Oxygen Spillover and Local W6+/W4+ Redox at MnOx@Na2WO4/SiO2 Interfaces: Thermodynamic–Kinetic Origin of Selective CH4 to C2 Oxidation Under Near-Ambient Pressure

A working-state model is proposed for the MnOx–Na2WO4/SiO2 catalyst in oxidative coupling of methane (OCM), where a Na2WO4-rich surface environment forms an adaptive interphase that buffers the effective interfacial oxygen chemical potential and stabilizes cooperative MnOx/Na–WOx/Mn–O–W motifs. A thermodynamic-kinetic scheme is developed that relates (1) reaction-induced surface enrichment (structural stabilization), (2) oxygen spillover (damping of local oxygen gradients), and (3) Mn ↔ W redox exchange as an electron-oxygen buffer channel. Ex situ XPS/EDS/EPR data indicate a dynamically stratified near-surface region with chemically heterogeneous environments of Mn, W, and O. The W 4f region remains dominated by the W6+ contribution in the presence of a minor reduced component after OCM. In oxygen-deficient mixtures (CH4/O2 > 4), interfacial reconstruction becomes more pronounced: Mn-centered Mars–van Krevelen chemistry determines CH4 activation and oxygen exchange, while the Na2WO4-rich phase ensures fast ion/oxygen transport. Observation of the EPR signal from W5+ ions in the tungstate matrix indicates the existence of reduced W intermediates at low oxygen potential. Optimization of C2 selectivity and stability is suggested to require maintaining the catalyst within the selective window of effective interfacial μO by adjusting CH2/O2 and contact time, as well as controlling the architecture of the Na–W–O/MnOx interfacial region.