Noble metal location in porous supports determined by reaction with phosphines

Different phosphines (PR3, R = phenyl, 4-methoxyphenyl) are demonstrated to be useful probe molecules for the spatial location and quantification of noble metal (NM) atoms within mesoporous and microporous support materials, such as silica A200, mesoporous SBA-15, and different zeolites Y. For this purpose, the high NMR sensitivity of 31P nuclei (spin I = ½, abundance of 100%) and the characteristic chemical shifts of phosphine complexes formed with noble metals were utilized. A straightforward method of physically mixing the reduced noble metal-containing catalysts with the solid phosphine powders and heating under well-defined conditions is shown. Complexation of triphenylphosphine (PPh3) and tris(4-methoxyphenyl)phosphine, P(PhOMe)3, with Pt, Rh, Pd, and Ru on open surfaces and in mesopores leads to characteristic 31P MAS NMR signals at δ = 33–41 ppm. Because of the different molecular diameters of PPh3 (0.72 nm) and P(PhOMe)3 (0.91 nm), adsorption of these two probe molecules allows to distinguish between noble metal atoms located at the outer surface of zeolite particles and those in secondary mesopores and in supercages of zeolites Y. For the first time this method enables the characterization of noble metals directly using probe molecules and 31P MAS NMR spectroscopy.