Photochemistry and UV/vis spectroscopy of hydrated vanadium cations, V+(H2O)n, n = 1-41, a model system for photochemical hydrogen evolution.

Jakob Heller,Milan Ončák,Christian Van Der Linde,Martin K Beyer,Dominik Muß,Tobias F Pascher

doi:10.1039/d1cp02382a

Abstract

Photochemical hydrogen evolution provides fascinating perspectives for light harvesting. Hydrated metal ions in the gas phase are ideal model systems to study elementary steps of this reaction on a molecular level. Here we investigate mass-selected hydrated monovalent vanadium ions, with a hydration shell ranging from 1 to 41 water molecules, by photodissociation spectroscopy. The most intense absorption bands correspond to 3d–4p transitions, which shift to the red from n = 1 to n = 4, corresponding to the evolution of a square-planar complex. Additional water molecules no longer interact directly with the metal center, and no strong systematic shift is observed in larger clusters. Evolution of atomic and molecular hydrogen competes with loss of water molecules for all V+(H2O)n, n ≤ 12. For n ≥ 15, no absorptions are observed, which indicates that the cluster ensemble is fully converted to HVOH+(H2O)n−1. For the smallest clusters, the electronic transitions are modeled using multireference methods with spin–orbit coupling. A large number of quintet and triplet states is accessible, which explains the broad features observed in the experiment. Water loss most likely occurs after a series of intersystem crossings and internal conversions to the electronic ground state or a low-lying quintet state, while hydrogen evolution is favored in low lying triplet states.

Highlights

The hydrogen evolution reaction will play an important role in future energy systems that rely on renewable energies.[1]
This indicates that the position of the oxygen atoms is not very well defined for this cluster size and that the water molecules undergo largeamplitude motions even at low temperatures in the coordination plane of the metal center for n = 3
They exhibit intense 3d–4p transitions, which continuously red-shift upon further solvation until vanadium reaches a saturated first solvation shell with four coordinated water molecules

Summary

Introduction

The hydrogen evolution reaction will play an important role in future energy systems that rely on renewable energies.[1]. Hydrogen evolution at metal centers exhibits a fascinating variability.[3]. Hydrogen atom formation is observed in Mg+(H2O)n in the gas phase within a relatively narrow size range.[4–6]. A series of quantum chemical studies indicated that the H atom is formed via the recombination of a proton with a hydrated electron, which forms spontaneously as soon as at least six water molecules are available.[7–11]. The corresponding aluminum species, Al+(H2O)n, eliminate H2 in a thermally activated reaction,[15,16] consistent with the preferred oxidation state +III of the metal center. Hydrogen evolution takes place in two steps, insertion of Al+ into an O–H bond via a concerted proton transfer results in the HAlOH+(H2O)nÀ1 structure, from which H2 evolves via a proton-hydride recombination, again mediated by concerted proton transfer through water wires.[17–19]

Methods

Results

Conclusion