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Imon Kalyan1 Anjali Pal1 Tarasankar Pal1

1, Indian Institute of Technology Kharagpur, Kharagpur, , India

Producing hollow nanoparticles of different size and shape is a challenging job. Galvanic replacement reaction (GRR) is considered as one of the reliable and reproducible synthesis technique to produce hollow nanoparticles. The number of available template materials for GRR is limited. In this work, As(0) nanoparticles are used as template materials which are capable of producing almost similar sized hollow gold nanoparticles (HGNPs). Here two different size ranges (viz., 50±7 and 70±10 nm) As(0) nanoparticles are synthesized by sodium borohydride reduction of arsenite under controlled pH (7-9), and controlled temperature (10° C and 40° C). Further, the reducing property of these As(0) nanoparticles are exploited to form two different sized HGNPs with average diameter 55±7 and 72±7 nm. These HGNPs are designated as AuNP1 and AuNP2. The controlled medium pH restricts the reduction of arsenite to As(0) and not to AsH3. The catalytic reduction of 4-nitrophenol (4-NP), which is although a toxic compound but used in many industries, to 4-aminophenol (4-AP), an industrially important compound, is a well studied model reaction. The size-dependent catalytic activities of AuNP1 and AuNP2 have been examined on the reduction of 4-NP to 4-AP in the presence of sodium borohydride. While both the nanoparticles exhibit excellent catalytic activity, the smaller particles (AuNP1) are observed to be more effective. The turn over frequency (TOF) of AuNP1 shows much higher value of 300 h-1 in terms of molar ratio of Au:4-NP:NaBH4 (1:50:50000) as compared to other gold catalysts used for 4-NP reduction. The reaction is carried out with various catalyst doses and various initial 4-NP concentrations. In all cases the reaction follows first order kinetics. The TOF for the catalytic reaction using AuNP1 and AuNP2 suggests that AuNP1 bears ~6 times higher catalytic activity compared to that of AuNP2. Both the nanocatalysts can be reused up to fourth cycle with good efficiency. In addition, it is also possible to control the size of As(0) by varying the reaction time which ultimately can lead to the formation of different sized HGNPs.

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