Computational physical chemistry
The broad area of the research activity in the group is computational physical chemistry. The main computational tool is Density Functional Theory based Molecular Dynamics (DFTMD or "Car-Parrinello"). Reactive species and the condensed phase environment are treated at strictly the same level of theory ("all-atom"). Using this fundamental approach we study reactivity in solutions, solids and at liquid/solid interfaces computing equilibrium constants, activation energies, electronic and vibrational spectra and other physical quantities such as dielectric properties. The DFTMD code we use is CP2K(http://www.cp2k.org) and occasionally also Quantum Espresso(http://www.quantum-espresso.org).
Computational electrochemistry is an application where all-atom DFTMD methods are particularly instructive and has become a major research topic for us. Our favourite model systems are the transition metal oxide electrodes used in energy conversion and storage. Proton coupled electron transfer (PCET) is a key process in these reactions. Unified treatment of oxidation/reduction and (de)protonation reactions is therefore crucial. Combining DFTMD and free energy perturbation (FTP) methods we have developed such a scheme. The central computational tool is reversible insertion of electrons and protons in periodic DFTMD model systems. Applying this method we can refer the computed redox free energies and electronic energy levels to the Standard Hydrogen Electrode (SHE) without having to introduce an additional interface with vacuum. We are using this approach to study the level alignment at electrochemical interfaces of typical transition metal oxides such as TiO2 and MnO2. The aim is to correlate the catalytic properties of the electrochemical interface to the electronic structure.
Figure: DFTMD model system of a TiO2 rutile (110) water interface at the point of zero net proton charge (PZC) consisting of 48 TiO2 units (Ti ions in yellow) and 71 H2O molecules in a periodically replicated orthorhombic MD cell (size 11.9 × 13.2 × 24.2 Å). The system has been reduced by inserting one electron and one proton. The excess electron visualized by the green spin density contours occupies a semilocalized orbital in the TiO2 slab. The proton is attached to the highlighted water molecule. The free energy of the double insertion gives the electron affinity of TiO2 (effectively the conduction band minimum) in contact with water relative to SHE.
Surface acidity and complexation
The surface of metal oxides exchanges protons with aqueous electrolytic solutions and binds also other ions. The excess surface charge is compensated by counterions in an electrical double layer. Surface acidity is therefore an important factor for the understanding of the reactivity. Since the solvation free energy of the proton is also the reference for acidity constants, the proton insertion method can be equally applied to compute the pKa of aqueous species in homogeneous solution as well as surfaces. We have used this method to compute surface pKa's of TiO2 and MnO2 and of main group oxides such as SiO2 (quartz) and Al(OH)3 (Gibbsite). Computational investigation of the properties of electric double layers and the effect on surface reactivity is a major challenge which is adressed in current research.
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