Stability of hydrogenation states of graphene and conditions for hydrogen spillover

The hydrogen spillover mechanism has been discussed in the field of hydrogen storage and is believed to have particular advantage over the storage as metal or chemical hydrides. We investigate conditions for practicality realizing the hydrogen spillover mechanism onto carbon surfaces, using first-principles methods. Our results show that contrary to common belief, types of hydrogenation configurations of graphene (the aggregated all-paired configurations) can satisfy the thermodynamic requirement for room-temperature hydrogen storage. However, the peculiarity of the paired adsorption modes gives rise to a large kinetic barrier against hydrogen migration and desorption. It means that an extremely high pressure is required to induce the migration-derived hydrogenation. However, if mobile catalytic particles are present inside the graphitic interstitials, hydrogen migration channels can open and the spillover phenomena can be realized. We suggest a molecular model for such a mobile catalyst which can exchange hydrogen atoms with the wall of graphene.open151

Analysis of differences in variables related to health and safety according to the employment type of Korean workers

Introduction: The purpose of this study was to understand the differences in variables related to health and safety according to the employment type of Korean workers, specifically to identify the differences by employment type on in health status, the likelihood of wearing protective gear when working, access to manuals on emotional expression, and access to information on risk factors related to health and safety. Methods: The secondary data of four items on employment type, health type of workers and safety among the 5th Korean Working Condition Survey conducted in 2017 in Korea was used in this study. The data of workers were processed by using SPSS/WIN 23.0 Program and R 3.1.2, and demographic characteristics were quantified as frequency and percentage.  Results: A total of 30,300 employed people were surveyed. The result shows that part-time workers have poorer health than full-time workers (c2 = 540.7155, p < 0.05), insufficiently wore protective gear (c2 = 24.8702, p < 0.05), had insufficient access to manuals on emotional expression (c2 = 27.7612, p < 0.05) and lacked information about risk factors (c2 = 185.0082, p < 0.05). Conclusion: Health and safety manager will need to have education and consultation, development of manual and perform an early intervention to improve safety environment as primary health care providers by understanding factors related to health and safety of part-time workers

Dynamical mean-field theory of Hubbard-Holstein model at half-filling: Zero temperature metal-insulator and insulator-insulator transitions

We study the Hubbard-Holstein model, which includes both the electron-electron and electron-phonon interactions characterized by $U$ and $g$, respectively, employing the dynamical mean-field theory combined with Wilson's numerical renormalization group technique. A zero temperature phase diagram of metal-insulator and insulator-insulator transitions at half-filling is mapped out which exhibits the interplay between $U$ and $g$. As $U$ ($g$) is increased, a metal to Mott-Hubbard insulator (bipolaron insulator) transition occurs, and the two insulating states are distinct and can not be adiabatically connected. The nature of and transitions between the three states are discussed.Comment: 5 pages, 4 figures. Submitted to Physical Review Letter