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 H₂/CO pressures in the 10^–5 mbar regime. Exposure of the Co(0001) to O₂(<i>g</i>) at 250 K leads to the formation of <i>p</i>(2 × 2) islands with a local coverage of 0.25 ML. Oxygen adsorption continues beyond 0.25 ML, reaching a saturation point of ∼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 ∼2.3 × 10^–5 mbar H₂(<i>g</i>). 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 ∼129 ± 7 kJ/mol for the flat Co(0001) and ∼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 × 10^–5 mbar and temperatures up to 630 K

Modeling the Surface Chemistry of Sugars: Glycolaldehyde on Rhodium (100)

It is important to understand the interaction of C–OH and CO functional groups of sugar with a catalytically active metal surface for selectively converting of biomass-derived molecules into useful chemicals. Glycolaldehyde (HOCH₂CHO), with its C–OH and CO functional groups, is the smallest molecule to model aspects of the chemistry of sugars on metal surfaces. Rhodium catalysts are candidates for activation of biomass-derived molecules. We have investigated the decomposition of glycolaldehyde on the Rh(100) surface using a combination of experimental surface science techniques (temperature-programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS)) and a computational method (density functional theory (DFT)). At low coverage, glycolaldehyde decomposition commences with O–H bond breaking upon adsorption at 100 K and proceeds via dehydrogenation and C–C bond breaking below room temperature, ultimately producing CO and hydrogen (synthesis gas). At high coverage a side reaction becomes apparent, involving C–O bond breaking. As a result, some methane and carbon formation are observed as well. Our findings on the decomposition of glycolaldehyde on Rh(100) suggest that sugars can be converted into synthesis gas on Rh surfaces, and, depending on the surface coverage, small hydrocarbons can be produced from sugar molecules, leaving the surface covered by surface carbon