Preparation of Ag@mSiO2 and Pt@mSiO2nano composites using trioctylmethyl ammonium hydrogen phthalate (TOMAHP) ionic liquid as reaction medium
Graphical abstract
Mechanism for Ag@mSiO2 and Pt@mSiO2 nano composites in TOMAHP ionic liquid medium.
Introduction
Nano particles (NPs) of noble metals with inert supports have received wide attention in recent past in various industrial applications due to their unique properties based on their catalytic activities [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Among them SiO2 supported noble metal NPs, such as, Ag(0) and Pt(0) are widely recognized as most important nano catalysts in both homogeneous and heterogeneous catalytic activities in various industrial applications, such as, waste nitric acid degradation, hydrogenation, ethanol oxidation etc. [11], [12], [13], [14], [15]. The catalytic activities of such NPs depend on the chemical nature of support, NPs size, shape, surface to volume ratio as well as quantum size effect. There are a number of methods available in literature for preparation of SiO2 supported noble metal NPs for various catalytic applications [2], [9], [11], [16], [17], [18]. One of the most common methods is the impregnation technique where platinum salt, H2PtCl6. 6H2O is mixed with SiO2 of average mess size of 200 and subsequently reduction of mixture with hydrazine hydrade results into Pt NPs [11], [19], [20], [21], [22]. However, the uncontrolled particle size, non-uniform distribution of metal NPs on to the inert support, less efficiency and complex chemical processes involved for preparation of such nano composites are the main drawbacks of the process [23]. In recent past, researchers have developed more efficient method called polyol method for preparation of supported metal NPs with desired sizes and shapes [2], [16], [17]. In this method, the metal NPs are prepared using a polymer at high pH in the presence of metal salt, reducing agent and ethylene glycol at high temperature. In the next step, the supported NPs are prepared by re-distributing the NPs in ethanol medium onto an inert substrate. The principle advantages of the process over earlier are the uniform distribution of metal NPs on to the support, narrow size distribution with desire size and shape [24], [25], [26]. The main drawbacks of the process are the use of more number of chemical steps as well as high temperature. Other techniques, such as sol-gel, chemical vapor deposition and electron beam evaporation are very complex and need high capital investment which make them difficult to recognize as popular synthetic routes for such nano composites preparation at normal laboratory conditions [16], [17], [18]. Now a days, ionic liquids have become useful reaction medium for green synthesis of size and shape controlled metal NPs with improve physical and chemical properties [27]. A large number of literatures are available on synthesis of transition metal NPs in various ionic liquids medium including imidazolium based functionalized and task specific ionic liquids [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]. The unique properties of ionic liquids such as low vapor pressure, high thermal and chemical stability, high ionic conductivity, high ionic density, molecular structure, high viscosity, density, polarity etc. make them unique reaction medium for nano material synthesis over conventional aqueous and organic solvents with desired particle size distributions and shapes [28], [30]. Mehdi et al. has prepared a number of transition metal NPs in imidazolium based functionalized ionic liquids [50]. The preference of imidazolium based compounds over other is due to high degree of solubility of various metal ions in imidazolium based ionic liquids. Singh et al. has described the synthesis of a number of noble metal NPs in task specific ionic liquids (TSILs) using NaBH4 as reducing agent dissolved in methanol [53], [54]. Kim et al. has developed size selective synthesis of gold and platinum NPs in novel thiol-functionalized ionic liquid medium using NaBH4 as reducing agent [55]. They have been able to prepare mono disperse metal NPs with a diameter of less than 10 nm.
Literature survey indicates that there is no method available for synthesis of SiO2 supported noble metal NPs in trioctylmethyl ammonium hydrogen phthalate (TOMAHP) TSILs. In the present work, simple chemical method for synthesis of Ag@mSiO2 and Pt@mSiO2 nano composites has been attempted using indigenously synthesized TSIL, TOMAHP, as reaction medium and NaBH4 as reducing agent. The Ag@mSiO2 and Pt@mSiO2 nano composites have been characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), small angle neutron scattering (SANS) as well as small angle X-ray scattering (SAXS) techniques. It has been shown that Ag NPs are well dispersed in Ag@mSiO2 composite whereas Pt NPs forms aggregate in the Pt@mSiO2 composite leading to poor dispersion of NPs.
Section snippets
Materials
SiO2 of mess size 200 was procured from Loba Chemicals, Mumbai, India. NaBH4 (A.R. grade, E-Mark), Methanol (A.R. grade, B.D.H), Ethanol (A.R. grade, BDH), H2PtCl6. 6H2O (Loba Chemicals) and AgNO3 (A.R. grade, B.D.H) were used without further purification. Aliquat@336 (S.D. Fine Chemicals; trioctylmethyl ammonium chloride, MW: 404.17), potassium hydrogen phthalate (Aldrich; 99% purity) and toluene (A.R. grade) were used for synthesis of TSIL.
Synthesis of task specific ionic liquid trioctylmethyl ammonium hydrogen phthalate
The TSIL, TOMAHP was synthesized using following
XRD studies
The X-ray diffraction is a powerful technique for determination of crystal structure, crystallite size and lattice parameter of unit cell of nano crystalline materials. The X-ray diffraction study of Ag@mSiO2 and Pt@mSiO2 nano composites were performed to evaluate the crystal structure and size of metal NPs embedded on SiO2 surface. Fig. 1, Fig. 2 shows the XRD profile of Ag@mSiO2 and Pt@mSiO2 nano composites where the intensity of different peaks is plotted against movement of detector angles
Conclusions
A novel, one step chemical method for preparation of Ag@mSiO2 and Pt@mSiO2nano composites has been developed using TOMAHP as reaction medium and NaBH4 as reducing agent. The AgNO3 and H2PtCl6·6H2O are highly soluble in TOMAHP due to presence of hydrogen phthalate anion and trioctylmethyl ammonium cation. Similarly, the support, SiO2 particles are well dispersed in TOMAHP medium due to its high viscosity. The TOMAHP acts as an excellent reaction medium as well as capping agent for synthesis of
References (73)
- et al.
Catal. Commun.
(2006) - et al.
Nano Today
(2011) - et al.
Catal. Comm.
(2010) - et al.
J. Mol. Catal. A Chem.
(2006) - et al.
J. Mol. Catal. A Chem.
(2005) - et al.
Appl. Catal. A General
(2006) - et al.
Appl. Surf. Sci.
(2000) - et al.
Fuel
(2003) - et al.
Mater. Lett.
(1995) - et al.
Electrochem. Acta
(2006)