Revealing the origin of high N₂ selectivity in NH₃ oxidation over copper-based acid catalysts via solid precursors...

The heterogeneity (acid sites) and stability (redox sites) of metal loss on metal-acid catalyst surfaces are the fundamental factors for the "trade-off" effect between activity and selectivity in the catalytic process with NH₃ as the activation precursor. However, the molecular-level impact of these surface characteristics on catalytic performance remains unclear and contentious. Herein, we propose a straightforward decomposition temperature control method, which operates below the single-layer dispersion threshold of solid sulfate/nitrate precursors. This method adjusts the micro-coordination electronic environment and work function of the active site by employing (2 H⁺)SO₄^2- species with negligible structural influence, thereby achieving precise control over the redox properties and acidity of metal-acid catalysts. In the NH₃ oxidation model reaction, ab initio molecular dynamics (AIMD) simulations show that even at 500 K, the (2 H⁺)SO₄^2- species can direct the dissociation of nearby adsorbed NH₃ into -NH₂ species. This is due to the (2 H⁺)SO₄^2- species can strip the electrons from the antibonding states below the Fermi level of the -NH₂ species via the high-position chemisorbed oxygen 2p orbitals, thereby reducing the electron state energy of the -NH₂ species through the -NH₂-1s-2p-(2 H⁺)SO₄^2- bond, even these Brønsted strong acid sites does not participate in the low-temperature NH₃ oxidation. This prevents the over-oxidation of -NH₂ species, anchoring the nearby high-selectivity i-SCR intermediate. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results confirm that abundant -NH₂ species at high temperatures can overcome the spatial steric hindrance of (2 H⁺)SO₄^2- species, enhancing the N₂ selectivity via the i-SCR mechanism (-NH₂ + NO_x/Nitrate → N₂ + H₂O).