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.