In situ XPS study of Pd(111) oxidation. Part 1: 2D oxide formation in 10~(-3) mbar O_2

摘要

The oxidation of the Pd(111) surface was studied by in situ XPS during heating and cooling in 3 x 10~(-3) mbar O_2. A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd_5O_4), the supersaturated Oads layer, dissolved oxygen and the ((67~(1/2)) x (67~(1/2)))R 12.2° surface structure. Exposure of the Pd(111) single crystal to 3 x 10~(-3) mbar O_2 at 425 K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O_(ads) layer. The supersaturated O_(ads) layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd_5O_4, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d_(5/2) components at 335.5 eV and 336.24 eV. During heating in 3 x 10~(-3) mbar O_2 the supersaturated O_(ads) layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600 K and 655 K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717 K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923 K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The "bulk" nature of the dissolved oxygen species was verified by depth profiling. During cooling in 3 x 10~(-3) mbar O_2, the oxidised Pd~(2+) species appeared at 788 K whereas the 2D oxide decomposed at 717 K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the ((67~(1/2)) x (67~(1/2)))R12.2° structure.