Microwave-assisted synthesis of ultra small bare gold clusters supported over Al2O3 and TiO2 as catalysts in reduction of 4-nitrophenol to 4-aminophenol

Abstract

In this work I present a novel method to prepare ultra small bare gold clusters (Aun) loaded over different supports. Size selected gold clusters protected by glutathione ligand (Au25(SG)18) was prepared and deposited over aluminium dioxide (Al2O3 Puralox SCCa-190HPV) and anatase titanium dioxide (TiO2) by impregnation method with doping percentage 0.5 and 1 wt %. Then the doped catalysts were irradiated 1 h by microwave synthesis labstation at 80 °C and 500 W to remove the glutathione ligands (GSH) and leave the bare gold clusters (Au25) supported over alumina and titania as a strategy for enhancing their catalytic activity and selectivity. The high resolution transmission electron microscope analysis confirmed the particle size of protected (1 wt % Au25(SG)18/Al2O3) and bare (1 wt % Au25/Al2O3) gold clusters over alumina have the same size distribution (∼1 nm). The TEM images of doped gold clusters over TiO2 before and after microwave treatment are clearer than alumina. This means the gold clusters over alumina and titania did not suffer from any agglomeration by microwave treatment. From nitrogen sorption isotherms at −196 °C for the protected and bare gold clusters over supports showed two different loop types of isotherms H3 and H1, respectively. Their specific surface area SBET, pore volume and average pore diameter were calculated. The pore size distribution of supported gold clusters before and after microwave treatment was measured. These results confirmed the ligands were removed from the supported gold clusters by microwave treatment. The pores in alumina and titania which were blocked by the protected clusters have become available. Reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in presence of NaBH4 was used as a test reaction to compare the catalytic activity of supported bare and protected gold clusters over alumina. The results showed that the 1 wt % Au25/Al2O3 catalyst exhibited the best catalytic performance in the reduction of 4-NP into 4-AP and revealed 100% conversion following 90 s stirring at room temperature. The extreme catalytic activity of the ultra small gold clusters attributed to their electronic properties and geometrical configuration.