Surface modification of γ-alumina by NaNO2, NaNO3, HNO2, HNO3 and H2SO4: A DFT-D approach

Abstract

In this study, the dissociative adsorption of NaNO₂, NaNO₃, HNO₂, HNO₃ and H₂SO₄ over (1 1 0) surface of γ-alumina non-spinel model were investigated through the dispersion corrected density functional theory (DFT-D) at PBE-D/DNP level of calculation. It was found that all of the species are dissociated to their ionic forms after adsorption and relaxation over the surface, i.e. Na⁺NO₂⁻, Na⁺NO₃⁻, H⁺NO₂⁻, H⁺NO₃⁻, H⁺HSO₄⁻ and 2H⁺SO₄²⁻. The Lewis acidity of alumina surface by addition of HNO₂, HNO₃ and H₂SO₄ is increased, while in the presence of NaNO₂ and NaNO₃, the acidity of catalyst is decreased. Theoretical calculations predict stronger dissociative adsorption of H₂SO₄ over the surface in compared to other compounds. The HNO₂ and HNO₃ mineral acids are better adsorbed over the surface than NaNO₂ and NaNO₃ salts. The better adsorption of nitrites than nitrates is due to the stronger electrostatic attractions. The order of NaNO₃ < NaNO₂ < HNO₃ < HNO₂ < H₂SO₄ for the dissociative adsorption energy of the title compounds is predicted.