A Concept of Chemical Potential Pumping Effect of Nonstoichiometric Oxides and the NO_x Sensing Mechanism of the Perovskite-Type La_0.5Sr_0.5FeO₃

Abstract

A concept of chemical potential pumping effect was introduced to describe the NO_x sensing mechanism of oxide catalysts with high electronic conductivity and large carrier density. When NO_x gas in a gas-stream decomposed on the surface of the catalyst oxides continuously, the high oxygen potential released by the decomposition of NO_x stationarily covered the whole surface of the oxide. Then, the bulk oxide was equilibrated with the steady-state high oxygen potential of the surface, which was much larger than that determined by the oxygen pressure of the surrounding gas-phase. This effect was confirmed by the conductivity enhancement of La₂CuO₄ and La_0.5Sr_0.5FeO₃ with NO₂ as well as nonstoichiometry variation of La_0.5Sr_0.5CoO₃ and EMF of galvanic cell with La_0.6Sr_0.4CoO_3−d. It was shown that the chemical potential pumping effect appears essentially on dense materials while the effect is hardly observed on porous material with large surface relative. On La_0.5Sr_0.5FeO₃ as well as on La₂CuO₄, NO₂ was found to decompose into NO and O₂, while NO was almost inactive on these oxides.