Isotopic study of hydrogen adsorption on magnesium oxide powders

Abstract

By measuring temperature-programmed desorption (t.p.d.) spectra of hydrogen adsorbed on MgO powders outgassed at 1123 K, eight different states of adsorbed species have been found. Species W₁ is reversibly adsorbed at 77 K and other irreversible species desorb with maximum rates at 190 (W₂), 229 (W₃), 293 (W₄), 327 (W₅), 460 (W₆), ca. 500 (W₇) and 608 K (W₈). All the species are hydrogen chemisorbed on different active sites. The numbers of active sites are > 1.7 (W₁), 32±3 (W₂+W₃), 23±3 (W₄+W₅), 1.9±0.5 (W₆+W₇) and 2.3±0.5 (W₈)× 10¹⁵ site m^–2 and the total number of active sites except for type W₁ amounts to ca. 60 × 10¹⁵ site m^–2.

A dominant species W₅ has been studied in detail. T.p.d. spectra, observed by using a mass-filter, of HD molecules have shown that molecular identity is conserved in the desorption process. This implies that the adsorbed species is immobile and localized on the surface and that pseudo-first-order kinetics controls the desorption process. The activation energy for H₂ desorption at the peak maximum is 99.4±0.6 kJ mol^–1, less than that for D₂ desorption by only 1.5 kJ mol^–1. The heat of adsorption is roughly estimated to be 90 kJ mol^–1. The values of isotope effects (H/D) observed for species W₅ on the sample having nearly half-coverage at 308 K are 2.2±0.2 and 0.71±0.02 for the adsorption rate and the equilibrium, respectively. It is revealed, on the basis of these isotope effects and available i.r. spectra on adsorption, that the formation of species W₅ is consistent with a heterolytic dissociation of hydrogen on a surface ion pair O^2–_LC—Mg²⁺_LC with very low coordination numbers. O—H and Mg—H bonds formed have no hydrogen bonding with neighbouring ions and behave as ‘free’ groups. Their frequencies, including bending modes, have been assigned.

Species W₂(and/or W₃), dominant in the lower-temperature region, also seems to be formed via the heterolytic dissociation of hydrogen. Its adsorption process requires a higher activation energy than that of species W₅, while the desorption process, controlled by first-order kinetics, requires a lower activation energy than that of species W₅.