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

Abstract

Molecular processes behind hydrogen evolution reactions can be quite complex. In macroscopic electrochemical cells, it is extremely difficult to elucidate and understand their mechanism. Gas phase models, consisting of a metal ion and a small number of water molecules, provide unique opportunities to understand the reaction pathways in great detail. Hydrogen evolution in clusters consisting of a singly charged metal ion and one to on the order of 50 water molecules has been studied extensively for magnesium, aluminum and vanadium. Such clusters with around 10-20 water molecules are known to eliminate atomic or molecular hydrogen upon mild activation by room temperature blackbody radiation. Irradiation with ultraviolet light, by contrast, enables hydrogen evolution already with a single water molecule. Here, we analyze and compare the reaction mechanisms for hydrogen evolution on the ground state as well as excited state potential energy surfaces. Five distinct mechanisms for evolution of atomic or molecular hydrogen are identified and characterized.

Master equation modeling of water dissociation in small ionic water clusters: Ag+(H2O)n, n = 4–6

“…[2][3][4][5][6] Delving into their gas-phase properties can substantially enhance our understanding of metal-ligand interactions, like ligand binding energies, [7][8][9] the onset of hydrogen bonding, [10][11][12] or photochemical hydrogen evolution. 13 In the gas phase, the analog of silver chloride precipitation can be observed by mass spectrometry, 14 and static calculations 15,16 as well as ab initio molecular dynamics simulations illustrating the formation of single-molecule precipitates in nite water clusters. 17 Binding energies for Ag + (H 2 O) n , n = 1-6, were previously measured by Holland and Castleman using high-pressure mass spectrometry.…”