CO adsorption and dissociation on Ru(0001) at elevated pressures

We have investigated the adsorption and dissociation of CO on Ru(0001) surfaces at pressures from ultra-high vacuum up to 1 Torr and temperatures from 293 to 575 K using Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS). At CO pressures above ~ 10− 6 Torr additional CO is adsorbed on the surface, leading to a CO coverage greater than the saturation coverage (0.66 ML) observed under UHV conditions. Binding energy shifts of the O 1s core level indicate that most of the additional adsorbed CO is located in bridge sites between two Ru atoms. At pressures above 10− 2 Torr the coverage of CO saturates at ~ 0.88 ML. Isobaric measurements at 0.04 Torr CO indicate that the bridge bonded CO is stable up to a temperature of ~ 350 K and desorbs entirely by ~ 400 K. Additional CO desorbs over the temperature range of ~ 450 K to ~ 485 K, decreasing the CO coverage to ~ 0.58 ML. Above 520 K we observe the build-up of carbon on the surface which we attribute to the dissociation of CO. At 575 K and 0.04 Torr CO the equivalent of ~ 3.8 ML of carbon is present on the Ru(0001) surface. Potential mechanisms for the formation of this large amount of carbon on the surface will be discussed.