Determining the Mechanism of Solution Metallic Nanocatalysis with Solid and Hollow Nanoparticles: Homogeneous or Heterogeneous

Metallic nanocatalysis in solution can take place either on the nanoparticle surface (heterogeneous mechanisms) or in solution by leached atoms or ions from the nanoparticle surface (homogeneous mechanism). So far, it has been difficult to distinguish between the two mechanisms. We have addressed this problem using the model reaction of the reduction of 4-nitrophenol (4-NP) by sodium borohydride (BH) in aqueous solution by metallic nanoparticles. We studied the reaction yield as a function of the free surface area of the nanoparticle after partial poisoning by 4-nitrothiophenol (4-NTP). Because of the fact both mechanisms will be dependent on the available active surface, we made use of the plasmonic properties of gold nanoparticles. For surface catalysis, if the products or intermediates have different dielectric value from that of the reactants, the surface plasmon extinction band would shift in the course of the reaction if solid or hollow plasmonic nanoparticles are used for the catalysis. This is observed for the reaction being examined in which hydrogen gas is produced causing a blue shift of the surface plasmon band of the hollow gold nanospheres (AuHNSs) used as a catalyst. Hollow nanoparticles can also be used to test for the involvement of the atomic surface leach mechanism. They have two plasmonic surfaces, the coupling between their surface fields increases by decreasing the wall thickness of the hollow nanoparticle if surface atom leaching takes place. This leads to a red shift of the nanocatalyst plasmon band. In the present study, AuHNSs did not show any red shift of the LSPR during or after the 4-NP-BH reduction. This agrees with a surface heterogeneous type mechanism not involving leached atoms in solution.