Fundamental Understanding of the Di-Air System: The Role of Ceria in NOx Abatement

Abstract

Temporal analysis of product (TAP) is used to investigate the effectiveness of CO, C3H6, and C3H8 in the reduction of a La–Zr doped ceria catalyst and NO reduction into N2 over this pre-reduced catalyst. Hydrocarbons are found to be substantially more effective in the reduction of this catalyst at high temperature (above 500 °C) as compared to CO. NO decomposes over oxygen anion defects created upon catalyst reduction. Deposited carbon, in case the catalyst is reduced by C3H6 or C3H8, acts as a delayed or stored reductant and is not directly involved in NO reduction. Instead the oxidation of deposited carbon by an oxygen species derived from lattice oxygen (re)creates the oxygen anion defects active in NO reduction. In situ Raman, in which NO is flown over C3H6 pre-reduced La–Zr doped ceria at 560 °C, additionally shows that re-oxidation of the La–Zr doped ceria catalyst starts prior to the oxidation of deposited carbon, which confirms our TAP findings that firstly NO re-oxidized the La–Zr doped ceria catalyst and that secondly the oxidation of deposited carbon only commences at a higher ceria oxidation state. These findings create a new perspective on the operating principle of Toyota’s Di-Air system.