Microwave-assisted synthesis of ultra small bare gold clusters supported over Al 2 O 3 and TiO 2 as catalysts in reduction of 4- nitrophenol to 4-aminophenol Mostafa Farrag Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt article info Article history: Received 1 April 2016 Received in revised form 22 May 2016 Accepted 19 June 2016 Available online 21 June 2016 Keywords: Microwave-assisted synthesis Bare gold clusters Au 25 (SG) 18 Micropores into mesoporous Reduction of 4-Nitrophenol abstract In this work I present a novel method to prepare ultra small bare gold clusters (Au n ) loaded over different supports. Size selected gold clusters protected by glutathione ligand (Au 25 (SG) 18 ) was prepared and deposited over aluminium dioxide (Al 2 O 3 Puralox SCCa-190HPV) and anatase titanium dioxide (TiO 2 ) 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 (Au 25 ) 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 % Au 25 (SG) 18 /Al 2 O 3 ) and bare (1 wt % Au 25 /Al 2 O 3 ) gold clusters over alumina have the same size distribution (~1 nm). The TEM images of doped gold clusters over TiO 2 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 S BET , 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 NaBH 4 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 % Au 25 / Al 2 O 3 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. © 2016 Elsevier Inc. All rights reserved. 1. Introduction Many ligands have been used to stabilize the monolayer pro- tected nanoclusters (MPCs) in solution [1e8]. MPCs have attracted significant research interest in recent years due to their physico- chemical properties and possible applicability in homogenous catalysis [9,10] and heterogeneous catalysis [1,2,9e14]. In homogeneous catalysis, we must use ligands to prevent the metal clusters from agglomeration in solution. The reactants will adsorb on a free adsorption sites on the metal center by insertion through the ligands. It is well known, the ligand can positively contribute in homogeneous catalytic reactions and is able to control the reaction selectivity by interacting with the adsorbed reactants [15] and participating within the catalytic cycles [16]. On the other hand, in heterogeneous catalytic reactions only the surface atoms contribute in the reaction. Since most of the coordination sites are blocked by adjacent metal atoms in addition to the ligands [14]. However, the supports increase the dispersion of adsorbed cata- lysts, and the electron transfers from the active metal oxides sup- ports and the adsorbed catalysts increase its catalytic activity [1,17,18]. So far many researchers used thiols protected metal clusters deposited over different active and inactive supports to activate E-mail address: mostafafarrag@aun.edu.eg. Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso http://dx.doi.org/10.1016/j.micromeso.2016.06.032 1387-1811/© 2016 Elsevier Inc. All rights reserved. Microporous and Mesoporous Materials 232 (2016) 248e255