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

DFT simulations are performed to understand the impact of H2O and CO on the NO interaction with Pd sites present at various locations in Pd/CHA passive NOx adsorbers. The presence of H2O 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 H2O, whereas monodentate nitrates are formed in the presence of H2O, which transform to bidentate nitrates upon H2O desorption. The detrimental effect of H2O 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. H2O 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 H2O in 6MR and 8MR, which could explain the reported increase in the NOx desorption temperature. The operating cycle of NO, including the transformation of Pd species and the effect of H2O and CO on NOx uptake and release, is proposed.