Observation and manipulation of subsurface hydride in Pd{111} and its effect on surface chemical, physical, and electronic properties

Abstract

We report the observation and manipulation of hydrogen atoms beneath the surface of a Pd{111} crystal by using low-temperature scanning tunneling microscopy. These subsurface hydride sites have been postulated to have critical roles in hydrogen storage, metal embrittlement, fuel cells, and catalytic reactions, but they have been neither observed directly nor selectively populated previously. We demonstrate that the subsurface region of Pd can be populated with hydrogen atoms from the bulk by applying voltage pulses from a scanning tunneling microscope tip. This phenomenon is explained with an inelastic excitation mechanism, whereby hydrogen atoms in the bulk are excited by tunneling electrons and are promoted to more stable sites in the subsurface region. We show that this selectively placed subsurface hydride affects the electronic, geometric, and chemical properties of the surface. Specifically, we observed the effects of hydride formation on surface deformation and charge and on adsorbed hydrogen on the surface. Hydrogen segregation and overlayer vacancy ordering on the Pd{111} have been characterized and explained in terms of the surface changes attributable to selective hydrogen occupation of subsurface hydride sites in Pd{111}.atomic manipulation ͉ catalysis ͉ palladium ͉ hydrogen ͉ scanning tunneling microscopy

Manipulating interfacial hydrogens at palladium via STM

“…4(b)). It is found that, for the system at hand, the function is slightly asymmetric with respect to the potential bias sign, which is consistent with the experimental observations by Sykes and co-workers 14 from which the transfer rates are inferred. For more information on the rate model, the reader is referred to the relevant literature.…”

“…14 The protuberances are most likely due to the clustering of hydrogens in the vicinity of the surface, with the topological feature mostly due to a hydrogen layer at the surface and to the partial population of the subsurface sites. This type of structures has also been inferred from experimental observations.…”

“…It is important to note that no time-resolved experimental results are available, but the timescales presented here are well below the experimental upper limit of a few minutes. Hence, the observed protuberances 14 should be considered as the asymptotic limit of the population evolution upon STM excitation. The depopulation of the initial state is quantitative for the unsaturated surfaces, while it remains far from quantitative for the saturated ones.…”

Hydrogen on and in Selected Overlayer Near-Surface Alloys and the Effect of Subsurface Hydrogen on the Reactivity of Alloy Surfaces

“…Beyond the possible hydrogen storage applications of such systems, subsurface and bulk hydrogen has been identified as a reaction intermediate for several catalytic reactions on monometallic catalysts, such as alkene hydrogenation on Pd [30,31] and alkene hydrogenation and methanation on Ni [32][33][34][35]. Additionally, subsurface hydrogen has been shown to affect the binding of other species on single crystals and nanoparticles [36]. Thus, a systematic understanding of the stability of subsurface hydrogen and its effect on the reactivity of NSAs would be helpful in establishing a better understanding of their catalytic behavior for hydrogen-related applications and in identifying alloys with promising catalytic properties.…”

Manipulation of subsurface carbon nanoparticles inBi2Sr2CaCu2O8+δusing a scanning tunneling microscope

“…Control at this scale was limited to surfaces until manipulation of individual dopants was first demonstrated in Pd when an STM tip was used to vertically transport H from bulk to subsurface sites by elevating the tip voltage [13].…”