Iron, titanium, vanadium, nickel, platinum and palladium are examples of transition metal catalysts. Transition metals and their compounds act as catalysts because their electronic configurations enable them to temporarily exchange electrons with reacting species.
The transition metals are the metals located in the middle section of the periodic table, called the d-block. Both transition metal and d-block element refer to the same category in the table, elements that have partially filled d-orbitals as their valence shells. Transition metals can be further subdivided into first, second and third row varieties.
Oxidation-reduction reactions require one molecule to lose electrons and another to gain electrons. Many oxidation-reduction reactions proceed at a very slow rate in the absence of catalysts because the likelihood of an oxidative species encountering a reductive species to exchange electrons is very low. The d-shell outer electrons of transition metal catalysts are easily lost and gained. They can lend electrons to the species undergoing reduction and take electrons from those undergoing oxidation. This facilitation of electron exchange speeds up the reaction. Transition metals are not used up when they function as catalysts. Instead, they undergo a temporary change in their oxidation numbers, which is restored at the end of the end of the reaction.