Colloidal Ag@Pt Core-Shell Nanoparticles for the Catalytic Reduction of Nitrophenol

Bimetallic nanoparticles (NPs) composed of two metal elements have attracted much interest due to the possibility of synergistically enhancing the catalytic performance of nanocatalysts. In this context, we have recently reported the seed-mediated growth of bimetallic core-shell NPs combining a plasmonic metal Ag as a core and a catalytic metal Pt as a shell with fine control of the core size and shell thickness. Here, we report a detailed study of the catalytic properties of our Ag@Pt core-shell NPs through a model reaction of the reduction of 4-nitrophenol (4-NP) in 4-aminophenol (4-AP) in the presence of NaBH4, where the Pt shell thickness can be tuned from 1 to 6 atomic layers by adjusting the ratio of the Pt precursor to Ag seed concentration. First, we optimized the catalytic performance of pure Ag NPs by showing the role played by protons. We showed that the addition of an aqueous HCl or H2SO4 solution improves the catalytic conversion rate due to the addition of H+. Furthermore, we showed that under light irradiation the presence of halide increases the catalytic efficiency of the Ag NPs by a photorecycling of Ag and the generation of hot electrons. We then evidenced that the bimetallic system is more catalytically active than its monometallic counterparts, highlighting how the synergy between the two metals crucially depends on both the thickness of the Pt shell and the size of the Ag core. The best conditions were found with four Pt atomic layers, which is attributed to a favorable electronic coupling between the Pt shell and the Ag core. Nevertheless, the effect of the irradiation on the catalytic activity of Ag@Pt NPs remains moderate, probably due to localized surface plasmon resonance damping by the Pt shell. Finally, the Ag@Pt NPs showed good stability and could be reused after running three recycling cycles, thus opening perspectives for the rational design of core-shell nanocatalysts with controlled shell morphologies.