Adsorption of [Cr(CN)₅NO]^3- on TiO₂ from solution studied by electron paramagnetic resonance spectroscopy

The adsorption of tetrabutylammoniumpentacyanonitrosylchromate(I), [(C₄H₉)₄N]₃[Cr(CN)₅NO], on TiO₂ from solution has been investigated by electron paramagnetic resonance (EPR) spectroscopy. From acetonitrile, the complex anion is adsorbed rapidly and binds strongly to the surface (Helmholtz double layer/molecular capacitor). All complex species were immobilized on the particle surface up to amounts corresponding approximately to charge compensation. For higher complex concentrations, the anion is adsorbed reversibly to give Langmuirian adsorption isotherms. [Cr(CN)₅NO]^3- is immobilized on the surface by second-sphere coordination via hydrogen bonds. This is confirmed by selective ¹³C enrichment of the axial CN^- ligand. After long-time equilibration, electrostatically adsorbed free-tumbling complex species (Gouy layer) are observed in addition to the immobilized species (Stern layer). No adsorption is observed from aqueous solution. The adsorption capacity is dependent on the surface charge determined by acid-base titration of an aqueous suspension of the pure oxide. This is shown by adsorption of [Cr(CN)₅NO]^3- on TiO₂ pretreated with acid (HNO₃) or base [KOH, (C₂H₅)₄NOH]. Ligand-exchange reactions occurring in the solid/solution interface were investigated in situ. The EPR linewidths are influenced by the sample preparation.