Ag/ZSM-5 traps for C₂H₄ and C₇H₈ adsorption under cold-start conditions

A series of Ag ion-exchanged ZSM-5 (Si/Al = 11.5) zeolites with different Ag loadings (0.5, 0.8, 1.6, 3.0, 4.6 wt%) were investigated for their ability to adsorb and desorb C₂H₄ and C₇H₈ under simulated diesel exhaust conditions. Ion-exchanging ZSM-5 zeolites with Ag (Ag(0.8)/ZSM-5) led to an increase in the adsorption capacities of C₂H₄ and C₇H₈ compared to HZSM-5 in the absence of H₂O. The C₂H₄ and C₇H₈ adsorption capacities of Ag(0.8)/ZSM-5 slightly decreased in the presence of H₂O, indicating that H₂O competes with both C₂H₄ and C₇H₈ for adsorption sites. Minor competitive adsorption between C₂H₄ and C₇H₈ was observed. Density functional theory calculations suggested that C₂H₄ is preferentially adsorbed in the sinusoidal channel of ZSM-5, while C₂H₄ and C₇H₈ are competing at the straight channels and the intersection of the sinusoidal and straight channels. With an increase in Ag loading from 0.5 to 4.6 wt% both C₂H₄ and C₇H₈ adsorption capacities increased from 38 to 163 and from 127 to 304 μmol/g, respectively. Temperature programmed desorption experiments indicated that an increase in Ag loading can lead to an increase in C₂H₄ and C₇H₈ desorption temperatures. During desorption the adsorbed C₇H₈ reacted to form CO, CH₂O and C₆H₆, while C₂H₄ desorbed as is. Reductive pretreatments at 200 and 600 °C over Ag(4.6)/ZSM-5 resulted to a 33 and 58% decrease in C₂H₄ adsorption capacity, respectively, compared to its oxidized form. The C₂H₄ adsorption capacity can be mostly recovered after oxidation treatments at 600 °C, indicating that Ag⁺ reduction is reversible in ZSM-5 zeolites. Reductive pretreatments did not alter the C₇H₈ adsorption capacity.