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.

Although H 2 O 2 has been often employed as a green oxidant for many CeO 2catalyzed reactions, the underlying principle of its activation by surface oxygen vacancy (V o ) is still elusive due to the irreversible removal of postgenerated V o by water (or H 2 O 2 ). The metastable V o (ms-V o ) naturally preserved on pristine CeO 2 surfaces was adopted herein for an in-depth study of their interplay with H 2 O 2 . Their well-defined local structures and chemical states were found facet-dependent affecting both the adsorption and subsequent activation of H 2 O 2 . It is concluded that a strong adsorption of H 2 O 2 on ms-V o may not guarantee its subsequent activation. The ms-V o can be only free for the next catalytic cycle when the electron density of surface Ce is high enough to reduce/break the O−O bond of adsorbed H 2 O 2 . This explains the 211.8 and 35.8 times enhancement in H 2 O 2 reactivity when the CeO 2 surface is changed from ( 111) and ( 110) to (100).

It is known that the interplay between molecules and active sites on the topmost surface of a solid catalyst determines its activity in heterogeneous catalysis. The electron density of the active site is believed to affect both adsorption and activation of reactant molecules at the surface. Unfortunately, commercial X‐ray photoelectron spectroscopy, which is often adopted for such characterization, is not sensitive enough to analyze the topmost surface of a catalyst. Most researchers fail to acknowledge this point during their catalytic correlation, leading to different interpretations in the literature in recent decades. Recent studies on pristine Cu2O [Nat. Catal. 2019, 2, 889; Nat. Energy 2019, 4, 957] have clearly suggested that the electron density of surface Cu is facet dependent and plays a key role in CO2 reduction. Herein, it is shown that pristine CeO2 can reach 2506/1133 % increase in phosphatase‐/peroxidase‐like activity if the exposed surface is wisely selected. By using NMR spectroscopy with a surface probe, the electron density of the surface Ce (i.e., the active site) is found to be facet dependent and the key factor dictating their enzyme‐mimicking activities. Most importantly, the surface area of the CeO2 morphologies is demonstrated to become a factor only if surface Ce can activate the adsorbed reactant molecules.

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2400% increase in CeO2 peroxidase-like activity can be easily achieved on the (100) surface due to the promoted H2O2 adsorption/activation.