Metalization-molecular dissociation

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Takemura K, Minomura S, Shimomura O, Fu]ii Y and Axe J D 1982 Structural aspects of solid iodine associated with metallization and molecular dissociation under high pressure Phys. Rev. B 26 998... 

Carbon monoxide is chemisorbed molecularly on some transition metals but dissociatively on others. An approximate borderline can be drawn through... 

Looking at the trends in dissociation probability across the transition metal series, dissociation is favored towards the left, and associative chemisorption towards the right. This is nicely illustrated for CO on the 4d transition metals in Fig. 6.36, which shows how, for Pd and Ag, molecular adsorption of CO is more stable than adsorption of the dissociation products. Rhodium is a borderline case and to the left of rhodium dissociation is favored. Note that the heat of adsorption of the C and O atoms changes much more steeply across the periodic table than that for the CO molecule. A similar situation occurs with NO, which, however, is more reactive than CO, and hence barriers for dissociation are considerably lower for NO. 

Another PES topology for molecular dissociation occurs when an intermediate molecularly chemisorbed state lies parallel to the surface between the physisorption well and the dissociated species as shown in Figure 3.2(b). This molecular state is usually described in terms of a diabatic correlation to a state formed by some charge transfer from the surface to the molecule [16]. In this case, there can be two activation barriers, V] for entry into the molecular chemisorption state of depth Wx and barrier V2 for dissociation of the molecularly chemisorbed state. This PES topology is relevant to the dissociation of some it bonded molecules such as 02 on metals, although this is often an oversimplification since distinct molecularly adsorbed states may exist at different sites on the surface [17]. In some cases, V < 0 so that no separate physisorbed state exists [18]. If multiple molecular chemisorption... 

Direct and indirect channels to molecular dissociation at metal and metal alloy surfaces... 

We will know review adsorption-desorption kinetic studies using molecular beams that provide direct insight in the elementary steps and accurate kinetic and energetic parameters. We will first take the case of NO on Pd/MgO(l 00) that has been recently studied in Marseilles [88-91]. This work will exemplify how it is possible to study the different aspects of the adsorption process on a complex surface adsorption and desorption from the clean surface, adsorption on the metal clusters by direct impingement or via a precursor state on the support, desorption from the metal particles, dissociation on the particles and removing of the dissociation products. 

Fujii et al.30 have reported experimental evidence for the molecular dissociation process in Br2 near 80 GPa. This transition, which is coincident with the onset of pressure-induced metallization, was first discovered in molecular/metallic iodine.3 A diatomic molecular crystal loses its molecular character in the limit when the intermolecular distance becomes equal to the intramolecular bond length. Fujii et al.30 applied the Herzfeld criterion to I2 and Br2 and estimated that the molar reffactivity reaches the atomic limit around 20 GPa in I2 and 80 GPa in B12. In both cases, the computed pressure coincides with that for molecular dissociation accompanied by metallization. 

The interaction of CO2 with group VIII metals was reviewed by Solymosi (29). At 80 K, the adsorption on a Rh field emitter exhibits an interesting crystal face dependency 30). In addition to molecular adsorption, dissociation occurs at an appreciable rate on the stepped surfaces around (111) and (100) at temperatures higher than 220 K. The field electron microscopy (FEM) patterns suggest that the surface structure of Rh has a striking influence on the ability of the metal to dissociate the CO2 molecule. From... 

A number of electrode processes involve an initial step of molecular dissociative adsorption at the electrode metal surface. Such reactions have important technological significance in the fields of fuel-cell and gas-battery development. For the cases of simple reactions involving, for example, H2 or Cl 2, these steps are the reverse of the final molecule-producing step in the corresponding gas evolution process. Examples are as follows ... 

---