Chemisorption of hydrogen on iron surfaces

The adsorption of hydrogen on Fe(110), (100) and (111) single crystal planes has been studied by means of low energy diffraction (LEED), thermal desorption spectroscopy (TDS), work function measurements and ultraviolet photoelectron spectroscopy (UPS). Isotope exchange experiments revealed the atomic nature of all species held at the surface above 140 K. The chemisorption bond is characterized by a bonding level with an ionization energy of 5.6 eV below the Fermi energy as identified by UPS which is derived from coupling the H 1s state to the valence states of the metal. Initial adsorption energies of 26, 24 and 21 kcal/mole were derived for the (110), (100) and (111) planes, respectively. The work function decreases with Fe(110) by 95 mV, whereas total increases by 75 and 310 mV were determined for the (100) and (111) surfaces, respectively. At saturation the (110) and (100) planes reveal the existence of two desorption states, whereas three states are observed with Fe(111). Whereas Fe(100) and (111) reveal no variation of the LEED pattern a series of ordered overlayer structures, ranging from c2 × 2 (or “2 × 1”) at θ = 12 to 1 × 1 at θ = 1, were observed with Fe(110). These structures can be interpreted in a straightforward manner in terms of subsequent filling of rows of adsorption sites along the [001]-surface direction whereby repulsive interactions are operating between particles in neighbouring rows. This model fits perfectly with the TDS data and enables the calibration of the absolute coverage (θsat = 1, i.e. a 1:1 ratio of H:Fe surface atoms). The initial sticking coefficient on Fe(110) is so = 0.16 and it was found that with this plane the variation of this quantity with coverage between θ = 0.1 and 1 obeys a simple Langmuir type law for dissociative adsorption on two adjacent vacant sites, viz. s = so (1 − θ)2.