Dissociative water adsorption

Water adsorption on alumina leads to molecular adsorbates [110]. There are some reports on the formation of isolated hydroxyl groups on regular sites [113, 114]. Whether they are the results of dissociative water adsorption is not completely clear, although there is a recent theoretical paper that predicts the dissociation of water on Al203(0001) by protonation of a surface oxygen, while the molecule previous to dissociation resides on a cation site [115]. There is no experimental verification of this result as yet. There are, however, explored routes to hydroxylate alumina surfaces either by exposure to atomic hydrogen or by a combination of aluminium deposition and subsequent hydrolyzation with water [116]. 

Dissociative water adsorption, which is a prerequisite for CO electro-oxidation, starts at more negative potentials on the composite Ru/Pt(lll) electrode than on the Pt(l 11) surface. A plausible explanation is that an activation of the Pt surface due to long-range electronic interactions with the Ru islands occurs, facilitating OH adsorption at lower potentials. However, because IR spectroscopy of COads is very sensitive to electronic influences, but gives no indications for such an effect, this explanation is rather unlikely. 

Odelius M (1999) Mixed molecular and dissociative water adsorption on MgO(lOO). Phys Rev Lett 82 3919-3922... 

FIGURE 3.15 (a) Perspective view of dissociative water adsorption on wollastonite 100 surface all shown... 

Using the static energy minimization technique, we have modeled most of the predominant surfaces and studied their active sites and coordination states and the stereochemistry of a-quartz. We have investigated adsorption behavior of molecular water, methanoic acid, and methylamine on pure a-quartz. Dissociated water adsorption depicting the situation of protonation and hydroxylation onto unsaturated surface has also been modeled. Using our knowledge of the persistent presence of surface hydroxyl group in aqueous media, we have studied adsorption of molecular water, methanoic acid, and methylamine on hydroxylated surface. 

It is evident from the adsorption energies presented in Table 3.5, that except for the 0001 and 1120 surfaces, dissociative water adsorption was more energetically favorable than other adsorbate molecules. The 1020 surface was the most reactive for associative adsorption, and the preference was for methylamine, followed by methanoic acid, followed by water. The lOll and lOlI surfaces did better when hydroxylated, but for associative adsorption the preference was more toward methylamine, followed by methanoic acid, followed by water. 

Water adsorption and dissociation on Ni( 111) and Nig(6 + 2) clusters have been studied by ab initio quantum-chemical calculations.744 "746... 

Many organic electrode processes require the adsorption of the electroactive species at the electrode surface before the electron transfer can occur. This adsorption may take the form of physical or reversible chemical adsorption, as has been commonly observed at a mercury/water interface, or it may take the form of irreversible, dissociative chemical adsorption where bond fracture occurs during the adsorption process and often leads to the complete destruction of the molecule. This latter t q)e of adsorption is particularly prevalent at metals in the platinum group and accounts for their activity as heterogeneous catalysts and as... 

Fig. 6-3S. Potential energy curves for water adsorption on metal surface in the states of molecules and hydrozjd radicals c = energy r = reaction coordinate solid curve = adsorption as water molecules and as partially dissociated hydroxj4 and hydrogen radicals broken curve = adsorption of completely dissociated oxygen and hydrogen radicals.

The calculated heats of dissociative adsorption of water, ammonia, and methyl alcohol (Fig. 19) are also presented in Table VII. The heat of water adsorption was 61 kcal/mol (CNDO/BW) or 43.7 kcal/mol (STO-3G), and the respective value for ammonia was 38.7 kcal/mol (CNDO/BW). If the pseudo-atoms A model the lattice silicon atoms, structure (d) is somewhat... 

The quantitative evaluation of defects is a well defined but hard to tackle problem for future studies that has to be taken on by our research community. Water adsorption is an example that lends itself to a study of the influence of defects, because at lower coverage the (100) cleavage planes of MgO and NiO do not dissociate water, while the presence of defects does induce water dissociation. 

Water adsorption has been studied quite extensively in the past, both on the TiO2(100) and the TiO2(110) surfaces [159, 160]. There is substantial evidence that water dissociates easily on the TiO2(100) surface leading to the formation of hydroxyl groups [161, 162]. On the TiO2(110) surface studies by... 

The hydration of acetylene takes place between acetylene and water both adsorbed on similar Cd Z"2 sites [14]. The increase of activity with Si/Al ratio suggests that heterolytically dissociated water is involved in hydration because this dissociative adsorption of water on bivalent metal zeohtes increases with Si/Al ratios as it has been observed for Ca- and Mg-Y,FAU [15]. 

The above picture of water/oxide interface does not obviously show the simultaneous, primary and secondary adsorption on non-dissociated water molecules. In their review, Etzler and Drost-Hausen wrote [89] Furthermore, as mentioned elsewhere in this paper (and other papers by the present author and associates), it is obvious that vicinal water is essentially unaffected by electrical double layers . Several properties of the vicinal water appear to be similar for various solid surfaces characterized by various point of zero charge (PZC) values (the paradoxical effect ). It is therefore to be expected that the contribution to the changes of the heat of immersion with changing pH, produced by the secondarily adsorbed vicinal water, is negligible. 

The chemical nature of the sites of alumina for water adsorption is not clearly understood. Chemisorption of water through dissociation into H and 0H ions which attach to the alumina surface, hydrogen bonding of water with surface oxygen and hydroxyl groups, van der Waals and pole-pole interactions between the water molecule and the alumina surface as well as condensation of water vapour in the mesopores of alumina are possible mechanisms [5]. 

For the study of adsorption to general oxide surfaces, the ideal ionic force-field would be consistent with a molecular mechanics model for dissociable water. Such a forcefield has indeed been developed and has been applied to the study of silicate (Rustad Hay, 1995) and iron(III) (Rustad et al., 1995) hydrolysis in solution, to bulk iron oxyhydroxide structures (Rustad et al., 1996a) and to the protonation of goethite surfaces (Rustad et al., 1996b). 

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