Effect of CO on hydrogen thermal desorption from Fe(100)

Abstract

The adsorption and coadsorption of H and CO was studied in order to illuminate the enhanced reactivity of H toward olefin hydrogenation induced by coadsorbed CO. The hydrogen saturation coverage on clean Fe(100) is 1.0 ± 0.1 ML, and H2 desorbs in two states, β1 and β2, with activation energies of 17 and 23 kcalmol, respectively. Low coverages of CO adsorbed on hydrogen-presaturated Fe(100) broaden and reduce the peak temperatures of the β1 and β2 desorption states. The dominant effect of coadsorbed CO is to weaken Fe-H bonds, and three new H2 desorption states are induced below 260 K at higher coverages of CO. No displacement of H2 by coadsorption of CO is apparent. The CO saturation coverage is uneffected by preadsorbed hydrogen, but the CO desorption features below 350 K are distinctly different than those for the clean surface. Desorption of CO above 350 K resembles that for CO adsorbed on the H-free surface. When H2 is adsorbed onto a CO-precovered surface at 100 K. the saturation coverage of hydrogen is reduced, and at the CO saturation coverage of 0.58 ML hydrogen adsorption is blocked. H2 desorption from surfaces precovered with a subsaturated adlayer of CO adsorbed at 100 K display only perturbed β1 and β2 states. CO apparently induces low temperature desorption of H2 only when it is adsorbed on top of a locally saturated H adlayer.