Impact of Solvation on the Structure and Reactivity of the Co3O4 (001)/H2O Interface: Insights From Molecular Dynamics Simulations

Abstract

The spinel Co3O4 has many beneficial properties for potential use in catalysis. In operando, water is always present and alters the properties of the catalyst. We have used ab initio molecular dynamics to understand the effect of water and solvation on the structure and reactivity of the Co3O4 (001) A-type and B-type surface terminations. Water adsorbs on both terminations via a partial dissociative mode, and the A-termination is seen to be more reactive. On this surface, a higher degree of dissociation is observed in the topmost layers of the crystal in contact with water. Water dissociates more frequently on the Co2+ sites (about 75%) than on the adjacent Co3+ sites, where the degree of dissociation is about 50%. Increasing water coverage does not change the degree of water dissociation significantly. OH− adsorption on the Co2+ sites leads to a reduction of the amount of reconstruction and relaxation of the surface relative to the clean surface at room temperature. Proton transfer within the water films and between water molecules and surface has localized character. The B-terminated interface is less dynamic, and water forms epitactic layers on top of the Co3+ sites, with a dissociation degree of about 25% in the contact layer.

Impact of Single-Pulse, Low-Intensity Laser Post-Processing on Structure and Activity of Mesostructured Cobalt Oxide for the Oxygen Evolution Reaction

“…Solvation of surface Co 3 O 4 by water is known to induce minor restructuring and hydroxylation, which improves OER activity. 10,58,59 The elevated particle temperature during PLPP is expected to increase this hydroxylation resulting most likely in the formation of α-Co(OH) 2 phase that has been reported to be composed of a portion of Co 2+ in tetrahedral geometry that increases the overall pre-edge intensity in XAS spectra and a weak octahedral contribution. 60 We hypothesize that this amorphous α-Co(OH) 2 could not be detected by XRD because the method is sensitive only to crystalline phase.…”

X-ray Absorption Near-Edge Structure (XANES) at the O K-Edge of Bulk Co3O4: Experimental and Theoretical Studies

“…The XAS study revealed the reduction of Co to Co during 2-propanol oxidation thus underlining the Co sites of Co O surfaces as the active sites for the catalytic reactions [ 3 ]. Similarly, dissociation of water on two different Co O (001) surface terminations was studied by Kox et al [ 10 ]. Here, it was observed that water dissociates more frequently on the Co sites than on the adjacent Co sites, which again underlines different reactivity of the Co ions in the spinel Co O surfaces.…”

On the Lattice Oxygen Evolution Mechanism: Avoiding Pitfalls

“…Given that the positive and negative charges compensate each other, it appears plausible to pull the metal cation from its active site stepwise into the electrolyte by replacing all removed oxygen species subsequently with solvating water molecules. On the other hand, ab initio molecular dynamics is a promising technique to elucidate in the presence of explicit water molecules whether the dissolution of the metal ion into solution takes place when the local environment of the active site is stepwise altered [27,28] …”