The adsorption of hydrogen on W(110) and Fe covered W(110) surfaces

Equilibrium values of isosteric desorption enthalpies Delta H and entropy changes Delta S for 2H((ads)) --> H-2(gas) on W(110) and Fe-1/W(110) were determined from equilibrium coverage vs temperature plots at a number of fixed H-2 pressures and plots of log P vs 1/T at fixed coverages. The latter were obtained from work function increments Delta phi, and Delta phi, vs absolute coverage data. The Delta S values also yield differential, and after integration over coverage, integral surface entropies. For H on W(110) Delta H-des decreases linearly from 35 Kcal mol(-1) of H-2 at low coverage to 29 Kcal mol(-1) of H-2 at theta = 0.5 and then remains virtually constant. Integral surface entropy exceeds that for an immobile, random lattice gas, presumably because of contributions from H-W vibrations, but is much less than for a 2-d translational gas at all coverages. For H on Fe-1/W(110) maximum attainable coverage is H/W congruent to 0.6. Delta H-des is roughly constant at 19-20 Kcal mol(-1) for H/W less than or equal to 0.3 but then rises to 30 Kcal mol(-1) of H-2. For H/W less than or equal to 0.3 surface entropy is somewhat above that of a 2-d translational gas, but concomitant with the increase in Delta H-des differential entropy drops to -35 cal K(-1)mol(-1) of H-2. LEED indicates no changes in the Fe layer associated with the enthalpy and entropy changes. It is hypothesized that these changes correspond to changes in H binding from sites on top of the Fe layer to locations between Fe atoms or between Fe and W. (C) 1997 Elsevier Science B.V.