CO₂ Adsorption and Reactivity on Rutile TiO₂(110) in Water: An <i>Ab Initio</i> Molecular Dynamics Study

Atomic-scale understanding of CO₂ adsorption and reactivity on TiO₂ is important for the development of new catalysts for CO₂ conversion with improved efficiency and selectivity. Here, we employ Car–Parrinello molecular dynamics combined with metadynamics simulations to explore the interaction dynamics of CO₂ and rutile TiO₂(110) surface explicitly treating water solution at 300 K. We focus on understanding the competitive adsorption of CO₂ and H₂O, as well as the kinetics of CO and bicarbonate (HCO₃^–) formation. Our results show that adsorption configurations and possible reaction pathways are greatly affected by proper description of the water environment. We find that in aqueous solution, CO₂ preferentially adsorbs at the bridging oxygen atom O_b, while Ti_5c sites are saturated by H₂O molecules that are difficult to displace. Our calculations predict that further conversion reactions include spontaneous protonation of adsorbed CO₂ and detachment of OH^– to form a CO molecule that is significantly facilitated in the presence of a surface Ti³⁺ polaron. In addition, the mechanisms of HCO₃^– formation in bulk water and near TiO₂(110) surface are discussed. These results provide atomistic details on the mechanism and kinetics of CO₂ interaction with TiO₂(110) in a water environment

Ab Initio Metadynamics Study of the VO₂⁺/VO²⁺ Redox Reaction Mechanism at the Graphite Edge/Water Interface

Redox flow batteries (RFBs) are promising electrochemical energy storage systems, for which development is impeded by a poor understanding of redox reactions occurring at electrode/electrolyte interfaces. Even for the conventional all-vanadium RFB chemistry employing V²⁺/V³⁺ and VO₂⁺/VO²⁺ couples, there is still no consensus about the reaction mechanism, electrode active sites, and rate-determining step. Herein, we perform Car–Parrinello molecular dynamics-based metadynamics simulations to unravel the mechanism of the VO₂⁺/VO²⁺ redox reaction in water at the oxygen-functionalized graphite (112̅0) edge surface serving as a representative carbon-based electrode. Our results suggest that during the battery discharge aqueous VO₂⁺/VO²⁺ species adsorb at the surface C–O groups as inner-sphere complexes, exhibiting faster adsorption/desorption kinetics than V²⁺/V³⁺, at least at low vanadium concentrations considered in our study. We find that this is because (i) VO₂⁺/VO²⁺ conversion does not involve the slow transfer of an oxygen atom, (ii) protonation of VO₂⁺ is spontaneous and coupled to interfacial electron transfer in acidic conditions to enable VO²⁺ formation, and (iii) V³⁺ found to be strongly bound to oxygen groups of the graphite surface features unfavorable desorption kinetics. In contrast, the reverse process taking place upon charging is expected to be more sluggish for the VO₂⁺/VO²⁺ redox couple because of both unfavorable deprotonation of the VO²⁺ water ligands and adsorption/desorption kinetics

Ab Initio Metadynamics Study of the VO₂⁺/VO²⁺ Redox Reaction Mechanism at the Graphite Edge/Water Interface

Redox flow batteries (RFBs) are promising electrochemical energy storage systems, for which development is impeded by a poor understanding of redox reactions occurring at electrode/electrolyte interfaces. Even for the conventional all-vanadium RFB chemistry employing V²⁺/V³⁺ and VO₂⁺/VO²⁺ couples, there is still no consensus about the reaction mechanism, electrode active sites, and rate-determining step. Herein, we perform Car–Parrinello molecular dynamics-based metadynamics simulations to unravel the mechanism of the VO₂⁺/VO²⁺ redox reaction in water at the oxygen-functionalized graphite (112̅0) edge surface serving as a representative carbon-based electrode. Our results suggest that during the battery discharge aqueous VO₂⁺/VO²⁺ species adsorb at the surface C–O groups as inner-sphere complexes, exhibiting faster adsorption/desorption kinetics than V²⁺/V³⁺, at least at low vanadium concentrations considered in our study. We find that this is because (i) VO₂⁺/VO²⁺ conversion does not involve the slow transfer of an oxygen atom, (ii) protonation of VO₂⁺ is spontaneous and coupled to interfacial electron transfer in acidic conditions to enable VO²⁺ formation, and (iii) V³⁺ found to be strongly bound to oxygen groups of the graphite surface features unfavorable desorption kinetics. In contrast, the reverse process taking place upon charging is expected to be more sluggish for the VO₂⁺/VO²⁺ redox couple because of both unfavorable deprotonation of the VO²⁺ water ligands and adsorption/desorption kinetics

Iron Dissolution from Goethite (α-FeOOH) Surfaces in Water by Ab Initio Enhanced Free-Energy Simulations

Dissolution of redox-active metal oxides plays a key role in a variety of phenomena, including (photo)­electrocatalysis, degradation of battery materials, corrosion of metal oxides, and biogeochemical cycling of metals in natural environments. Despite its widespread significance, mechanisms of metal-oxide dissolution remain poorly understood on the atomistic level. This study is aimed at elucidating the long-standing problem of iron dissolution from Fe­(III)-oxide, a complex process involving coupled hydrolysis, surface protonation, electron transfer, and metal–oxygen bond cleavage. We examine the case of goethite (α-FeOOH), a representative phase, bearing structural similarities with many other metal (hydr)­oxides. By employing quantum molecular dynamics simulations (metadynamics combined with the Blue Moon ensemble approach), we unveil the mechanistic pathways and rates of both nonreductive and reductive dissolution of iron from the (110) and (021) goethite facets in aqueous solutions at room temperature. Our simulations reveal the interplay between concerted internal (structural) and external (from solution) protonations as essential for breaking Fe–O bonds as well as for stabilizing intermediate configurations of dissolving Fe. We demonstrate specifically how Fe­(III) reduction to Fe­(II) yields higher dissolution rates than the proton-mediated pathway, whereas the most rapid dissolution is expected for these two processes combined, in agreement with experiments

Actinide Dioxides in Water: Interactions at the Interface

A comprehensive understanding of chemical interactions between water and actinide dioxide surfaces is critical for safe operation and storage of nuclear fuels. Despite substantial previous research, understanding the nature of these interactions remains incomplete. In this work, we combine accurate calorimetric measurements with first-principles computational studies to characterize surface energies and adsorption enthalpies of water on two fluorite-structured compounds, ThO₂ and CeO₂, that are relevant for understanding the behavior of water on actinide oxide surfaces more generally. We determine coverage-dependent adsorption enthalpies and demonstrate a mixed molecular and dissociative structure for the first hydration layer. The results show a correlation between the magnitude of the anhydrous surface energy and the water adsorption enthalpy. Further, they suggest a structural model featuring one adsorbed water molecule per one surface cation on the most stable facet that is expected to be a common structural signature of water adsorbed on actinide dioxide compounds

Mechanisms of Degradation of Na₂Ni[Fe(CN)₆] Functional Electrodes in Aqueous Media: A Combined Theoretical and Experimental Study

Prussian blue analogues (PBAs) are versatile functional materials with numerous applications ranging from electrocatalysis and batteries to sensors and electrochromic devices. Their electrochemical performance involving long-term cycling stability strongly depends on the electrolyte composition. In this work, we use density functional theory calculations and experiments to elucidate the mechanisms of degradation of model Na2Ni[Fe(CN)6] functional electrodes in aqueous electrolytes. Next to the solution pH and cation concentration, we identify anion adsorption as a major driving force for electrode dissolution. Notably, the nature of adsorbed anions can control the mass and charge transfer mechanisms during metal cation intercalation as well as the electrode degradation rate. We find that weakly adsorbing anions, such as NO3–, impede the degradation, while strongly adsorbing anions, such as SO42–, accelerate it. The results of this study provide practical guidelines for electrolyte optimization and can likely be extrapolated to the whole family of PBAs operating in aqueous media