Surface hydroxyl group dominating aerobic oxidation of methane below room temperature†

Abstract

Direct oxidation of methane (DOM) using molecular oxygen (O₂) and hydrogen (H₂) is currently considered to be triggered by in situ produced H₂O₂ or free hydroxyl radicals (˙OH). However, the role of the surface hydroxyl group in the DOM that is in situ formed from O₂ and H₂ has long been ignored. Herein, we provide experimental evidence that DOM using H₂ and O₂ over titanium silicate-supported single Pd atoms coated with an ultrathin N-doped carbon (Pd₁/TS-1@CN) catalyst is dominated by a surface hydroxyl group instead of H₂O₂ or free ˙OH. Furthermore, the direct bonding between Pd atoms with the pyrrolic nitrogen of the coating layers reinforces the bonding strength of Pd₁ and framework oxygen, forming a unique N₁–Pd₁–O₂ configuration that considerably boosts the stability of isolated Pd active sites and their capability to stably generate a surface hydroxyl group from H₂ and O₂. Therefore, Pd₁/TS-1@CN yields a liquid oxygenate productivity of 647 μmol g_cat⁻¹ h⁻¹ with 100% selectivity at 15 °C and high stability over 30 cycles with no activity loss. Our findings regarding the catalytic role of the surface hydroxyl group in DOM and its stabilization strategy open up a new avenue for designing advanced catalysts for the DOM using O₂ under mild reaction conditions.