Electrochemical and XPS evidence of the aqueous formation of Mo ₂ O ₅

XPS was employed to detect the surface species deposited on an activated platinum electrode following cathodic polarization in deaerated 0.1 M (NH 4 ) 6 Mo 7 O 24 · 4H 2 O solution. At – 160 mV (saturated calomel electrode, SCE), evidence was found of the possible dissociation of polymolybdate to molybdate with subsequent surface adsorption. At –340 mV (SCE) molybdate was observed to reduce to Mo 5+ in the form of Mo 2 O 5 . At – 540 mV (SCE), Mo 4+ was the stable species, existing in the form of tetravalent oxyhydroxide MoO(OH) 2 according to XPS analysis. No further reduction from Mo 4+ to lower‐valent states was observed when cathodically polarized to – 900 mV (SCE). Mo 2 O 5 was found to be unstable in air, oxidizing readily to MoO 3 .