The role of AgOAl entities in adsorption of NCO species and reduction of NOx

• The models of Ag/Al2O3 catalyst are constructed based on NMR and EXAFS.
• AgOAlocta and AgOAltetra are the predominant AgOAl entities on the Ag/Al2O3 surface.
• Vital NCO intermediates bonded on AgOAltetra entity is favorable to reduce NO2 energetically and kinetically.
• CNO species, as an isomer of NCO species, exhibit similar activity to reduce NO2.

In order to investigate the selective catalytic reduction of NOx by hydrocarbons, Ag/Al2O3 models were constructed by means of density functional theory (DFT) and experimental results. The geometrical structures and vibrational frequencies obtained at B3LYP levels of DFT were compared with the corresponding experimental results. EXAFS results suggest silver ion species were predominant on the Al2O3 surface. Two forms of interaction between Ag and the Al2O3 support can be deduced, namely AgOAlocta (octahedral, AlO6) and AgOAltetra (tetrahedral, AlO4). The AgOAlocta interaction is energetically favorable, which can be corroborated by both NMR results and DFT calculation. However, vital NCO intermediate species prefer to adsorb on the AgOAltetra entity. In the reduction of NO2, NCO species close to AgOAltetra are more favorable than those on AgOAlocta, both energetically and kinetically. Thus AgOAltetra rather than AgOAlocta should be considered as the active site during the HC-SCR process. The CNO species, as an isomer of NCO, was also examined by theoretical methods, and it was found to be more kinetically favorable to reduce NOx over the site of the AgOAltetra entity than NCO species.

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