Self-Terminating Surface Reconstruction Induced by High-Index Facets of Delafossite for Accelerating Ammonia Oxidation Reaction Involving Lattice Oxygen.

Ammonia (NH 3 ) is a promising hydrogen (H 2 ) carrier for future carbon-free energy systems, due to its high hydrogen content and easiness to be liquefied. Inexpensive and efficient catalysts for ammonia electro-oxidation reaction (AOR) are desired in whole ammonia-based energy systems. In this work, ultrasmall delafossite (CuFeO 2 ) polyhedrons with exposed high-index facets are prepared by a one-step NH 3 -assisted hydrothermal method, serving as AOR pre-catalysts. The high-index CuFeO 2 facet is revealed to facilitate surface reconstruction into active Cu-doped FeOOH nanolayers during AOR processes in ammonia alkaline solutions, which is driven by the favorable Cu leaching and terminates as the 2p levels of internal lattice oxygen change. The reconstructed heterostructures of CuFeO 2 and Cu-doped FeOOH effectively activate the dehydrogenation steps of NH 3 and exhibit a potential improvement of 260 mV for electrocatalytic AOR at 10 mA cm -2 compared to the pre-restructured phase. Further, density functional theory (DFT) calculations confirm that a lower energy barrier of the rate-determining step (*NH 3 to *NH 2 ) is presented on high-index CuFeO 2 facets covered with Cu-doped FeOOH nanolayers. Innovatively, lattice oxygen atoms in Fe-based oxides and oxyhydroxide are involved in the dehydrogenation steps of AOR as a proton acceptor, broadening the horizons for rational designs of AOR catalysts.