(442c) Pre-Synthesized Nanoparticles Are Not Always Catalytically Active: Importance of Pretreatment for the Removal of Stabilizing Polymer Monolayers and Growth-Control Additives

A number of methods are now available for the synthesis of catalytically-relevant metal and metal oxide nanoparticles. These methods generally involve the use of molecular or polymeric phases to stabilize the particles in solution. These protecting agents prevent aggregation in solution and remain on the particle surface after inclusion into high-surface area or planar supports. They generally hinder the ability of the nanoparticle to catalyze reactions because they are so strongly-bound to the surface and must be removed completely to realize the full catalytic potential of the metal or the elucidation of true structure-function relationships in catalysis.

We demonstrate the difficulty associated with the complete removal of the PVP from the surface of nanoparticles included into a high-surface area mesoporous silica matrix. Colloidal Pt nanoparticles (1.7?7 nm) were synthesized by solution-based reduction methods in the presence of high molecular weight PVP to create nearly monodisperse PVP-protected Pt nanoparticles and encapsulated in mesoporous silica by direct participation in the hydrothermal synthesis. Thermal analysis of the removal of PVP under oxidative and reductive methods demonstrated that oxidative conditions leading to the total combustion of PVP were the most efficient for PVP removal as determined through a carbon mass balance. These materials proved to be catalytically ineffective without significant improvement over the starting material protected by PVP, even though elemental analysis suggested > 90% of the carbon from PVP was removed. The calcination-reduction protocol that led to the most active catalysts involved cyclic oxidation-reduction treatments at mild temperatures (373 K); these catalysts were an order of magnitude more active for the hydrogenation of ethylene than a catalyst calcined for 12 h at 673 K. The decreased activity for the materials calcined at high temperatures is not due to a decrease in active surface area because of sintering, but potentially to the densification of the PVP monolayer during pretreatment.

The introduction of foreign metal ions to colloidal metal nanoparticle synthetic procedures leads to control of particle shape. We demonstrate that the addition of small amounts of Ag⁺ to a polyol synthesis of Pt nanoparticles leads to exquisite shape-control, but the intrinsic activity for ethylene hydrogenation of the shape-controlled Pt particles is significantly less than spherical nanoparticles. The difference in reactivity is not related to the influence of shape, but rather a negative influence of residual silver on the catalytic kinetics. The decrease in rate correlated with the amount of Ag retained in the nanoparticles. X-ray absorption spectroscopy proved Ag was in the form of small clusters (<1-1.5 nm) located on the surface of the Pt nanoparticles. The selective etching of Ag with HNO₃ is employed in solution to remove Ag from the surface of the Pt nanoparticles supported on mesoporous silica. After etching, the removal of Ag from the nanoparticle surface was verified with elemental analysis and x-ray absorption spectroscopy. The shape of the particles remained unchanged after etching. After etching, the shape-controlled nanoparticles had catalytic activities on a per surface atom basis comparable to spherical Pt nanoparticles for the hydrogenation of ethylene. The ability to remove Ag and maintain the size and shape of the original nanoparticle enables reaction rates and selectivity to be measured as a function of particle shape.

These examples demonstrate that the removal of stabilizing polymer monolayers and growth-controlling additives is a crucial step to create pre-synthesized nanoparticles that are catalytically active.