Investigation of Ce/BEA as a passive NOx adsorber: 1. The occurrences of cerium and adsorption behavior

Ce/BEA has great potential as a passive NOx absorber (PNA) to reduce the NOx emissions during the cold start phase especially in diesel engine for its high resistance to CO and suitable NOx desorption temperature range. However, as a novel material, its physical and chemical properties still remain unknown. In this work, Ce/BEA was synthesized by ion-exchange method to identify the active species in NOx adsorption, compared with aggregated Ce species in Ce/SiAlOx prepared by impregnation method. X-ray diffraction (XRD), high resolution-transmission electron microscopy (HR-TEM) and aberration-corrected-scanning transmission electron microscopy (AC-STEM) were performed to confirm the isolated single atom formation of Ce species in Ce/BEA. UV–vis diffraction reflection spectra (DRS) and X-ray photoelectron spectroscopy (XPS) were conducted to find the difference of chemical environment between the isolated Ce species and aggregated ones. PNA performance evaluation and temperature programmed reduction of hydrogen (H2-TPR) results show the high NOx storage capacity and active oxygen species quantity in Ce/BEA. It is also found that the main NOx adsorption species are Ce4+–O* rather than Ce3+. NOx-diffuse reflectance infrared Fourier transform spectroscopy (NOx-DRIFTS) confirms that the bidentate nitrite species are the critical NOx adsorption intermediates and the conception of NOx adsorption and desorption processes is proposed.