Hydrogen on iron

Machocki, A. 1991. Formation of carbonaceous deposit and its effect on carbon monoxide hydrogenation on iron-based catalysts. Appl. Catal. 70 237-52. 

THE SURFACE CONCENTRATION OF HYDROGEN ON IRON DURING HYDROGEN EVOLUTION... 

The heat of adsorption of hydrogen on iron at room temperature is almost identical with that of hydrogen on nickel in its dependence of... 

The heat of adsorption of hydrogen on iron at —183° is shown in Fig. 17, where it is compared with the heat of adsorption of hydrogen on iron at room temperature as represented by the solid curve. It is seen that the heat of adsorption of hydrogen on iron at —183° stays essentially constant until the surface is completely covered, at which point it drops to very low values. This gives a very important clue with regard to the mobility of the adatoms on the surface—that is, their ability to migrate from site to site. 

Figure 1.27 A mixed potential plot for the bimetallic couple of iron and zinc. The figure also explains the higher corrosion rate of iron than zinc in hydrochloric acid solution. Despite the more positive reduction potential of iron, the evolution of hydrogen on iron has a high exchange current density (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

Fig. 23. Reaction rate constants for the adsorption of hydrogen on iron clusters of size n (number of atoms). From Ref. 17.

Type 1 isotherms exhibit prominent adsorption at low relative pressures p/po (the relative pressure p/po is defined as the equilibrium v or pressure divided by the saturation vapor pressure) and then level off. Type 1 isotherm is usually considered to be indicative of adsorption in micropores (e.g., adsorption of benzene on microporous active carbon) or monolayer adsorption due to the stror adsorbent-adsorbate interactions (which may be the case for chemisorption, which involves chemical bonding between adsorbate and the adsorbent surface, e.g., adsorption of hydrogen on iron). In the case of nonpolar gases commonly used for charactmzation of porous solids (nitrogen, argon) [10, 12, 13, 17, 56], chemisorption is unlikely and therefore e I reflects usually adsorption on microporous solids. However, type I isotherms may also be observed for mesoporous materials with pore size close to the micropore range. In particular, in the case of adsorption of N2 at 77 K or Ar at both 77 K and 87 K in cylindrical pores, a type I isotherm would have to level off below the relative pressure of about 0.1 for the material to be exclusively microporous, as inferred fi-om tile results of recent studies of siliceous and carbonaceous ordered mesoporous materials (OMM) [57-59]. Consequently, when a type 1 isotherm does not level off below the relative... 

Calculate the hydrogen overpotential of an iron electrode and the corrosion current of iron for pH between 2 and 8. The potential of the iron electrode at unit hydrogen concentration (pH = 0), F= —0.75 V vs. SCE. The Tafel slope for the hydrogen evolution reaction is foc = 0.12 V/decade. The exchange current density of hydrogen on iron ipe is 10 " K/cvcl. Assume that the corrosion potential is —0.5225 V vs. SHE. 

Determine the corrosion potential and corrosion rate of an iron pipe carrying 1sulfuric acid at 0.2 m/s at 25°C. Assume that the entire iron surface acts as cathode, that Tafel slopes are 0.100V, and that the exchange current densities for Fe/Fe + and for hydrogen on iron are 10 and 10 A/m respectively. 

Typical catalytic reactions that have been investigated, in some detail, using this approach include hydrocarbon conversion on platinum and modified platinum surfaces (isomerization, hydrogenolysis, hydrogenation, dehydrogenation and cyclization), dehydrosulfurization on molybdenum, ammonia synthesis on iron, and carbon monoxide hydrogenation on iron. 

We notice that by increasing the pressure Pa while keeping Pb constant we should see a succession in the first three cases. This is what is observed in the reaction between ammonia and deuterium (heavy hydrogen) on iron, as shown by the rate variation curves in Figure 13.5 with the partial pressures of ammonia and deuterium. 

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