Titration of chemisorbed oxygen by hydrogen on Ru(001)

The titration of oxygen chemisorbed on Ru(001) by H 2 at low pressures is reported. For near-saturation doses of O 2 , the subsequent H 2 titration is characterized by three distinct features: first, a long induction time, then a rapid reaction and, finally, a slow reaction. Electron spectroscopies, AES and XPS, were used to follow the decay of the oxygen signal. The first two features are accounted for by a model which involves dissociative adsorption of hydrogen and blockage of dissociation sites. The third feature is accounted for in terms of an inactive form of chemisorbed oxygen. All three regions are approximately first order in hydrogen pressure. The rate in the second region, at a fixed oxygen signal, is sensitive to the initial amount of oxygen present. This is ascribed to variations in the size and degree of perfection of (1 × 2) domains. With increasing temperature, the break between the second and third regions becomes less pronounced. This is accounted for in terms of an activated (6.5 ± 1.0 kcal mole −1 ) conversion of inactive to active oxygen. The induction times are very sensitive to small changes in surface oxygen concentration caused by electron stimulated desorption of oxygen.