Why 3d-series of elements acts as good catalyst?

Explanation

(a) Transition metals and their compounds exhibit catalytic activity due to
(i) Ability to adopt to multiple oxidation states
(ii) Transition metals can form complexes with reactant molecules using vacant d-orbitals. This lowers the energy of activation of the reaction and increases rate of the reaction.
(iii) Due to the presence of large surface area in finely divided state.
(b) Due to `d^(0)` configuration of `Ti^(4+)`, it is colourless where `Cr^(3+)` has `d^3` configuration facilitating d-d transition.