Constrained-Orbital Density Functional Theory. Computational Method and Applications to Surface Chemical Processes

Simulating Electron Transfer Reactions in Solution: Radical-Polar Crossover

Single-electron transfer (SET) promotes a wide variety of interesting chemical transformations, but modeling of SET requires a careful treatment of electronic and solvent effects to give meaningful insight. Therefore, a combined constrained density functional theory and molecular mechanics (CDFT/MM) tool is introduced specifically for SET-initiated reactions. Mechanisms for two radical-polar crossover reactions involving the organic electron donors tetrakis(dimethylamino)ethylene (TDAE) and tetrathiafulvalene (TTF) were studied with the new tool. An unexpected tertiary radical intermediate within the TDAE system was identified, relationships between kinetics and substitution in the TTF system were explained, and the impact of the solvent environments on the TDAE and TTF reactions were examined. The results highlight the need for including solvent dynamics when quantifying SET kinetics and thermodynamics, as a free energy difference of >20 kcal/mol was observed. Overall, the new method informs mechanistic analysis of SET-initiated reactions and therefore is envisioned to be useful for studying reactions in the condensed phase.

Systematic Characterization of Electronic Metal-Support Interactions in Ceria-Supported Pt Particles

Electronic metal-support interactions affect the chemical and catalytic properties of metal particles supported on reducible metal oxides, but their characterization is challenging due to the complexity of the electronic structure of these systems. These interactions often involve different states with varying numbers and positions of strongly correlated d or f electrons and the corresponding polarons. In this work, we present an approach to characterize electronic metal-support interactions by means of computationally efficient density functional calculations within the projector augmented wave method. We describe Ce3+ cations with potentials that include a Ce4f electron in the frozen core, overcoming prevalent convergence and 4f electron localization issues. We systematically explore the stability and chemical properties of different electronic states for a Pt8/CeO2(111) model system, revealing the predominant effect of electronic metal-support interactions on Pt atoms located directly at the metal-oxide interface. Adsorption energies and the reactivity of these interface Pt atoms vary significantly upon donation of electrons to the oxide support, pointing to a strategy to selectively activate interfacial sites of metal particles supported on reducible metal oxides.

An Efficient Multilayer Approach to Model DNA-Based Nanobiosensors

In this work, we present a full computational protocol to successfully obtain the one-electron reduction potential of nanobiosensors based on a self-assembled monolayer of DNA nucleobases linked to a gold substrate. The model is able to account for conformational sampling and environmental effects at a quantum mechanical (QM) level efficiently, by combining molecular mechanics (MM) molecular dynamics and multilayer QM/MM/continuum calculations within the framework of Marcus theory. The theoretical model shows that a guanine-based biosensor is more prone to be oxidized than the isolated nucleobase in water due to the electrostatic interactions between the assembled guanine molecules. In addition, the redox properties of the biosensor can be tuned by modifying the nature of the linker that anchor the nucleobases to the metal support.

Oxidation-State Constrained Density Functional Theory for the Study of Electron-Transfer Reactions

We propose a new constrained density functional theory (CDFT) approach which directly controls the oxidation state of the target atoms. In this new approach called oxidation-state constrained density functional theory (OS-CDFT), the eigenvalues of the occupation matrix obtained from projecting the Kohn-Sham wave functions onto the valence orbitals are constrained to obtain the desired oxidation states. This approach is particularly useful to study electron transfer problems in transition metal-containing systems due to the multivalent nature of the transition metal ions. The calculation of the forces on the ions and of the coupling constant was implemented under the OS-CDFT scheme to allow efficient and accurate study of electron transfer reactions. We demonstrated the application of this method in the study of different electron transfer reactions including the aqueous ferrous-ferric self-exchange reaction, polaron hopping in the TiO₂ anatase and bismuth vanadate, and photoexcited electron transfer in the sapphire.

Energy Decomposition Analysis for Metal Surface-Adsorbate Interactions by Block Localized Wave Functions

The energy decomposition analysis based on block localized wave functions (BLW-EDA) allows one to gain physical insight into the nature of chemical bonding, decomposing the interaction energy in (1) a "frozen" term, accounting for the attraction due to electrostatic and dispersion interactions, modulated by Pauli repulsion, (2) the variationally assessed polarization energy, and (3) the charge transfer. This method has so far been applied to gas- and condensed-phase molecular systems. However, its standard version is not compatible with fractionally occupied orbitals (i.e., electronic smearing) and, as a consequence, cannot be applied to metallic surfaces. In this work, we propose a simple and practical extension of BLW-EDA to fractionally occupied orbitals, termed Ensemble BLW-EDA. As illustrative examples, we have applied the developed method to analyze the nature of the interaction of various adsorbates on Pt(111), ranging from physisorbed water to strongly chemisorbed ethylene. Our results show that polarization and charge transfer both contribute significantly at the adsorption minimum for all studied systems. The energy decomposition analysis provides details with respect to competing adsorption sites (e.g., CO on atop vs hollow sites) and elucidates the respective importance of polarization and charge transfer for the increased adsorption energy of H₂S compared to H₂O. Our development will enable a deeper understanding of the impact of charge transfer on catalytic processes in general.

Addressing global uncertainty and sensitivity in first-principles based microkinetic models by an adaptive sparse grid approach

In the last decade, first-principles-based microkinetic modeling has been developed into an important tool for a mechanistic understanding of heterogeneous catalysis. A commonly known, but hitherto barely analyzed issue in this kind of modeling is the presence of sizable errors from the use of approximate Density Functional Theory (DFT). We here address the propagation of these errors to the catalytic turnover frequency (TOF) by global sensitivity and uncertainty analysis. Both analyses require the numerical quadrature of high-dimensional integrals. To achieve this efficiently, we utilize and extend an adaptive sparse grid approach and exploit the confinement of the strongly non-linear behavior of the TOF to local regions of the parameter space. We demonstrate the methodology on a model of the oxygen evolution reaction at the Co₃O₄ (110)-A surface, using a maximum entropy error model that imposes nothing but reasonable bounds on the errors. For this setting, the DFT errors lead to an absolute uncertainty of several orders of magnitude in the TOF. We nevertheless find that it is still possible to draw conclusions from such uncertain models about the atomistic aspects controlling the reactivity. A comparison with derivative-based local sensitivity analysis instead reveals that this more established approach provides incomplete information. Since the adaptive sparse grids allow for the evaluation of the integrals with only a modest number of function evaluations, this approach opens the way for a global sensitivity analysis of more complex models, for instance, models based on kinetic Monte Carlo simulations.

Lewis-Brønsted Acid Pairs in Ga/H-ZSM-5 To Catalyze Dehydrogenation of Light Alkanes

The active sites for propane dehydrogenation in Ga/H-ZSM-5 with moderate concentrations of tetrahedral aluminum in the lattice were identified to be Lewis-Brønsted acid pairs. With increasing availability, Ga⁺ and Brønsted acid site concentrations changed inversely, as protons of Brønsted acid sites were exchanged with Ga⁺. At a Ga/Al ratio of 1/2, the rate of propane dehydrogenation was 2 orders of magnitude higher than with the parent H-ZSM-5, highlighting the extraordinary activity of the Lewis-Brønsted acid pairs. Density functional theory calculations relate the high activity to a bifunctional mechanism that proceeds via heterolytic activation of the propane C-H bond followed by a monomolecular elimination of H₂ and desorption of propene.