Figure shows that higher temperatures always lead to higher methane formation rates. With regard to the pellet size, however, it can be seen that there is an optimum with medium pellet sizes. For small pellets, the methane formation rate approaches the intrinsic kinetics, while it approaches zero for larger pellets. The observed maximum formation rate is caused by the slight diffusion limitations, which reduces the selfinhibition of the reaction rate by CO. At 513 K, for instance, this leads to a 1.35-fold effective methane formation rate for the optimal pellet size of 1.75 mm with respect to intrinsic conditions. With higher temperatures, the maximum shifts to smaller pellets, since the higher activity inherently involves a stronger diffusion limitation, while the improvement in effective methane formation rate with respect to intrinsic conditions diminishes. A comparison with other CO/CO2 ratios in the appendix (Figure A.3 and Figure A.5) shows that a higher CO content shifts the optimum further towards larger pellets, which underlines the inhibition by CO as main reason for the non-monotonic impact of pellet size on effective methane formation rate. The observed maximum in methane formation rate is in good agreement with the findings by Roberts and Satterfield , while potential multiplicity mentioned by the authors is absent under the conditions of the present work. In addition, figure shows the corresponding reactant selectivity for CO from which the reactant selectivity of CO2 can be derived via equation (CO formation rate in Figure A.7). A decreasing reactant selectivity of CO can be observed with increasing both temperature and pellet diameter. However, the selectivity in the investigated range is much more sensitive to the pellet size than to the temperature. For example, a significant CO2 conversion cannot be achieved by temperature variation, if small pellets with sizes below 1.5 mm are used. On the contrary, CO2 hydrogenation can hardly be avoided in large pellets (compare