Effect of H₂O and CO on the NO Operating Cycle in Pd/CHA Passive NO_x Adsorbers

Abstract

DFT simulations are performed to understand the impact of H 2 O and CO on the NO interaction with Pd sites present at various locations in Pd/CHA passive NO x adsorbers. The presence of H 2 O facilitates the reduction of [PdOH−PdOH] +2 and [Pd−O−Pd] +2 by NO in 8-membered rings (8MR) but increases the activation barrier in 6MR. Bidentate nitrates are formed in the absence of H 2 O, whereas monodentate nitrates are formed in the presence of H 2 O, which transform to bidentate nitrates upon H 2 O desorption. The detrimental effect of H 2 O on NO binding over [PdOH−PdOH] +2 and [PdOH] + is mitigated in the presence of CO. Additionally, CO preferentially reduces the Pd(II) species when both CO and NO are coadsorbed on [Pd−O−Pd] +2 , resulting in the formation of Pd +1 for further NO uptake. H 2 O does not significantly impact the reduction of Pd(II) species by CO.Pd +1 sites can also reoxidize to Pd(II) species under an oxidizing atmosphere. However, the reoxidation of Pd +1 is inhibited in the presence of H 2 O in 6MR and 8MR, which could explain the reported increase in the NO x desorption temperature. The operating cycle of NO, including the transformation of Pd species and the effect of H 2 O and CO on NO x uptake and release, is proposed.