Hydrogen Manipulated Below Surface of Palladium

Scientists at Penn State University in University Park, Pennsylvania, have manipulated hydrogen atoms into stable sites beneath the surface of a palladium crystal, creating a structure predicted to be important in metal catalysts, hydrogen storage and fuel cells.

Observations of the effects of the resulting subsurface hydrides – hydrogen atoms with a partial negative charge – confirmed the existence of the stable sites, which had been predicted but previously had neither been deliberately assembled nor directly observed. The research was led by Paul S. Weiss, distinguished professor of chemistry and physics at Penn State.

After moving absorbed hydrogen atoms to just below the crystal surface, the researchers were able to observe how the presence of the hydride in specific sites within a metal crystal affects the chemical, physical and electronic properties of the metal. Understanding these effects could advance efforts to improve chemical reactions involving metal catalysts. In addition, the subsurface hydride may provide a model material for application in hydrogen storage and fuel cells. The ability to prepare the subsurface hydride provides a research tool for these applications.

The researchers carried out the experiments in a low-temperature scanning tunneling microscope (STM) under ultrahigh vacuum by exposing the crystal to a hydrogen atmosphere. They removed excess hydrogen from the surface by cycles of exposure to heat and oxygen. After the surface had been cleaned, the researchers were able to use electrons from the STM tip to move hydrogen atoms that had been absorbed into the bulk metal up into the stable subsurface sites. As the hydride formed underneath the surface of the material, Weiss and his team observed that the surface of the crystal distorted, the positive charge of palladium atoms above them increased, and interactions occurred with hydrogen atoms on the surface of the palladium crystal.

This research was funded by the Air Force Office of Scientific Research, with additional support from the Army Research Office, the National Science Foundation and the Office of Naval Research.