From the external surface the CO molar fraction reaches zero very quickly and consequently the CO methanation rate also drops to zero. Once the CO fraction approaches very small values, the CO2 hydrogenation rate increases sharply as CO inhibition is removed. Deep inside the catalyst pellet, the CO2 reaction rate drops again, as the CO2 fraction continues to decrease, as well. Although the decreasing CO hydrogenation rate is compensated by increasing CO2 methanation to a certain extent, the overall CH4 formation rate is not fully compensated and therefore lower compared to the small pellets. Interestingly, a significant profile in CO molar fraction is simulated for 𝑧 = 1 (figure ), which underlines diffusion