Formation of ammonia from ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ by Haber's process using Fe is an example of

The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis.

${\mathrm{N}}_{2\left(\mathrm{g}\right)}+3{\mathrm{H}}_{2\left(\mathrm{g}\right)}\stackrel{\mathrm{Fe}\left(\mathrm{s}\right)}{\to }2{\mathrm{NH}}_{3\left(\mathrm{g}\right)}$

The reactants are in gaseous state while the catalyst is in solid state.