The Interaction of Hydrogen with the Ni(977) Surface


Figure 1

The motivation to explore interactions of hydrogen with surfaces originates from several sources. Fundamentally, being the most simple adsorbate, hydrogen is the model atom for analyzing gas-surface interactions. In chemical physics, topics ranging from heterogeneous catalysis to synthesis, e.g. the Fischer-Tropsch reaction, involve hydrogen as either a reactant or product. Hydrogen research is also essential in materials science and metallurgy for both hydrogen's mischief and its benefits. The most common damage caused by hydrogen in transition metals is embrittlement. Microscopically, this is because hydrogen always prefers surface sites over the bulk in transition metals. Consequently, at any given temperature, the surface coverage can be considerably greater than the bulk concentration. Large scale cracking and embrittlement therefore ensue, because once a microscopic fissure is initiated by mechanical stress, it will entice other hydrogens. Much hydrogen research, however, is motivated by hydrogen's utility. Interest has been increasing regarding the use of some alloys to dissolve or store gaseous hydrogen. In addition, the progress of fuel cell technology necessitates a precise understanding of the underlying reaction mechanisms in hydrogen-surface interactions, especially adsorption.
Figure 2

Phase diagrams are central to experiments involving adsorbates, and although the hydrogen on Ni(111) phase diagram is well understood, it should not be assumed that the phase behavior will be identical on Ni(977). Possibly the most interesting aspect of this diagram is the order-disorder transition, which occurs at a surface temperature of 270 K for Ni(111) (Figure 1). We have located this transition on the H/Ni(977) phase diagram by varying the substrate temperature at a fixed coverage. Our results (Figure 2) show a transition at 310 K, a much higher temperature than for H/Ni(111). This reversible phase transition is second order and is best fit with TC = 310 K and b = 0.12, indicative of two-dimensional Ising behavior. Stabilization of the ordered phase is attributed to pinning from the step edges.
Figure 3

Another topic of interest in our lab is hydrogen diffusion. Heterogeneous diffusion undertakes a crucial role in dynamic surface phenomena including catalytic reactions, epitaxial crystal growth, and associative desorption. Using helium atom scattering, we are able to study diffusion mechanisms by measuring the Doppler broadening of a diffusely scattered "quasielastic" peak in time-of-flight spectra. There are several diffusion models which serve to predict this width as a function of parallel momentum transfer. The simplest of these is isotropic random continuous motion on a smooth surface (Figure 3a). If the substrate is not flat, but rather provides corrugation for the adatoms, diffusion should take place in the form of discrete jumps (Figure 3b or 3c). Each of these models predicts a different dependence of the diffuse quasielastic peak width as a function of parallel momentum transfer. Furthermore, once the appropriate diffusion model is identified, we will be able to measure the diffusion constant and thermodynamic barrier to diffusion for hydrogen on Ni(977).

More analysis is required to explain the differences between the H/Ni(111) and H/Ni(977) phase diagrams. Work also remains to be done on hydrogen's diffusive motion on a vicinal surface. Further diffusion experiments are currently underway, which will hopefully lead to a definitive selection among the proposed adatom diffusion models.



76. "Influence of steps on the interaction between adsorbed hydrogen atoms and a nickel surface"

    Aubrey T. Hanbicki, S.B. Darling, D.J. Gaspar, and S.J. Sibener, J. Chem. Phys. 111 9053-9057 (1999) Abstract




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