The process that does not occur at the interface of phases is :

1.  Crystallisation

2.  Heterogeneous catalysis

3.  Homogeneous catalysis

4.  Corrosion

 

 
 
(3) HINT: In homogeneous catalysis reactant and catalyst both are in same phase.
 
Explanation: Homogeneous catalysis does not occur at the interface of phases as in case of homogeneous catalysis reactant and catalyst have same phase and their distribution is uniform throughout.