AVS 51st International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuM

Paper SS1-TuM9
Nanoporous Pd Films Grown Via Ballistic Deposition: Structural And Catalytic Properties

Tuesday, November 16, 2004, 11:00 am, Room 210B

Session: Catalytic Reactions: The Role of Surface Steps and Structure
Presenter: J. Kim, Pacific Northwest National Laboratory
Authors: J. Kim, Pacific Northwest National Laboratory
Z. Dohnálek, Pacific Northwest National Laboratory
B.D. Kay, Pacific Northwest National Laboratory
Correspondent: Click to Email
Hydrogenation of ethylene was studied on dense and nanoporous Pd films using molecular beams and temperature programmed desorption (TPD). Porous films were deposited on a Mo(100) substrate at temperatures ranging from 22 to 300 K. Due to limited surface mobility at such low temperatures, the growth is dictated by a ballistic deposition mechanism. The adsorption properties of the films were characterized as a function of Pd deposition angle as well as growth and post-annealing temperatures using N@sub 2@ physisorption. The highest surface area of ~ 80 m@super 2@/g was observed for films grown at 85° and 22 K. Upon annealing to 300 K, the films lose ~40 % of their area, further annealing to 500 K leads to complete densification. Reactivity studies show that nanoporous Pd, covered with H and subsequently saturated with ethylene at 22 K, is highly efficient catalyst for ethylene hydrogenation. This is in sharp contrast with dense Pd films where only trace amounts of ethane were observed. As seen from the reaction yields on films preannealed to various temperatures, the number of catalytically active sites decreases faster than the area of the films. Additionally, cycling of the ethylene hydrogenation reaction also leads to a decrease of ethane production due to poisoning of the active sites.