Defects hydrogen adsorption

The most important features of both the reflectance and the photolumi-nescence spectra have been explained by the preceding model since it is based on ideal surface structures essentially determined by (001) planes. Thus, several likely possibilities, such as the presence of surface defects, impurities, and remaining adsorbates, the relaxation of the planes exposed at the surface, the impurity-induced reconstruction of the surfaces, and changes in the force constants, have been excluded (80). A more detailed model is needed in which the ion pair of the metal cation and oxygen anion can be taken into account on the basis of such experimental evidence as the hydrogen adsorption on MgO obtained by Coluccia and Tench (65) and Ito et al. (90). 

Color is not unique in adsorbing hydrogen rapidly and irreversibly. ESB signals in irradiated porous glass attributed to holes trapped at a boron impurity center and at some sort of Si-0 defect disappeared irreversibly on addition of hydrogen (81a). Similar sites might be anticipated in silica gel as well, and could be responsible for part of the hydrogen adsorption. 

From a mechanistic point of view, there are at least two different sites for hydrogen adsorption. The desorption from the positions binding less strongly sets in at about 400 °C already, while from those bound more tightly, it only occurs above 600 °C. The latter sites presumably are those inside the tubes, whereas the weaker binding is related to positions on the outer wall. Existing defects should also contribute to hydrogen adsorption, yet chemisorption should be considered in this case. 

S. Letardi, M. Celino, E. Cleri and V. Rosato, Atomic hydrogen adsorption on a Stone-Wales defect in graphite . Surface Science, 496, 33 (2002). 

The deposits having lesser surface defects, which act as the active sites of hydrogen adsorption, exhibited higher current efficiency. The presence of gelatin inhibited the H2 evolution and significantly improved the current efficiency (see Fig. 8). 

We will start this section with the study of the hydrogen adsorption on different Pd-Au nanostructures. Figure 1.13 shows the considered systems that include planar surfaces, defects, adatoms, and nanostructures. 

N. (2009) Effect of vacancy defects in graphene on metal anchoring and hydrogen adsorption. Appl. Phys. Lett, 94, 173102. 

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