STM investigation of the coadsorption and reaction of oxygen and hydrogen on Ni(110)

The interaction and reaction between oxygen and hydrogen on Ni(110) has been studied by STM. For O and H surface coverages below 1/2 MI, coadsorption experiments at 300 K indicate that there is a large local repulsion between the differing species, which consequently phase separate into (2 x 1)-O and (1 x 2)-H structural domains. However, if the O surface concentration is at or above 1/2 ML, adsorption of H is inhibited due to a site blocking mechanism. It is revealed that the titration reaction of a pre-oxygenated Ni(110) surface (theta(O) less than or equal to 2/3 ML) at 473 K initiates preferentially at step edges running along the <[1(1)over bar0]> direction and subsequently progresses across flat terraces. Details concerning the overall titration rate, as a function of the initial oxygen coverage, are shown to be dominated by this local reaction mechanism. The interaction of hydrogen with the higher density (9 x 5)-O phase (theta(O) approximate to 1 ML) is markedly different from that of the lower density phases; relatively small amounts of hydrogen induce a structural phase transformation in which the metastable (9 x 5)-O is converted to domains of(3 x 1)(f)-O and NiO(100) phases.