Adsorbed hydrogen atoms

Much work has been undertaken to understand the steps and intermediates by which the reaction occurs on the heterogeneous catalyst surface. However, the exact mechanism is not fully established. One approach assumes a first-step adsorption of carbon monoxide on the catalyst surface followed by a transfer of an adsorbed hydrogen atom from an adjacent site to the metal carbonyl (M-CO) ... 

Entry from the aqueous phase The mechanism of electrochemical production of hydrogen on steel in aqueous solution has received much attention. It is accepted that the reaction occurs in two main stages. The hrst of these is the initial charge transfer step to produce an adsorbed hydrogen atom. In acid solution this involves the reduction of a hydrogen ion ... 

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 ... 

Alternatively the adsorbed hydrogen atom may participate in a second electrochemical reaction, known as electrochemical desorption ... 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

Finally, the electrode potential may affect the overall process by determining the state of oxidation of the electrode surface. It is well known that m aqueous solution a platinum electrode has a bare surface only over the narrow potential range from approximately -t-0-4 V to -tO-8 V versus N.H.E. at more cathodic potentials it is covered by adsorbed hydrogen atoms while at more anodic potentials it is covered by... 

Most often, these radicals are unstable and can exist only while adsorbed on the electrode, although in the case of polycyclic aromatic compounds (e.g., the derivatives of anthracene), they are more stable and can exist even in the solution. The radicals formed first can undergo a variety of chemical or electrochemical reactions. This reaction type is the analog of hydrogen evolution, where electron transfer as the first step produces an adsorbed hydrogen atom, which is also a radical-type product. 

The current-producing steps (those producing electrons) are the ionization of adsorbed hydrogen atoms and the anodic formation of new species from water molecules ... 

Therefore, electrons adjust their state to any instant position of the proton and solvent polarization in both the initial (hydroxonium ion) and final (adsorbed hydrogen atom) states. The proton in the hydroxonium ion sees an average electron cloud but feels any instant configuration of solvent polarization. 

Adsorbed hydrogen atoms which are the intermediates of reaction 8 can be used for the hydrogenation of ethylene and acetylene. The light driven reaction occurs according to the following scheme ... 

Fig. 3.4. ZnO film conductivity as a function of amount of adsorbed hydrogen atoms. 1 - Film temperature -196 C filament temperature 1,000 C 2 — Film temperature -1% C filament temperature 1,100 C...

Further investigations of the above discussed effects show that, at fixed temperature of the oxide film (catalyst), the jump in the electric conductivity first increases in amplitude, as the portion of alcohol vapor admitted into the vessel increases. On further increase of the admitted portion of alcohol, the jump amplitude reduces (starting with the pressure of 3.6-10 2 Torr). At the pressure of 3.2-10 Torr, the jump in the electric conductivity of the zinc oxide film is less pronounced. Finally, at still higher pressures, it disappears (Fig.4.9). This effect is not unexpected. On our mind, it is associated with the fact that, as the concentration of alcohol vapor increases, the sum of the rate of interaction of the vapor with adsorbed hydrogen atoms and the rate of surface recombination of hydrogen atoms at the time instant of production becomes higher than the chemisorption rate of these atoms. The latter is responsible for the increase of the electric conductivity of the semiconductor oxide film via the reaction... 

It has been already mentioned in preceding section that in process of ordering of disordered adsorbents the energy get released which is sufficient to brake the bonds in the surface compounds. Therefore, the emission of initially adsorbed active particles due to disorder relaxation should be studied in disorder surfaces. It is very convenient to use for such studies the amorphous antimony with adsorbed hydrogen atoms. The properties of thin antimony films have been studied in substantial detail due to their use in manufacturing of photocathodes [12]. 

However, this formulation yields very little information on the actual course of the reaction, i.e. of which partial processes it consists. This set of partial processes is termed the mechanism of the electrode reaction. In the first case, the electrode reaction at some electrodes involves the formation of an adsorbed hydrogen atom, followed by the recombination reaction ... 

Van der Meerakker [28] gives a general mechanism for electroless processes involving various reductants. The mechanism assumes the formation of adsorbed hydrogen atoms, which can then be oxidized or desorbed as H2 gas. In the case of Co(II) reduction by hypophosphite, the relevant mechanism is as follows ... 

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