Figure compares the determined CO reactant selectivity for simulation and experiment at 𝑇 = 556 K and different CO/COx ratios 𝑧. It can be seen that the CO reactant selectivity is always higher for the small (175 µm) compared to the large (800 µm) pellets, both in simulations and experiments. In addition, experiments at high 𝑧 values exhibit a CO selectivity exceeding 1, which means that more CO is consumed than CH4 is formed. This can be explained by formation small amounts of higher hydrocarbons, as discussed in our previous work, which are not considered in the model . Note that the formation of higher hydrocarbons increases the numerator in eq. ( ), while the denumerator is unaffected, which eventually leads to an overestimation of the experimental CO reactant selectivity. In addition, reverse water-gas-shift reaction may occur in the experiment, which is not considered in the kinetic model. Nevertheless, the decrease in reactant selectivity for 𝑧 < 0.5 is evident for both simulation and experiment, which supports the postulated shift from CO to CO2 hydrogenation with declining CO/COx ratio. In the methanation of mixtures with a high CO/COx ratio 𝑧, the choice of pellet size therefore has a lesser effect on the reactant selectivity as in the case of low CO/COx ratios.