Molecular and Dissociative Adsorption

The magnitude of AZ/a s can tell us whether a diatomic molecule remains intact upon adsorption or dissociates into its constituent atoms. For adsorption as a diatomic molecule, AH. ds is defined as the energy needed to break the bond between the substrate (M) and the adsorbed molecule (X2), MX2  

If the diatomic molecule dissociates upon adsorption, the heat of adsorption for this purpose is defined as 

From the knowledge of (which is always positive) and the binding energy of the gas-phase molecule, the energy of the surface chemical bond is given by 

We will discuss the surface-structure sensitivity of chemical binding of adsorbed atoms and molecules when we review the properties of the surface chemical bond in Chapter 6. 

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Yildirim, T., J. Iniguez, S. Ciraci, Molecular and dissociative adsorption of multiple hydrogen molecules on transition metal decorated C60. Phys. Rev. B Condens. Matter Mater. Phys. 72(15), 153403 (4 pages), 2005. 

B. Yoon, H. Hakkinen, and U. Landman, Interaction of O2 with gold clusters molecular and dissociative adsorption, J. Phys. Chem. A 107,4066-4071 (2003). 

The classical dynamics treatment for CO on Ni 001) yields results which are in qualitative agreement with these findings. Approximately 80% of the CO molecules that eject are found to eject intact, without rearrangement. The formation of NiCO and NuCO clusters have been observed to form over the surface via reactions of Ni atoms and CO molecules. No evidence has been found for NiC and NiO clusters in the calculations. The ion bombardment approach, then, is a very sensitive probe for distinguishing between molecular and dissociative adsorption processes. 

Kazansky VB, Serykh AI, Pidko EA (2004) DRIFT study of molecular and dissociative adsorption of light paraffins by HZSM-5 zeolite modified with zinc ions methane adsorption. Journal of Catalysis 225(2) 369-373... 

The adsorption of diatomic molecules on a metal surface may be considered as a competition between molecular and dissociative adsorption... 

Figure 4.40 shows schematically three possible forms of potential energy curves for molecular and dissociative adsorption as the molecule approaches the surface. The molecularly adsorbed state can be considered as a precursor state for dissociation. The dotted curves represent molecular adsorption with a heat of adsorption... 

It is often observed that molecular adsorption prevails at lower temperatures and that dissociative adsorption occurs at higher temperature. This may be caused by kinetics the activation energy for dissociative adsorption is then too high for dissociation at lower temperature. It could also have a thermodynamic reason. If the number of surface sites where adsorption can take place is equal for molecular and dissociative adsorption, the surface can accommodate twice as many molecules in the molecular state than in the dissociative state. Hence, molecular adsorption will prevail if the heat of dissociative adsorption is not very much higher than the heat of molecular adsorption. The entropy change upon... 

Methanol adsorption at Mo03(010), (001), and (100) surfaces has been studied in cluster models using the semi-empirical ENHT method [230]. The calculations suggest that both molecular and dissociative adsorption may occur at the different surfaces. The stability of molecular adsorption complexes increases from the (100) surface to (001) and (010) if the latter contains oxygen... 

The TiOiCllO) surface facilitates both molecular and dissociative adsorption of formic acid. The dissociative adsorption of formic acid (to form surface formates) is mediated by surface oxygen anions low-energy electron diffraction studies have shown that formate is ordered into (2x1) domains with formates bridging the surface titanium cations. This ordered layer is disrupted on heating formate may recombine with surface hydroxyl groups to desorb formic acid (reverse of reaction 2) or it may decompose to form the dehydration products CO and H2O, as well as small amounts of CO2 and H2 [43]. 

McGill PR, Idriss H (2008) Ab initio study of surface acid-base reactions. The case of molecular and dissociative adsorption of ammonia on the (Oil) surface of rutile TiO, Langmuir 24 97... 

Kazansky V B, Serykh A I, van Santen R A and Anderson B G (2001), On the nature of the sites of dihydrogen molecular and dissociative adsorption in ZnHZSM-5. II. Effects of sulfidation , Catal Lett, 74, 55. 

Molecular and dissociative adsorption can be understood as acid-base processes. Molecules adsorbing without dissociation always bind to one or several metal cations. NH3 and pyridine are the most commonly used probes for determining the acid site strength of oxides. 

Ab initio simulation methods (like the Car-Parrinello method) are currently so much limited in system size (typically substantially less than 200 atoms) and simulation time (a few picoseconds), that they have only been applied to vacuum systems, for example, to the investigation of molecular and dissociative adsorption of water on a magnesium oxide surface [159, 160], and not to an entire metal-solution interface. However, these methods hold substantial promise for the future. 

A few recent calculations [82,83] have found a mixed molecular and dissociative adsorption. Dissociation is supposed to occur on the low-coordinated surface atoms [68, 79, 84-88], on steps [75,76,86], and when surfaces are reconstructed [89], eventually leading to the presence of facets with other orientations [90]. 

Nitric Oxide. Nitric oxide (NO) is a very active molecule because it has one electron in its relatively low lying LUMO orbital. As a result, it dissociates more readily than CO and it can more easily adopt tilted configurations. Thus, both molecular and dissociative adsorptions of NO are observed on )/-Mo2N (79). In addition to the NO, N2 and N2O are also observed indicating occurrence of decomposition and disproportionation reactions of NO. 

Molecular and dissociative adsorption of oxygen has been revealed by vibrational spectroscopies such as IRAS and electron energy loss spectroscopy (EELS) over... 

The spectroscopic investigations performed by Sleight and coworkers [14-17] demonstrated that methanol chemisorption on M0O3 at room temperature results in a combination of molecular and dissociative adsorption mechanisms. The first mechanism can be considered as a physical adsorption since the methanol molecules adsorb intact on the surface. Dissociative adsorption is a chemisorption process that involves the formation of metal-methoxy (M-OCH3) groups. Further,... 

Table 2.3 Some theoretical data for the molecular and dissociative adsorption of hydrogen on gold single-crystal and cluster surfaces.

It is possible to explain the adsorption phenomena for the molecular and dissociative adsorption at energetic level. For the dissociative adsorption, the energy level is low. However, when the energetic barrier is too low, molecules can easily be adsorbed. Thus, the energy barrier may predict qualitatively the chemisorbed state, according to the Lennard-Jones diagram [1, 17]. 

Molecular and dissociative adsorption can be understood as acid/base processes. 

L. Benco, T. Bu6ko, J. Hafner and H. Toulhoat. A density functional theory study of molecular and dissociative adsorption of H2 on active sites in mordenite. /. Phys. Chem. B 109, 2005, 22491-22501. 

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