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Abstract

Oxidation of the Pd(111) surface was studied by in situ XPS during heating and cooling in 3x10-3 mbar O2. Number of adsorbed/dissolved oxygen species was identified by the in situ XPS technique such as the 2D surface oxide (Pd5O4), the supersaturated Oads layer, dissolved oxygen and the (√67x√67)R12.2°.
Exposure of the Pd(111) single crystal to 3x10-3 mbar O2 at 425 K led to appearance of the 2D oxide phase, which was in equilibrium with the supersaturated Oads layer. The supersaturated Oads layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd5O4, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d5/2 components at 335.5 eV and 336.24 eV. During heating in 3x10-3 mbar O2 the supersaturated Oads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with 2D oxide between 600 K and 655 K. The 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. Substantial signal of oxygen assigned to the dissolved species was detected even at 923 K. Dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The “bulk” nature of the dissolved oxygen species, Oγ, was verified by depth profiling.
During the cooling ramp in 3x10-3 mbar O2, the palladium oxidised states appeared at 788 K whereas the 2D oxide decomposed at lower temperature during heating ramp. The surface oxidised states exhibited an inverse hysteresis. The palladium oxidised state observed during cooling was assigned to a new oxide phase, probably (√67x√67)R12.2°.