Immobilization of size variable Au nanoparticles on surfactant-modified silica and their catalytic application toward 4-nitrophenol reduction: A comparative account of catalysis

Abstract

Galvanic replacement reaction (GRR) is a widely used powerful synthesis technique to produce different hollow nanoparticles. In the present work, As(0) nanoparticles in two different size ranges (e.g. 50±7 and 70±10 nm) are synthesized by sodium borohydride reduction of arsenite under controlled pH, and subsequently they are used as templates for GRR reaction to produce two different sized hollow gold nanoparticles (HGNPs), which are designated as RHGNPs and BHGNPs (because of their red and blue colors respectively), with average diameter 55±7 and 72±7 nm, respectively. The unique feature of the reduction of arsenite to As(0) and not to AsH3, is the control of medium pH at 7-9. Now the use of RHGNPs and BHGNPs in homogeneous phase restricts them to recollect and reuse properly. So a soft solid template, cationic surfactant-modified silica (SMS) having positive charge in the surface has been considered to ‘adsolubilize’ the RHGNPs and BHGNPs. The ionic attraction between negatively charged surface of both the HGNPs (which arises due to the in-situ stabilization by surface adsorbed H2AsO4⁻) and the positively charged SMS holds them tightly. SMS after the adsorption of RHGNPs on to it looks pinkish-red and the material is designated as RG-SMS. Similarly, SMS after the adsorption of BHGNPs looks bluish-purple and the material is called BG-SMS. This is a unique procedure to prepare RG-SMS and BG-SMS. The catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of sodium borohydride has been studied using RG-SMS and BG-SMS (i.e. red and blue HGNPs immobilized in the solid phase) as the catalyst to examine the size effect. While both the nanoparticles exhibit excellent catalytic activity, the smaller particles (Au present in RG-SMS) are observed to be more effective than the larger ones (Au present in BG-SMS). The reaction is carried out with varying catalyst doses and initial 4-NP concentrations and the reaction follows first order kinetics in all cases. The first order rate constant values obtained are 0.0667 min−1 and 0.0257 min−1 for RG-SMS and BG-SMS respectively at a dose of 0.041 g L−1 and at 4-NP concentration 9.9 × 10−5 M. The turnover frequency (TOF) obtained at a dose 0.041 g L−1 are 0.0159 per minute and 0.0079 per min for the catalytic reaction using RG-SMS and BG-SMS, respectively. The results suggest that RG-SMS bears ∼2 times higher catalytic activity than that of BG-SMS. The reusability of both catalysts up to the 4th cycle has been checked. The catalytic activity of the HGNPs in the solution phase and the solid phase (i.e. on SMS) are compared. It has been observed that solution phase catalysts are better in terms of activity but solid supported catalysts are better in terms of their shelf-life and ease of application.