Vibrational and thermodynamic properties of H2 adsorbed on MgO in the 300-20 K interval

The adsorption of H-2 on high surface area, sintered and smoke MgO samples fully characterized by HRTEM and AFM microscopies has been investigated in the 300-20 K temperature interval by FTIR spectroscopy On high surface area MgO, dissociative adsorption of H-2 has been observed with formation of reversible (absorbing at 3454 and 1325 cm(-1)) and irreversible (absorbing at 3712 and 1125 cm(-1)) OH and MgH species already reported in previous studies at 300 K. Cooling the MgO/H-2 system down to 20 K results in the irreversible formation at about 200 K of new OH (absorbing at 3576-3547 cm(-1)) and MgH (absorbing at 1430-1418 cm(-1)) surface groups never observed before. The spectra recorded at 20 K in H-2 atmosphere also show absorptions in the 4800-4000 cm(-1) frequency interval undoubtedly due to molecularly adsorbed species. Decreasing the MgO surface area results in the disappearance of all of the spectroscopic manifestations due to the hydride and hydroxyl groups formed upon dissociative adsorption of hydrogen, whereas those due to H-2 adsorbed in molecular form are maintained (although with much reduced intensity). This behavior is the consequence of the reduction, revealed by HRTEM and AFM, of the concentration of surface defects (cationic and anionic sites located on edges, corners, steps, inverse edges and inverse corners). On the basis of the morphological characterization and of the IR spectroscopic studies, it is concluded that the sites responsible for the H-2 dissociative adsorption are mainly inverse steps "coupled" with edges and corners, whereas more usual "isolated" defects (edges, steps, and corners) adsorb hydrogen only in molecular form. The specific adsorption energy for the formation of molecular Mg-nC(2+)...H-2 adducts on (corners; 7.5 kJ/mol), Mg-4C(2+) (edges; 4.6 kJ/mol), and Mg-5C(2+) (on (100) planes; 3.6 kJ/mol) coordinatively unsaturated sites has been also calculated from the temperature dependence of the intensity of the related IR bands (v(HH) mode).