Photochemical Hydrogen Evolution at Metal Centers Probed with Hydrated Aluminium Cations, Al⁺(H₂O)_n, n=1–10

Abstract

Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n = 2 reveal that solvation shifts the intense 3s-3p excitations of Al + into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S 1 to T 2 takes place, and photochemical hydrogen formation starts on the T 2 surface, which passes through a conical intersection, changing to T 1 . On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n = 8. Clusters with n = 9 or 10 occur exclusively as HAlOH + (H 2 O) n-1 and are transparent in the investigated energy range. For n = 4-8, a mixture of Al + (H 2 O) n and HAlOH + (H 2 O) n-1 is present in the experiment.

Infrared Multiple Photon Dissociation Spectroscopy Confirms Reversible Water Activation in Mn+(H2O)n, n ≤ 8

“…28 This water activation occurs in a similar size range for Al + (H 2 O) n and V + (H 2 O) n , for which we recently probed the transition from M + (H 2 O) n to HMOH + (H 2 O) n -1 by ultraviolet–visible (UV–vis) spectroscopy. 29 , 30 …”

Thermally Activated vs. Photochemical Hydrogen Evolution Reactions–A Tale of Three Metals

“…Details on data analysis and derivation of photodissociation cross sections are found in the original publications. [36][37][38] Accurate computational approaches are indispensable for the description of the electronic structure and reactivity of the clusters. For ground state and IR calculations, density functional theory (DFT) and coupled cluster (CC) approaches are among the most popular ones.…”

“…We recently revisited this system and were able to measure photodissociation spectra down to 225 nm (up to 5.51 eV) for [Al(H 2 O) n ] + , n = 1-10. [37] For n = 1, only one data point at 225 nm was obtained, for which we identified the photochemical loss of a hydrogen atom. For n = 2-8, strong absorption is observed starting around 4.5 eV, with cross sections of the order of 10 À 17 cm 2 .…”

“…Reproduced under CC-BY license from Ref. [37] © 2021 The Authors. long water chains are energetically competitive, and are populated under the conditions of the experiment.…”

Size-Specific Infrared Spectroscopic Study of the Reactions between Water Molecules and Neutral Vanadium Dimer: Evidence for Water Splitting

“…Water splitting is a renewable and environmentally friendly energy source. − Since surface-supported single atoms/clusters have been demonstrated to boost the catalytic performance in many applications, , studies of the reactions between water molecules and isolated metal atoms/clusters would help to unravel the microscopic mechanisms of water splitting at the molecular level. , Pioneering spectroscopic studies of the reactions of water molecules with ionic metals have been carried out, based on the advantages of straightforward detection and size selection. − The conventional products are the M + (H 2 O) n solvation complexes, in which the water molecules are weakly bound to the ionic metals . A few insertion complexes of HMOH + (H 2 O) n are accessible for M = Al, V, Mn. − …”