A3.4 Transition Metal Compounds
Transition metal compounds often work well as catalysts because the reaction involves the variable oxidation states of the elements.
First the reactant is absorbed or adsorbed into/onto the catalyst.
The catalysed reaction occurs in two steps (in either order):
1. 1.
The oxidation of the transition metal atom/ion to a higher oxidation state,
2. 2.
And the reduction of the transition metal atom/ion – therefore, overall, the transition metal atom/ion is unchanged.
In the process the oxidation reaction (that its being catalysed) has occurred in two steps each with a lower activation energy than the original uncatalysed reaction (similar to the scheme for homogenous catalysts above.)
An example of this in action is the reaction of persulfate ions with iodide ions:
The overall reaction is:
${\textsf{S}_\textsf2\textsf{O}_\textsf8^\textsf{2-}}_\textsf{(aq)} {\textsf{+2I}^\textsf{-}}_\textsf{(aq)} \rightarrow {\textsf{2SO}_\textsf4^\textsf{2-}}_\textsf{(aq)}{\textsf{+I}_\textsf2}_\textsf{(aq)}$
This reaction is very slow at room temperature but it speeds up if either
$\textsf{Fe}^\textsf{2+}$
or
$\textsf{Fe}^\textsf{3+}$
Step 1: the persulfate ions oxidise
$\textsf{Fe}^\textsf{2+}$
to
$\textsf{Fe}^\textsf{3+}$
:
$\textsf{2Fe}^\textsf{2+}_\textsf{(aq)}+{\textsf S_\textsf2\textsf O_\textsf 8^\textsf{2-}}_\textsf{(aq)} \rightarrow {\textsf{2SO}_\textsf4^\textsf{2-}}_\textsf{(aq)}+ {\textsf{2Fe}^\textsf{3+}}_\textsf{(aq)}$
Step 2: the
$\textsf{Fe}^\textsf{3+}$
oxidises
$\textsf{I}^\textsf{-}$
to
$\textsf{I}_\textsf{2}$
and is reduced back to
$\textsf{Fe}^\textsf{2+}$
$\textsf{2Fe}^\textsf{3+}_\textsf{(aq)} + \textsf{2I}^{-}_\textsf{(aq)} \rightarrow \textsf{2Fe}^\textsf{2+}_\textsf{(aq)}+ {\textsf{I}_\textsf2}_\textsf{(aq)}$
The reactions are possible because of the electrode potentials of the Redox reactions involved – shown below:
Electrode Potential
$\textsf{E}^\textsf0$
/ Volts
${\textsf{S}_\textsf2\textsf O_\textsf 8^\textsf{2-}}_\textsf{(aq)}+\textsf{2e}^- \rightarrow {\textsf{2SO}_\textsf4^\textsf{2-}}_\textsf{(aq)}$
+2.01
$\textsf{Fe}^\textsf{3+}_\textsf{(aq)}+ \textsf{e}^{-} \rightarrow \textsf{Fe}^\textsf{2+}_\textsf{(aq)}$
+0.77
${\textsf{I}_\textsf2}_\textsf{(aq)}+\textsf{2e}^\textsf{-} \rightarrow \textsf{2I}^{-}_\textsf{(aq)}$
+0.54