The reaction-diffusion within the catalyst pellet during methanation of CO/CO2 mixtures is described by a one-dimensional spherical model introduced in detail in our previous work . The discretization proceeds along the spatial domain 𝑦 from the center of the spherical pellet to the external surface (see figure ). At each discrete position an overall mass balance (equation ( )) and a molar balance for each component (equation ( )) is established. This set of material balance equations appropriately considers chemical reaction, material transport and pressure changes induced by volume contraction during reaction. In accordance with the experiments, isothermal conditions are assumed together with absence of external mass transfer limitations. Note that steady-state balances are given here according to the assumptions and the scope of the paper, while the unsteady-state balances are in the focus of our previous work .