Synergy of Ag and Yttria-stabilized Zirconia catalysts during Diesel soot oxidation
Résumé
Motivated by increasingly stringent emissions regulations, Diesel Particulate Filters (DPFs) have seen widespread used as the only technically and economically feasible means for meeting current and future Particulate Matter (PM) emissions limits. DPFs present high filtering efficiency (>99%) but must be periodically regenerated due to soot particles accumulation [1]. EURO 6 standards require the utilization of a NOx catalytic after-treatment device [2], preferentially placed between the Diesel Oxidation Catalyst (DOC) and the DPF [2]. Therefore, NO2 cannot anymore be utilized as an oxidant for soot combustion. This makes crucial the development of effective catalysts for soot combustion with oxygen, combining (i) activity at low temperatures and (ii) an excellent stability at high temperatures for complying the necessary durability. We have recently reported that Yttria-stabilized Zirconia (YSZ), a pure oxygen ion ceramic conductor without any redox property, is a promising catalyst to continuously oxidize soot with oxygen in Diesel exhaust conditions [3]. Isotopic Temperature-Programmed Oxidation (TPO) [3] and isothermal catalytic oxidation experiments [4] using labeled oxygen 18O2 demonstrated the key-role of bulk oxygen species in the soot oxidation process, when YSZ/soot were in tight contact mode. It was proposed that the ignition of the soot oxidation on YSZ involves a fuel-cell-type electrochemical mechanism at the nanometric scale. The efficiency of this process strongly depends both on the YSZ/soot contact and also on the oxygen partial pressure. The tight catalyst/ soot contact is not representative of the contact obtained inside DPFs. The target of this study is to further improve the YSZ soot oxidation efficiency when the catalyst/soot mixture is in the loose contact. Silver efficiency for soot oxidation is based on its ability to deliver oxygen from the lattice/surface to the soot particle in a wide temperature range [5], combining that with its high mobility, during the oxidation process, makes it a good candidate for our goal. Therefore, Ag nanoparticles were deposited on YSZ from a Ag colloidal suspension. The catalytic performances for soot oxidation of these catalysts were compared with those of bare YSZ and Ag/?-Al2O3 as a reference material both in tight and loose contact mode. In addition, different characterizations (TEM, TPR, XPS and isotopic exchange) were also conducted to elucidate the Ag/YSZ interactions.