the result is a less efficient reduction process. For the high-pressure fuel-rich cases the catalytic-gaseous chemical coupling is weak enough to be neglected such that the reduction can be conducted separately for higher reduction efficiency. The reduced mechanisms reproduced well the major gaseous species concentrations, gas temperatures and homogeneous ignition distances obtained with the detailed mechanisms, thus demonstrating the capacity of the applied method in reducing catalytic/gas-phase reaction mechanisms. In addition, it is shown that for fuel-lean stoichiometries the reduction could provide a rapid indication if gas-phase combustion is ignited, without the need of full simulations. The reduced mechanisms are expected to facilitate large-scale simulations, with fidelity, for the design and thermal management of practical catalytic combustion systems. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.