No abstract

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 Mg 3C 2+ (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 (ν(HH) mode).

Cu+-exchanged Si/Al 11:1 chabazite has been studied ab initio using the periodic CRYSTAL03 computer code with Hartree−Fock and the hybrid B3LYP Hamiltonians to characterize the structures and energetics of the Cu+ ion sitting preference and its interaction with H2. Two sites (I and IV) have been found to be stable for Cu+ ion:  site I, the most stable one, envisaging coordination in a six-membered zeolite ring and site IV in which the Cu+ ion sits in the largest eight-membered ring. Interaction of H2 gives adsorption energies at B3LYP of −13 and −56 kJ/mol for sites I and IV, respectively. The B3LYP bathochromic harmonic H2 frequency shifts are 847 and 957 cm-1 for adsorption at sites I and IV, respectively, in good agreement with the shifts measured (1030 and 1081 cm-1) in the Cu−ZSM-5 system in which Cu+ ion is, respectively, three and bi-coordinated by the oxygen atoms of the zeolite framework. Analysis of the components of the adsorption energy, carried out within the cluster approach, revealed that charge transfer from the Cu(3dπ) orbital through the antibonding H2(σu) and orbital polarization play a significant role in the H2 adsorption energy, and cause the large bathochromic H2 frequency shift.

Cu(I) ions in Cu-ZSM-5 form Cu+(H2) complexes, stable at room temperature and sub-atmospheric H2 pressure, which do not have any homogeneous analogue except for matrix-isolated [Cu(eta2-H2)Cl]. Comparison with the unstable Na+(H2) adducts formed in the parent Na-ZSM-5 zeolite allow the conclusion that the Cu(I)/H2 bond is governed by sigma-pi overlap forces.