How one-photon can induce water splitting into hydrogen peroxide and hydrogen by aluminum porphyrins. Rationale of the thermodynamics

Abstract

An alternative approach of two-electron water splitting into hydrogen peroxide and hydrogen by one-photon was developed to bypass the photon-flux density bottleneck in artificial photosynthesis catalysed by molecular catalysts. Here we report how one-photon can induce the water splitting by earth abundant aluminum porphyrins and how the thermodynamics is rationalized. A simple energy correlation between the input energy of one-photon excitation of aluminum porphyrin species (E₀₀ = 2.03 eV) and output energy from the splitting of two water molecules into hydrogen peroxide and hydrogen (3.54 eV) reveals a requirement for additional energy from the surrounding heat bath to drive the catalytic cycle. We have estimated the activation energies for both the two-electron oxidation of water and hydrogen evolution by Arrhenius analysis in the temperature range of 295–346 K. The activation energy for the two-electron oxidation of water catalysed by AlTMPyP was found to be pH dependent (pH 7.0, 50.6 kJ mol⁻¹ to pH 12.5, 26.5 kJ mol⁻¹), which reflects that the reaction is driven by both water and hydroxide ions as the substrates. Under the pH 12.5 conditions the thermodynamic correlation was well rationalized by comparing the total energy input with the energy stored in the reaction forming H₂O₂ and H₂.