AVS 47th International Symposium
    Surface Science Thursday Sessions
       Session SS1+MC-ThM

Paper SS1+MC-ThM8
The Oxidation of Pd(100)

Thursday, October 5, 2000, 10:40 am, Room 208

Session: Oxide Applications and Oxidation
Presenter: G. Zheng, Yale University
Authors: G. Zheng, Yale University
E.I. Altman, Yale University
Correspondent: Click to Email
Motivated by interest in Pd as a catalyst for oxidation reactions and the catalytic combustion of CH@sub 4@, the interaction of Pd(100) with O@sub 2@ was studied using LEED, STM, and TPD. Initial exposure of Pd(100) to O@sub 2@ at room temperature resulted in a (2x2) structure that saturated at an oxygen coverage of 0.25 ML ( 1 ML = 1 O/Surface Pd ) following 30 L exposure. Further exposure to O@sub 2@ at room temperature resulted in a mixture of (2x2) and p(5x5) LEED patterns, which persisted until the oxygen coverage reached a saturation value of 0.68 ML. Increasing the exposure temperature to 475 K resulted in the p(5x5) structure in the oxygen coverage range of 0.60-0.80 ML, which transformed into a (@sr@5x@sr@5)R27° pattern beyond 0.80 ML. TPD experiments revealed four oxygen desorption features from Pd(100) following exposure to O@sub 2@, including peaks at 800 K, 700 K and 650 K, as well as a low temperature oxygen shoulder at 600 K. The highest temperature peak corresponds to the (2x2) chemisorption structure, and the other two peaks fall into the regime where p(5x5) and (@sr@5x@sr@5)R27° patterns appear. The three peaks saturated in sequence, and the low temperature shoulder showed up before saturation of the 650 K peak. These results indicate at least four distinct oxygen states on Pd(100). Upon annealing, the (@sr@5x@sr@5)R27° LEED pattern transformed into the (2x2) pattern without stepping through the p(5x5), and the p(5x5) structure converted into the (2x2) pattern. The temperature dependence of oxygen up-take was also studied. Oxygen was found to have the highest sticking coefficient on Pd(100) at 550 K. STM studies revealed the atomic structures of various oxygen phases on Pd(100), as well as their relative reactivity towards reduction by in-situ monitoring of the surface during exposure to CO and propylene.