Significance of the surface silica/alumina ratio and surface termination on the immersion freezing of ZSM-5 zeolites

Heterogeneous ice nucleation in the atmosphere impacts climate, but the magnitude of the effect of ice clouds on radiative forcing is uncertain. Surfaces that promote ice nucleation are varied. Because O, Si, and Al are the most abundant elements in the Earth's crust, understanding how the Si : Al ratio impacts the ice nucleation activity of aluminosilicates through exploration of synthetic ZSM-5 samples provides a good model system. This paper investigates the immersion freezing of ZSM-5 samples with varying Si : Al ratios. Ice nucleation temperature increases with increasing surface Al content. Additionally, when ammonium, a common cation in aerosol particles, is adsorbed to the zeolite surface, initial freezing temperatures are reduced by up to 6 °C in comparison to proton-terminated zeolite surfaces. This large decrease in ice nucleation activity in the presence of ammonium suggests that the cation can interact with the surface to block or modify active sites. Our results on synthetic samples in which the surface composition is tunable gives insight into the role of surfaces in heterogeneous ice nucleation processes in the atmosphere. We emphasize the importance of examining surface chemical heterogeneities in ice nucleating particles that could result from a variety of aging pathways for a deeper understanding of the freezing mechanism.