Unravel the mechanism of soot oxidation over silver-supported Yttria-Stabilized Zirconia through isotopic experiments

SSCI-VIDE+CARE+ABO:PVENational audience1. Introduction Particulate Matter (PM) emission abatement for passenger Diesel vehicles has become more and more stringent over the past years. From 2009, Euro 5 standards have permitted to generalize Diesel Particle Filter (DPF) to reduce PM emissions. DPF proved to be an efficient way to trap soot particles in the exhaust, the main drawback being the necessity to regenerate the filter regularly as soot collection slowly obstructs the filter, increasing the pressure drop. Therefore, an efficient catalyst for the DPF regeneration at low temperature, using only oxygen as an oxidant, is strongly required. Our group has shown that Yttria-Stabilized Zirconia (YSZ), a well-known pure oxygen ionic conductor, can oxidize soot [1-3] with lattice oxygen active species with a concomitant gaseous oxygen replenishment into the lattice. This study aims to investigate the mechanism of soot oxidation over bare YSZ and silver-supported YSZ through labelling oxygen adsorption and exchange, CO2 adsorption and exchange as well as isotopic oxidation of soot. 2. Experimental A series of silver-impregnated YSZ catalysts was prepared by wet impregnation followed by calcination at 700°C. Catalysts displayed silver loading ranging from 0 to 7.5wt%. Samples were characterized by XRD, XPS and TEM. Performances toward soot oxidation were measured by running TPO (temperature-programmed oxidation) of a mixture of model soot (Printex U) and catalyst with various partial pressures of oxygen. Similar experiments were performed with labelled O2 (Temperature-programmed Isotopic exchange, TPIE). Isothermal oxygen exchange (IOE) experiments were also conducted with C16O2 over previously exchanged samples with labelled oxygen. 3. Results and discussionAg/YSZ catalysts display high thermal stability and maximal activity for soot oxidation for remarkably low loading of Ag (1 wt.%) thanks to the ability of silver to disperse into small metallic nanoparticles. The mobility of silver particles allows contact improvement between the catalyst and soot. Oxygen partial pressure was shown to impact performances; the activity drop when decreasing the oxygen partial pressure from 5 to 1% was found to be stronger in the presence of silver than over bare YSZ, thus indicating that silver plays a role into oxygen replenishment. As YSZ is a non-reducible oxide, soot oxidation must involve the integration of gaseous oxygen into the lattice to replace lattice oxygen species consumed to oxidize soot. This is most probably the rate-determining step of the overall combustion process. This replenishing of YSZ is promoted by Ag nanoparticles, as confirmed by TPIE. Their presence on YSZ strongly promotes the oxygen dissociative adsorption, as well as its integration into the YSZ lattice. CO2 exchange occurred at low temperatures and could be considered as an artefact when performing isotopic soot oxidation. This underlines that the oxygen mechanism via surface carbonation must be carefully analysed to clearly conclude the origin of the active oxygen species. Isothermal oxygen exchange experiments performed at 400°C with labelled oxygen on YSZ and Ag/YSZ in both tight and loose contact modes with the model soot clearly show that the largest amount of lattice oxygen is detected in the CO2 produced during the first 20 min of the soot combustion process in comparison with the CO2 route exchange. This indicates that YSZ bulk oxygen species are largely involved in soot oxidation. Figure 1. Mechanism for soot ignition over Ag/YSZ, oxidation of soot to CO and CO2 on YSZ (1) and in contact with Ag (2), O2 adsorption and dissociation over silver nanoparticles (3).4. ConclusionsIsotopic experiments proved that active oxygen species for soot oxidation originate from the YSZ lattice, despite the fact that gaseous CO2 exchange also occurs over the catalyst. Silver nanoparticles promote soot oxidation activity by activating the dissociative adsorption and lattice integration of gaseous oxygen. References 1. E. Obeid, L. Lizarraga, M.N. Tsampas, A. Cordier, A. Boréave, M.C. Steil, G. Blanchard, K. Pajot, P. Vernoux, Continuously regenerating Diesel Particulate Filters based on ionically conducting ceramics, J. Catal. 309 (2014) 87–96.2. E. Obeid, M.N. Tsampas, S. Jonet, A. Boréave, L. Burel, M.C. Steil, G. Blanchard, K. Pajot, P. Vernoux, Isothermal catalytic oxidation of diesel soot on Yttria-stabilized Zirconia, Solid State Ionics, 262 (2014) 253–256.3. A. Serve, A. Boreave, B. Cartoixa, K. Pajot, P. Vernoux, Synergy between Ag nanoparticles and yttria-stabilized zirconia for soot combustion, Appl. Catal. B Environ. 242 (2019) 140–149