Unravelling Phosphorus-Induced Deactivation of Pd-SSZ-13 for Passive NOx Adsorption and CO Oxidation

Phosphorus (P) originating from lubricant oil additives or biofuels is an emerging chemical poison in catalytic systems for automotive exhaust after-treatment. Here, we demonstrate that P-poisoning led to severe deactivation of small-pore Pd-SSZ-13 zeolites (with CHA framework) as passive NOx adsorbers (PNA) and CO oxidation catalysts for cold-start exhaust purification applications. Deactivation mechanisms of P-poisoning were unraveled by comparatively examining the P-free and P-loaded Pd-SSZ-13 zeolites using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), temperature-programmed reduction by hydrogen (H-2-TPR), CO pulse adsorption, temperature-programmed desorption using NH3 as a probe molecule (NH3-TPD), ultraviolet/visible light (UV/vis) spectroscopy, and in situ diffuse relectance infrared Fourier transform spectroscopy (DRIFTS). The loss of isolated Pd sites-namely, [Pd(OH)]+ and Pd-2. (located in the eight- and six-membered rings of CHA framework, respectively)-was revealed to be largely responsible for the deactivation of Pd-SSZ-13 in passive NOx adsorption and catalytic CO oxidation. In situ DRIFTS studies using NO or CO as a probe molecule suggest that [Pd(OH)](+) was more susceptible to P-poisoning than Pd2+. Specifically, P-poisoning led to a migration of [Pd(OH)](+) from cationic exchange sites to the zeolite surface, forming inactive metaphosphate (i.e., [Pd(OH)]+PO3-) and bulk PdOx species at high temperatures. In contrast, P-poisoning of Pd2+ sites proceeded via a sequential transformation to [Pd(OH)](+) first, and then to [Pd(OH)]+PO3- and bulk PdOx. This study provides a comprehensive mechanistic understanding on the deactivation of Pd-SSZ-13 by P-poisoning, and may guide the design of high-performance, phosphorus-resistant Pd-zeolite catalysts for cold-start exhaust after-treatment.

Automated summary

This study demonstrates that phosphorus (P) originating from lubricant oil additives or biofuels can lead to severe deactivation of Pd-SSZ-13 zeolites in automotive exhaust after-treatment systems. The loss of isolated Pd sites, specifically [Pd(OH)]+ and Pd2+, due to P-poisoning was found to be the primary reason for deactivation. In situ studies suggest that [Pd(OH)]+ is more susceptible to P-poisoning than Pd2+, leading to migration of [Pd(OH)]+ to the zeolite surface and subsequent formation of inactive metaphosphate and bulk PdOx species.