Eley-Rideal reaction

Experimental probes of Born-Oppenheimer breakdown under conditions where large amplitude vibrational motion can occur are now becoming available. One approach to this problem is to compare theoretical predictions and experimental observations for reactive properties that are sensitive to the Born-Oppenheimer potential energy surface. Particularly useful for this endeavor are recombinative desorption and Eley-Rideal reactions. In both cases, gas-phase reaction products may be probed by modern state-specific detection methods, providing detailed characterization of the product reaction dynamics. Theoretical predictions based on Born-Oppenheimer potential energy surfaces should be capable of reproducing experiment. Observed deviations between experiment and theory may be attributed to Born-Oppenheimer breakdown. 

An interesting body of work merits the reader s attention concerning the Eley-Rideal reaction of H-atom abstraction of chlorine atoms adsorbed on gold38-41 ... 

The energetics of the Eley-Rideal reaction (A E —230 kJ mol-1) are well established.42 Here, the highly exoergic reaction forming gas-phase HC1 was probed by time-of-flight velocity measurements,39,41 scattering angular distributions,39,41 and state-selective laser spectroscopy.39-41... 

Despite the large exoergicity, less than about 100 kJ mol-1 appears as HC1 translation, rotation or vibration. On average, 60kJmol-1 appears as HC1 translation, 30 kJ mol-1 appears as HC1 vibration (peaks at v = 1) and 10 k.l mol 1 as rotation.39 It was argued in Ref. 39 that despite possessing similar energetics, the Eley-Rideal reaction is qualitatively different than the gas-phase reaction ... 

Eley-Rideal reaction, wherein a gas-phase species reacts directly with an adsorbed intermediate without having to be bound to the surface itself thus,... 

Figure 3.37. HD (v,J) flux F(v,J) produced by the Eley-Rideal reaction of D+H/Cu(lll) and H+D/Cu(lll) as a function of the vibrational state v and rotational energy Er(E ). From Ref. [422].

Many experiments (especially electrochemical studies) are performed under wet conditions, and the presence of an electrolyte may change the overall energetics of gas phase reaction mechanisms. Of course, the ORR reaction creates water molecules as final reaction products. Eley-Rideal reaction mechanisms (which will be discussed in the next Section) also rely on the electrolyte as a source of hydrogen atoms. Although results from gas-phase calculations are sometimes used to interpret experiments performed in solution, we believe that at least some treatment of the water solvent is required to obtain relevant results. 

Figure 5. Eley-Rideal reaction pathways. Relative energies are in eV and do not include zero-point energy contributions. H atoms are assumed to come from electrolyte, individually adsorbed atoms reside on fee sites, and all other species reside on on-top sites.

Stability was investigated for n = I and n = 2 in both the Langmuir-Hinshelwood and the Eley-Rideal cases. The results indicate that the Eley-Rideal reaction mechanism tends to stabilize the system whereas the Langmuir-Hinshelwood mechanism easily leads to oscillatory behavior. A serious problem with this analysis, however, is that the physical interpretation of the notion of a surface temperature in the first few layers of catalyst remains unclear. 

On balance then the evidence favours the hypothesis that the selectivity in ammonia oxidation is determined by competition between NH 3 and O2 molecules for active step sites and not by the relative rates of NO desorption and reaction with ammonia. Conditions can be found favouring complete coverage of active sites by N atoms (<200 °C, desorption rate limited), or by NH3 molecules (200—500 °C, Eley-Rideal reaction with gaseous oxygen) or by O atoms (500— 1000 °C, Eley-Rideal reaction with gaseous ammonia). Above 1000 °C, simple NH3 decomposition supervenes, perhaps with oxidation of the hydrogen thus liberated. 

Persson et al. studied theoretically the Eley-Rideal reaction in which an incident species reacts directly with an adsorbate to form a product molecule that promptly leaves the surface. As an example of this reaction they studied the H + H—>H2 reaction on the Cu(lll) surface. The calculational approach was very similar to that discussed above for the CO + O CO2 reaction on Pt(lll). They studied different conflgurations of two H atoms on the Cu(lll) surface, shown in Figure 15, and calculated the total energy as a function of the structural parameters also shown in the flgure. Moreover, they studied also an isolated H2 molecule. 

In the CO oxidation on Au-SV-graphene, the Langmuir—Hinshelwood reaction proceeds with a low barrier of 7.2 kcal mol and the following Eley- Rideal reaction has an even lower barrier of 4.2 kcal moH [168]. CO is adsorbed first in the Langmuir-Hinshelwood reaction and has equal to —35.6 kcal mol , while O2 has Ej, of -31.2 kcal mol . A similar sequence was proposed as in the case of Pt [169]. 

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