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Quantitative Distinction between Noble Metals Located in Mesopores from Those on the External Surface

Carolin Rieg, Daniel Dittmann, Zheng Li, Robert Lawitzki, Katrin Gugeler, Sarah Maier, Guido Schmitz, Johannes Kästner, Deven P. Estes,
Published: 12 July 2021

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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