Influence of Pd Speciation on the Adsorption-Desorption Properties of Pd/BEA Hydrocarbon Traps for Diesel Emissions
Ryan Zelinsky, University of Virginia
Pd/BEA hydrocarbon traps were synthesized using different techniques and Pd loadings. The resulting catalysts were characterized using H2 TPR and CO pulse chemisorption to identify Pd speciation and dispersion respectively. Consistent with previous studies, lower Pd mass loading led to higher ion exchange and Pd utilization. Results from H2 TPR allowed identification of Pd2+ ionic species in several different chemical environments, as well as Pd particles. Higher mass loading resulted in more particle formation and reduced Pd2+ peak sizes. A set of hydrocarbon trap experiments using a simulated diesel exhaust mixture were conducted over several different Pd/BEA samples with varied Pd speciation and loading. Temperature programmed desorption and temperature programmed oxidation were used to characterize the trapping mechanisms. Ethylene was used as a surrogate molecule for short chain alkenes which served as a probe for Pd utilization. In addition to ethylene single feed, NO and CO co-feeds were introduced to investigate their impact on adsorption and desorption behavior. The results were correlated to the Pd speciation and mass loading.