Bonding dissociative adsorption

For alkali modified noble and sp-metals (e.g. Cu, Al, Ag and Au), where the CO adsorption bond is rather weak, due to negligible backdonation of electronic density from the metal, the presence of an alkali metal has a weaker effect on CO adsorption. A promotional effect in CO adsorption (increase in the initial sticking coefficient and strengthening of the chemisorptive CO bond) has been observed for K- or Cs-modified Cu surfaces as well as for the CO-K(or Na)/Al(100) system.6,43 In the latter system dissociative adsorption of CO is induced in the presence of alkali species.43... 

Similar to the case of CO, the dissociation propensity of NO depends largely on the substrate, following the same general trends. Alkali introduction on metal substrates promotes the dissociative adsorption of NO, both by weakening the N-O intramolecular bond and by stabilizing the molecular state which acts as a precursor for dissociation. 

In our previous work [11], it has been shown that the reduction of NO with CH4 on Ga and ln/H-ZSM-5 catalysts proceeds through the reactions (1) and (2), and that CH4 was hardly activated by NO in the absence of oxygen on these catalysts. Therefore, NO2 plays an important role and the formation of NO2 is a necessary step for the reduction of NO with CH4. In the works of Li and Armor [17] and Cowan et al. [18], the rate-determining step in NO reduction with CH4 on Co-ferrierite and Co-ZSM-5 catalysts is involved in the dissociative adsorption of CH4, and the adsorbed NO2 facilitates the step to break the carbon-hydrogen bond in CH4. It is suggested that NO reduction by use of CH4 needs the formation of the adsorbed NO2, which can activate CH4. 

The N,N-dimethylpyrrolidine did not react at the same conditions that is, the tetrasubstituted nature of the hydrazine functionality prevents the cleavage of the N-N bond. It is likely that the rupture of the N-N bond needs the dissociative adsorption (N-H bond rupture) of at least one nitrogen atom. 

We consider two cases (see Fig. A.13). First, the metal has a work function that is between electron affinity (the energy of the o -level) and the ionization potential (the energy of the o-level) of the molecule. Upon adsorption, the levels broaden. However, the occupation of the adsorbate levels remains as in the free molecule. This situation represents a rather extreme case in which the intramolecular bond of the adsorbate molecule stays about as strong as in the gas phase. The other extreme occurs if both the a-level and the o -1evel fall below the Fermi level of the metal. Because the antibonding G -level is filled with electrons from the metal, the intramolecular bond breaks. This is the case for hydrogen adsorption on many metals. Thus, a low work function of the metal and a high electron affinity of the adsorbed molecule are favorable for dissociative adsorption. 

The dipole moment of the adsorbed water molecules is estimated to be = 0.22 D (unit of D = 3.36 x 10 ° C m) from the slope of the observed curves shown in Fig. 5-25. Since this dipole moment is nearly one tenth of the dipole moment of gaseous water molecules (m = 1.84 D), the dipole of the adsorbed water molecules on the silver surface is suggested to be aligned almost parallel to the metal surface by forming hydrogen-bonded two-dimensional clusters of water molecules. On the other hand, bromine molecules are in the state of dissociative adsorption on the silver surface, producing adsorbed bromine atoms which receive electrons... 

The associative mechanism resembles a conventional radical (hydrogen atom) substitution reaction where the 7T-bonded benzene molecule is attacked by a hydrogen atom formed by the dissociative adsorption of water or hydrogen gas. The activation energy in this process is essentially due to the partial localization of one tt electron in the transition complex 21, 31). The transition state differs, however, from conventional substitution reactions by being 77-bonded to the catalyst surface ... 

The low probability for dissociative adsorption of HCOOH on Ag is clear indication that adsorption was activated. For such a process, one would expect the dissociative sticking probability to depend on the structure of the surface, since alignment of the O-H bond with the metal surface... 

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