Atom Recombination

The condensed-phase atomic recombination process has been extensively studied from both experimental and theoretical points of view. This apparently simple reaction is actually rather complex, and our knowledge of the process is still very far from being complete. We have already referred to this type of reaction several times to illustrate certain features of the kinetic theory formulation. We now give a more detailed and coherent discussion. 

Since we are concerned with recombination in the liquid phase, the appropriate potential for the description of atom-atom interactions is the potential of mean force fV(r) = -kgTlng(r), schematically displayed in Fig. 4.1. If we adopt the model described in Section VI, in which the solvent-solvent and solute-solvent forces are approximated by hard-sphere interactions, then 

In view of these solvent structure effects, it is convenient to classify the recombination events into primary and secondary processes. Primary recombination processes are those in which the recombination takes place before the atoms separate to a distance roughly equal to the first maximum in the mean potential (i.e., recombination in the solvent cage ). Secondary recombination involves the recombination of solvent-separated-atom pairs. 

Part of the stimulus for research in this area comes from the possibility of probing the dynamics of such processes on short time scales by using picosecond lasers. The standard pulse-and-probe experiments will measure the entire time profile of the recombination and photodissociation processes. An interpretation of such results therefore requires a consideration of the dynamics on several potential energy surfaces for both the primary and secondary recombination processes. The very short time behavior is often obscured by experimental problems (laser rise times etc.), but the secondary recombination process is more easily studied. 

Theoretical treatments often focus on either the primary or secondary processes. The traditional approaches that make use of a simple diffusion equation for the pair dynamics are necessarily restricted to a description of 

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Trimoleciilar reactions require the simultaneous encounter of tliree particles. At the usually low particle densities of gas phase reactions they are relatively unlikely. Examples for trimoleciilar reactions are atom recombination reactions... 

In fact, the bimolecular mechanisms are generally more likely. Even the atom recombination reactions sometimes follow a mechanism consisting of a sequence of bimolecular reactions... 

Instead of concentrating on the diffiisioii limit of reaction rates in liquid solution, it can be histnictive to consider die dependence of bimolecular rate coefficients of elementary chemical reactions on pressure over a wide solvent density range covering gas and liquid phase alike. Particularly amenable to such studies are atom recombination reactions whose rate coefficients can be easily hivestigated over a wide range of physical conditions from the dilute-gas phase to compressed liquid solution [3, 4]. 

Although the transition to difhision control is satisfactorily described in such an approach, even for these apparently simple elementary reactions the situation in reality appears to be more complex due to the participation of weakly bonding or repulsive electronic states which may become increasingly coupled as the bath gas density increases. These processes manifest tliemselves in iodine atom and bromine atom recombination in some bath gases at high densities where marked deviations from TronnaF behaviour are observed [3, 4]. In particular, it is found that the transition from Lto is significantly broader than... 

Otto B, Schroeder J and Tree J 1984 Photolytic cage effect and atom recombination of iodine in compressed gases and liquids experiments and simple models J. Chem. Phys. 81 202... 

The approach is ideally suited to the study of IVR on fast timescales, which is the most important primary process in imimolecular reactions. The application of high-resolution rovibrational overtone spectroscopy to this problem has been extensively demonstrated. Effective Hamiltonian analyses alone are insufficient, as has been demonstrated by explicit quantum dynamical models based on ab initio theory [95]. The fast IVR characteristic of the CH cliromophore in various molecular environments is probably the most comprehensively studied example of the kind [96] (see chapter A3.13). The importance of this question to chemical kinetics can perhaps best be illustrated with the following examples. The atom recombination reaction... 

Two thermocouples, Em at x = 0 and Ex at a distance x, permit the monitoring of the atomic hydrogen concentration change along the side-tube. The atoms recombining on the thermocouple tip covered by the active catalyst evolve the heat of reaction and thus the thermoelectric power becomes a relative measure of the concentration of atoms in the gas phase. Finally, one obtains for the direct use in an experimental work the following equation... 

Coefficients of H Atom Recombination, y, at —78°C on Nickel or Nickel-Copper Foils and Their Respective 0-Hydride Phases... 

Fig. 10. Coefficient of H atom recombination on Ni-Cu alloy catalysts as a function of the alloy composition, at 20°C. A, on Ni-Cu foils (59), O, on Ni-Cu evaporated films af ter their previous homogenization at 400°C (65,65a) d, on Ni-Cu foils after a multiple hydrogen absorption-desorption treatment (64a).

Fig. 13. Arrhenius plots of the kinetics of H atom recombination on a Ni77Cu23 alloy film catalyst. Above room temperature—active NiCu film with low activation energy. Below room temperature—film deactivated owing to a 0-hydride phase formation activation energy markedly increased. After Karpinski el al. (65).

Fig. 14. Arrhenius plot of the kinetics of H atom recombination on a rich in copper alloy film catalyst Ni20Cu80. Within the whole range of temperature the linear relationship holds activation energy constant. After Karpinski (65a).

The hydride phase may be present in a catalyst as a result of the method of its preparation (e.g. hydrogen pretreatment), or it may be formed during the course of a given reaction, when a metal catalyst is absorbing hydrogen (substrate—e.g. in H atom recombination product—e.g. in HCOOH decomposition). The spontaneous in situ transformation of a metal catalyst (at least in its superficial layer) into a hydride phase is to be expected particularly when the thermodynamic conditions are favorable. 

Low-temperature, photoaggregation techniques employing ultraviolet-visible absorption spectroscopy have also been used to evaluate extinction coefficients relative to silver atoms for diatomic and triatomic silver in Ar and Kr matrices at 10-12 K 149). Such data are of fundamental importance in quantitative studies of the chemistry and photochemistry of metal-atom clusters and in the analysis of metal-atom recombination-kinetics. In essence, simple, mass-balance considerations in a photoaggregation experiment lead to the following expression, which relates the decrease in an atomic absorption to increases in diatomic and triatomic absorptions in terms of the appropriate extinction coefficients. 

A dilute I2/CCI4 solution was pumped by a 520 nm visible laser pulse, promoting the iodine molecule from its ground electronic state X to the excited states A,A, B, and ti (Fig. 4). The laser-excited I2 dissociates rapidly into an unstable intermediate (I2). The latter decomposes, and the two iodine atoms recombine either geminately (a) or nongeminately (b) ... 

One way to relate the concentration of an intermediate to other concentrations is by assuming that earlier reversible steps have equal forward and reverse rates. The proposed mechanism begins with the decomposition of Bf2 molecules into Br atoms, most of which recombine rapidly to give Br2 molecules. At first, Br2 molecules decompose faster than Br atoms recombine, but the Br atom concentration quickly becomes large enough for recombination to occur at the same rate as decomposition. When the two rates are equal, so are their rate Rate of decomposition = Rate of recombination expressions Rr2] =. 1 [Br] ... 

On a homogeneous surface, the rate of the atom recombination step will be proportional to the square of the atom surface concentrations ... 

Gdowski GE, Farr JA, Madix RJ. 1983. Reactive scattering of small molecules from platinum crystal surfaces D2CO, CH3OH, HCOOH and the nonanomalous kinetics of hydrogen atom recombination. Surf Sci 127 541. 

Gas phase third-order reactions are rarely encountered in engineering practice. Perhaps the best-known examples of third-order reactions are atomic recombination reactions in the presence of a third body in the gas phase and the reactions of nitric oxide with chlorine and oxygen (2NO T Cl2 -> 2NOC1 2NO + 02 -> 2N02). 

Surprisingly, despite requiring two analyte molecules to produce one S2 molecule, the kinetics of the chemiluminescent reaction are first order with respect to the sulfur compound. This can be explained if every H2S or CH3SH molecule is consumed in the reaction and every S atom recombines to form S2, through the use of an excess of OCIO to maintain pseudo-first-order reaction conditions [81]. The limit of detection for this analysis was found to be 3 ppbv for H2S. 

Thus, upon heating, CH30(a) and H(a) recombination occurred, and no hydrogen atom recombination was observed. 

A CH4 pyrolysis mechanism appears to be consistent with our observation that preheating improves partial oxidation selectivity. First, higher feed temperatures increase the adiabatic surface temperature and consequently decrease the surface coverage of O adatoms, thus decreasing reactions lOa-d. Second, high surface temperatures also increase the rate of H atom recombination and desorption of H2, reaction 9b. Third, methane adsorption on Pt and Rh is known to be an activated process. From molecular beam experiments which examined methane chemisorption on Pt and Rh (79-27), it is known that CH4 must overcome an activation energy barrier for chemisorption to occur. Thus, the rate of reaction 9a is accelerated exponentially by hi er temperatures, which is consistent with the data in Figure 1. 

A similar mechanism was proposed earlier by Adamson for photoreduction of Co(NH3)5Br2 +. 48 An observed 4>red of 1.97 for Co(NH3)5I2+ predicts a quantum efficiency of 0.97 for the primary process [reaction (20)]. The mechanism also predicts that 4>red will depend upon the Co(NH3)5I2+ concentration and inversely upon the intensity of the irradiating light in the case where recombination of I atoms is important. Support for the mechanism of Haim and Taube came from the observation that upon flash photolysis of Co(NH3)5I2 + solutions with 370-mp. light, unusually short-lived transient I atoms were observed.62 This was taken to indicate that paths [reaction (21), for example] other than I atom recombination accounted for loss of I atoms in this system. 

Motz, H., and H. Wise, Diffusion and Heterogeneous Reaction. III. Atom Recombination at a Catalytic Boundary, . /. Chem. Phys., 32, 1893-1894 (1960). 

I.R. Hurle, "Measurements of Hydrogen-Atom Recombination Rates Behind Shock Waves , llthSympCombstn (1967), 827-36... 

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