Non-thermal Plasma-assisted NO _x Reduction over Na-Y Zeolites: The Promotional Effect of Acid Sites

 

The effect of acid sites on the catalytic activities of a series of H⁺-modified Na-Y zeolites was investigated in the non-thermal plasma assisted NO _x reduction reaction using a simulated diesel engine exhaust gas mixture. The acid sites were formed by NH ⁺₄ ion exchange and subsequent heat treatment of a NaY zeolite. The catalytic activities of these H⁺-modified NaY zeolites significantly increased with the number of acid sites. This NO _x conversion increase was correlated with the decrease in the amount of unreacted NO₂. The increase in the number of acid sites did not change the NO level, it stayed constant. Temperature programmed desorption following NO₂ adsorption showed the appearance of a high temperature desorption peak at 453 K in addition to the main desorption feature of 343 K observed for the base Na-Y. The results of both the IR and TPD experiments revealed the formation of crotonaldehyde, resulting from condensation reaction of adsorbed acetaldehyde. Strong adsorptions of both NOx and hydrocarbon species are proposed to be responsible for the higher catalytic activity of H⁺-modified Na-Y zeolites in comparison to the base NaY material