Catalyst and activator for exothermic reaction

Described herein is a combination of a catalyst and an activator and method of use for causing an exothermic reaction in a low temperature environment. The mixture can include sodium nitrite, an ammonium-based compound, a catalyst that comprises an oxidizer, and an activator and can be injected into a fluid flow path. The fluid flow path can be a pipeline, a flowline, a wellbore, or a subterranean formation. The mixture can cause an exothermic reaction in the fluid flow path and remove, using the exothermic reaction, a damaging material from the fluid flow path.