Nanoscale Chemical Imaging of Basic Sites Distribution on Catalytically Active Mg–Al Mixed Oxide Particles

Abstract

The acid–base properties of catalytic materials play a crucial role in facilitating chemical transformations. Nanoscale structural heterogeneities within these catalysts can significantly affect the distribution, type, and strength of their acid–base sites, thereby influencing both localized and overall catalytic reactivity. In this study, high spatial-resolution chemical imaging of basic sites on supported Mg–Al mixed oxide (MgAlO_x) particles, which serve as catalysts for aldol condensation reactions, was achieved using atomic force microscopy–infrared (AFM-IR) nanospectroscopy measurements while using formic acid as a chemical probe for surface basic sites detection. This approach enabled us to identify the distribution, geometry, and strength of basic sites with nanoscale precision. It was revealed that platelet MgAlO_x particles predominantly exhibit a uniform bidentate adsorption of formic acid, whereas aggregates display a heterogeneous distribution of both monodentate and bidentate adsorption modes, indicating differences in the distribution, geometry, and strength of the basic sites. Additionally, upon exposure to formic acid, smaller particles underwent phase reconstruction, transitioning into cubic-like structures characterized by distinct bidentate adsorption of formic acid. This transformation was attributed to the rehydration and intercalation of formate species. The insights gained by conducting high spatial resolution nanospectroscopy measurements highlight the correlation between flat surfaces, characterized by a low density of surface defects, and a homogeneous distribution of basic sites, with a dominant bidentate adsorption mode of formic acid. These results emphasize the critical role of high spatial resolution chemical imaging in unraveling the link between structural features and acid–base functionality in catalytic materials.