Electrodeposition of Platinum in Some Amide-Type Ionic Liquids Containing Bis(acetylacetonato)Platinum(II)

Abstract

Platinum (Pt) is widely used as a catalyst for various chemical and electrochemical reactions, such as water electrolysis, fuel cell and hydrogenation of organic species. Pt nanoparticle is often employed for many chemical reactions including methanol oxidation, conversion of exhaust gas, and so on. Recently, aprotic ionic liquids have been expected to be alternative media for electrodeposition of metals due to their wide electrochemical potential window, acceptable ionic conductivity, and negligible vapor pressure. Electrodeposition of Pt from halogeno-Pt source has been reported in the amide-type ionic liquid so far.1 Bis(acetylacetonato)metal complexes have attracted intensive interest as a precursor for the preparation of metal nanoparticles. Recently, we have reported electrodeposition of palladium from bis(acetylacetonato)palladium(II) in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA).2 In the present study, electrochemical behavior of bis(acetylacetonato)platinum(II) (Pt(acac)2) has been investigated in the bis(trifluoromethylsulfonyl)amide (TFSA–) ionic liquids composed of 1-butyl-1-methylpyrrolidinium (BMP+), 1-hexyl-1-methylpyrrolidinium (HMP+) and 1-decyl-1-methylpyrrolidinium (DMP+). The ionic liquids were prepared by the metathesis reactions of LiTFSA with the dialkylpyrrolidinum bromides, BMPBr, HMPBr and DMPBr in deionized water, separated by solvent extraction, finally dried under vacuum at 100 °C for 24 hours. The water content of each ionic liquid was found to be < 10 ppm by Karl Fischer titration. Pt(acac)2 was dissolved to give pale yellow transparent liquids in BMPTFSA, HMPTFSA and DMPTFSA by agitating for several hours at 60 °C. Tetra-coordinated Pt(acac)2 complex was found to exist in the ionic liquids by UV-visible spectroscopy. Cyclic voltammetry measurements of a glassy carbon electrode were conducted in BMPTFSA, HMPTFSA and DMPTFSA containing 5 mM Pt(acac)2 at various temperatures. Cyclic voltammograms showed a well-defined cathodic current peak around –1.7 V. Potentiostatic cathodic reduction of the ionic liquids containing 5 mM Pt(acac)2 at –1.7 V at 130 °C resulted in formation of metallic Pt. Thus, the cathodic current peak around –1.7 V was assignable to reduction of Pt(acac)2 to Pt(0). The reduction of Pt(acac)2 was suggested to involve formation of some intermediate species of Pt by cyclic voltammetry. Granular shaped deposit was observed on the glassy carbon substrate by scanning electron microscopy (SEM) after potentiostatic electrolysis in BMPTFSA containing 5 mM Pt(acac)2 at –1.7 V. The existence of Pt in the deposit was confirmed by energy dispersive X-ray spectroscopy (EDS). The appearance of the electrolytes turned to reddish black from pale yellow, indicating formation of Pt nanoparticles in the ionic liquids after electrolysis.1-4 Transmission electron microscopy (TEM) of the electrolytes after electrolysis confirmed formation and dispersion of Pt nanoparticles in the ionic liquids. The d-spacing values of the nanoparticles obtained in BMPTFSA, HMPTFSA and DMPTFSA were estimated to be 0.224, 0.229 and 0.225 nm, respectively, which were close to that of 0.227 nm for Pt(111). Pt nanoparticles obtained in the ionic liquids were suggested to form by disproportionation of the intermediate species of Pt. The average sizes (in diameter) of the Pt nanoparticles obtained after electrolysis in BMPTFSA, HMPTFSA and DMPTFSA were found to be 2.6 ± 0.7, 4.1 ± 1.2 and 4.3 ± 0.7 nm, respectively. Therefore, TFSA–-based ionic liquids could be promising media to prepare Pt and Pt nanoparticles with controlled size by electrolysis. Acknowledgement:Part of this work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.