Adsorption behaviors of CO on W(1 1 0) and Mo(1 1 0) surfaces in the β-state are still not clear

Adsorption structure of CO on W and Mo at above ～800 K (β-CO) has been extensively studied in the history of surface science. Most of the previous studies concluded that CO is dissociated in the β-CO, and a tilted structure plays a role as a precursor state of the dissociation. We have recently studied valence band spectra of the β-CO on W(1 1 0), oxygen-precovered W(1 1 0) and Mo(1 1 0) using synchrotron radiation. CO-derived states with binding energies close to those of the 4σ-CO can be observed, implying a non-dissociative chemisorption in this high-temperature state. We suggest that still some additional works need to be done in order to understand adsorption structure of β-CO completely.