Coadsorption phases of CO and oxygen on Pd(111) studied by scanning tunneling microscopy

The adsorption of CO on an oxygen precovered Pds111d surface was investigated between 60 and 300 K. Applied methods were variable temperature scanning tunneling microscopy sSTMd and video STM to analyze the coadsorption structures. The STM data are compared with simulated STM images for the various surface phases in order to identify the appropiate structural model for each case. Low-energy electron diffraction and reaction isotherms by means of mass spectrometry were used to correlate the phases with the reaction yielding CO2. The video-STM data recorded during CO adsorption at 300 K on the s232dO phase show a fast phase transition into the sÎ33Î3dR30O structure, followed by reaction to CO2. The reaction only starts after completion of the phase transition, indicating that the sÎ33Î3dR30O structure plays a crucial role for the reaction. At temperatures between 170 and 190 K the phase transition is slow enough to be monitored with STM. The experimental images of both the s232dO and the sÎ33Î3dR30O structures are well reproduced by the simulations. Further CO adsorption caused a second phase transition into a ps231dO structure. The STM simulations strongly support a pure oxygen ps231d structure, rather than a mixed O+CO structure, in contrast to previous experimental work. The CO molecules form the same structures between the O islands that are known from the pure Pds111d/CO system. At lower temperatures, between 110 and 60 K, a so far unknown s232d phase was observed. The formation of this structure, and its imaging by the STM, show that it constitutes a mixed ps232dO+CO structure, where the oxygen atoms remain unchanged, and the CO molecules occupy hcp sites between the O atoms.