Reaction-diffusion processes in multiscale catalytic porous media are found in a wide range of scientific areas as, for example, electrochemical energy conversion and storage devices, geological systems and bioengineering. The dependency of effective transport properties on reaction rate has been long debated in the literature, and traditionally ignored in emerging fields, such as polymer electrolyte fuel cells (PEFCs). In this work, a 1D upscaling method is presented to evaluate the effective properties (effective diffusivity and catalyst utilization) of PEFC catalyst layers featuring first-order kinetics. Unlike Whitaker's closure method, the present algorithm is easy to implement and well suited for porous media with arbitrarily complex 3D geometries. The numerical results show that the normalized effective diffusivity and catalyst utilization are not passive geometrical properties but are influenced by the reaction-diffusion coupling when the Thiele modulus is higher than 1. This effect can be important at high current densities in the cathode catalyst layer of state-of-the-art PEFCs.