Catalytic Property of Transition Metals

Transition metals and their compounds are among the most important and most widely used catalysts in the chemical industry, in laboratory chemistry, and in biological systems. Two structural features of transition metals, working together, explain why.

Variable oxidation states. Because a transition metal can readily interconvert between two (or more) stable oxidation states, it can accept electrons from one reactant and donate them to another, forming an unstable intermediate compound partway through the catalytic cycle and then regenerating its original oxidation state at the end -- effectively providing an alternative, lower-activation-energy reaction pathway that does not require the two reactants to interact directly with one another. Iron, for instance, cycles between $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$ in its role catalysing the reaction between iodide and peroxodisulfate ions (developed further in a worked exercise later in this chapter), and vanadium cycles between the $+5$ and $+4$ oxidation states ($\text{V}_2\text{O}_5 \rightleftharpoons \text{VO}_2$) in the industrial oxidation of sulfur dioxide.

Large surface area and adsorption. When used in the finely divided metallic or oxide form, transition metals present a large active surface area on which gaseous or dissolved reactant molecules can be adsorbed. Adsorption onto the metal surface weakens the bonds within the adsorbed reactant molecule (by donating electron density from the metal's partially filled $d$ orbitals into the reactant's antibonding orbitals, or vice versa), lowering the activation energy needed for the reaction to proceed, and also holds the reacting molecules close enough together, in the right orientation, to react more readily than they would in the free gas or solution phase. …