Pd-Support Oxides Interaction Defined Redox Selectivity In Pd-Ce_0.7Zr_0.3O₂-Al₂O 3 Three-way Catalysts

To meet the increasingly strict regulations for automotive exhaust emissions, fundamental studies of the three-way catalysts (TWCs) are highly demanded. The present work is going to analyze the Pd-support oxides interactions defined redox activity and sites selectivity in Pd-Ce_0.7Zr_0.3O₂-Al₂O₃ catalysts, providing insights for the development of advanced TWCs.According to texture analysis, in comparison with that of Pd-Al₂O₃ interface, Pd particles are mainly stayed in oxidized states on ceria-zirconia, leaving ceria-zirconia slightly reduced. The chemical bonding of Pd-O-Ce is the major contributor to this. Pd-support interactions have been determined to be the major factor that influences the overall catalytic performance and ageing resistance.Pd-Ce_0.7Zr_0.3O₂ shows much lower oxidation activity than the detached Pd particles and Ce_0.7Zr_0.3O₂. The oxygen spillover effect has been proposed accordingly, lattice oxygen from ceria-zirconia support oxides maintains the redox couples of Pd2+/Pd1+/Pd0. The intermediate Pd0 species not only facilitate CO adsorption, but effectuate CO2 production by utilizing the [O] to turn itself into Pdn+. The improved oxygen buffering effect and CO conversion are limited under 300 oC due to kinetics, and are limited to Ce4+ reducibility approaches to 50 % because of thermodynamics.On Pd-Al₂O₃ interface, electron rich Pd clusters benefit the adsorption and dissociation of NO, where the dissociated [O] can be utilized for the simultaneous C-H debonding and oxidation. Pd-Ce_0.7Zr_0.3O₂ only favors CO oxidation in TWCs'light-off tests, which is a structural sensitive reaction, and the strong oxdative interface frustrates the subsequent reduction of NO2δ+ species.In combination of the conducted characterizations, it is observed that Pd particles are able to migrate freely over the support oxides in Pd-Ce_0.7Zr_0.3O₂-Al₂O₃ catalysts during 950 oC 10 h hydrothermal aging, bringing similar overall interface properties. More severe sintering of ceria-zirconia happens during the aging, and results in the more drastic decrease of light-off activity of Pd-Ce_0.7Zr_0.3O₂ parts.