Development and Impact of Precisely Tunable Mesoporous Silica-Encapsulated Core-Shell Nanoparticles

Ellis Hammond-Pereira

doi:10.7273/000005110

This work concerns the development of a precisely-tunable silica-encapsulated gold core-shell nanoparticle (CSNP) catalyst, and the impacts of tuning on catalytic performance. Confining nanoparticles within a porous structure is a valuable technique to reduce agglomeration, helping to preserve their typically high catalytic activity despite harsh reaction conditions. Further investigations reveal that beyond maintaining particle size, the presence of a pore structure itself fundamentally improves catalysis. Research elucidating this effect is hindered by the nature of traditional porous catalysts, whose nanoparticles grow to match the diameter of the pore. Diffusing nanoparticles random distances into existing pores also obfuscates the concept of pore length. Absent methods to control these morphological parameters individually, the specific mechanism by which pore constraint improves catalysis has remained unclear. CSNPs present an opportunity to investigate the effects of pore geometry on catalysis, unconstrained by either of these problems. Pores are freely scalable without influencing nanoparticle size, and exhibit concrete, measurable pore lengths. The catalytic improvement of pore constraint, comparing CSNPs to non-porous control catalysts, is presented. The presence of a pore environment improved both benzyl alcohol conversion and selectivity compared to a control catalyst using the same support material. CSNPs exhibited higher activity and comparable selectivity to supported nanoparticles whose strong metal-support interactions intrinsically aid performance. The ability to individually tune the pore length and pore diameter of silica-encapsulated gold core-shell nanoparticles via seeded encapsulation method is demonstrated. This technique facilitated the synthesis of CSNP samples with increasing pore length, and CSNP samples with increasing pore diameter. When benchmarked using benzyl alcohol oxidation, CSNPs with longer pores increased catalytic activity exclusively through the formation of the desired aldehyde product. By contrast, increasing pore diameter increased the formation of the aldehyde and ester products unselectively. The isolated nature of these orthogonal studies, uniquely enabled by the updated CSNP synthesis, demonstrates with clarity the impact of pore-constraint on catalysis. The ability to influence catalytic behavior on an active surface, exclusively via manipulation of a molecule's path to said surface, is a powerful tool, one merely suggested by preceding literature. The direct observation of such behavior is unprecedented, and has immediate applicability across the catalysis field. Furthermore, these findings align with several molecular confinement studies which observe the formation of concentric microphases within comparable pore environments. Not only does this provide a concrete path for future work, but also has the potential to connect two fundamentally related fields (molecular confinement and porous catalysis), which largely operate in isolation.

Development and Impact of Precisely Tunable Mesoporous Silica-Encapsulated Core-Shell Nanoparticles

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