Synthesis of Ru-based catalysts for CO2 methanation and experimental assessment of intraporous transport limitations

Aiming at the optimization of the efficiency of the CO2methanation process at industrially relevant conditions on Ru/Al2O3catalysts, the effect of the size of the catalytic pellet on the catalyst performance has been investigated. For this purpose, γ-Al2O3spheres of various diameters (100, 800 and 2300 μm) have been impregnated so to achieve a 0.5 wt. % Ru loading. Interestingly, the dry impregnation method has led to the preparation of eggshell catalysts on supports bigger than 100 μm. This indicates a strong interaction between the γ-Al2O3support and the Ru precursor, which is thus deposited only on the outer shell of the spherical pellets. By changing this interaction by tuning the acidity of the impregnating solution with an inorganic acid, it is possible to effectively control the thickness of the catalytic layer, and homogeneously impregnated pellets can eventually be obtained. The impregnated pellets have been tested in process conditions of industrial interest for CO2methanation, under kinetically controlled regime (i.e. after crushing and sieving). No differences in CO2conversion nor in CH4selectivity have been observed between the powdered homogeneously impregnated pellet and the powdered eggshell pellet when HNO3is used to modify the pH of the impregnating solution, indicating that the acid introduction does not affect the catalyst performance. Subsequently, homogeneously impregnated pellets have been tested to assess the role of internal diffusional limitations. No significant differences in the catalyst activity have been observed when varying the dimension of the pellets, indicating the absence of relevant intraporous diffusional limitations with a Ru loading of 0.5 wt. %. Evidences have been collected also on the role of Cl ions remaining on the catalyst surface when using RuCl3as precursor and/or HCl as acidifying compound.