Differentiating surface metal cations on pristine metal oxides by trimethylphosphine-assisted 31P NMR for catalytic correlation

Facet engineering of metal oxides tailors surface chemistry and catalytic performance. This stems from the varying Lewis acidity (or electron density) of surface metal cations based on their host facet, which affects reactant adsorption/activation and product desorption. However, correlating these surface properties with observed activities is limited by conventional surface tools. Commercial X-ray photoelectron spectroscopy (XPS) has low surface sensitivity, while ammonia-temperature programmed desorption (TPD) and pyridine-infrared (IR) spectroscopy cannot distinguish surface metal cations and quantify them across different facets. This complicates the establishment of reliable correlations with observed activity, leading to different interpretations among researchers over the years. Given this, the use of nuclear magnetic resonance (NMR) combined with NMR-active probes for surface characterization of metal oxides has recently gained significant attention. Among various probes, 31P active-trimethylphosphine (TMP) stands out for its ability to distinguish the Lewis acidity of surface metal cations across different facets and provide their individual concentrations. While this technique has effectively accounted for facet activity in various metal oxides, no review has comprehensively explored the correlation between shape-dependent properties and observed activities across different reactions. Herein, we provide an integrated discussion on the surface characteristics of well-defined metal oxides (e.g., TiO2, ZnO, HNb3O8, and CeO2) revealed by this technique and conventional surface tools, as well as their effectiveness in establishing catalytic correlations across various reactions, including photochemical, redox, and acid-catalyzed processes. © 2026 The Author(s). Published by Elsevier B.V.