Theoretical evidence of the relationship established between the HO radicals and H 2 O adsorptions and the electroactivity of typical catalysts used to oxidize organic compounds

Adsorption grand potential of OH on metal oxide surfaces

CONTEXT

Describing chemical processes at solid-liquid interfaces as a function of a fixed electron chemical potential presents a challenge for electronic structure calculations and is essential for understanding electrochemical phenomena. Grand Canonical Density Functional Theory (GCDFT) allows treating solid-liquid interfaces in such a way that studying the influence of a fixed electron potential arises naturally. In this work, GCDFT is used to compute the adsorption grand potential (AGP), a key parameter for understanding and predicting the behavior of adsorbates on surfaces. We focused on the adsorption of an OH molecule on three metallic surfaces commonly used in electrochemical processes, such as the oxygen evolution reaction (OER). Our study aims to offer insights into how AGP can be used to compare adsorption strengths under different fixed electron chemical potentials, which is crucial for designing efficient electrode materials. By determining the average number of electrons self-consistently under varying chemical potentials, we showed how one can distinguish between electron acquisition and depletion during the adsorption process, offering a deeper understanding of the adsorbate-surface interactions.

METHODS

The approach used in this work employs the Kohn-Sham-Mermin formulation of the Grand Canonical Density Functional Theory. The computations were performed using the periodic open-source density functional theory software, JDFTx, with the Garrity-Bennett-Rabe-Vanderbilt library of ultrasoft pseudopotentials. Calculations were made using truncated Coulomb potentials and the auxiliary Hamiltonian method with the PBE exchange-correlation functional, along with DFT-D2 long-range dispersion corrections. The implicit solvation model CANDLE was used to describe the electrolyte with a 1 M concentration.

Hydroxyl radicals in anodic oxidation systems: generation, identification and quantification

Anodic oxidation has emerged as a promising treatment technology for the removal of a broad range of organic pollutants from wastewaters. Hydroxyl radicals are the primary species generated in anodic oxidation systems to oxidize organics. In this review, the methods of identifying hydroxyl radicals and the existing debates and misunderstandings regarding the validity of experimental results are discussed. Consideration is given to the methods of quantification of hydroxyl radicals in anodic oxidation systems with particular attention to approaches used to compare the electrochemical performance of different anodes. In addition, we describe recent progress in understanding the mechanisms of hydroxyl radical generation at the surface of most commonly used anodes and the utilization of hydroxyl radical in typical electrochemical reactors. This review shows that the key challenges facing anodic oxidation technology are related to i) the elimination of mistakes in identifying hydroxyl radicals, ii) the establishment of an effective hydroxyl radical quantification method, iii) the development of cost effective anode materials with high corrosion resistance and high electrochemical activity and iv) the optimization of electrochemical reactor design to maximise the utilization efficiency of hydroxyl radicals.

Surface morphology controls water dissociation on hydrated IrO2 nanoparticles

Iridium oxide is a highly efficient catalyst for the oxygen evolution reaction, whose large-scale application requires decreasing the metal content. This is achieved using small nanoparticles. The knowledge of the water-IrO₂ nanoparticle interface is of high importance to understand the IrO₂ behavior as electrocatalyst in aqueous solutions. In this contribution, DFT (PBE-D2) calculations and AIMD simulations on IrO₂ nanoparticle models of different sizes ((IrO₂)₃₃ and (IrO₂)₁₁₅) are performed. Results show that two key factors determine the H₂O adsorption energy and the preferred adsorption structure (molecular or dissociated water): metal coordination and hydrogen bonding with oxygen bridge atoms of the IrO₂ surface. Regarding metal coordination, and since the tetragonal distortion existing in IrO₂ is retained on the nanoparticle models, the adsorption at iridium axial vacant sites implies stronger Ir-H₂O interactions, which favors water dissociation. In contrast, Ir-H₂O interaction at equatorial vacant sites is weaker and thus the relative stability of molecular and dissociated forms becomes similar. Hydrogen bonding increases adsorption energy and favors water dissociation. Thus, tip and corner sites of the nanoparticle, with no oxygen bridge atoms nearby, exhibit the smallest adsorption energies and a preference for the molecular form. Overall, the presence of rather isolated tip and corner sites in the nanoparticle leads to lower adsorption energies and a smaller degree of water dissociation when compared with extended surfaces.

Electrocatalytic oxidation of ciprofloxacin by Co-Ce-Zr/γ-Al2O3 three-dimensional particle electrode

In this work, Co-Ce-Zr/γ-Al₂O3₃ particle electrodes were prepared for the efficient degradation of ciprofloxacin (CIP). Co-Ce-Zr/γ-Al₂O₃ particle electrodes were analyzed with a scanning electron microscope (SEM), X-Ray Diffraction (XRD), X-Ray Fluorescence Spectrometer (XRF), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDS). According to the results, significant amounts of Co₃O₄, CeO₂, and ZrO₂ were formed on the Co-Ce-Zr/γ-Al₂O₃ particle electrodes. It was shown that when the conditions of the reaction system were at pH=6, conductivity of 4 ms/cm, current of 0.2 A, initial pollutant concentration of 100 mg/L, and material dosage of 15 g, CIP could be completely degraded within 40 min, and the energy consumed in the reaction was 41.3 kWh/kg CIP. The rate of total organic carbon (TOC) removal by Co-Ce-Zr/γ-Al₂O₃ particle electrodes was recorded to be approximately 52.6%. Using a response surface methodology, we explored the optimal operating conditions. At the same time, we also explored the influence of inorganic anions in water and actual water medium on the rate of CIP removal. In addition, the ESR data proved that the main active substance in the reaction system was ·OH. The degradation intermediates were investigated, and the possible mechanism was proposed. Thus, this research provided a new solution for the treatment of antibiotic-containing wastewater.

Electronic structure behavior of PbO2, IrO2, and SnO2 metal oxide surfaces (110) with dissociatively adsorbed water molecules as a function of the chemical potential

A detailed analysis of the electronic structure of three different electrochemical interfaces as a function of the chemical potential (μ) is performed using the grand canonical density functional theory in the joint density functional theory formulation. Changes in the average number of electrons and the density of states are also described. The evaluation of the global softness, which measures the tendency of the system to gain or lose electrons, is straightforward under this formalism. The observed behavior of these quantities depends on the electronic nature of the electrochemical interfaces.

Water Adsorption on MO2 (M = Ti, Ru, and Ir) Surfaces. Importance of Octahedral Distortion and Cooperative Effects

Understanding metal oxide MO₂ (M = Ti, Ru, and Ir)-water interfaces is essential to assess the catalytic behavior of these materials. The present study analyzes the H₂O-MO₂ interactions at the most abundant (110) and (011) surfaces, at two different water coverages: isolated water molecules and full monolayer, by means of Perdew-Burke-Ernzerhof-D2 static calculations and ab initio molecular dynamics (AIMD) simulations. Results indicate that adsorption preferably occurs in its molecular form on (110)-TiO₂ and in its dissociative form on (110)-RuO₂ and (110)-IrO₂. The opposite trend is observed at the (011) facet. This different behavior is related to the kind of octahedral distortion observed in the bulk of these materials (tetragonal elongation for TiO₂ and tetragonal compression for RuO₂ and IrO₂) and to the different nature of the vacant sites created, axial on (110) and equatorial on (011). For the monolayer, additional effects such as cooperative H-bond interactions and cooperative adsorption come into play in determining the degree of deprotonation. For TiO₂, AIMD indicates that the water monolayer is fully undissociated at both (110) and (011) surfaces, whereas for RuO₂, water monolayer exhibits a 50% dissociation, the formation of H₃O₂ ^- motifs being essential. Finally, on (110)-IrO₂, the main monolayer configuration is the fully dissociated one, whereas on (011)-IrO₂, it exhibits a degree of dissociation that ranges between 50 and 75%. Overall, the present study shows that the degree of water dissociation results from a delicate balance between the H₂O-MO₂ intrinsic interaction and cooperative hydrogen bonding and adsorption effects.

Parametric Mathematical Modelling of Cristal Violet Dye Electrochemical Oxidation Using a Flow Electrochemical Reactor with BDD and DSA Anodes in Sulfate Media

An important issue in electrochemical oxidations of pollutant compounds, like organic dyes, is identifying a suitable correlation between operational conditions and electrochemical process performance. In such sense, this work deals with the parametric modelling of direct electrochemical incineration of crystal violet (CV) dye in a FM01-LC flow electrochemical reactor with a plastic spacer configuration using boron doped diamond (BDD) and dimensionally stable (IrO2 and IrO2-SnO2-Sb2O5) anode plates. Mathematical model takes into account the fluid dynamics effects by the use of FM01-LC reactor considering mass transport rate of organic compound (R) from bulk solution to electrode surface, characterized by a dispersion coefficient and Pe number. The effect of strong oxidants produced in the electrode surface can be neglected since the characteristic time constant reaction of pollutants with such oxidants is lower than those describing the diffusion of organic compound to the electrode surface. Model parameters were estimated throughout a fitting method of the experimental data. The model proposed here predicted a 99.7 removal percentage of CV with boron doped diamond and IrO2-SnO2-Sb2O5 anodes obtained experimentally, meanwhile a 79 % removal with the IrO2 anode was reached at Re = 2204 during an electrolysis time of 7200 s for both cases. In the case of IrO2 anodes, complex interactions between hydroxyl-radical and electrode surface provokes an intermediate kinetic process, with an effectiveness factor of 0.59. When BDD and IrO2-SnO2-Sb2O5 anodes were used, the removal process mediated by hydroxyl-radicals absorbed in electrode surface was fully limited by mass transport.