(111b) Incipient Formation of Methanol Dehydrogenation Active Sites during the Copper-Copper Oxide Phase Transition

Methanol dehydrogenation occurs in oxidative (ODH) or anaerobic (ADH) conditions, generating H₂O on oxide catalysts or H₂ on metal catalysts, respectively; taking copper oxide nanoparticles (CuO_x) as an example, depending on the contacting fluid environment, CuO_x may catalyze either ODH or ADH reactions where structures reduce from oxide (CuO and Cu₂O) to metal (Cu⁰) states with decreasing oxygen chemical potential at the surface.^[1] During the oxide-to-metal phase transition, active sites strained at the metal-oxide interfaces could be more reactive than their stable counterparts,^[2] but current transient methods used to probe these sites cannot accurately couple the intermediate states with their reactivity. Here, we uncover the reactivity of metastable CuO_x active sites, circumventing the limitation of transient techniques by coupling rigorous kinetic assessments—free of heat and mass transport corruptions—with operando spatially-resolved x-ray absorption spectroscopy (XAS) under oxygen-lean integral conditions. As reactor residence time increases, Cu²⁺ reduces to Cu¹⁺ and Cu⁰ because H₂O desorption in ODH Mars-van Krevelen catalytic cycles removes lattice oxygen atoms to reduce the Cu state; we reveal that all Cu states in this phase transition (CuO_x from x=0–1) are a single-valued function of the instantaneous oxygen-to-methanol ratio. On intermediate CuO_x surfaces, instantaneous CH₃OH ADH rates are five-fold higher than their steady state counterparts (for O₂/CH₃OH of 0.016, 7.4 vs. 1.3 mmol (g-atom-Cu_surf s)^-1, 5 kPa CH₃OH, 503 K), because surface Cu⁰ sites are polarized at the Cu⁰-Cu₂O-CuO interfaces. Taken together, these findings paint a mechanistic picture of oxide-to-metal phase transitions and illustrate the critical role of strained active sites for promoting dehydrogenation catalysis.

[1] Broomhead, W. T.; Chin, Y.-H. Catalysis, Royal Society of Chemistry, 2024, 35, 69–105.

[2] Ruiz Puigdollers, A.; Schlexer, P.; Tosoni, S.; Pacchioni, G. ACS Catalysis 2017, 7, 6493–6513.