Adsorption of hydrogen on Pd(100)

Abstract

The energetic, kinetic and structural properties of hydrogen chemisorbed on a Pd(100) surface were studied by means of thermal desorption, work function and LEED measurements. Under the applied conditions no interference with bulk dissolution occurs and dissociative adsorption gives rise to a continuous increase of the work function by up to 0.20 eV. The dipole moment of the adsorbate complex is constant up to θ ≈ 0.9 and then increases until saturation at θ ≈ 1.35 (at 170 K) is reached. The formation of a second adsorbed state at high coverages manifests itself also by a low-temperature shoulder in the thermal desorption spectra and in the variation of the isosteric heat of adsorption, E ad, with coverage: E ad remains practically constant ( 24.5 kcal mole ) up to θ ≈ 0.9 and then decreases. The sticking coefficient is initially rather high ( s 0 ≈ 0.5) and varies with coverage in a way which can be described by a precursor-state model. The preexponential factor for desorption is about 10 −2 cm 2 atom −1 s −1. Desorption follows second order kinetics only at very low coverages, at high θ it exhibits quasifirst order. This effect is attributed to the existence of lateral interactions between adsorbed hydrogen atoms which manifest themselves also in the appearance of a c(2 × 2) LEED pattern at low temperatures. The “extra” diffraction spots attain their maximum intensity at θ = 0.5, and a structural model is proposed whereafter in this phase the H atoms occupy next-nearest neighboring adsorption sites with local fourfold symmetry. Order-disorder transitions were followed by recording the intensity of the half-order spots as a function of temperature at various coverages. The resulting phase diagram exhibits a critical temperature T c = 260 K at θ = 0.5 and is slightly asymmetric with respect to this coverage. The data are analysed in terms of a lattice gas model and estimates for the pairwise interaction energies yield repulsion between nearest neighbors ( w 1 = 0.5 kcal mole ) and attraction between next-nearest neighbors ( w 2 = −0.3 kcal mole ). The additional operation of non-pair-wise interactions is made responsible for the asymmetric shape of the phase diagram. Whereas the adsorbed layer is obviously localized at T ⩽ 270 K, a detailed analysis of the adsorption entropy reveals that for T ⩾ 370 K a rather good description can be obtained with a model of delocalized two-dimensional translation.