Trace Amount of Water Boosting Highly Selective Coupling of Methane to Ethane via a Noble-Metal Free Ni2+-doped MgO/Al2O3 Photocatalyst

Direct and selective coupling of methane into high-value added products under mild conditions is a grand challenge since the activation of the inert C-H bonds (439 kJ mol − 1 ) and the inhibition of over-oxidation are simultaneously required. Herein, we report the fabrication of a noble-metal free Ni 2+ -doped MgO/Al 2 O 3 photocatalyst (NiMgAl-x, x represents calcination temperature) through facile topotactic conversion of the corresponding NiMgAl-layered double hydroxides (denoted as NiMgAl-LDH). EXAFS spectroscopy demonstrates that for NiMgAl-x, the Ni 2+ ions are isomorphically doped into MgO cell and no NiO phase is formed. By involving trace amounts of water, photo-oxidation of methane to ethane can be accomplished by using NiMgAl-800 as catalyst in a flow reactor with a high production rate (454.30 µmol g − 1 h − 1 ) and a superior selectivity (97.8%), which was the highest yield among all the noble-metal free photocatalysts reported so far. Detailed characterizations reveal the electron-enriched surface oxygens of NiMgAl-800 generated during the topotatic process lead to the enhanced photocatalytic activity. In-situ DRIFT experiments and DFT calculations show that the introduced H 2 O molecules can modulate the Gibbs free energy of ŸCH 3 towards a coupling conductive pathway instead of production of CO 2 , resulting in an increased C 2 H 6 selectivity and inhibited CO 2 production.