Labeling and Probing the Silica Surface Using Mechanochemistry and ¹⁷ O NMR Spectroscopy*

In recent years, there has been increasing interest in developing cost-efficient, fast, and user-friendly ¹⁷ O enrichment protocols to help to understand the structure and reactivity of materials by using ¹⁷ O NMR spectroscopy. Here, we show for the first time how ball milling (BM) can be used to selectively and efficiently enrich the surface of fumed silica, which is widely used at industrial scale. Short milling times (up to 15 min) allowed modulation of the enrichment level (up to ca. 5 %) without significantly changing the nature of the material. High-precision ¹⁷ O compositions were measured at different milling times by using large-geometry secondary-ion mass spectrometry (LG-SIMS). High-resolution ¹⁷ O NMR analyses (including at 35.2 T) allowed clear identification of the signals from siloxane (Si-O-Si) and silanols (Si-OH), while DNP analyses, performed by using direct ¹⁷ O polarization and indirect ¹⁷ O{¹ H} CP excitation, agreed with selective labeling of the surface. Information on the distribution of Si-OH environments at the surface was obtained from 2D ¹ H-¹⁷ O D-HMQC correlations. Finally, the surface-labeled silica was reacted with titania and using ¹⁷ O DNP, their common interface was probed and Si-O-Ti bonds identified.