Passive NOx adsorption using zeolite supported Pd catalysts has been considered one of most effective methods for low-temperature NO storage in the diesel engine emission control. In the present work, first-principles density functional theory calculations were used to study NO adsorption at the highly dispersed Pd sites, i.e., monomeric PdIIO and dimeric [PdII-O-PdII]2+ in the HBEA zeolite. It has been found that each PdIIO and [PdII-O-PdII]2+sites could bind with three and four NO molecules, respectively. Upon adsorption, NO could be oxidized to NO2, resulting in the reduction of PdII to Pd0 for PdIIO and PdII to PdI, for [PdII-O-PdII]2+, respectively. With increasing NO coverage at both sites, NO oxidation becomes more facile with low activation barriers of 13 and 22?kJ/mol. At the [PdII-O-PdII]2+ site, the binding of formed NO2 becomes weak at high NO coverage. Both PdIIO and [PdII-O-PdII]2+sites are protonated to the more stable [PdIIOH]+ and [PdIIOH]+/[PdIIOH]+ sites in the presence of H2O or neighboring BAS sites. Finally, we studied the CO effect on the NO adsorption in Pd/HBEA. With co-fed CO in the NOx mixture, both PdIIO and [PdII-O-PdII]2+ could be reduced to Pd° and [PdI-PdI]2+ sites. Compared to the NO adsorption, our calculations show that CO adsorption is slightly stronger at the [PdII-O-PdII]2+site while it is slightly weaker at the PdIIO site. CO oxidation, rather than NO oxidation readily occurs if NO and CO co-adsorbed. The reduced PdI site and the [PdIIOH]+/[PdIIOH]+ site enhances NO adsorption.
Revised: July 22, 2020 |
Published: January 5, 2019