The popup help in vanilla for the inert catalyst instructs the player to put it in a furnace to reactivate it, but that isn't possible - the only inputs to furnaces are stone/iron ore / copper ore /prismite ore and iron plate.

voidcraft 2.3.0

The inert catalyst just need to wait like 30 seconds.

It spoils into an active catalyst.

If you aren't using Space Age, then catalysts are restored by smelting rather than spoilage because vanilla Factorio doesn't support spoilage.

Anyway, I wasn't able to reproduce this. I made a new game with only base and Voidcraft selected (i.e., not space-age) and as expected the inert catalyst was restored by putting it in a furnace.

Ran into same issue with inert catalyst being non smeltable and non spoiling. Seemed to come from having quality mod installed without space age mod as removing quality allowed the inert catalyst to become smeltable.

voidcraft 2.4.3
quality 2.0.72

Ok, this will be fixed in Voidcraft 2.5.