Coordination Inversion of the Tetrahedrally Coordinated Ru_4f Surface Complex on RuO₂(100) and Its Decisive Role in the Anodic Corrosion Process

Abstract

The c(2 × 2) reconstruction of RuO 2 (100) leads to unusual and flexible surface functionality in the form of a tetrahedrally coordinated Ru 4f surface complex with distinct chemical properties that are important for the anodic activity and stability in water electrolysis. We employ first-principles methods based on density functional theory calculations to elucidate the hybridization of this Ru 4f surface species and its rich coordination chemistry. Under oxygen evolution reaction (OER) conditions, the consecutive breaking of structural Ru 4f −O back bonds is shown to proceed via the back side attack of Ru 4f (akin to the Walden inversion) by dissociative water adsorption and the release of two proton/electron pairs. This inversion step of the coordination environment of a surface transition metal cation is considered to be the key step in the anodic dissolution process. Oxygen evolution over Ru 4f on RuO 2 (100)−c(2 × 2) competes with its dissolution. From ab initio thermodynamics, the inherent OER activity over Ru 4f is shown to be lower than that of Ru surface sites on the unreconstructed RuO 2 (100), while the dissolution propensity of the Ru 4f surface complex is higher. This finding calls into question the frequently stated hypothesis that higher activity is correlated with a lower stability of the active center.

Staged Training of Machine-Learning Potentials from Small to Large Surface Unit Cells: Efficient Global Structure Determination of the RuO2(100)-c(2 × 2) Reconstruction and (410) Vicinal

“…Indeed, {410} vicinals exhibiting the two-vertex bound Ru 4f motif, cf. Figure , also have a more pronounced share at these most O-rich conditions, supporting the suggestion of Hess and Over that these species play a role in the anodic corrosion process. Now that structural models are established for this (110)/(100) facet edge, future detailed mechanistic studies may hence develop an atomic-scale understanding of this degradation, as well as of the connection of this edge with the improved chlorine evolution reaction selectivity reported for the high-temperature, O-poor calcined RuO 2 nanoparticles of Jirkovský et al…”

“…These complexes are bound to the surface via either two or three O vertices. In particular, the two-vertex motif has recently been proposed as an intermediate species in the anodic corrosion process . Its increased occurrence in particular in the structure class that is most stable at the higher chemical potentials, Δμ O > −0.17 eV, would then point to a particular role of the (110)/(100) facet edge in the decomposition process.…”

The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni–Cr–Mo Alloys

“…This higher oxidation state complex can then be dissolved through a coordination inversion process. [ 76 ] This can explain the drastic change in surface chemistry observed for the Ni–Cr–Mo alloy at the onset of OER where Mo 4+ oxide, which is catalytically active [ 36 ] and present in the oxide film as shown in Figure 3, takes part in the OER mechanism similar to that of IrO 2 and RuO 2 and is dissolved and redeposited as MoO 3 on the surface as observed in the AP‐XPS data shown in Figure 4. This explanation is also in line with the chemical state of the dissolved Mo species, which was found to be MoO 3, as shown in Figure 5, as well as the detected chemical state in the layer of corrosion products in Figure 6, where MoO 3 was detected.…”

“…[40] IrO 2 and RuO 2 , well-studied model systems for OER, have been found to degrade through the dissolution of a metal oxide complex of a higher oxidation state, namely IrO 4 2− , IrO 3 , or RuO 4 2− . [72][73][74][75][76][77] During OER, the surface atoms of Ir or Ru oxide that take part in the reaction are further oxidized from the 4+ oxidation state to a 6+ oxidation state complex in the OER catalytic cycle. This higher oxidation state complex can then be dissolved through a coordination inversion process.…”

Hollow Nickel Nanochains Coated with Nickel-Iron Hydroxide for Oxygen Evolution

“…To address this issue, catalysts have been utilized for water splitting. The best electrocatalysts for water splitting are platinum for HER and metal oxides, such as ruthenium and iridium oxides, for the OER. , However, owing to their high cost and scarcity, these precious metal catalysts are unsuitable for mass production and commercialization.…”