In a new study published in CCS Chemistry, researchers have uncovered how a small number of water molecules can trigger structural transformation in neutral hydrated metal hydroxide clusters, providing new insights into hydration-driven changes in catalytic metal-oxygen frameworks.

The hydration of multinuclear metal hydroxides is ubiquitous in processes ranging from catalytic conversion and metal corrosion to the synthesis and operation of functional materials. During hydration, water molecules can reorganize hydrogen-bonding networks, which may further alter the core skeleton and local coordination environment of metal hydroxides, leading to specific structural transformations.

Understanding these hydration-induced structural changes is critical for revealing the nature of active sites at real solid-liquid catalytic interfaces. However, studying neutral clusters remains challenging because, unlike ionic species, they are difficult to detect and mass-select.

To tackle this challenge, a research team led by Profs. JIANG Ling and LI Gang from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) investigated the structural evolution of neutral hydrated binuclear strontium hydroxide clusters.

The researchers found that only three water molecules are sufficient to trigger the configuration transformation of the core skeleton from Sr₂(μ₂-OH)₂(η¹-OH) to Sr₂(μ₂-OH)₃, which provides new insights into the hydration mechanisms of metal-oxygen frameworks in catalysts.

Previously, the team developed a neutral-cluster infrared spectroscopy endstation based on infrared-vacuum ultraviolet (IR-VUV) spectroscopy, integrating infrared (IR) excitation with vacuum ultraviolet (VUV) threshold ionization technology. This platform enables highly sensitive detection, structural characterization, and reactivity studies of mass-selected neutral clusters.

By combining this platform with a tabletop vacuum ultraviolet (VUV) light source, the researchers obtained size-selected IR spectra of neutral Sr₂(OH)₃(H₂O)ₙ (n = 1–5) clusters, , allowing researchers to investigate hydration-induced structural evolution.

Experimental IR spectra via the 193 nm vacuum ultraviolet laser, identified structures of neutral Sr₂(OH)₃(H₂O)₃ clusters, and a Sr-based perovskite model. (Image by JIANG Shuai)

Combining the experimental spectra with quantum chemical calculations and ab initio molecular dynamics (AIMD) simulations, the researchers found that clusters with hydration numbers n ≤ 2 adopt the Sr₂(μ₂-OH)₂(η¹-OH)(H₂O)_n configuration, whereas those with n ≥ 3 evolve into the Sr₂(μ₂-OH)₃(H₂O)_n configuration, revealing a striking configuration transformation occurring at n = 3.

This structural transition is driven by pronounced deformation of the Sr₂(μ₂-OH)₂(η¹-OH) core skeleton induced by the third water molecule, generating substantial deformation energy.

Through the subsequent rearrangement of the hydrogen-bonding network in the Sr₂(μ₂-OH)₃(H₂O)_n configuration, the Pauli repulsion energy is effectively reduced, thereby stabilizing the structure and making this configuration the global minimum on the potential energy surface.

According to the researchers, the present system serves as an ideal model for studying metal-oxygen frameworks and hydration processes, offering a new approach for systematically investigating hydration-driven lattice rearrangement and active-site regulation in catalysts.