Michael Busch

Understanding the structure and chemical bonding in water dimers is central to the study of many (photo-)electrochemical oxidation reactions. Two structures of the water dimer radical cation, namely, proton-transfer and hemi-bonded structures, have been suggested using density functional theory (DFT) and coupled cluster singles, doubles, and perturbative triples [CCSD(T)]. Both structures are identified by us as local minima, and their relative stability strongly depends on the level of theory. The exact exchange correlates linearly to the energy difference between both local minima. DFT functionals with less than 20 percent exact exchange predict the hemi-bonded structure to be more stable, while more than 20 percent of the exact exchange stabilizes the proton-transfer structure. The latter structure is also confirmed by CCSD(T) benchmark computations. These computations, furthermore, indicate that the oxidized water dimer consists of a hydronium cation (H3O+) and an HO· radical. These results are reproduced by DFT functionals with more than 50% of exact exchange (BHandH, M06-2X, and M06-HF). The transition barrier for the interconversion from the proton-transfer to the hemi-bonded structure is 0.6 eV, while the reverse reaction has a barrier of 0.1 eV.

CO 2 reduction is typically performed at neutral pH. Under these conditions CO 2 is in equilibrium with H 2 CO 3 , HCO 3 − and CO 3 2− . However, despite their presence so far most studies solely focus on the contribution of CO 2 while carbonate species as alternative reactants are generally neglected. Using density functional theory (DFT) modelling we explore the possible contribution of these carbonate species to the overall CO 2 reduction activity for a Fe porphyrin model catalyst. Considering only reaction Gibbs free energies, we find the reduction of carbonic acid (H 2 CO 3 ), bicarbonate (HCO 3 − ) and CO 2 to be equally likely. However, owing to a very high activation barrier for the initial adsorption of CO 2 onto the catalyst, bicarbonate and carbonic acid reduction are found to be several orders of magnitude faster. These data are used to model the pH dependence of the reaction rates of the different reactants. These results confirm that carbonic acid and bicarbonate are the most likely reactants independent of the pH and reactor setup.