Valence-State-Dependent Surface Lattice Oxygen in CeO₂‑Modified VPO Catalysts: Elucidating the Mechanism of n‑Butane...

The selective oxidation of n-butane to maleic anhydride (MA) over the vanadium phosphorus oxide (VPO) catalyst stands as a prominent example of industrial selective alkane functionalization. Despite its commercial maturity, the ambiguous relationship between the catalytic performance and vanadium valence states (V_ox) or surface lattice oxygen (O_S‑L) persists in generating debate and uncertainty regarding the identity of the active sites in VPO catalysts. Herein, we reconcile this debate by demonstrating that the catalytic efficiency is governed by valence-state-dependent O_S‑L rather than by merely the O_S‑L content. Using time-resolved H₂-TPR experiments, we successfully distinguished two distinct types of valence-state-dependent O_S‑L: V⁵⁺-O_S‑L (O_S‑L related to the V⁵⁺ phase) and V⁴⁺-O_S‑L (O_S‑L related to the V⁴⁺ phase). Their specific functions were then elucidated through a combination of in situ reactant-specific H₂-TPR experiments (utilizing n-butane, 1,3-butadiene, or propylene as probes) and ¹⁸O-isotopic labeling studies. The results clearly demonstrate that V⁵⁺-O_S‑L is identified as the site for the initial hydrogen abstraction from n-butane, while V⁴⁺-O_S‑L facilitates the crucial epoxidation step to form MA. Furthermore, by employing CeO₂ as a modulator, we precisely tune the V⁵⁺-O_S‑L/V⁴⁺-O_S‑L ratio through the formation of Ce-O-V bonds, as evidenced by the X-ray absorption near edge (XANES) and extended X-ray absorption fine structure (EXAFS) analysis. An optimal V⁵⁺-O_S‑L/V⁴⁺-O_S‑L ratio for MA synthesis is 2.22-2.82; otherwise, it would be more conducive to a nonselective reaction. This work introduces a novel design paradigm for the development of high-performance VPO catalysts, offering a comprehensive framework for the rational design of heterogeneous oxidation catalysts via precise control of valence-specific oxygen chemistry.