Atomic-Scale Views of Water-Assisted Carbonation on Wollastonite (CaSiO₃)

Abstract

Mineral carbonation of silicate oxides like wollastonite (CaSiO₃) plays a crucial role in Earth’s long-term carbon cycle and offers a promising strategy for permanent CO₂ sequestration [1]. However, the atomistic mechanisms governing these mineral–CO₂ interactions remain poorly understood [2], hindered by the intrinsic complexity and insulating nature of silicate surfaces. The conventional mechanism for mineral carbonation is dissolution-precipitation, where mineral-derived cations react with bicarbonate species in an aqueous solution to form solid carbonates. Nevertheless, recent theoretical studies have also suggested the existence of direct chemisorption pathways for carbonate formation [3]. Here, we reveal a fundamental step of carbonation through a direct chemisorption pathway on the lowestenergy (100) surface of wollastonite. Our approach combines atomically resolved non-contact atomic force microscopy (nc-AFM) using a qPlus sensor and functionalized tips [4] under ultrahigh vacuum (UHV) conditions with density functional theory (DFT) calculations and AFM simulations [5]. Our study shows that water vapor is released from the mineral during UHV cleavage, and the first water molecule per unit cell adsorbs without a barrier into an exceptionally stable configuration not reported in previous work [6]. This creates a hydrated surface, which is also expected to happen at ambient conditions. The presence of adsorbed water is crucial for promoting the spontaneous chemisorption of CO₂ and resulting surface carbonate formation. Our findings provide the first direct, atomic-scale evidence of a water-assisted carbonation pathway on a silicate oxide proceeding from the gas phase, independent of aqueous dissolution.