Interaction of Hydrogen with a Cobalt(0001) Surface

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Abstract The interaction of hydrogen and deuterium with the Co(0001) surface has been studied in UHV between 90 and 500 K by means of LEED, temperature-programmed thermal desorption (TPD) and work function change (ΔΦ) measurements. Hydrogen adsorbs spontaneously and dissociatively in two atomic binding states denoted as β 1 and β 2 with a high initial sticking probability. The adsorption energies E(β 1) and E(β 2) are 80 and 100 kJ/mol, respectively, if a first-order desorption kinetics is assumed (a second-order kinetic analysis yields unreasonably low values for both the desorption energies and frequency factors). At sufficiently low temperatures, the adsorbed H atoms form a faint (2 × 2) LEED superstructure, which is best developed after an exposure of ∼20 L. From the temperature dependence of the fractional-order beam intensity at different exposures we determine the critical temperature of the (2 × 2) phase as ∼ 243 ( ± 10) K. Similar to the H-on-Ni(111) system the existence range of the (2 × 2) phase in the temperature–coverage plane is asymmetric; i.e., below the critical coverage Θ crit the respective long-range order has a higher range of stability than above Θ crit. By analogy with H/Ni(111), we assume a similar honeycomb H structure also for the H-on-Co(0001) system and suggest the critical coverage Θ crit to be 0.5, i.e., half a monolayer of H atoms. The H-induced work function change is surprisingly small; it decreases, forms a shallow minimum of −18 meV after ∼20 L exposure around the optimum coverage of the (2 × 2) phase and reaches a saturation value of −10 meV. Our data are discussed and compared with previous work on H/Co(0001) and other close-packed transition metal surfaces, especially with the H-on-Ni(111) system.