Surface Functionalities, Speciation, and Strength of Brønsted Acid Sites from a ³¹P-¹⁰⁹Ag NMR Tag

Understanding the atomic structure, speciation, and reactivity of surface sites is critical for establishing structure-activity relationships in heterogeneous catalysis. A longstanding objective in this field is the simultaneous identification of Brønsted acid sites (BASs) and the quantification of their acidity. Although BASs often govern the overall reactivity of catalysts and specific support effects such as bifunctionality, achieving this objective has remained a grand challenge. In this work, we selectively tag reactive surface functionalities on materials, such as -OH groups spanning a wide range of acidity, using an organosilver(I) compound that incorporates ¹⁰⁹Ag and ³¹P reporter nuclei at natural abundance. This ³¹P-¹⁰⁹Ag surface tag, which can be visualized by transmission electron microscopy (TEM) when applicable, provides dual NMR probes, enabling a high-resolution characterization of BASs by solid-state nuclear magnetic resonance (SSNMR) spectroscopy. Leveraging Dynamic Nuclear Polarization Surface Enhanced NMR spectroscopy (DNP-SENS), 2D ³¹P-{¹⁰⁹Ag} J-correlated NMR spectroscopy enables the detection and resolution of different surface functionalities (e.g., reactive -OH and -NH groups) and their speciation (e.g., terminal O vs bridging O sites) across various levels of surface complexity. Rationalizing the complementary NMR signatures of ³¹P-¹⁰⁹Ag tags using Density Functional Theory (DFT) calculations and constructing a 2D map enables the precise evaluation of surface acidity across diverse sites and materials. The simultaneous identification of surface structures and quantitative assessment of their properties or reactivities advance the molecular-level understanding of the surface sites and chemistry in functional materials, paving the way for rational design, in heterogeneous catalysis.