Promoters, hydrogen entry into metals

The entry of hydrogen into metals is promoted by various compounds, especially those involving elements from Groups VB (P, As, and Sb) and VIB (S, Se, and Te) of the periodic table. Some anions such as CN , CNS , and 1 and certain carbon compounds such as CS2, CO, and CSN2H4 also promote hydrogen entry. 

The amount of the hydrogen that is liberated on or near a metal surface, which then enters the metal, varies according to the environment and condition of the metal. The main factor that promotes the entry of hydrogen into a metal is the presence on the metal of a surface poison such as sulfide or other species, which inhibit the hydrogen recombination reaction. 

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