Identification of Main Active Sites and the Role of NO2 on NOx Reduction with CH4 over In/BEA Catalyst: A Computational Study

Erhao Gao,Hua Pan,Li Wang,Jun Chen,Yao Shi

doi:10.3390/catal10050572

Abstract

The main active sites and the catalytic process in selective catalytic reduction of NOx 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.

Highlights

The selective catalytic reduction (SCR) of NOx by hydrocarbons has attracted much interest in the past years [1,2]
The main active sites that are responsible for the CH4 -SCR over In-containing zeolites are still up for debate
Quantum chemistry calculation based on density functional theory (DFT) has been a powerful tool to elucidate details regarding the geometry and electronic structures of the active sites, which are essential to get a deeper understanding of the active sites and the catalytic process

Summary

Introduction

The selective catalytic reduction (SCR) of NOx by hydrocarbons has attracted much interest in the past years [1,2]. A number of zeolite-supported metal oxides (Pt [6], Co [7], Fe [8], Mo [9], In [10,11,12,13,14,15], etc.) catalysts have been explored, among which the indium-containing zeolites were the prominent one with excellent CH4 -SCR activities. The main active sites that are responsible for the CH4 -SCR over In-containing zeolites are still up for debate. Proposed that intrazeolitic [InO]+ was the active site on In/ZSM-5 catalyst during the process of SCR. Lónyiet et al [12,17] found that [InOH]2+ species originated from H2 O dissociation were responsible for initiating the steps of the SCR process. Quantum chemistry calculation based on density functional theory (DFT) has been a powerful tool to elucidate details regarding the geometry and electronic structures of the active sites, which are essential to get a deeper understanding of the active sites and the catalytic process

Methods

Results

Conclusion