Symmetrical configuration of hydrogen atoms in two-dimensional ice discovered in China for the first
Academician Wang Enge, Professor Jiang Ying, and Chen Ji of Peking University cooperated with Professor Guo Jing from the School of Chemistry of Beijing Normal University, etc., using high-resolution qPlus atomic force microscopy technology, took the atomic-level resolution image of protons in the water layer for the first time, and found that Eigen and Zundel's two configurations of hydrated protons can exist stably on solid surfaces, and further confirm the new configuration of hydrogen atom symmetry in surface two-dimensional ice at room temperature and atmospheric pressure caused by full quantum effect. The work, titled "Visualizing Eigen / Zundel cations and their interconversion in monolayer water on metal surfaces", was published in the top international academic journal "Science" on July 15.
In this work, the researchers co-deposit hydrogen atoms and water molecules on the surface of different metals (Au, Cu, Pt, Ru). The hydrogen atoms undergo charge transfer with the metal substrate to form hydrogen ions, which further combine with water molecules spontaneously. A two-dimensional hydrogen bond network is formed. In order to be able to distinguish water molecules and hydrated protons from real space, the researchers developed a new generation of qPlus non-intrusive atomic force microscopy (qPlus-AFM) based on the detection of hydrated sodium ions in 2018 (Nature 557, 701 (2018)). ), and improved its detection sensitivity and imaging resolution to ~2 pico-N and ~20 picometres (the best international level), respectively, "seeing" the atomic structure of hydrated proton monomer (H3O+) and its configuration by Eigen for the first time. A two-dimensional hexagonal hydrogen-bonding network formed by the self-assembly of hydrated protons (bottom).
By increasing the hydrogen ion doping concentration, the Eigen-configuration hydrated protons are transformed into Zundel-configuration hydrated protons (Fig. 2B). High-resolution AFM images of hydrated protons in Zundel configuration can directly distinguish the protons being shared by two water molecules to form a symmetrical hydrogen bond configuration. First-principles path-integrated molecular dynamics (PIMD) results show that nuclear quantum effects induce quantum delocalization of hydrogen nuclei, which promotes the formation of symmetric hydrogen bonds and stabilizes the Zundel configuration at room temperature. This is also the first time that the concept of hydrated protons was proposed more than 100 years ago. The microstructure of hydrated protons was observed in real space for the first time, and a new two-dimensional ice species with a symmetrical configuration of hydrogen atoms maintained at normal pressure at room temperature was discovered. state.
On this basis, the researchers controlled the proton transfer through the AFM tip, and found that two Eigen configuration hydrated protons can be combined into a Zundel configuration hydrated proton, and an extra proton is transferred from the water layer to the solid surface ( H*), forming the Zundel+H* configuration (below). This is a novel proton co-transfer process that goes beyond the known fundamental steps of the hydrogen evolution reaction on the electrode surface. Further study found that there is a hydrated proton concentration-dependent Eigen-Zundel transition on the Au (111) surface, while different concentrations of hydrated protons on the Pt (111) surface are more inclined to form the Zundel configuration (Fig. 3D). This means that when the hydrated proton concentration is low, the Zundel configuration hydrated protons in the Pt (111) surface water layer and the H* adsorbed on the solid surface mainly generate H2 through the Heyrovsky reaction path (H+ + e- + H* → H2); When the concentration of hydrated protons increases, the coverage of H* adsorbed on the surface increases accordingly, thereby opening a new Tafel reaction pathway (2H* → H2) for hydrogen production. These images show that the full quantum effect is helpful for understanding the microscopic mechanism of efficient hydrogen production at Pt electrodes, and also provides a new idea for improving the hydrogen production efficiency by improving electrode materials.