H atom recombination

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. 

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. 

Under reducing conditions hydrogen atoms are the main chain carriers. Data from fuel-rich flames indicate that SO2 is efficient in catalyzing H-atom recombination. The H-atom removal presumably involves the following reaction sequence ... 

The compound responsible for the 265 m/z absorption has not yet been identified. The obvious assignment to sym-diethylhydrazine is contradicted by the absorption spectrum of an authentic sample of the hydrazine (supplied by Merck, Sharp and Dohme, Montreal), which shows no sign of a peak at 265 m/z. Instead, the spectrum in ethylamine solvent has a broad maximum at 358 m/z and rises steeply near the solvent cutoff at 250 m/z. We assume, therefore, that in Reaction 2, the ethylamine radical is scavenged by the solvent, to yield other ultimately stable products. In liquid ammonia, hydrogen and amide appear to be the only decomposition products, implying that the process H + NH8 - H2 + NH2 does not occur. The relative inefficiency of this process compared to H atom recombination in liquid ammonia is also indicated by radiation chemical studies (6). 

Understanding the mechanism for H2 formation from H atoms in space is an old and important problem in astrophysics. It is generally accepted that H atom recombination takes place on interstellar dust particles [105]. The exact composition of these dust particles is not known, but there is spectroscopic evidence that they contain graphitic components, and this has stimulated several electronic structure studies of the H-graphite interaction [86, 89, 106-109]. The recent DFT studies of Sidis and co-workers [108, 109] and Sha and Jackson [89], who modeled graphite as a coronene molecule and a slab, respectively, are in general agreement. We review the results of these studies in Section 3.1. 

A recent estimate89 of the rates of decomposition of CH4, formed by the activation process, H + CH8, at a temperature of 2000 °K. would appear too low by at least a factor of 100, and the conclusion regarding catalysis of H atom recombination by methyl, based on this estimate, would not be supported. 

Another method of measuring the relative quantum yield of the radical decomposition process (eq. 22) was also devised recently (144). This involves HNO chemiluminescence photoexcitation spectroscopy. When an H atom recombines with an NO molecule, an electronically excited HN0 ( A A" ) is formed. Fluorescence emission from HNO occurs at 762 nm. The HNO chemiluminescence in a low-pressure 1 10 mixture of H2CO and NO is proportional to the H-atom quantum yield from the photolysis of H2CO. The photoexcited HNO (red) chemiluminescence excitation spectrum of a H2CO/NO mixture obtained with a tunable laser at high resolution is shown in Fig. 2 together with an absorption spectrum and a H2CO fluorescence (blue) excitation spectrum (237). The relevant reaction scheme is as follows ... 

Figure 5.3 Welding Torch Hydrogen gas is piped into a special tube in which many of the H2 molecules are separated into atoms by an electrical discharge. The gas flow is adjusted to the rate at which most of the H atoms recombine into molecules just as they exit from the torch. The heat produced at that point is intense enough to weld pieces of steel together.

Whereas in oxidative addition electrons are used to form new bonds between metal centre and adsorbate fragment, in the reverse reaction reductive elimination of the adsorbate donates electrons to the metal centre. The H-atoms recombine to form H2 that desorbs. 

Hydrogen is a known major product, but experimentally it has not been determined whether H2 and H atoms are formed simultaneously, or if the H2 results from H atom recombination. If H2 is produced by molecular elimination, one would expect different reactions in combustion chemistry than if only H atoms were present initially. 

Considerable work is directed at the study of H-atom recombination relevant to RF plasma discharges (Kae-Nune et al, 1996 Perrin et al, 1998). Use is made of temperature-programmed desorption (TPD) (Zangwill, 1988 Bruch et al, 1997) in conjunction with threshold ionization mass spectrometry. The surfaces studied... 

In the 1970s and 1980s both the clean and H-covered Si surfaces were characterized by diffraction and spectroscopic methods, but only in the last decade have there been reproducible studies of chemical kinetics and dynamics on well-characterized silicon surfaces. Despite the conceptual simplicity of hydrogen as an adsorbate, this system has turned out to be rich and complex, revealing new principles of surface chemistry that are not typical of reactions on metal surfaces. For example, the desorption of hydrogen, in which two adsorbed H atoms recombine to form H2, is approximately first order in H coverage on the Si(lOO) surface. This result is unexpected for an elementary reaction between two atoms, and recombi-native desorption on metals is typically second order. The fact that first-order desorption kinetics has now been observed on a number of covalent surfaces demonstrates its broader significance. 

Interestingly, the yield of the hydrogenated products is substantially higher than the yield of hydrogen evolved in the absence of the unsaturated compounds. This was attributed to the function of surface-activated ethylene or acetylene in scavenging metal-associated H atoms In the absence of the unsaturated substrates, surface-associated H atoms recombine with valence-band holes. This recombination process competes with H-atom dimerization. In the presence of ethylene or acetylene, hydrogen atoms are trapped by the hydrogenation pathway, and consequently, the recombination process is diminished. 

A much higher efficiency of H2O as collision partner is observed in H atom recombination, where kjgg( ifi)lktgg AT) is 20 at room tempera-ture and at flame temperatures of 1000-2000 K. The temperature dependence of recombination rate constants is expressed in Table 1.1 in the form... 

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