Nanoconfinement Facilitates The Precipitation of Iron Hydroxides in Porous Amorphous Aluminum Oxides

In nature, nanoscale spaces where molecules are confined are ubiquitous, including within nanoparticle aggregates, biofilms, and fractures in rocks, leading to unique size-dependent chemical and physical interactions. Nanoconfinement also offers unique opportunities to manipulate and control reaction pathways and the properties of materials and substances in energy storage and conversion, carbon capture, catalysis, water purification and nanoelectronics. The well known tendency of nanoconfinement to lower the dielectric constant of water and limit diffusive mass transport rates leads to a common assumption that reactions are slower compared with their counterparts in bulk solution. Here, we report that the nanoconfinement can instead promote mineral replacement reactions. We examined the proton-promoted dissolution rate of porous amorphous aluminum oxide matrices of varying pore diameter when contacted with ferric chloride solution at acidic pH. Consumption of protons within the pores coupled to Al³⁺ release leads to oversaturation with respect to ferric hydroxides. Consequently, 2-5 nm Al-doped ferrihydrite and akageneite nanoparticles heterogeneously nucleated on the dissolving surfaces of aluminum oxides pores, based on various solid characterization including SEM-EDS, TEM-EDS, XRD, and Mossbauer spectroscopy. Furthermore, the surface area normalized dissolution rates of the pores coupled to precipitation of ferric hydroxides exhibit an exponential increase of two orders of magnitude as pore diameter linearly decreases from 200 nm to 30 nm. This increasing rate trend of dissolution-reprecipitation with decreasing pore size aligns quantitively with the principle of heterogenous nucleation of ferric hydroxides on curved surfaces. These results underscore the notion that nanoconfined space can accelerate dissolution-reprecipitation reactions by reducing the energy barrier to heterogenous nucleation of secondary phase, primarily due to the smaller pore size. Our results shield light on the underestimated and significant role of confined spaces in mineral replacement reactions that occur across a wide range of natural and materials synthesis environments.