Catalytic behavior of nickel nanoparticles stabilized by lower alkylammonium bromide in aqueous medium

Due to widespread applications as catalysts metals precipitated from aqueous solutions has continued to be a thoroughly investigated subject [1]. The nanoscale metallic particles are of current interest in the field of catalysis, microelectronics, biology and material science [2], [3], [4], [5]. The role of the nanometals as catalysts has been specifically discussed [6], [7]. Among the various chemical approaches developed for the preparation of colloidal metal, the reduction of metal salts is the most common. Reducing agents including hydrogen, hydrazine, alcohols, LiAlH4 or NaBH4 and many others have been used to prepare metal colloids in the nanometric size range [2], [3], [4], [5].

Among the various kinds of metal nanoparticles, stabilization of nickel, copper, and iron is relatively difficult in the aqueous medium because of oxide or hydroxide formation. Nickel nanoparticles have attracted much attention because of applications in catalysis, medical diagnostics and magnetic applications. Until now, few papers have been reported on the preparations of nickel nanoparticles or those of their alloys. Such preparations have been mostly performed in organic media to avoid the formation of nickel oxide or nickel hydroxide [8], [9], [10], [11], [12], [13]. The syntheses of Ni nanoparticles in aqueous solution using CTAB and tetradodecylammonium bromide have been reported, but nanoparticles are only separated by using a permanent magnet [14].

In the present study, we report for the first time the heterogeneous synthesis of monodisperse nickel nanoparticles from nickel salt in aqueous solution of CTAB and lower alkylammonium bromide in the presence of hydrazine hydrate as reducing agent. Platinum ion (Pt/Ni; 1:100,000) has been used as nucleating agent for the growth of nickel nanoparticles. The nanoparticles have been separated by precipitation and centrifugation. The particle sizes, structure, capping behavior and thermal stability have been characterized by TEM, XRD, EDS, TGA–DSC and FTIR. The synthesis process of Ni nanoparticles in the presence of CTAB alone also produces a mixture of Ni(OH)2 and Ni nanoparticles.

The catalytic behavior of nanoparticles has been studied for the reduction of p-nitrophenol in aqueous medium using hydrazine hydrate as reducing agent. The kinetics of the reaction has been monitored using a UV–vis spectrophotometer for the disappearance of nitrophenolate ion and formation of p-aminophenol. The catalytic reduction has been studied at different temperatures: 20, 40, 50 and 60 °C. The solid phase catalyst remains active at the end of reaction and can be easily separated from the reaction mixture.