UC Davis

Air-stable catalysts consisting of tantalum nitride nanoparticles represented as a mixture of Ta _x N _y and TaO _x N _y with diameters in the range of 0.5 to 3 nm supported on highly dehydroxylated silica were synthesized from TaMe₅ (Me = methyl) and dimeric Ta₂(OMe)₁₀ with guidance by the principles of surface organometallic chemistry (SOMC). Characterization of the supported precursors and the supported nanoparticles formed from them was carried out by IR, NMR, UV-Vis, extended X-ray absorption fine structure, and X-ray photoelectron spectroscopies complemented with XRD and high-resolution TEM, with dynamic nuclear polarization surface enhanced NMR spectroscopy being especially helpful by providing enhanced intensities of the signals of ¹H, ¹³C, ²⁹Si, and ¹⁵N at their natural abundances. The characterization data provide details of the synthesis chemistry, including evidence of (a) O₂ insertion into Ta-CH₃ species on the support and (b) a binuclear to mononuclear transformation of species formed from Ta₂(OMe)₁₀ on the support. A catalytic test reaction, cyclooctene epoxidation, was used to probe the supported nanoparticles, with 30% H₂O₂ serving as the oxidant. The catalysts gave selectivities up to 98% for the epoxide at conversions as high as 99% with a 3.4 wt% loading of Ta present as Ta _x N _y /TaO _x N _y .