Gold nanoparticles supported on mesostructured oxides for the enhanced catalytic reduction of 4-nitrophenol in water

In this work, Au nanoparticles supported on aluminum oxide (Au/ANT) and titanate (Au/TNT) nanotubes were synthesized for their use as catalysts in the reduction of 4-nitrophenol to produce 4-aminophenol with NaBH₄ as the reducing agent. The catalysts were prepared with a 0.5 % metal loading employing the nanotube supports modified with 3-aminopropyl-trimethoxysilane (APTMS) to provide plentiful anchoring sites to trap the Au nanoparticles and prevent their agglomeration. All materials were characterized using a range of analytical techniques, and it was found that Au zero-valent nanoparticles were homogenously supported on the inner/outer surfaces of the nanotubular-structured carriers. Both catalytic systems were highly active and selective in the reduction of 4-nitrophenol, giving TOF values of 20,561 and 19,560 h⁻¹ for Au/TNT and Au/ANT, respectively. The excellent catalytic activity was attributed to the highly dispersed Au clusters on the support surfaces through enhanced functionalization with APTMS, and the confinement effect of the nanotubular carriers. Furthermore, Au/TNT exhibited a high operational stability for the reduction of 4-nitrophenol reaching 10 catalytic cycles with only a moderate decrease in the conversion level after the seventh cycle, which was attributed to a degree of metal leaching. Finally, the catalytic reduction performance of the Au/TNT catalyst was tested in different nitroarene-substituted pharmaceuticals, and revealed a high activity (>99 % after 60 min of reaction) and selectivity toward production of the desired substituted anilines, thereby highlighting the potential of this catalyst for application in these processes.