Catalytic activity of γ-Al2O3(110) in the NO + H2 reaction: a DFT study

Abstract

In this work, the interaction of the surface of γ-Al₂O₃(110) with NO and H₂ was studied using density functional theory calculations. Free γ-Al₂O₃(110) adsorbs NO and binds H atoms, but repels the H₂ molecule. A triplet of low-coordinated O^II–Al^III–O^II atoms provides the catalytic activity of γ-Al₂O₃(110) along the path: (i) the adsorption of NO/Al^III is followed by the binding of H₂ to form a hydroxylamine derivative NHOH through an intermediate NO/Al^III + 2 × H/O^II complex; (ii) recombination of NHOH with the release of N₂ through an intermediate NHOH/Al^III + NHOH/Al^IV or adsorption of NO followed by the release of N₂O through the intermediate NHOH/Al^III + NO/Al^IV; the pathway ends with the regeneration of γ-Al₂O₃(110). The calculated adsorption heats ensure the diffusion of H atoms from the deposited Pt to the surface (110), initiating the formation of the NH₂/Al^III + H/O^II complex, which releases NH₃ endothermically and is stable enough to inhibit stage (ii) of the above reaction pathway. An excess of O₂ in the NO + H₂ mixture excludes H/Pt and eliminates inhibition. The formation of oxynitrides is suppressed, but not excluded by more exothermic surface processes. The N-doped conductivity of bulk and surface oxynitrides Al₃₂O₄₇N and the dependence of the heat of adsorption of H atoms on the band gap width were revealed, which suggests a relationship between the band gap width and catalytic activity.