Leaching of Ba²⁺ in NO_x Traps

Abstract

It has been found that crystalline BaCO₃ replaces crystalline BaAl₂O₄ in prototypical NO _x traps upon exposure to liquid water. This phenomenon can be understood in terms of acid–base chemistry, where Ba²⁺ is leached from the aluminate, precipitating as a carbonate, under the influence of weak carbonic acid naturally present due to the equilibrium between CO₂ in the air and the water. Consequences of such a redistribution of Ba²⁺ in a NO _x trap could include a decrease in surface area of the trapping material and a loss of contact between the trapping material and the catalytic components, such as Pt. Indeed, preliminary testing has shown that exposure of fully formulated NO _x traps to liquid water results in the loss of approximately half of the initial trapping capacity.