ADSORPTION AND DECOMPOSITION OF AMMONIA ON FE(110)

The interaction of NH3 with a Fe(110) surface was studied by means of LEED, AES, UPS, work function and thermal desorption measurements. Below room temperature non-dissociative adsorption takes place into two states with bond strengths of about 17 and 10 kcal/mole leading to the formation of a partially disordered overlayer with a maximum coverage of θ = 1 6 (≙ 2.9 × 1014 molecules/cm2) and a maximum work function decrease by 2.4 eV. The dipole moment of the adsorbate is independent of coverage (2.2 Debye) and even larger than that of the free NH3 molecule, which indicates that chemisorption is predominantly taking place through coupling of the N lone electron pair to the metal. NH3,ad is characterized by valence ionization potentials of 6.7 and 11.2 eV below EF. The adsorption kinetics follows a simple first-order rate law with an initial sticking coefficient S0 = 0.16. At 350 K a stable surface intermediate characterized by ionization potentials of -5.2 and -8.4 eV and a 2 × 2-LEED pattern is formed which is identified with NHad. Above 400 K complete dissociation and desorption of H2 takes place. The remaining adsorbed N-atoms recombine and desorb as N2 only above 850 K. In contrast to the previously investigated Fe(111) plane adsorbed ammonia exhibits no isotopic exchange with adsorbed deuterium which is attributed to somewhat different rate constants of the consecutive steps of dissociation and recombination. This effect is also made responsible for the fact that with Fe(111) and (100) presumably NH2,ad is observed as a transient intermediate, whereas with Fe(110) NHad may be isolated on the surface. Preadsorption of Nad influences the adsorption energy and sticking coefficient of NH3, but has no strong influence on the totally adsorbed amount. © 1979.