No abstract

The adsorption and desorption of CO have been studied on the W(111) surface in the temperature range of 300 and 1100 K by x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and thermal desorption spectroscopy. Saturation adsorption of CO at 300 K reveals four desorption states at 410, 850, 1000, and 1150 K, called α, β1, β2, and β3 states, respectively. The desorption kinetic order of CO in the β3 state is followed by the first order, indicating that CO in the β3 state seems to be nondissociative, rather than dissociative. The inequalities of XPS and UPS obtained following adsorption of CO and O2 under the same condition support a nondissociative adsorption of CO in the β3 state. Based on calculations for CO adsorbed on W43 cluster models using atom-position and electron delocalization molecular orbit, the changes in binding energy and in intensity of XPS and UPS can be attributed to a lying-down species of nondissociatively adsorbed CO in the β3 state. As a result, the binding energies of O(1s) and C(1s) for adsorbed CO in the α state is characterized by 529.8 and 282.7 eV, respectively, whereas those for nondissociatively adsorbed CO in the β3 state is characterized by 530.1 and 282.4 eV, respectively.

The adsorption of CO on W(110) surfaces was studied using thermal desorption spectroscopy (TDS), and core and valence level spectroscopy. At 120 K, CO forms a tilted structure at lower coverages (α1), whereas it adsorbs normal to the surface at higher coverages (α2). Tilted structures have been suggested to be precursors of dissociative chemisorption; however, experimental evidence is provided for the non-dissociative chemisorption of CO at temperatures above 900 K (which is referred to as the β-state): TDS shows first order desorption kinetics. The core and valence level spectra of O/W(110) and those of β-CO/W(110) are different. Most importantly, the 4σ molecular orbital of CO can be identified in the valence level spectra of the β-CO.