The Static and Dynamic Lattice Changes Induced by Hydrogen Adsorption on NiAl(110)

Static and dynamic changes induced by adsorption of atomic hydrogen on the NiAl(110) lattice at 130 K have been examined as a function of adsorbate coverage. Adsorbed hydrogen exists in three distinct phases. At low coverages the hydrogen is itinerant because of quantum tunneling between sites and exhibits no observable vibrational modes. Between 0.4 ML and 0.6 ML, substrate mediated interactions produce an ordered superstructure with c(2x2) symmetry, and at higher coverages, hydrogen exists as a disordered lattice gas. This picture of how hydrogen interacts with NiAl(110) is developed from our data and compared to current theoretical predictions.Comment: 36 pages, including 12 figures, 2 tables and 58 reference

Interaction of nitrogen with iron surfaces

The adsorption of N2 on clean Fe(100) and Fe(111) single-crystal surfaces was studied in the temperature range 140–1000 K by means of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), thermal-desorption spectroscopy (TDS) and work-function measurements (Δφ). Above room temperature, only dissociative adsorption takes place, leading to increases in work function of 0.33 and 0.25 eV on Fe(100) and (111), respectively, and is mainly identified with UPS by the appearance of a chemisorption level derived from N2p-states at about 5 eV below the Fermi level. At 500 K, the initial rate of adsorption is faster by about a factor of 20 on the (111) plane, the initial sticking coefficient, however, being very small (10−7–10−6) on both surfaces. The initial activation energies for adsorption are about 5 and 0 kcal/mole on Fe(100) and Fe(111), respectively, and increase with coverage in both cases. The mean activation energies for desorption were estimated to be 58(100) and 51 kcal/mole (111), so that nearly equal values for the strength of the M−N bond result. A simple ordered c2 × 2 structure is formed on Fe(100) which is completed at θ = 0.5 and for which a model is proposed wherein the N atoms are located in fourfold sites on the unreconstructed Fe(100) surface, leading to a configuration similar to that in the (002) plane of (fcc) Fe4N. Several independent observations strongly indicate that the Fe(111) surface reconstructs. A whole series of complex LEED patterns (depending on N bulk and surface concentrations and on the conditions of heat treatment) is formed with this plane which are interpreted in terms of the formation of hexagonal layers of “surface nitrides” which have a thickness of about 2 atomic layers and most probably are related to the (111) plane of Fe4N. Desorption of N2 (being found to be a first-order rate process) is regarded as equivalent to the decomposition of the “surface nitrides.” The close similarity to the kinetics of decomposition of (bulk) Fe4N indicates identical mechanisms for both processes. Although the bulk solubility of N is very small under the chosen experimental conditions, this process interferes with the adsorption and desorption measurements and was analyzed in some detail, mainly by isotopic exchange. Evidence for the existence of a weakly bound (probably molecular) species was found with Fe(111) only at the lowest temperatures (140 K) and under a steady-state pressure of 4 × 10−4 Torr of N2. This species causes a decrease in the work function and is rapidly pumped off. Its adsorption energy is estimated to be in the range between 5 and 10 kcal/mole

Chemisorption of hydrogen in 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 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

Interaction of ammonia with Fe(111) and Fe(100) surfaces

The adsorption and decomposition of NH3 on clean and nitrogen covered Fe(111) and Fe(100) surfaces has been studied by means of UPS, AES, LEED, thermal desorption and work function measurements. Molecularly adsorbed ammonia is characterized by valence ionization potentials of 7.4 and 11.8 eV below the Fermi level which are derived from the 3a1-and leorbitals of free NH3. NH3,ad decreases the work function by about 2 eV and is presumably coupled to the surfaces through the nitrogen atom, the adsorption energies being in the range of 10–12 kcal/mole. Even at 160 K slow chemical transformation takes place. At 320 K the last species containing N-H bonds are lost from the surface, either by desorption of NH3 or by complete dissociation into Nad + Had. The latter process is strongly suppressed by the presence of preadsorbed atomic nitrogen. At least one intermediate species (presumably NH2,ad) could be identified by characteristic variations of the photoelectron spectra. This may recombine with adsorbed hydrogen (as verified by isotope-exchange experiments) leading to NH3-desorption around room temperature. Repeated high-temperature treatment with ammonia caused some facetting of the Fe(100) plane

Vivianite formation and distribution in Lake Baikal sediments

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