Evolution of palladium speciation on a β-zeolite based passive NOx adsorber: effects of catalyst pretreatment and aging

Robert  Pace, University of Kentucky Center for Applied Energy Research

Evolution of palladium speciation on a β-zeolite based passive NOx adsorber: effects of catalyst pretreatment and aging

Robert B. Pace, Yaying Ji, Mark Crocker

University of Kentucky Center for Applied Energy Research

Palladium containing zeolites have shown promise for the mitigation of automotive hydrocarbon and NOx emissions at low temperatures.1-2 In the current work, CO was used as a probe molecule to identify the Pd species present on a β zeolite with a Pd/Al ratio of 0.09. Pd-CO stretching bands were observed using DRIFTS, Pd speciation being monitored after a range of pretreatments in order to identify and evaluate the stability of the palladium species present.  Catalyst pretreatment under Ar at 500 °C followed by CO adsorption at 25 °C revealed the presence of multiple CO bands which can be assigned to CO adsorbed on Pd in different oxidation states (both metallic Pd0 and ionic Pd+ and Pd2+).3 Reduction of the catalyst at 100, 300, and 500 °C under 10% H2 for 1 h showed progressive loss of ionic Pd species as the temperature was increased, reduction at 500 °C yielding CO bands associated with primarily metallic palladium.  In order to examine the effect of repeated reduction/oxidation cycles, the catalyst was exposed to the reducing conditions described above, followed by oxidation of the catalyst at 500 °C under air for one hour.  Re-oxidation of the reduced catalyst demonstrated remarkable recovery of ionic Pd species in addition to the appearance of new Pd-CO bands which could be associated with either Pd2+ or somewhat less likely, Pd3+ species.4  After three cycles, the relative intensities of the Pd-CO bands stabilized, with primarily metallic Pd being present on the reduced catalyst, and Pd0, Pd2+ being the primary Pd-CO bands observed after oxidation. These results are indicative of good catalyst stability over time and will inform further catalyst development.

  1. Zheng, Y.; Kovarik, L.; Engelhard, M. H.; Wang, Y.; Wang, Y.; Gao, F.; Szanyi, J., Low-Temperature Pd/Zeolite Passive NO x Adsorbers: Structure, Performance, and Adsorption Chemistry. The Journal of Physical Chemistry C 2017, 121 (29), 15793-15803.
  2. Chen, H.-Y.; Collier, J. E.; Liu, D.; Mantarosie, L.; Durán-Martín, D.; Novák, V.; Rajaram, R. R.; Thompsett, D., Low Temperature NO Storage of Zeolite Supported Pd for Low Temperature Diesel Engine Emission Control. Catalysis Letters 2016, 146 (9), 1706-1711.
  3. Aylor, A. W.; Lobree, L. J.; Reimer, J. A.; Bell, A. T., Investigations of the Dispersion of Pd in H-ZSM-5. Journal of Catalysis 1997, 172 (2), 453-462.
  4. Naccache, C.; Primet, M.; Mathieu, M., Study of hydrogen and carbon monoxide interactions with palladium-Y zeolite by ESR and IR Spectroscopy. ACS Publications: 1973.