Modeling the Effect of the Electrolyte on Standard Reduction Potentials of Polyoxometalates

The electrochemistry of transition metal oxide systems is gaining much interest in the context of energy storage. Yet, predicting the redox behavior of such systems remains very challenging for computational chemistry. In this work, we examined instead a computational strategy for related nano-sized molecular transition metal polyoxoanions, as such polyoxometalates (POMs) can be treated at manageable computational costs. As an example, we addressed the effects of an aqueous electrolyte at the atomic scale for estimating the standard reduction potentials Mn­(IV/III) and Mn­(III/II) of the tri-Mn-substituted W-based Keggin ion. The electrolyte model involves explicitly solvated Li⁺ counterions and accounts for the fluctuating aqueous medium, described in first-principles molecular dynamics simulations. After equilibration, the systems showed different local structures of the electrolyte around the POM, depending on the oxidation state of the Mn centers. These varying local structures affect the Mn reduction potentials differently for the redox couples under study. Hybrid DFT calculations yield rather accurate absolute redox potentials for Mn, in good agreement with experiment, i.e., within 0.1 eV. This is in strong contrast to analogous results from an implicit solvation model, where redox potentials were notably underestimated, whereas models with counterions added, but without explicit solvation, notably overestimated the redox potentials, by up to 1 eV. Only by taking into account the full atomistic structure of the multicomponent system, solute, and surrounding electrolyte is one able to estimate the electrochemical properties of nanostructured transition metal oxide systems with acceptable accuracy

Metal-Supported Metal Clusters: A Density Functional Study of Pt₃ and Pd₃

The geometric, energetic, and electronic properties of metal-supported metal clusters were examined computationally by applying a method based on density functional theory to model systems. To explore lattice strain effects on these systems, Pt₃ and Pd₃ clusters adsorbed on Au(111) and Cu(111) were studied. The geometric and electronic properties of these small metal-supported clusters were found to differ from the corresponding overlayer system. The d-band centers of the adsorbed clusters on Au(111) were calculated to be very similar to those of the adsorbed clusters on Cu(111), indicating that the support has only a minor effect on the d-band center of the adsorbed clusters. In contrast, the gap between the local d-band centers of Pt and Pd overlayers and the Fermi energy is reduced from −2.41 eV (Pt) and −2.10 eV (Pd) on Cu(111) to −1.41 eV (Pt) and −1.22 eV (Pd) on Au(111)

Does the Preferred Mechanism of a Catalytic Transformation Depend on the Density Functional? Ethylene Hydrosilylation by a Metal Complex as a Case Study

For an extended set of density functionals (BP86, BLYP, B3LYP, B3PW91, PBE, PBE0, mPWPW, MPW1K, M06-L, M06, MPW3LYP, TPSS) we explored the key steps of four mechanisms of ethylene hydrosilylation (Glaser–Tilley, Chalk–Harrod, modified Chalk–Harrod, and σ-bond metathesis) by a Rh­(I) catalyst, previously studied at the B3LYP level. The Chalk–Harrod and the σ-bond metathesis mechanisms were determined to be preferred for all these functionals. The preference among these two mechanisms and the corresponding highest relative barriers (6.6–11.8 kcal·mol^–1) depend on the functional. To a certain extent, the differences in the description of the reaction can be traced back to the correlation part of the functionals. For the most notable functional-dependent barrier, similar values were calculated when the LYP correlation functional and the functionals M06-L and M06 were employed, but distinctively different values resulted from the functionals PBE, PW91, and TPSS

Toward a Reliable Energetics of Adsorption at Solvated Mineral Surfaces: A Computational Study of Uranyl(VI) on 2:1 Clay Minerals

We developed an efficient computational protocol for studying adsorption at solvated solid surfaces by a quantum mechanical method. We combine first-principles molecular dynamics at low temperature with simulated annealing and optimization steps to allow relaxation of the solvent structure without strongly perturbing the geometry of adsorption complexes. On the example of uranyl­(VI) adsorption at the (110) edge surface of smectite minerals we show by density functional calculations using periodic slab models that our approach yields more reliable energies than direct optimization. In this way we were able to identify the preferred adsorption complex at this smectite surface. By decomposing the complex formation energies into deprotonation energies of the surface and adsorption energies as well as by a charge analysis of the adsorption sites, we rationalize this result as well as the composition and the structures of less stable adsorbed species. Our computational results are compatible with available experimental structural data of uranyl­(VI), adsorbed at montmorillonite

Monovalent Cation-Exchanged Natrolites and Their Behavior under Pressure. A Computational Study

Recently natrolite was shown to be an auxetic material that is able to exchange extra-framework Na⁺ cations with other mono-, di-, and trivalent cations. Under pressure up to several GPa, these cation-exchanged natrolites undergo superhydration and/or phase transformations in the cation–water arrangement. Using density functional theory we studied in silico the ion exchange in natrolites. First we optimized the structures of Li⁺-, Na⁺-, K⁺-, Rb⁺-, and Cs⁺-exchanged natrolites at ambient conditions and compared the resulting lattice energies to that of the hypothetical H-form of natrolite. Of all natrolites, the smallest formal exchange energy was found for Na-NAT, in agreement with the natural occurrence of this material. Then we modeled the effect of pressure on Na-, Rb-, and Cs-natrolites, addressing (<i>i</i>) the incorporation of water ligands into the zeolite framework, accompanied by an increase in volume; and (<i>ii</i>) the changes in the cation–water arrangement within the zeolite pores. The computational models reproduce reasonably well the critical pressure, at which these phenomena occur, and, in the case of Cs-NAT, point toward a cation displacement model for its structural transition under pressure

O₂ Activation and Catalytic Alcohol Oxidation by Re Complexes with Redox-Active Ligands: A DFT Study of Mechanism

As a contribution to understanding catalysis by transition metal complexes with redox-active ligands (here: catecholate – cat), we report a computational study on the mechanism of a catalytic cycle where (i) O₂ is activated at the metal center of the catecholate complex [Re^V(O)­(cat)₂]⁻ to yield [Re^VII(O)₂(cat)₂]⁻, which (ii) subsequently is applied to oxidize alcohols. We were able to identify the steps where the redox-active ligands played a crucial role as e^– buffer. For O₂ homolysis, a series of sequential 1e^– steps leads to superoxo and bimetallic intermediates, followed by facile cleavage of the bimetallic peroxo O–O linkage. The <i>trans–cis</i> isomerization of <i>trans</i>-[Re^V(O)­(cat)₂]⁻ is the crucial step of O₂ activation, with an absolute free energy barrier of 16.8 kcal mol^–1 in methanol. Due to the ionic character of intermediates, all reaction barriers of O₂ activation are significantly lowered in a polar solvent, thus rendering O₂ homolysis kinetically accessible. With computational results for the activation barriers of all elementary steps as well as the calculated solvent effects, we are able to rationalize all pertinent experimental findings. For catalytic alcohol oxidation, we propose a novel cooperative mechanism that involves two units of the metal complexes, ruling out the reaction via a seven-coordinated active oxidant, as previously hypothesized. We present in detail calculated energies and barriers for the reaction steps of the oxidation of methanol as model alcohol as well as the energetics of crucial steps of the experimentally studied oxidation of benzyl alcohol, both transformations for methanol as solvent

O₂ Activation and Catalytic Alcohol Oxidation by Re Complexes with Redox-Active Ligands: A DFT Study of Mechanism

As a contribution to understanding catalysis by transition metal complexes with redox-active ligands (here: catecholate – cat), we report a computational study on the mechanism of a catalytic cycle where (i) O₂ is activated at the metal center of the catecholate complex [Re^V(O)­(cat)₂]⁻ to yield [Re^VII(O)₂(cat)₂]⁻, which (ii) subsequently is applied to oxidize alcohols. We were able to identify the steps where the redox-active ligands played a crucial role as e^– buffer. For O₂ homolysis, a series of sequential 1e^– steps leads to superoxo and bimetallic intermediates, followed by facile cleavage of the bimetallic peroxo O–O linkage. The <i>trans–cis</i> isomerization of <i>trans</i>-[Re^V(O)­(cat)₂]⁻ is the crucial step of O₂ activation, with an absolute free energy barrier of 16.8 kcal mol^–1 in methanol. Due to the ionic character of intermediates, all reaction barriers of O₂ activation are significantly lowered in a polar solvent, thus rendering O₂ homolysis kinetically accessible. With computational results for the activation barriers of all elementary steps as well as the calculated solvent effects, we are able to rationalize all pertinent experimental findings. For catalytic alcohol oxidation, we propose a novel cooperative mechanism that involves two units of the metal complexes, ruling out the reaction via a seven-coordinated active oxidant, as previously hypothesized. We present in detail calculated energies and barriers for the reaction steps of the oxidation of methanol as model alcohol as well as the energetics of crucial steps of the experimentally studied oxidation of benzyl alcohol, both transformations for methanol as solvent

Does the Preferred Mechanism of a Catalytic Transformation Depend on the Density Functional? Ethylene Hydrosilylation by a Metal Complex as a Case Study

For an extended set of density functionals (BP86, BLYP, B3LYP, B3PW91, PBE, PBE0, mPWPW, MPW1K, M06-L, M06, MPW3LYP, TPSS) we explored the key steps of four mechanisms of ethylene hydrosilylation (Glaser–Tilley, Chalk–Harrod, modified Chalk–Harrod, and σ-bond metathesis) by a Rh­(I) catalyst, previously studied at the B3LYP level. The Chalk–Harrod and the σ-bond metathesis mechanisms were determined to be preferred for all these functionals. The preference among these two mechanisms and the corresponding highest relative barriers (6.6–11.8 kcal·mol^–1) depend on the functional. To a certain extent, the differences in the description of the reaction can be traced back to the correlation part of the functionals. For the most notable functional-dependent barrier, similar values were calculated when the LYP correlation functional and the functionals M06-L and M06 were employed, but distinctively different values resulted from the functionals PBE, PW91, and TPSS

Monovalent Cation-Exchanged Natrolites and Their Behavior under Pressure. A Computational Study

Recently natrolite was shown to be an auxetic material that is able to exchange extra-framework Na⁺ cations with other mono-, di-, and trivalent cations. Under pressure up to several GPa, these cation-exchanged natrolites undergo superhydration and/or phase transformations in the cation–water arrangement. Using density functional theory we studied in silico the ion exchange in natrolites. First we optimized the structures of Li⁺-, Na⁺-, K⁺-, Rb⁺-, and Cs⁺-exchanged natrolites at ambient conditions and compared the resulting lattice energies to that of the hypothetical H-form of natrolite. Of all natrolites, the smallest formal exchange energy was found for Na-NAT, in agreement with the natural occurrence of this material. Then we modeled the effect of pressure on Na-, Rb-, and Cs-natrolites, addressing (<i>i</i>) the incorporation of water ligands into the zeolite framework, accompanied by an increase in volume; and (<i>ii</i>) the changes in the cation–water arrangement within the zeolite pores. The computational models reproduce reasonably well the critical pressure, at which these phenomena occur, and, in the case of Cs-NAT, point toward a cation displacement model for its structural transition under pressure

Modeling Polaron-Coupled Li Cation Diffusion in V₂O₅ Cathode Material

The transport of intercalated Li cations in oxide materials comprises two aspects, ion diffusion and migration of an associated small polaron. We examined computationally these two aspects of Li transport in vanadium pentoxide (V₂O₅) cathode material in a consistent fashion, using a DFT+U approach. Exploring various migration scenarios at low Li concentrations, we determined barriers of ∼0.3 eV, mostly due to polaron migration. In consequence, intercalating Li atoms, at low concentrations, migrate in the interlayer region of V₂O₅ as quasi-particles where Li cations remain closely associated with their valence electrons, where a small polaron structure forms around the reduced vanadium center