A catalyst system comprising a transition metal compound on a solid carrier, wherein the solid carrier is a surface-reacted calcium carbonate comprising ground natural calcium carbonate (GNCC) or precipitated calcium carbonate (PCC), and at least one water-insoluble calcium salt other than calcium carbonate, and wherein the surface-reacted calcium carbonate shows: (i) a specific surface area of from 15 to 200 m 2 /g measured using nitrogen and the BET method according to ISO 9277:2010; (ii) an intra-particle intruded specific pore volume in the range of from 0.1 to 2.3 cm 3 /g calculated from mercury porosimetry measurement, wherein the intra-particle intruded specific pore volume is determined as defined in the description; and (iii) a ratio of the at least one water-insoluble calcium salt to calcite, aragonite and/or vaterite in the range of from 1:99 to 99:1 by weight, wherein the surface-reacted calcium carbonate is a reaction product of ground natural calcium carbonate (GNCC) or precipitated calcium carbonate (PCC) treated with CO 2 and one or more H 3 O + ion donors, wherein the CO 2 is formed in situ by the H 3 O + ion donors treatment and/or is supplied from an external source, wherein the transition metal is selected from titanium, zirconium, ruthenium, palladium, silver, osmium, platinum, gold, and mixtures thereof, and the transition metal compound is a molecular transition metal compound, a transition metal salt, a metallic transition metal compound including the elemental transition metal or a transition metal complex, and wherein the at least one water-insoluble calcium salt is defined as a calcium salt, which, when 100 g thereof is mixed with 100 g deionised water and filtered on a filter having a 0.2 μm pore size at 20 °C to recover the liquid filtrate, provides less than or equal to 1 g of recovered solid material following evaporation at 95 to 100 °C of 100 g of said liquid filtrate at ambient pressure. (i) a specific surface area of from 15 to 200 m 2 /g measured using nitrogen and the BET method according to ISO 9277:2010; (ii) an intra-particle intruded specific pore volume in the range of from 0.1 to 2.3 cm 3 /g calculated from mercury porosimetry measurement, wherein the intra-particle intruded specific pore volume is determined as defined in the description; and (iii) a ratio of the at least one water-insoluble calcium salt to calcite, aragonite and/or vaterite in the range of from 1:99 to 99:1 by weight, wherein the surface-reacted calcium carbonate is a reaction product of ground natural calcium carbonate (GNCC) or precipitated calcium carbonate (PCC) treated with CO 2 and one or more H 3 O + ion donors, wherein the CO 2 is formed in situ by the H 3 O + ion donors treatment and/or is supplied from an external source, wherein the transition metal is selected from titanium, zirconium, ruthenium, palladium, silver, osmium, platinum, gold, and mixtures thereof, and the transition metal compound is a molecular transition metal compound, a transition metal salt, a metallic transition metal compound including the elemental transition metal or a transition metal complex, and wherein the at least one water-insoluble calcium salt is defined as a calcium salt, which, when 100 g thereof is mixed with 100 g deionised water and filtered on a filter having a 0.2 μm pore size at 20 °C to recover the liquid filtrate, provides less than or equal to 1 g of recovered solid material following evaporation at 95 to 100 °C of 100 g of said liquid filtrate at ambient pressure. <EOS>