Pure atomic oxygen adsorbed on Pd(111) surface usually forms (2×2)O ordered structure. Coadsorbed hydrogen enhances the diffusion of oxygen atoms and might induce structural phase transition (2×2)O→(√3×√3)O above 150 K. At higher temperature coadsorbed hydrogen and oxygen system demonstrates [1] a remarkable effect in an interval of 10-20 K. At 210 K the (√3×√3)O structure transforms back to (2×2)O when adsorption of additional hydrogen atoms is terminated. With further increase of temperature the water formation is observed at 220 K which is manifested by decrease of ordered oxygen clusters.
Here we propose the kinetic modelling of this experiment. We introduce oxygen and hydrogen interactions and kinetic processes leading to the phase transition (2×2)O→(√3×√3)O on Pd(111). We also demonstrate how reverse phase transition (√3×√3)O→(2×2)O occurs due to hydrogen dissolution to the bulk. The processes considered in our simulation are oxygen and hydrogen diffusion, hydrogen dissolution and segregation back to the surface as well as the reaction between O and H. The formation of water is assumed to consist of two steps: rate limiting reaction O+H→OH and fast OH+H→H2O reaction followed by immediate H2O desorption from the surface. The simulations are performed by kinetic Monte Carlo method using standard model dynamics on the hexagonal lattice [2]. The interaction parameters between oxygen atoms as well as between O and H atoms were chosen to satisfy the occurrence of experimentally observed ordered structures.
1. T.Mitsui, M.K.Rose, E.Fomin, D.F.Ogletree, M.Salmeron, Surface Science 511, 259 (2002).
2. E.E.Tornau, V.Petrauskas, G.Zvejnieks, Catalysis Today, 116, 62-68 (2006). |