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

摘要

Highly dispersed low-coordinate Pd sites on SiO2 are fabricated by grafting the Pd-II PCP-pincer complex ((PCP)-P-iota Bu)Pd OH ((PCP)-P-iota Bu = 2,6-C6H3(CH2P`Bu-2)(2)) on SiO2 followed by calcination with ozone (100 degrees C) and reduction with H-2 (300 degrees C). The chemisorption process and structure of this organometallic complex on SiO2 is established by solution phase H-1 and P-31 NMR and solid-state P-31 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.