Effect of Reduction and Re-oxidation on Pd Speciation in Pd-zeolite Passive NOx Adsorbers

Robert  Pace, University of Kentucky Center for Applied Energy Research

Effect of Reduction and Re-oxidation on Pd Speciation in Pd-zeolite Passive NOx Adsorbers

Robert B. Pace,1,2 Yaying Ji,1 Mark Crocker,1,2 Trevor Lardinois,3 Raj Gounder3

1University of Kentucky Center for Applied Energy Research

2Department of Chemistry, University of Kentucky

3Charles D. Davidson School of Chemical Engineering, Purdue University

Pd-exchanged zeolites have been extensively investigated for their performance as Passive NOx Adsorbers (PNAs), though much remains to be understood about the deactivation of these materials in the presence of exhaust gas components such as H2, H2O, and CO.1 In this study, in-situ diffuse reflectance IR spectroscopy was employed to identify the Pd speciation of both Pd-CHA and Pd-BEA zeolites after pretreatment under a range of conditions, CO adsorption being employed to visualize the Pd distribution. Exposure of Pd-BEA to ppm levels of water was found to lead to the reduction of super-electrophilic Pd2+CO = 2212, 2189 cm-1), whereas this Pd species was stable on Pd-CHA even after exposure to water at high temperature (500 °C). Prior work has  attributed this to the smaller CHA pore that serves to stabilize this species with respect to Pd-hydrolysis.2  The effects of pretreatment in CO were also examined, the production of large Pd particles being observed along with a CO adsorption site that represents a potential intermediate in Pd sintering.3 Re-oxidation in air after reduction by CO resulted in a decrease in the intensity of ionic Pd-CO bands compared to the untreated catalyst, suggesting some Pd was lost to sintering during CO exposure at high temperatures.4  In contrast, reduction in hydrogen followed by re-oxidation in air led to better recovery of ionic Pd and the absence of the CO adsorption site associated with sintering. Further, smaller Pd particles were observed when the catalyst was reduced in hydrogen (as compared to reduction in CO), and smaller particles are more readily re-dispersed as ionic Pd than the larger ones formed after by CO reduction.  These results indicate that CO will be especially problematic for the long-term stability of these materials as it promotes Pd sintering to a greater degree than hydrogen or water.4, 5

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