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, TS (300-1400 K), we found the bulk oxygen concentration CO 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 CO from 0.03 to 1.5 at. %. The sticking coefficient α H2 was found to depend on TS but not on the gas temperature. The effect of CO 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 CO nor on TS. 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.