Insights into Hydrogen Gas Environment-Promoted Nanostructural Changes in Stressed and Relaxed Palladium by Environmental Transmission Electron Microscopy and Variable-Energy Positron Annihilation Spectroscopy

Abstract

Environmental transmission electron microscopy (ETEM) and variable-energy positron annihilation spectroscopy (VEPAS) are used to observe hydrogen-induced microstructural changes in stress-free palladium (Pd) foils and stressed Pd thin films grown on rutile TiO substrates. The microstructural changes in Pd strongly depend on the hydrogen pressure and on the stress state. At room temperature, enhanced Pd surface atom mobility and surface reconstruction is seen by ETEM already at low hydrogen pressures ( p < 10 Pa). The observations are consistent with molecular dynamics simulations. A strong increase of the vacancy density was found, and so-called superabundant vacancies were identified by VEPAS. At higher pressures, migration and vanishing of intrinsic defects is observed in Pd free-standing foils. The Pd thin films demonstrate an increased density of dislocations with increase of the H pressure. The comparison of the two studied systems demonstrates the influence of the mechanical stress on structural evolution of Pd catalysts.

Hydrogen-related defects in titanium dioxide at the interface to palladium

“…However, changes of the S parameter are found in the Pd over-layer (E < 13 keV). This effect is discussed by Roddatis et al [16].…”

“…Metal/oxide systems using titanium dioxide (TiO 2 ) as a support have been used in a broad range of technological applications where they serve as model systems for fundamental process studies in surface science [1,2], catalysis [3][4][5][6][7][8][9][10][11][12][13][14][15][16], the field of resistive switching [17][18][19], as well as for Schottky contacts [20][21][22][23]. One central question in these studies is how chemical and physical properties of the metal/TiO 2 interface are influenced by an electric field [19], by metal-support interaction [11], or by defect concentrations [21,22].…”

Investigating Helium Bubble Nucleation and Growth through Simultaneous In-Situ Cryogenic, Ion Implantation, and Environmental Transmission Electron Microscopy

“…Although very little microscopy work has been done on fcc metals implanted with He at cryogenic temperatures, He bubble formation has been studied in aged Pd tritide, PdT 0.6 . Tritium decay does not induce displacement damage in the lattice, so the vacancy concentration is limited to thermal vacancies, although H can stabilize vacancies, and formation of β-phase hydride can cause microstructural changes [18]. Bubbles are at equilibrium when the He pressure inside the bubble equals the surface tension of the host material, p = 2 γ / r , where p is the He pressure, γ is the surface energy of the host material, and r is the bubble radius [19].…”

Atomic Origin for Hydrogenation Promoted Bulk Oxygen Vacancies Removal in Vanadium Dioxide

“…A previous work has found that H dopants in metal oxide materials may promote the aggregation and migration of oxygen vacancies. 27,36 Therefore, we expect that in H−VO 2−δ the effusion of hydrogen dopants upon environmental heating could drive the removal of interior V O defects. Here we simulated the migration of V O in a 2 × 2 × 2 VO 2−δ supercell with or without H-doping by using the Climbing-Image Nudge Elastic Band (CI-NEB) method to locate the transition states along the V O migration pathway.…”