A cluster DFT study of NH3 and NO adsorption on the (MoO2)2+/HZSM-5 surface: Lewis versus Brønsted acid sites

Abstract

A systematic DFT study was carried out to investigate NH3 and NO adsorption on both Lewis and Brønsted acid sites of (MoO2)2+/HZSM-5 catalyst by using cluster models. The adsorption energy results indicate that NH3 could strongly adsorb on both Lewis and Brønsted acid sites in the form of coordinated NH3 and NH4+, respectively, whereas NO represents poorer adsorption ability. It is also found that Lewis and Brønsted acid sites are competitive energetically for NH3 adsorption. According to the difference in the proposed mechanisms for NH3 adsorption on different acid sites, particular attention has been focused on the first dissociation of coordinated NH3 for Lewis acid site and the effect of Mo site on the introduction of NO for Brønsted acid site. For the coordinated NH3 on Lewis acid site, the more electron donation from NH3 is, the greater its adsorption stability is and the higher active its H atoms are. In addition, DOS results show that stability of the H atoms is enhanced by interacting with framework oxygen and especially the H atoms chemical-bonded with framework oxygen. For the NH4+ on Brønsted acid site, reduced-state Mo5+ holds stronger reducibility and oxidizability than terminal oxygen, which is suggested to play a key role in adsorption and activation of NOx together with the adsorbed NH4+.