Proton Acceptor near the Active Site Lowers Dramatically the O–O Bond Formation Energy Barrier in Photocatalytic Water Splitting

Abstract

The O–O bond formation process via water nucleophilic attack represents a thermodynamic and kinetic bottleneck in photocatalytic water oxidation because of the considerably high activation free energy barrier. It is therefore of fundamental significance and yet challenging to find strategies to facilitate this reaction. The microscopic details of the photocatalytic water oxidation step involving the O–O bond formation in a catalyst–dye supramolecular complex are here elucidated by density functional theory-based Car–Parrinello molecular dynamics simulations in the presence of an extra proton acceptor. Introducing a proton acceptor group (OH–) in the hydration shell near the catalytic active site accelerates the rate-limiting O–O bond formation by inducing a cooperative event proceeding via a concerted proton-coupled electron-transfer mechanism and thus significantly lowering the activation free energy barrier. The in-depth insight provides a strategy for facilitating the photocatalytic water oxidation and for improving the efficiency of dye-sensitized photoelectrochemical cells.

Tuning the Proton‐Coupled Electron‐Transfer Rate by Ligand Modification in Catalyst–Dye Supramolecular Complexes for Photocatalytic Water Splitting

“…In particular, the proton H i diffuses into the solvent bulk via a “chain” of hydrogen‐bonded water molecules, which can be well described by the Grotthuss mechanism [25] (see Supporting Information, section S8). This mechanism has been already observed in our previous works [6,15] . The reaction coordinate d(O i ←O ii ) is then further shortened to 1.6 Å to better explore the complete free energy profile along this reaction pathway and no back reaction occurs (see Supporting Information, section S9).…”

“…An accurate description of the PCET reaction and corresponding free energy profile requires an explicit inclusion of the water environment as it is crucially involved in the reaction process [6,11,15] . Therefore, an orthorhombic box of dimensions 25.1×17.7×14.4 Å 3 with periodic boundary conditions containing the WOC‐dye solute L0‐L3 together with 162 explicit water molecules is used in the DFT‐MD simulations performed with the CPMD program [20] .…”

“…In the context of PCET reactions in artificial photosynthesis, the photocatalytic water splitting cycle in a WOC‐dye supramolecular complex [(cy)Ru II bpy(H 2 O)] 2+ ‐NDI (cy= p ‐cymene, bpy=2,2′‐bipyridine, NDI=2,6‐diethoxy‐1,4,5,8‐diimide‐naphthalene) has recently been systematically investigated in silico , providing the driving force and the energy barrier of each PCET step by DFT‐based molecular dynamics (DFT‐MD) simulations [6,14] . The computed energy barrier (Δ G *=15.9 kcal mol −1 ) and corresponding reaction rate ( k =15.7 s −1 ) confirm that the third catalytic PCET step involving the O−O bond formation is indeed the kinetic bottleneck of the entire catalytic water oxidation half‐reaction, which would increase the possibility of charge recombination and thus lower the quantum yield [6,15] …”

“…[6,14] The computed energy barrier (ΔG* = 15.9 kcal mol À 1 ) and corresponding reaction rate (k = 15.7 s À 1 ) confirm that the third catalytic PCET step involving the OÀ O bond formation is indeed the kinetic bottleneck of the entire catalytic water oxidation half-reaction, which would increase the possibility of charge recombination and thus lower the quantum yield. [6,15] In this work we explore the possibility of enhancing the rate of this critical PCET step in the WOC-dye complex [(cy)Ru II bpy (H 2 O)] 2 + À NDI by modifying the bipyridine ligand that is covalently bound to the NDI dye (see Scheme 1). Specifically, a series of alkyl groups varying in size and mass were introduced in the bpy residue near the CÀ N bond connecting the WOC and the NDI dye (L0-L3 in Scheme 1).…”

Theoretical calculation of a TiO2-based photocatalyst in the field of water splitting: A review

“…However, photoelectric emission spectroscopy data show that the O 2p energy level of water species is below the valenceband maximum (VBM) of TiO 2 [87], revealing the irrationality of the traditional four-step reaction mechanism from a thermodynamic perspective. Subsequently, through further experimental technical measurement and characterization, two new mechanisms, the redox photooxidation (RP) mechanism [88,89] and nucleophilic (NA) reaction mechanism [90,91], have been proposed in succession. In both of these mechanisms, the O atoms on the TiO 2 lattice are the active sites for trapping holes, and not the O atoms on the water molecules.…”