Low temperature NO adsorption over Pd-based catalysts for cold start application

Abstract

The abatement of NOx arising from automobiles becomes an important societal problem due to detrimental effect on human health and environment. Therefore, global regulations on NOx emission have been continuously issued and reinforced. To fulfill the legislations, various NOx reduction technologies such as three-way catalysis (TWC), selective catalytic reduction (SCR), and NOx storage reduction (NSR) systems have been successfully developed and applied. Among them, SCR and NSR have been adapted to address NOx <br/> emission in diesel engines. However, both systems require high temperature (above 200 °C) to acquire proper NOx reduction efficiency. Recently, in order to control the NOx emission during cold start, low temperature NO adsorption materials, namely, cold start catalysts (CSC) or passive NOx adsorbers (PNA) having the ability of storing NOx at low temperature and releasing them at high temperature where SCR and NSR properly operate draw significant attention. <br/> To have proper function as the low temperature NO adsorption catalyst, it must have not only catalytic ability (such as high NO storage capacity and proper NOx desorption temperature) but also durability under diesel exhaust conditions. In this work, various materials such as PGM(active metal), metal-oxide and zeolite(support) were investigated to identify the possibility for the low temperature NO adsorption catalyst. Especially, to evaluate the potential for the practical application of catalyst, exhaust gas conditions (such as <br/> hydrothermal aging and sulfur aging) causing catalytic deactivation were applied.<br/> At first, hydrothermally aged Pd/CeO2 was employed for NO adsorption at low temperature (80 – 160 °C). For comparison, Pt or Pt-Pd supported on CeO2 or Al2O3 catalysts were also evaluated. Analysis of the hydrothermally aged catalysts clearly demonstrate that Pt and/or Pd on CeO2 is more resistant to sintering than that on Al2O3 support. NO adsorption/desorption results indicate that CeO2-based catalysts have superior NO adsorption ability than Al2O3-based catalysts. In addition, PGM/CeO2 catalysts exhibit the desirable desorption temperature for cold start application. DRIFT spectra of adsorbed NOx species on Pd/CeO2 during NO adsorption/desorption demonstrate that NOx desorption peaks at 250, 300, and 450 °C derive from weakly bound nitrite, nitro-nitrito species, and nitrate species, respectively. Based on the activity test and characterization, it can be mentioned that Pd in Pd/CeO2 plays an important role in generating additional NO adsorption site arising from the interaction between Pd and CeO2 and helping the oxidation from adsorbed nitrite to nitrate. Therefore, Pd/CeO2 can be a good candidate as low temperature NO adsorption catalyst for cold start application. <br/> Next, to investigate the effect of sulfur aging (SA) and regeneration (DeSOx) on the changes in NO adsorption ability and physicochemical properties of catalysts, both treatments were applied to Pd/CeO2 and Pd/Ce0.58Zr0.42O2 (Pd/Ce58) catalysts. SA treatment on both catalysts leads to the negligible NO <br/> adsorption ability since it gives rise to form thermodynamically more stable Ce(SO4)2 than adsorbed NOx species. Structural and textural analysis after regeneration treatment exhibits that Pd/Ce58 maintains the crystalline size with similar pore size distribution, while Pd/CeO2 does not. In addition, the former <br/> sample recovers its own Pd distribution after regeneration, although the latter does not. However, the fact that both regenerated catalysts do not restore the NO adsorption ability to the level of HTA ones indicates that either textural property or Pd dispersion cannot account for the deterioration of NO adsorption ability after sulfur aging and regeneration. Instead, H2-TPR analysis on both samples indicates that the intimate interaction between Pd and ceria is disappear after regeneration, as proved by the absence of the simultaneous reduction of CeO2 and PdO below 20 °C. Hence, it can be concluded that the sulfur aging and the subsequent regeneration has negative effect on the interaction between PdO and CeO2, resulting in the irreversible degradation in the NO adsorption ability at low temperature. This result implies that the application of Pd/Ce based catalyst to practical condition is difficult.<br/> To obtain the hydrothermal stability and sulfur resistance, zeolite-based catalyst was employed. Fresh Pd/SSZ-13 did not possess good NO adsorption capacity irrespective of the preparation methods such as incipient wetness impregnation, wet impregnation, ion-exchange, and solid state ion exchange. However, after hydrothermal aging (HTA) treatment, Pd/SSZ-13 could obtain enhanced NO adsorption ability at low temperature, which will be suitable for the application of cold start catalyst. H2-TPR and XAFS results clearly indicate that PdO mainly existed over the fresh sample, while HTA treated sample contained ionic Pd species, demonstrating the redistribution of PdO into highly dispersed ionic Pd species within the SSZ-13 structure arising from HTA treatment. DRIFT results display the formation of two nitrosyl complexes adsorbed on Pd2+ ions, which are directly related to two desorption peaks of NOx at 250 and 400 °C. All combined results provide the unambiguous evidence about the generation of Pd ions in SSZ-13 zeolite induced by HTA <br/> treatment, which play as the active sites for NO adsorption at low temperature.