Synthesis of New Oxide Solid Solutions of High Redox Activities and Their Application to Automotive Exhaust Catalysts

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Because of recent environmental regulations, reduction of the redox temperature and addition of high thermal stability are urgent subjects for oxide solid solutions employed in the automotive exhaust catalysts.In this study, low temperature reduction behavior of CeO_2-ZrO_2-Bi_2O_3 solid solutions was promoted by the addition of silver, which is an oxygen permeable component. The mechanism of the behavior was attributed to the synergetic effects of the partial solution of silver into the CeO_2-ZrO_2-Bi_2O_3 lattice and the surface deposition of silver on the solid solution. The reactivity of oxygen in the bulk of the catalyst greatly improves the soot combustion activities at low temperatures.Furthermore, we prepared and characterized a CeO_2-ZrO_2-Bi_2O_3 solid solution supported on La-stabilised γ-Al_2O_3 (CZB/Al_2O_3), which has the potential to be a key material in advanced catalytic converters. The low temperature redox activities demonstrated by the CZB/Al_2O_3 catalyst, which is effective at low temperatures below 100 ℃ even after calcination at 1000 ℃, have never been attained in conventional catalysts without help of precious metals. By the deposition of the CeO_2-ZrO_2-Bi_2O_3 solid solution on the surface of γ-Al_2O_3, particle size of the solid solution considerably decreased to a nanoscale, and the dynamic oxygen storage capacity became 7.5 times of the bulk CeO_2-ZrO_2-Bi_2O_3 catalyst without the γ-Al_2O_3 support (at 600 ℃). Studies using CH_4 oxidation, as a model reaction, showed that the redox activities of such catalysts at low temperatures correlates with their catalytic activities.