Deactivation of phosphorus-poisoned Pd/SSZ-13 for the passive adsorption of NOx

Journal article, 2022

Automotive catalysts can be exposed to various poisonous substances that can cause physical or chemical deactivation. One of such poisons is phosphorous, which originates from lubricant oils. This study focuses on the phosphorus deactivation of Pd/SSZ-13 used as a passive NOx adsorber (PNA). A clear deactivation caused by phosphorus was observed, and it was increased by increasing the content of phosphorous. It was concluded that phosphorous can cause both physical and chemical deactivation. This was evident from XPS analysis, where the presence of phosphorus pentoxide (P2O5) causes physical deactivation whereas metaphosphate (PO3-) and phosphate (PO43-) cause chemical deactivation. Also, it was shown that metaphosphates (PO3-) become the dominant phosphorous species at higher P concentrations. Lesser amounts of O2 were released in P-poisoned Pd/SSZ-13, as was found in oxygen TPD when increasing the P concentration, due to the presence of more PO3- species. Furthermore, XRD and 27Al NMR analyses revealed that phosphorus also interacted with alumina in the zeolite framework by forming Al-O-P species; this was also supported by SEM-EDX, where there was a clear overlap of P with Al and Pd spectra. DRIFTS analysis showed that OH groups in contact with the zeolite structure became contaminated by phosphorus and caused a chemical deactivation of Pd/SSZ-13. It was also found that, during multiple cycles, the PNA capacity decreased for phosphorus-poisoned samples. This was caused by the transformation of P2O5, which causes physical blocking, to PO3-, which interacts chemically with the palladium species.