Improvement of NOx uptake/release over Pd/Beta by propylene: shielding effect of intermediates on adsorbed NOx species

Abstract

Passive NOx adsorbers (PNAs) are capable of trapping NOx at low temperature and releasing the trapped NOx into the gas circuit at higher temperatures, where downstream NOx reduction catalysts are activated. Hydrocarbons have a significant effect on the performance of PNAs, nonetheless research in this area has been overlooked. Here the chemistry of NOx adsorption and desorption in the presence of C₃H₆ was studied. For different pore-size zeolites (BEA, MFI and CHA), the addition of C₃H₆ always increased the NOx adsorption capacity at a low temperature and raised the NOx desorption temperature. Spectroscopic and computational investigations were performed using the model Pd/Beta to unravel the relevant mechanism. Fourier transform infrared (FTIR) spectra indicated that more Pd⁺ was formed in the presence of C₃H₆, which contributed to higher NOx storage capacity. An intermediate Pd–NC₃H₆O was probed and its evolution procedure was modeled by density functional theory (DFT) calculations. The results showed that a shielding effect of Pd–NC₃H₆O on Pd⁺–NO improved the NOx desorption temperature. This investigation has important implications for how short-chain olefins and even more complex gas mixtures affect the NOx adsorption and desorption performance of Pd/zeolite.