Molecular probes and MAS NMR spectroscopy
We present the successful distinction of noble metal sites located in mesopores from their external surface counterparts. Three methods are compared, namely increased probe molecule size, solid-state diffusion, and selective coverage of mesopore surfaces. The latter method proved suitable as a general approach not only for the shown quantitative investigation of noble metals but also for acid sites.
Triphenylphosphine (TPP) is intruduced as suitable probe molecule for acid sites. We show that it can be combined with ammonia for the quantitative investigation of acid sites location. This is proven by application on amorphous silica-alumina, MCM-22 zeolite, and desilicated 10-MR zeolties (ZSM-5 and TNU-9).
Arylphosphines of different size are applied for the quantitative investigation of noble metals (ruthenium, rhodium, palladium, platinum). A distinction between external surface and secondary mesopores/supercages is presented.
We show that Triphenylphosphine binds stoichiometrically with noble metals platinum and rhodium located on the surface of solid catalysts.The stoichiometry between TPP and noble metal reproducibly varies from support to support.