Nitrogen is not an activating atmosphere, it just prevents the carbon from being oxidized by air. It is extremely important to do it in most cases, except with H3PO4 for which activation can be done in air, as the phosphate products "protect" the carbon during this step, provided that the temperature is not too high (below 580°C). Have a look at the attached file.

Alain

I agree with Alain's answer.

If carbon is activated while heating under air for enough time you end up having no more carbon left

Sorry I forgot to add a note:

Based on our own experience I do not recommend using phosphoric acid as it gets sticky and not easy to remove after activation. It penetrates through the activated carbon pores and gets stuck over there.

Thank you both for your answers.