The effect of nanoparticle size on catalytic activity has been studied widely and is attributed to variations in geometric and electronic properties at the nanoscale regime. Smaller nanoparticles are usually more reactive than larger ones due to increasing proportions of edge and corner sites with lower coordination numbers than terrace sites. However, retaining thermodynamically unstable surfaces at the nano regime is challenging, especially during catalysis. Traditional procedures used to prepare nanoparticles with precise and monodisperse size are lengthy and utilize capping agents, whose partial removal during the catalytic reaction can result in aggregation. Herein, we produced catalytic Pd nanoparticles with controlled and monodisperse sizes by confining them in the pores of ordered mesoporous silica nanoparticle (MSN) supports. We synthesized MSN with different pore widths (MSN-x, x = 2, 3, 4, 5 nm) using alkyltrimethyl-ammonium bromide surfactant templates with different chain lengths and varying amounts of the swelling agent mesitylene. Pd was incorporated inside the pores of MSN by stirring them in aqueous solutions of Pd(NH3)4Cl2 followed by reduction under H2 flow. ICP-OES, HAADF-STEM analysis, and CO pulsed chemisorption analysis indicated the amount of Pd retained by the materials increased with their pore widths and that the Pd nanoparticles were slightly smaller than the pores. These results strongly suggest that regulating MSN pore width is an effective tool to control nanoparticle size. We studied the effect of Pd size on its catalytic activity for the Suzuki-Miyaura coupling and the hydrogenation of phenol. The activity of Pd/MSN-2 per surface Pd atom was almost twice as high compared to the other catalysts. The higher activity of the smallest particles (Pd/MSN-2) was attributed to the increased number of low-coordinated sites and higher back bonding capacity of the catalyst. Our result suggests that ordered mesoporous supports can be used as simple tools to produce ligand-free size-controlled catalysts to investigate structure-sensitive reactions.

Facile control of Pd nanoparticle catalytic activity via confinement in ordered mesoporous support

ACS Spring Meeting 2021

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