Adsorption of deuteroporphyrin-IX on nickel from neutral and alkaline aqueous solutions

The adsorption, protective, and passivating effects of deuteroporphyrin (H2P) on nickel in neutral and alkaline buffer solutions have been studied.Deuteroporphyrin adsorption isotherms were obtained on the reduced nickel surface at E=-0.65 V and on the oxidized surface at E=0.2 V.The highest values of the free energy of adsorption, 0 ads,max , G obtained at E=-0.65 V, amount to 0 a,max G =61.4 kJ/mol for pH 7.4 and 0 a,max G =59.5 kJ/mol for pH 9.2.These values indicate the chemisorption interaction of deuteroporphyrin with the metal surface.In chloride borate buffer solutions with pH 7.4 and 9.2, deuteroporphyrin stabilizes the passive state of nickel at concentrations that are three orders of magnitude lower than the concentration of the chloride ion.The thicknesses of conditional deuteroporphyrin monolayers on the oxidized and reduced nickel surfaces in neutral and alkaline buffer solutions have been calculated.The orientation of the deuteroporphyrin anion on the electrode surface has been determined.XPS studies have shown that H2P forms a thin protective layer on the surface due to the formation of a bond between the surface and carboxylic groups of molecules.Adsorption is directly indicated by the presence of peaks related to porphyrin ring nitrogen atoms in the nitrogen spectrum of the sample after treatment, and is indirectly, by an increase in the proportion of the hydroxide layer in the Ni2p3/2 spectrum.