Direct Synthesis of Low-Coordinate Pd Catalysts Supported on SiO2 via Surface Organometallic Chemistry

Highly dispersed low-coordinate Pd sites on SiO2 are fabricated by grafting the PdII PCP-pincer complex (tBuPCP)Pd–OH (tBuPCP = 2,6-C6H3(CH2PtBu2)2) on SiO2, followed by calcination with ozone (100 °C) and reduction with H2 (300 °C). The chemisorption process and structure of this organometallic complex on SiO2 is established by solution-phase 1H and 31P NMR and solid-state 31P CPMAS NMR spectroscopy, XPS, DRIFTS, and AC-HAADF-STEM. The CO adsorption properties of the Pd centers reveal a surprisingly high fraction of adsorption sites where CO is bound in a linear fashion, indicative of low-coordinate Pd. Furthermore, enhanced selectivity of these catalyst centers in aerobic alcohol oxidation versus a control catalyst argues that these low-coordinate sites are the catalytically active sites.