Dissociative chemisorption surface defects

Strictly speaking, the hydroxyl group, OH, does not necessarily belong to the category of point defects it results from the dissociative chemisorption of water or hydrogen on the surface of an oxide. The reason why we are including... 

Because pit formation is activated by elastic strain from extended defects (which can be deliberately introduced), we can exert some degree of control over the size and density of the pits and, therefore, the available population of undercoordinated Mo. To test the influence that these sites have on the chemisorption of alcohols, a series of model surfaces were created with different densities of undercoordinated Mo associated with pits and then reacted with methanol saturated N2 at 330 °C. These conditions lead to the formation of the H, Mo03 phase, which was used as a quantitative and local indicator of the dissociative chemisorption reaction. The images in Fig. 13 illustrate selected results from these experiments. In Fig. 13a and b, it is clear that the acicular H,jMo03 precipitates are growing from the lateral walls of the surface pits [75]. 

As a final point, which illustrates the importance of surface processes to semiconductor behaviour, Neave et al. [346] and Kiinzel and Ploog [347] have shown that deep level incorporation during the growth of GaAs films from beams of Ga and arsenic is dependent on the arsenic species used. The deep levels are believed to be associated with intrinsic defects and films prepared from As4, in which a pairwise interaction is involved, contain a higher concentration of three specific deep centres than those prepared from As2 where only simple dissociative chemisorption occurs. 

H2 and H adsorption on simple metal oxides has been studied even less than that on transition metal oxides. H2 adsorption onto various defects on the MgO(lOO) surface has been treated theoretically using defect lattice techniques, including the relaxation of the lattice around the defects. Surface defects (F- and V-centers and self-trapped holes) were all found to activate dissociative chemisorption, resulting in the formation of OH radicals [127, 128]. This is in general agreement with the observed catalytic activity of MgO after the creation of F-centers by X-rays... 

CO is adsorbed molecularly on smooth Ni surfaces but dissociatively at steps. The probability of dissociative chemisorption is generally higher at surface defects such as steps and edges. 

Compositional effects and mechanisms. Am Mineral 84 345-356 Bimker BC, Haaland DM, Michalske TA, Smith WL (1989a) Kinetics of dissociative chemisorption on strained edge-shared surface defects on dehydroxylated sihca. Smf Sci 222 95-118 Bimker BC, Haaland DM, Ward KJ, Michalske TA, Smith WL, Balfe CA (1989b) Infrared spectra of edge-shared silicate tetrahedra. Surf Sci 210 406-428 CahnRW (1986) Melting and the surface. Nature 323 668-669... 

This scheme summarizes the properties of the sites, that is, of a pair of silicon radicals associated with two anomalously reactive oxygen atoms — but provides little information concerning the geometry, apart from the requirement that the two silicon atoms must be closely spaced. Dissociative chemisorption of several molecules — that is, H2O, CH3OH, O2, etc. — takes place on dehydroxylated silica [116] as compared with nonpretreated surfaces, and this can be primarily associated with the presence of Si defects, as depicted by structure R. 

Chemisorption may also proceed by a mechanism involving an electron transfer between the adsorbate and the substrate (oxidation- reduction or redox interaction). It is the case for adsorbates such as O2 or CI2 that are strong electron acceptors. O2 can be molecularly or dissociatively adsorbed, CI2 is dissociatively adsorbed. The redox reactions that involve electronic carriers are expected to occur preferentially on semiconducting or metallic oxides. On wide-bang-gap insulators these reactions are promoted by surface defects such as ion vacancies, which may act as sources or sinks for electrons. 

The relative rates of undissociative 02 chemisorption vs. dissociative 02 chemisorption, which depend in general terms on the presence of defective Pd sites on the surface (Figure 8.13). 

In conclusion, the combined experimental and theoretical study of methanol adsorbed on MgO films with different defect densities allows for a better identification of the surface sites responsible for the MgO reactivity. On the inert terrace sites only physisorption is observed. Molecular chemisorption, activation, and heterolytic dissociation occur on irregular sites. The low-coordinated Mg-O pairs of ions located at edges and steps can lead to strongly activated and even dissociated methanol molecules. Adsorption of CHsO" and H+ fragments seems to be preferred over dissociation into and OH ... 

The relative ease with which hydrogen chemisorbs on the surface of a metal oxide surface mainly depends on the chemical nature of the oxide and on the O-vacancies. Thus, hydrogen adsorbs dissociatively on a perfect titanium oxide surface [10,11]. The energetically most favorable mode for the adsorption of atomic hydrogen is the adsorption on the outermost O atom, accompanied by the reduction of a Ti atom. In this mode, protons are formally adsorbed while an equivalent amount of Ti(IV) atoms are reduced to Ti(III). Theoretical calculations have demonstrated that H adsorption is less favorable on a defective surface than on a perfect surface. However, the best adsorption mode for the atomic chemisorption on a defective surface is heterolytic adsorption, which involves two different adsorption sites one H+/0= and one H on the surface. This adsorption mode is best on irreducible oxides such as MgO however, it is less favorable than adsorption on the perfect Ti02 surface [10]. The heat of atomic adsorption in all cases is very weak and dissociation onto the surface is unlikely. The molecular adsorption (physisorption), thus, remains the most stable system. 

To summarize, recent studies of hydrogen chemisorption, Hg/ Dg exchange and hydrogenation reactions on oxide-supported gold catalysts clearly show that the Au nanoparticles are able to activate the dissociation of hydrogen molecules. Likewise, they support the theory that adsorption only occurs on defective Au surface atoms at corners and edges of the nanoparticles. A second conclusion drawn from some of these studies is that the dissociation of Hg on supported gold nanoparticles could be an activated process. ... 

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