Enhanced carbon monoxide poisoning resistance of Pd/BEA by integrating with CeO₂ for low temperature NO adsorption: The role of CeO₂ on oxidizing carbon monoxide to CO₃²⁻

Passive NO_x adsorption (PNA) represents a promising technology for controlling NO_x emissions during cold starts. However, Pd/zeolite catalysts, the latest generation of PNA materials, suffer from serious deactivation under CO concentrations, owing to the CO induced aggregation of highly dispersed Pd sites to PdO particles. In this work, we found that the resistance to CO poisoning of Pd/BEA can be enhanced by integrating with CeO₂ through physical mixing. The resulting CeO₂ + Pd/BEA composite catalyst demonstrated a notably higher NO_x storage capacity (with a NO_x/Pd ratio of 1.3) compared to Pd/BEA (0.8). This improvement can be attributed to the interaction between CeO₂ and the BEA support, which promoted the formation of nitrates on CeO₂ during NO adsorption at 80 °C. More importantly, CeO₂ effectively shielded Pd²⁺ ions from reduction by CO, with 88 % of Pd²⁺ ions remaining after CO exposure, significantly outperforming Pd/BEA, which retained only 66 %. Spectra characterization and DFT results indicate that the defects in CeO₂ strongly trapped CO (−0.973 eV), and the oxidation of trapped CO to CO₃²⁻ by adsorbed oxygen (O_ads) was exothermic (−1.35 eV), which reduced the exposure of Pd²⁺ ions to CO and inhibited the reduction and agglomeration of Pd²⁺ ions. While the composite sample still exhibited a slight reduction in NO_x storage capacity after CO poisoning, with the NO_x/Pd ratio decreasing from 1.3 to 0.9. The DFT simulation results indicate that the Ce-O_ads bonds in CeO₂ are weakened during the decomposition of the CO₃²⁻ intermediate. This weakening could potentially facilitate the release of O_ads, thereby reducing the O_ads content as evidenced by spectral evidence. The decrease of O_ads diminished nitrate formation on the CeO₂ component in composite sample, resulting in the decline of NO_x storage capacity after CO poisoning.