Molecular-Orbital-Based Verification of Effective Water Photo Splitting to HOOH As a Precursor of Oxygen and Hydrogen on Pt-Loaded TiO2
Authors: Yanagida Shozo , Yanagisawa Susumu , Yamashita Koichi , Jono Ryota , Segawa Hiroshi 
Published: February 2020

Density functional theory based molecular modeling (DFT/MM) (B3LYP/6-31G*) of molecular complexes that are induced by van der Waals and Coulomb interaction lead us to theoretical understandings of physical and chemical phenomena especially when they are effective and efficient in electronic function. Recently, DFT/MM verified that in efficient water splitting in photoelectrochemical TiO2-based cells, electron-transfer oxidation of water yields HOOH on UV-light irradiated and biased-TiO2 anode.1)

With regards to water photo splitting using Pt-loaded anatase TiO2 (Pt/TiO2), Arakawa and Sayama reported that efficient Pt/TiO2-catalyzed water photo splitting occurs in carbonate solution, giving stoichiometric amounts of O2 and H2.2) On careful examination of their data, concentration of hydroxide ion (HO-) as source of O2 and of water molecules as source of H2 seem to balance by using slightly alkali solution (pH=11) that is buffered by carbonate ion. In addition, they proposed two photolysis mechanisms via HOOH or peroxocarbonate for oxygen formation. However, it is not cleared how HOOH forms initially. Successive two-electron transfer oxidation is hardly conceivable on Pt/TiO2 because of very low energy potential of HOMO of one-electron-oxidized water molecule.

We now verify and predict that, in Arakawa and Sayama’s photocatalytic water splitting, one-electron-transfer water oxidation leads to formation of OH radical (HO.) and then its coupling reaction to HOOH, and that exothermic platinum-mediated H2 evolution is driving force for the effective photo splitting of water.

Hydrated hydroxyl ions, e.g., HO-/H2O and HO-/(H2O)2 are considered to undergo one-electron-oxidization on TiO2. We verified on the basis of DFT/MM of their electron transfer oxidations that, all oxidations will occur endothermically, and the two-electron transfer oxidations will be hardly occurred because more energy is necessary to withdraw electron on HOMO (E=-7.2 and -8.2 eV) of respective one-electron-oxidized specie, [HO-/H2O].- and [HO-/(H2O)2].-. The DFT/MM also rationalizes larger endothermic energy as much as 5 times of those of one-electron-oxidation (Figure 1).

We extended DFT/MM to one-electron oxidation of [HO-/H2O] on anatase TiO2 model, [HO-/H2O/OHTi9O18H] (Figure 2). [HO-/H2O] interacts with [OHTi9O18H] exothermically (DE= -32.35 kcal/mol), and HOMO energy becomes lowered to HOMO=-2.23eV compared to HOMO=1.06eV of [HO-/H2O]. DFT/MM of one-electron-oxidized [HO-/H2O/OHTi9O18H].- reveals that spin (radical) locates on hydrated [HO./H2O/OH] site on Ti9O18H. Accumulation of hydroxyl radical (HO.) will yield hydrogen peroxide by coupling of HO. on photo-irradiated TiO2 surface.

It is known that sodium percarbonate is an adduct of sodium carbonate and hydrogen peroxide with formula 2Na2CO3•3H2O2.3) Carbonate-stabilized hydrogen peroxide (HOOH) undergoes disproportionation reaction to yield triplet oxygen, 3O2 and H2O via auto-electron transfer and proton transfer at hydrogen-bonded HOOH dimer, [(H2O2) 2] as depicted in Figure 3.

DFT/MM is extended to understand photo-reductive formation of H2 on Pt of Pt/TiO2. Platinum metal may prefer to less polar H2O species rather than polar ones. As shown in Figure 4, symmetrical nonpolar cyclic H2O trimer [np(H2O)3] is molecular-modeled for feasibility study of successive electron transfer reduction. All of intermediary [np(H2O)3].- and [np(H2O)3]..--, which have no hydrogen bonding, are shown with electron density. As LUMO configuration and spin density are also informative for electron-transfer reduction, LUMO configuration is shown for the starting [np(H2O)3] and spin density for others.

Figure 4 shows that reductive electron transfer to LUMO orbital, and spin density is delocalized at center of [np(H2O)3].- and [np(H2O)3]..--. Interestingly, the elongated O-H bond in SPE[np(H2O)3]..--, where oxygen atoms are fixed and calculated, is suggestive as transient state of coupling of hydrogen radical to H2. Large endothermic energy, DE=+45.94 kcal/mol and DE=+178.27 makes us understand difficulty of successive photo-reduction of water under UV light irradiance in the absence of any catalysts.

We succeeded in molecular modeling of platinum cluster as Pt6. DFT/MM is then farther extended to van der Waals complex of [np(H2O)3] with Pt6 (Figure 5). Molecular orbital-driven alignment of np(H2O)3 with Pt6 occurs exothermically (DE=-43.06 kcal/mol) and LUMO energy, -3.65eV is low enough to accept electron effectively for reduction. One-electron accepted [np(H2O)3/Pt6].- and two-electron accepted [np(H2O)3/Pt6]..-- are both calculated to form exothermically, giving DE=-77.21 and DE=-65.63 kcal/mol, respectively. Close examination of [np(H2O)3/Pt6]..-- suggests that, although spin density locates exclusively at Pt6, all hydrogen of water orients to Pt6 and HOMO energy is very positive, HOMO(1.66eV). We now understand that Pt clusters work as mediator for effective exothermic and efficient successive electron transfer for H2 evolution, and simultaneously, accelerate photocatalytic water splitting to HO. and HOOH.

References

1) S.Yanagida, S.Yanagisawa, K. Yamashita, R. Jono and H. Segawa, Molecules
2015, 20(6), 9732-9744; doi:10.3390/molecules20069732.

2) K. Sayama and H. Arakawa, J. Chem. Soc., Faraday Trans., 1997, 93(8), 1647È1654

3) Wikipedia, search “Sodium percarbonate”

Figure 1