Protolysis Reaction on Pyrophyllite Surface Molecular Models: A DFT Study.

Area of Science:

• Geochemistry
• Materials Science
• Computational Chemistry

Background:

• Mineral dissolution is crucial for geochemical processes in soils and sediments.
• Clay minerals, like pyrophyllite, play a significant role in these processes.
• Understanding dissolution mechanisms at the atomic scale is essential.

Purpose of the Study:

• To investigate the atomic-scale mechanisms of pyrophyllite dissolution under acidic conditions.
• To simulate protolysis reactions at different pyrophyllite edge surfaces using Density Functional Theory (DFT).
• To identify the most reactive sites and understand the role of protons and hydronium ions.

Main Methods:

• Employed Density Functional Theory (DFT) to model pyrophyllite dissolution.
• Constructed molecular cluster models for four distinct edge surfaces: {100}, {010}, {110}, and {130}.
• Analyzed interactions of protons and hydronium ions with oxygen sites on these edges.

Main Results:

• Bridge oxygens, particularly those bonded to Si and Al, are the most reactive sites for protonation.
• The {110} edge showed the least reactivity, while {100}, {010}, and {130} edges were highly reactive.
• Hydronium ions induced comparable or greater structural changes than protons, facilitating dissolution.

Conclusions:

• Protonation of bridge oxygens is the rate-limiting step in phyllosilicate dissolution.
• Octahedral cations are preferentially released over tetrahedral cations during dissolution.
• Edge reactivity is critical, and water plays a key role in proton transfer and protolysis.