NH3 adsorption and competition with H2O on a hydroxylated aluminosilicate surface

Area of Science:

• Geochemistry
• Surface Science
• Atmospheric Chemistry

Background:

• The interaction of ammonia (NH3) with aluminosilicates is crucial but poorly understood due to experimental challenges with insulating materials.
• A knowledge gap exists regarding NH3's role in ice nucleation on silicate dust, with its precise function remaining debated.

Purpose of the Study:

• To investigate the fundamental interactions between NH3 and microcline feldspar (KAlSi3O8), a key ice-nucleating mineral.
• To elucidate the adsorption behavior of NH3 on the microcline (001) surface and its interplay with surface hydroxyls and water.

Main Methods:

• Atomically resolved non-contact atomic force microscopy (nc-AFM)
• X-ray photoelectron spectroscopy (XPS)
• Density functional theory (DFT)-based calculations

Main Results:

• NH3 and H2O adsorb molecularly at identical sites on the microcline (001) surface, forming hydrogen bonds with surface silanol (Si-OH) and aluminol (Al-OH) groups.
• Adsorption energies for NH3 and H2O are closely matched.
• NH3 is readily replaced by H2O on the surface.

Conclusions:

• NH3 and H2O compete for the same adsorption sites on microcline feldspar.
• The findings challenge the hypothesis that ice nucleation on microcline involves an ordered H2O layer forming on pre-adsorbed NH3 molecules.