Effect of silica-to-alumina ratio on hydrocarbon adsorption–desorption properties of Ag/ZK-5 Zeolites for gasoline cold-start emission control

Abstract

Cold-start hydrocarbon (HC) emissions from gasoline engines remain a critical challenge in automotive exhaust purification, as three-way catalytic converters remain inactive below their light-off temperature. Ag-modified zeolites are promising HC trap materials, yet the structure–performance relationship regulated by the silica-to-alumina ratio (SAR) requires further systematic clarification. In this work, Ag/ZK-5 zeolites with SAR values of 2.5, 5, 10, and 20 were fabricated via the ion-exchange method, and their adsorption–desorption properties for ethylene and toluene were evaluated in combination with multi-technique characterizations and in-situ DRIFTS analysis. The results demonstrate that SAR exerts a well-defined dual regulatory effect on two interconnected levels: the zeolite framework (crystallinity, textural properties, and surface acidity) and the supported silver species (loading, chemical state, and dispersion degree). The SAR2.5 sample achieves the highest ethylene adsorption capacity of 623 μmol/g owing to its abundant ionic Ag⁺ sites, but suffers from severe silver agglomeration and limited toluene uptake. The SAR5 sample delivers the optimal comprehensive performance, with a toluene adsorption capacity of 850 μmol/g and desorption temperatures for both hydrocarbons higher than those of both the lower-SAR SAR2.5 and the higher-SAR SAR10/SAR20 counterparts, which benefits from its well-preserved crystallinity, highly dispersed Ag⁺ active sites, and balanced pore structure and surface acidity. High-SAR samples (SAR10, SAR20) exhibit inferior adsorption performance for both hydrocarbons due to framework amorphization and poor silver dispersion. In-situ DRIFTS characterizations reveal that ethylene adsorption follows a π-complexation pathway with partial oxidative transformation, while toluene adsorption is driven by the synergism of O–H···π hydrogen bonding and π-complexation. This work provides experimental guidance for the rational design of zeolite-based HC traps for cold-start emission control.