AVS 56th International Symposium & Exhibition
    Surface Science Tuesday Sessions
       Session SS2-TuM

Paper SS2-TuM1
Surface Structure of the Pd-H System

Tuesday, November 10, 2009, 8:00 am, Room N

Session: Reactions on Metals and Bimetallics
Presenter: K.F. McCarty, Sandia National Laboratories
Authors: K.F. McCarty, Sandia National Laboratories
B. Santos, Universidad Autónoma de Madrid, Spain
T. Herranz, Intituto de Química-Física "Rocasolano", Spain
J.I. Cerda, Instituto de Ciencias de Materiales de Madrid, Spain
J.M. Puerta, Instituto de Ciencias de Materiales de Madrid, Spain
J. de la Figuera, Intituto de Química-Física "Rocasolano", Spain
Correspondent: Click to Email
Palladium hydride, PdH, is the model transition-metal hydride. Surface-science studies of PdH are scarce for two reasons. First, hydrogen pressures in the 10’s of torr range are needed to form PdH near room temperature. Second, the expansion of the lattice that occurs during hydride formation tends to destroy Pd single crystal. To overcome this latter problem, we use thin epitaxial films of Pd(111) on Ru(0001) and W(110) to examine the surface science of the Pd-H system. To achieve an effective higher hydrogen pressure, we expose the Pd films to atomic H. We use low-energy electron diffraction (LEED) to characterize the Pd surface after H exposure. By performing the LEED in a low-energy electron microscope (LEEM), we are able to collect diffraction data from the same, atomically flat regions before and after H exposure. Detailed analysis of the LEED IV curves shows that exposure to atomic H at about 200K expands the interlayer spacing of the top two Pd layers but does not affect deeper layers. We interpret these changes as the formation of a surface hydride. Exposure to atomic H while cooling from above room temperature also produces the surface hydride. The lack of diffusion deeper into the film suggests that an energy barrier inhibits diffusion past the second Pd layer. The synthesis of a bulk hydride was attempted by exposing a Pd film to 40 torr of hydrogen and then reducing the pressure after cooling. The resulting material presented sharp, 3-fold diffraction patterns with an IV response different than the surface hydride. We will discuss the structure of this material, which we tentatively assign to a bulk PdH.