Identification of main active sites and the role of NO₂ on NO_x reduction with CH₄ over in/BEA catalyst: A computational study

The main active sites and the catalytic process in selective catalytic reduction of NO_x by CH4 (CH₄-SCR) on In/BEA catalyst were investigated by density functional theory (DFT) using a periodic model. The [InO]⁺ and [InOH]²⁺ moieties were constructed in the channel of periodic BEA zeolite representing the Lewis and Brønsted acid sites. The electronic structures [InO]⁺ and [InOH]²⁺ were analyzed, and it was found that the [InO]⁺ group were the main active sites for CH₄ activation and NO/NO₂ adsorption in the CH₄-SCR process. CH₄ molecules could be activated on the O site of the [InO]⁺ group in In/BEA, which was resulted from the strong interactions between the C-p orbital of the CH4 molecule and the O-p orbital of the [InO]⁺ group. CH₄ activation was the initial step in CH4-SCR on In/BEA catalyst. NO₂ molecules were essential in the SCR process, and they could be produced by NO reacting with gaseous O₂ or the O atom of the [InO]⁺ group. The presence of NO₂ could facilitate the key intermediate nitromethane (CH₃NO₂) formation and lower the reaction barrier in the SCR process.