Hana Aizawa, Tom A. J. Welling, Shin Saito, Hikaru Namigata, Keishi Suga, Kanako Watanabe, Daisuke Nagao
Hollow nanoreactors have been shown to have superior performance compared to conventional supported catalytic nanoparticles under certain conditions, but clear design rules have not been established. Here, hollow mesoporous silica particles incorporating gold nanoparticles with well-designed different inner diameters (void sizes), shell thicknesses, and gold loadings were used to investigate the effect of these factors on the catalytic performance of hollow nanoreactors. As a model reaction, the reduction of 4-nitrophenol was used. Experimentally, it was found that nanoreactors with an inner diameter of 885 nm outperformed those with a 275 nm inner diameter when keeping the gold concentration per void volume equal. When fitting the experimental data using a diffusion-reaction model, smaller nanoreactors operated in the reaction-limited regime due to a comparatively higher ratio of inner shell surface area to gold surface area, while equalizing the concentration of gold in the system. Large nanoreactors with a thicker shell or higher gold loading were diffusion limited instead. Moreover, the larger nanoreactors with thin shells showed an increased reaction rate per gold surface area. The nanoreactors with a Damköhler number, which describes the ratio between reaction rate and mass transfer rate, of approximately 1 outperformed reaction-limited and diffusion-limited nanoreactors. These nanoreactors had a slightly decreased reactant concentration (10–30%) within the void, which boosted the reaction rate. For Damköhler numbers around 1, this increase in reaction rate overcomes the decrease in mass transport, leading to a net increase in the apparent reaction rate. These results showed that the balance between reaction rate and mass transfer represented by the Damköhler number can be tuned using the inner diameter, shell thickness, and gold loading to optimize the apparent reaction rate of hollow nanoreactors.