Surface oxides on Pd(111): STM and density functional calculations

Abstract

The formation of one-layer surface oxides on Pd͑111͒ has been studied by scanning tunneling microscopy ͑STM͒ and density functional theory ͑DFT͒. Besides the Pd 5 O 4 structure determined previously, structural details of six different surface oxides on Pd͑111͒ will be presented. These oxides are observed for preparation in oxygen-rich conditions, approaching the thermodynamic stability limit of the PdO bulk oxide at an oxygen chemical potential of −0.95 to − 1.02 eV ͑570-605 K, 5 ϫ 10 −4 mbar O 2 ͒. Sorted by increasing oxygen fraction in the primitive unit cell, the stoichiometry of the surface oxides is Pd 5 O 4 , Pd 9 O 8 , Pd 20 O 18 , Pd 23 O 21 , Pd 19 O 18 , Pd 8 O 8 , and Pd 32 O 32 . All structures are one-layer oxides, in which oxygen atoms form a rectangular lattice, and all structures follow the same rules of favorable alignment of the oxide layer on the Pd͑111͒ substrate. DFT calculations were used to simulate STM images as well as to determine the stability of the surface oxide structures. Simulated and measured STM images are in excellent agreement, indicating that the structural models are correct. Since the newly found surface oxides are clearly less stable than Pd 5 O 4 , we conclude that Pd 5 O 4 is the only thermodynamically stable phase, whereas all newly found structures are only kinetically stabilized. We also discuss possible mechanisms for the formation of these oxide structures.

Oxygen interaction with the Pd(112) surface: From chemisorption to bulk oxide formation

“…8(d)]. The structure of the surface oxide observed in our STM measurements resembles closely that of the Pd 8 O 8 structure that has been reported to form on the Pd(111) surface upon oxidation at 5 × 10 −4 mbar O 2 at temperatures between 570 and 605 K. 27 This finding correlates very well with the HRCLS observations: the topmost spectra in Fig. 7 were measured after exposure to 623 K and 1 × 10 −3 mbar oxygen.…”

Oxidation at the Subnanometer Scale

“…The structures of the oxidized gold clusters are clearly different from Cu and Ag. The preference of two-dimensional structures is to a large extent preserved and the ground states are planar with the exception of (Au 2 O) 3 and (Au 2 O) 6 . Furthermore, the importance of a linearly coordinated Au atoms is clearly visible.…”

Pd(110) surface oxide structures investigated by STM and DFT

“…This concerns in particular the Pd(1 1 1) and Pd(1 0 0) surfaces [11][12][13][14][15][16]. The understanding of adsorption phases of oxygen on low index faces of palladium is important for the design of realistic palladium catalyst such as oxide supported palladium nanoparticles [17,18].…”