Oxygen Adsorption and Water Formation on Co(0001)

Oxygen adsorption and removal on flat and defective Co(0001) surfaces have been investigated experimentally using scanning tunneling microscopy, temperature-programmed and isothermal reduction, synchrotron X-ray photoemission spectroscopy, and work function measurements under ultrahigh vacuum conditions and H-2/CO pressures in the 10(-5) mbar regime. Exposure of the Co(0001) to O-2(g) at 250 K leads to the formation of p(2 x 2) islands with a local coverage of 0.25 ML. Oxygen adsorption continues beyond 0.25 ML, reaching a saturation point of similar to 0.39 ML O-ad, without forming cobalt oxide. Chemisorbed oxygen adlayers can be reduced on both flat and defective Co(0001) surfaces by heating in the presence of similar to 2.3 x 10(-5) mbar H-2(g). The onset of the oxygen removal as water during temperature-programmed reduction experiments (1 K s(-1)) is at around 450 K on flat Co(0001) and 550 K on defective Co(0001). By evaluation of isothermal reduction experiments using a kinetic model, the activation energy for water formation is found to be similar to 129 +/- 7 kJ/mol for the flat Co(0001) and similar to 136 +/- 7 kJ/mol for the defective Co(0001). Adsorbed oxygen cannot be reduced by CO (g) on flat and defective Co(0001) using CO pressures' up to 1 x 10(-5) mbar and temperatures up to 630 K.