Methane Oxidation over Cu /[CuOH] Pairs and Site-Specific Kinetics in Copper Mordenite Revealed by Operando Electron Paramagnetic Resonance and UV/Visible Spectroscopy.

Cu-exchanged mordenite (MOR) is a promising material for partial CH oxidation. The structural diversity of Cu species within MOR makes it difficult to identify the active Cu sites and to determine their redox and kinetic properties. In this study, the Cu speciation in Cu-MOR materials with different Cu loadings has been determined using operando electron paramagnetic resonance (EPR) and operando ultraviolet-visible (UV/Vis) spectroscopy as well as in situ photoluminescence (PL) and Fourier-transform infrared (FTIR) spectroscopy. A novel pathway for CH oxidation involving paired [CuOH] and bare Cu species has been identified. The reduction of bare Cu ions facilitated by adjacent [CuOH] demonstrates that the frequently reported assumption of redox-inert Cu centers does not generally apply. The measured site-specific reaction kinetics show that dimeric Cu species exhibit a faster reaction rate and a higher apparent activation energy than monomeric Cu active sites highlighting their difference in the CH oxidation potential.