Adsorption of hydrogen on nickel single crystal surfaces

Abstract

Adsorption of hydrogen on Ni(111), (100), and (110) surfaces was studied by means of LEED; energy loss spectroscopy, flash desorption, and work function measurements, as well as by the technique of laser‐induced thermal desorption. Noticeable variations of the LEED intensities for Ni(111) and (100) indicate the formation of disordered adsorbed layers, whereas with Ni(110) the formation of ``streaked'' diffraction patterns and (at high coverages) of a 1×2 structure were observed. Hydrogen adsorption causes a strong damping of the intensity of the surface plasmon excitation and the appearance of an electron loss peak around 15 eV. Flash desorption experiments revealed for Ni(111) and Ni(100) the existence of β₁ and β₂ states, the former being only filled after completion of the β₂ state. Desorption from the β₂ state follows a second‐order rate law for Ni(111) and Ni(100), but is of first order with Ni(110). Maximum increases of the work function $\mathrm{\Delta \; \phi }$ by 0.195, 0.170, and 0.530 eV were observed with Ni(111), (100), and (110), respectively. Adsorption isotherms ${\mathrm{\Delta \; \phi =f(p}}_{{\mathrm{H}}_2})$ for Ni(111) and Ni(100) may be closely described up to medium coverages on the basis of a model that assumes second‐order desorption and first‐order adsorption kinetics. The validity of this model was further proved by measurements of the variation of coverage with time. The adsorption isotherms for Ni(110) have a unique shape indicating the occurrence of two‐dimensional condensation due to attractive interactions in the adsorbed layer. Isosteric heats of adsorption at low coverages of 23 kcal/mole for Ni(111) and Ni(100) and of 21.5 kcal/mole for Ni(110) were evaluated. With Ni(110) the onset of condensation is characterized by an increase of E_ad by about 2 kcal/mole. A steplike decrease of E_ad at medium coverages accompanies the formation of the β₁ states.