Quantitative Distinction between Noble Metals Located in Mesopores from Those on the External Surface
Abstract: We compare three methods for quantitatively distinguishing the location of noble metals (NM) in mesopores from those found on the external particle surface. MCM-41 and SBA-15 with NM either preferentially located in mesopores or on the external surface were prepared and characterized by TEM. We quantified arylphosphines forming complexes with NM by 31 P MAS NMR spectroscopy. Phosphine/NM ratios drop from 2.0 to 0.2 when increasing the probe diameter from 1.08 to 1.54 nm. The reaction between NM and triphenylphosphine (TPP) within 3.0 nm MCM-41 pores takes due to confinement effects multiple weeks. In contrast, external NM react with TPP instantly. A promising method is filling the pores using the pore volume impregnation technique with tetraethylorthosilicate (TEOS). TPP loading revealed that 66% of NM are located on the external surface of MCM-41. The pore filling method can be used in association with any probe molecule, also for the quantification of acid sites.
Keywords: External Surface / Mesoporous materials / Platinum / solid-state NMR spectroscopy / Surface Chemistry
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