Effects of bulk impurity concentration on the reactivity of metal surface: Sticking of hydrogen molecules and atoms to polycrystalline Nb containing oxygen.

Nonmetallic impurities segregated onto metal surfaces are able to drastically decrease the chemical reactivity of metals. In the present paper, effects of bulk impurities on the reactivity of metallic surfaces were investigated in a wide temperature range on an example of the sticking of hydrogen molecules and atoms to Nb [polycrystalline, with mainly (100)] containing solute oxygen. At all the investigated surface temperatures, T_S (300–1400 K), we found the bulk oxygen concentration C_O to have a strong effect on the integral probability, α_H2, of dissociative sticking of H₂ molecules followed by hydrogen solution in the metal lattice: α_H2 monotonically decreased by orders of magnitude with increasing C_O from 0.03 to 1.5 at. %. The sticking coefficient α_H2 was found to depend on T_S but not on the gas temperature. The effect of C_O on α_H2 is explained by the presence of oxygen-free sites (holes in coverage) serving as active centers of the surface reaction in the oxygen monolayer upon Nb. In contrast to H₂ molecules, H atoms were found to stick to, and be dissolved in, oxygen-covered Nb with a probability comparable to 1, depending neither on C_O nor on T_S. This proves that, unlike H₂ molecules, H atoms do stick to be dissolved mainly through regular surface sites covered by oxygen and not through the holes in coverage. [ABSTRACT FROM AUTHOR]