Recombination reaction of hydrogen

Studies on the hydrogen electrode reaction (HER) could probably be traced back to the time of Faraday, who observed loss with time of a mixed gas of H2 and O2 once produced and kept in a closed one-compartment electrolysis cell. This was a typical example of what is now called electrocataly-sis.< - > A parallelism between the hydrogen overpotential and relative rate of catalytic recombination reaction of hydrogen atoms on various metals was already noted by Bonhoeffer in 1924/ and the HER was thus accepted as a means by which one may obtain information on the catalytic characteristics of various metallic materials. 

Troe, J. The thermal dissociation/recombination reaction of hydrogen peroxide H202(-hM)=20H (-I-M) III. Analysis and representation of the temperature and pressure dependence over wide ranges. Combust. Flame 158, 594—601 (2011)... 

Historically, electrocatalytic science developed from investigations into cathodic hydrogen evolution, a reaction that can be reahzed at many metals. It was found in a number of studies toward the end of the nineteenth century that at a given potential, the rate of this reaction differs by severaf orders of magnitude between metals. In one of the first theories of hydrogen evofution, the recombination theory of hydrogen overvoltage, the rate of this reaction was finked directfy to the rate of the catalytic... 

In the analysis of Iyer et al, tH,ji is not assumed to be very much less than iV, so that iV at steady state equals the sum of the currents representing the Tafel recombination reaction and hydrogen permeation... 

The effectiveness of catalysts A and B to desulfurize SRN and blend naphtha was investigated and the results are shown in Table 4. Figure 4, which shows the performance of catalyst A, illustrates that it is easier to desulfurize SRN than blend naphtha. The results also confirmed higher HDS performance with blend naphtha than SRN with both catalysts. This could be due to the refractive material in the hydrocracked fraction of the blend naphtha. With blend naphtha and catalyst A the minimum total sulfur ofO.69 ppm was obtained at 320°C, while with SRN the minimum was 0.37 ppm at 300°C. Above these temperatures, the occurrence of H2S-alkene recombination reactions increased the total sulfur. Nickel-molybdenum catalysts are known to reduce recombination reactions by hydrogenating alkenes. Higher temperatures and very active catalysts can cause cracking at the reactor outlet allowing alkenes production[13],... 

Fig, 6.7. a Site time yield in the processes described in the figure as a function of radius of the metal cluster involved in the catalytic processes. No particle size effect is observed down to small clusters, b Particle size effect for Pt clusters dispersed on Si02 in the recombination reaction of oxygen and hydrogen, c Particle size effect for Fe clusters dispersed on MgO for the ammonia synthesis at atmospheric pressure and 570° K. d Relationship between catalytic activity, dispersion of the metal cluster on support and bulk metal character of the cluster for metals of the group VIII... 

A reaction of hydrogen atoms with the CHj groups of a surrounding polymer chain and formation of secondary alkyl radicals and molecular hydrogen will compete with this reaction. Secondary aUcyl radicals either recombine or disproportionate but the probability of both reactions is not so different now. Not only disproportionation of... 

Thus, when amines are used, the oxidation chains can be terminated not only on account of the reaction of hydrogen transfer according to reaction (4), but also by reactions of disproportionation (5) and cross recombination (7). 

In this case the seeond step is a chemical recombination reaction of adsorbed hydrogen atoms produced in the first step, which according to the reaction stoichiometry must proceed twice. The two mechanisms presented here and the rate-determining steps can be distinguished by measuring Tafel slopes. For a more detailed discussion of this point the reader may refer to the literature [3-6]. 

DispEcement. In many of the appHcations of chelating agents, the overall effect appears to be a displacement reaction, although the mechanism probably comprises dissociations and recombinations. The basis for many analytical titrations is the displacement of hydrogen ions by a metal, and the displacement of metal by hydrogen ions or other metal ions is a step in metal recovery processes. Some analytical pM indicators function by changing color as one chelant is displaced from its metal by another. 

The amount of the hydrogen that is liberated on or near a metal surface, which then enters the metal, varies according to the environment and condition of the metal. The main factor that promotes the entry of hydrogen into a metal is the presence on the metal of a surface poison such as sulfide or other species, which inhibit the hydrogen recombination reaction. 

The second stage of the reaction to produce molecular hydrogen may occur through either of two mechanisms. In the first of these, known as chemical desorption or chemical recombination, two adsorbed hydrogen atoms combine to produce a hydrogen molecule ... 

---