Tuning the reduction power of visible-light photocatalysts of gold nanoparticles for selective reduction of nitroaromatics to azoxy-compounds-Tailoring the catalyst support

It is a challenge to attain high selectivity to azoxy-compounds in the reduction of aromatic nitro compounds as azoxy-compounds can be readily reduced to azo- and aniline compounds. We proposed a new solution to this challenge: gold nanoparticles (AuNPs) on hydrotalcite (HT) support were used to catalyze the reduction under visible light irradiation at ambient temperature; and phosphate (PO43-) and metal (such as Ga3+, Zn2+) ions were incorporated into the HT support to moderate the reduction power of the catalyst avoiding the formation of azo- and aniline compounds. Very high azoxy-compound selectivity was achieved under green mild conditions. The photocatalytic activity strongly depends on the incident light wavelength and intensity, and we can further enhance the catalytic efficiency of this photocatalytic process by slight increase in reaction temperature (e.g. 10-20 degrees C). Moreover, high stability and recyclability of the catalyst were also observed under the investigated conditions. A plausible reaction mechanism was proposed based on the experimental results and literatures. The introduction of both phosphate and metal ions into HT support can prevent the further reduction of azoxy-compounds to azo-compounds or anilines and achieve excellent selectivity to azoxy-compounds. This study reveals that we may engineer a product chemoselectivity by tailoring the supporting materials, and may present a new strategy towards the development of versatile heterogeneous photocatalysts. (C) 2017 Elsevier B.V. All rights reserved.