(611a) The Stability of Site-Isolated Pt-SBA15 Materials From Tris(tert-butoxy)Siloxy Complexes of Pt(II) and Pt(IV) During the Hydrogenation of Alkenes

The stabilization of isolated catalytic sites on a solid matrix is an approach to synthesize catalysts that lend themselves to one extreme of structure in the determination of structure-function relationships. Earlier investigations have focused primarily on site-isolated early transition metal centers grafted onto metal oxide supports. Here we demonstrate the synthesis of Pt siloxy compounds and grafting to mesoporous silica via a thermolytic molecular precursor approach, their structural characterization by x-ray absorption spectroscopy (both near edge and extended variants), as well as their catalytic activity for low temperature (and room temperature) ethylene hydrogenation. Temperature programmed reduction and extended x-ray absorption fine structure (EXAFS) spectroscopy demonstrated the removal of organic ligands from the grafted Pt(II) complex occurred at 373 K. After reduction, the single site catalyst was highly active for room temperature ethylene hydrogenation, but with time-on-stream, the isolated Pt atoms aggregated together to form Pt nanoparticles. After reduction at 373 K and cooling to sub-ambient temperatures, the single site Pt catalysts were active for ethylene hydrogenation and appeared to remain stable under these conditions. Sub-ambient EXAFS confirmed the stability and site isolation of the atomically-dispersed Pt enabling us to assess on the mechanism of ethylene hydrogenation on single Pt atoms.