Not exactly a powder but Zyklon B was liquid hydrocyanic acid (HCN) in a porous granulated Material https://de.m.wikipedia.org/wiki/Zyklon_B#/media/Datei%3ACyklonb.JPG
It did not need to be activated, however in winter gas Chambers needed to be heated to ensure the acid would go into its gaseous state fast enough.
Not a series but Death of Stalin is a super funny movie that is actually closer to reality than one might think.