The chemisorption and coadsorption of water and oxygen on Cs-dosed Cu(110)

Abstract

The adsorption of H 2O and the coadsorption of oxygen and H 2O on Cu(110) containing overlayers of Cs have been studied with thermal desorption mass spectroscopy (TDS), X-ray photoelectron spectroscopy, Auger electron spectroscopy, and work function measurements. On clean Cu(110), H 2O adsorbs at 110 K and desorbs intact at 175 K, with no measurable dissociation probability (< 0.5%). In the presence of Cs a, the heat of adsorption of H 2O is increased by ∼ 50%, and the cleavage of OH bonds during TDS becomes highly probable when θ Cs * exceeds ∼ 0.5 monolayers (i.e., the work-function minimum). In this case, hydrogen adatoms are produced which associatively desorb as H 2 at ∼ 440 K (or ∼ 200 K for Cs multilayers). In addition, a CsOH a species is produced which decomposes at ∼ 510 K., probably via the evolution of gaseous (CsOH) 2 dimers. When Cs a and O a are both present on the surface, the cleavage of OH bonds to produce surface hydroxyl species proceeds already at 110 K, considerably colder than in the presence of either Cs a or O a alone. In addition, Cs a allows the production of approximately twice as much OH a than with O a alone. This OH a desorbs as H 2O at ∼ 350–420 K via the disproportionation reaction 20H a→ H 2O g+O a, considerably warmer than in the absence of Cs. It is postulated that this Cs-induced stabilization of OH a may be responsible for the promotional effect of Cs in water-gas shift catalysis over Cu. While H 2O a has a dipole pointing toward the surface, OH a creates a substantial dipole moment pointing away from the Cu. Upon dosing H 2O to Cs/O/Cu(110), the CsOH a species is also produced, and it decomposes at a slightly higher temperature than in the absence of O a (∼ 500–580 K).