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 S1 to T2 takes place, and photochemical hydrogen formation starts on the T2 surface, which passes through a conical intersection, changing to T1. 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+(H2O) n‐1 and are transparent in the investigated energy range. For n=4–8, a mixture of Al+(H2O) n and HAlOH+(H2O) n‐1 is present in the experiment.
Highlights
Metal centers with their redox capabilities play a key role in hydrogen evolution, for example, as catalysts in polymer electrolyte membrane electrolyzers[1] or photo-electrochemical water splitting.[2]
Quantum chemistry predicts transitions of Al+ (H2O) and the inserted HAlOH+ only above 6.0 eV, but the rising flank of the first Al+(H2O) band, which peaks at 6.3 eV, may be responsible for the experimentally observed data points: Simplified spectrum modeling using linearized reflection principle predicts the intensity of 1 × 10À 18 cm2 at 5.6 eV for Al+(H2O), Ia, in good agreement with the experiment
All three molecular orbitals derived from the Al+ [Ne] 3s3p configuration have lower excitation energy compared to the atomic ion (7.42 eV),[47] which can be explained by the destabilization of the Al 3s orbital in the Al+(H2O) ground state due to an anti-bonding interaction with the water molecule
Summary
Metal centers with their redox capabilities play a key role in hydrogen evolution, for example, as catalysts in polymer electrolyte membrane electrolyzers[1] or photo-electrochemical water splitting.[2] Evidence for naturally occurring Al+(H2O)n, n 3, was found in mass spectrometry of the ionosphere.[3] Hydrated metal ions in the gas phase are ideal model systems to study the elementary steps of formation of atomic or molecular hydrogen, in particular the relevant charge transfer processes.[4,5] In many cases, the unusual + I oxidation state can be obtained in water clusters, sometimes within a limited size regime, resulting in species of the composition M+(H2O)n, with, for example, M=Mg, Ca, Al, V, Mn, Fe, Co.[6,7,8,9,10,11,12] Some of these systems, like M=Mg, Al, V, are known to eliminate atomic or molecular hydrogen upon heating by room temperature blackbody radiation,[8,9,13,14,15] always with very specific dependence on cluster size. In the case of Al+(H2O)n, H/D exchange experiments with
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
