Differentiating Surface Ce Species among CeO2 Facets by Solid-State NMR for Catalytic Correlation

Altering the exposed facet of CeO2 nanocrystallites and hence the control of surface chemistry on the nano level have been shown to significantly change their performances in various catalytic reactions. The chemical state of surface Ce, which is associated with Lewis acidity and hence the adsorption/activation energy of reactants on the surface, is expected to vary with their hosted facets. Unfortunately, traditional surface tools fail to differentiate/quantify them among hosted facets and thus have led to different interpretations among researchers in the past decades. Herein, probe-assisted nuclear magnetic resonance is employed for the surface investigation of different CeO2 facets. They not only allow differentiation of the surface Ce atoms between hosted facets at high resolution but can also provide their corresponding concentrations. The as-established facet fingerprint of CeO2 can thus report on the facet distribution/concentration of a given CeO2 sample. Dephosphorylation and H2O2 reduction were tested as probe reactions to demonstrate the importance of obtaining comprehensive surface Ce information for the active site identification and the rational design of CeO2-based catalysts. Around 1000 and 4500% increase in activity of those reactions can be easily achieved on pristine CeO2 without further surface engineering when its terminal facet is wisely chosen. Our results thus imply that the basic surface knowledge of even a simple catalyst can be more important than the continuous development of their fancy derivatives without clear guidance.