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

Abstract

Equilibrium values of isosteric desorption enthalpies ΔH and entropy changes ΔS 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 Δø, and Δø vs absolute coverage data. The ΔS values also yield differential, and after integration over coverage, integral surface entropies. For H on W(110) ΔH des decreases linearly from 35 Kcal mol −1 of H 2 at low coverage to 29 Kcal mol −1 of H 2 at θ = 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 ≅ 0.6 . ΔH des is roughly constant at 19–20 Kcal mol −1 for H W ≤ 0.3 but then rises to 30 Kcal mol −1 of H 2. For H W ≤ 0.3 surface entropy is somewhat above that of a 2-d translational gas, but concomitant with the increase in ΔH des differential entropy drops to −35 cal K −1mol −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.