The interaction of hydrogen with alloy surfaces

The interaction of hydrogen with metal surfaces has been a subject of interest to the scientific community for many years as a testing ground for theoretical concepts governing surface reactivity. Conventional wisdom has been that metals which have a low density of d-like holes at the Fermi level will not dissociate molecular hydrogen, however, a more recent theory, developed in conjunction with the research presented in this thesis, claims the relevant parameter for spontaneous dissociation is the depth of the d-band. The (110) and (111) terminations of the ordered alloy NiAl are model systems for probing the nature of gas surface interactions. NiAl(110) supplies 50% Ni and 50% Al on a local scale while NiAl(111) provides domains of 50% Ni and 50% Al to incoming gases. This thesis presents a thorough experimental investigation of the interaction of hydrogen with both surfaces, and couples the experimental observations with existing theoretical calculations to provide insight into the nature of the gas-surface interaction. Through a vigorous application of a multitude of techniques, a striking contrast between the chemistry of the H/NiAl(110) and H/NiAl(111) systems is discovered. There is an activation barrier over the (110) surface, while none exists over NiAl(111), and on NiAl(110) hydrogen acts as an effective potential influencing the entire surface identically, while on NiAl(111) adsorption is very simple, with hydrogen bonding only to well localized sites on the Ni terraces. These interaction differences originate from subtle differences of the local electronic properties. While the density of states for bulk NiAl, NiAl(110) and the Al terminated NiAl(111) domains are virtually identical, the Ni terminated NiAl(111) surface has a feature shifted slightly closer to the Fermi level, and can be shown to be responsible for the increased reactivity of this surface. This study shows that the chemistry of a multicomponent surface can not be understood simply as a combination of chemistry from each constituent. In this case, each NiAl surface does not behave like Ni, Al or even bulk NiAl