ADSORPTION OF [CR(CN)5NO]3- ON TIO2 FROM SOLUTION STUDIED BY ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY

The adsorption of tetrabutylammoniumpentacyanonitrosylchromate(I), [(C 4H9)4N]3[Cr(CN)5NO], on TiO2 from solution has been investigated by el ectron paramagnetic resonance (EPR) spectroscopy. From acetonitrile, t he complex anion is adsorbed rapidly and binds strongly to the surface (Helmholtz double layer/molecular capacitor). All complex species wer e immobilized on the particle surface up to amounts corresponding appr oximately to charge compensation. For higher complex concentrations, t he anion is adsorbed reversibly to give Langmuirian adsorption isother ms. [Cr(CN)5NO]3- is immobilized on the surface by second-sphere coord ination via hydrogen bonds. This is confirmed by selective C-13 enrich ment of the axial CN- ligand. After long-time equilibration, electrost atically adsorbed free-tumbling complex species (Gouy layer) are obser ved in addition to the immobilized species (Stern layer). No adsorptio n is observed from aqueous solution. The adsorption capacity is depend ent on the surface charge determined by acid-base titration of an aque ous suspension of the pure oxide. This is shown by adsorption of [Cr(C N)5NO]3- on TiO2 pretreated with acid (HNO3) or base [KOH, (C2H5)4NOH] . Ligand-exchange reactions occurring in the solid/solution interlace were investigated in situ. The EPR linewidths are influenced by the sa mple preparation.