Adsorbate hydrogen

CoAsS, are also used as sources. The ore is roasted and Co is precipitated as the hydroxide and then reduced to Co with carbon (hep below 417 - C, cep to m.p.). The metal is silvery white and readily polished. It dissolves in dilute acids and is slowly oxidized in air. Adsorbs hydrogen strongly. The main use of cobalt is in alloys. Cobalt compounds are used in paints and varnishes, catalysts. Cobalt is an essential element in the diet. World production 1976 32 000 tonnes metal. 

Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

Iron adsorbs hydrogen readily and is a hydrogenation catalyst. 

Catalysts reduced with formaldehyde contain no adsorbed hydrogen and are less pyrophoric. 

Only recently has a mechanism been proposed for the copper-cataly2ed reaction that is completely satisfactory (58). It had been known for many years that a small amount of carbon dioxide in the feed to the reactor is necessary for optimum yield, but most workers in the field beHeved that the main reaction in the formation of methanol was the hydrogenation of carbon monoxide. Now, convincing evidence has been assembled to indicate that methanol is actually formed with >99% selectivity by the reaction of dissociated, adsorbed hydrogen and carbon dioxide on the metallic copper surface in two steps ... 

Catalysts reduced with formaldehyde carry no adsorbed hydrogen and are less pyrophoric. Barium carbonate as a support may sometimes be advantageous in that the neutrality of the h3 drogenation mixture may be maintained. Barium sulfate or barium carbonate may be a better support than carbon, which may, in some instances, so strongly adsorb the derived product that recovery is difficult or incomplete. Palladium may be more completely and easily recovered from a spent catalyst where carbon rather than barium sulfate is the support. In general, the submitter prefers a catalyst prepared according to procedure C. 

An interesting field of application is the protection of tantalum against hydrogen embrittlement by electrical connection to platinum metals. The reduction in hydrogen overvoltage and the shift of the free corrosion potential to more positive values apparently leads to a reduced coverage by adsorbed hydrogen and thereby lower absorption [43] (see Sections 2.1 and 2.3.4). 

The UCKR.ON test problem assumes the simplest uniform surface implicitly, because adsorbed hydrogen coverage is directly proportional to the partial pressure of gaseous hydrogen and adversely affected by the partial pressure of the final products. Such a simple mechanism still amounts to a complex and unaccustomed rate expression of the type solved by second order algebraic equations. 

Cg = eoneentration of vaeant sites Chs = eoneentration of adsorbed hydrogen... 

Phase diagrams of adsorbed He particles show interesting features which have many similarities to the phase diagrams of adsorbed hydrogen systems. 

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 ... 

However, when the second stage in the hydrogen evolution reaction is electrochemical desorption, the rate of this reaction is increased as the potential falls, and the adsorbed hydrogen concentration may remain constant or fall, according to the detailed electrochemistry. This results in curves such as that shown in Fig. 8.38 for steel in sodium chloride solution. 

Whether the adsorbed hydrogen is produced from the gas phase or from aqueous solution, it appears that the presence of hydrogen atoms distorts the crystal structure of the metal surface, and this results in a surface solubility which is higher than that of the bulk. The depth of this distortion is not clear, but it seems possible that the distorted zone may play an important part in initiating brittle-fracture processes. 

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. 

This equilibrium is of importance in providing diagnostic criteria for the mechanism of the h.e.r., since the rate of permeation of hydrogen through a thin iron membrane can provide information on the coverage of the surface with adsorbed hydrogen. 

The anode compartment contains a reference electrode and counterelectrode and by means of a potentiostat the anode side is maintained at a constant potential. The coverage of adsorbed hydrogen on the cathode side will depend on the current density i and the nature of the electrolyte solution, and the cell can be used to study the effect of a variety of factors (composition and structure of alloys, pH of solution, effect of promoters and inhibitors) on hydrogen permeation. 

The first equation was derived by assuming that the rate-controlling step is the reaction of one molecule of adsorbed C02 with two molecules of dissociated adsorbed hydrogen. The second equation, which correlates almost as well, is based on the assumption that the rate-determining step is the reaction of one molecule of adsorbed C02 with two molecules of adsorbed hydrogen. This indicates that, in this particular case, it was not possible to prove reaction mechanisms by the study of kinetic data. 

Extensive studies are still needed on hydrogen-metal surface interactions, leading to various forms of adsorbed hydrogen of different specific reactivity with the metal catalyst surface. Nevertheless, one can conclude on the basis of the experimental evidence presented that certain facts al-... 

As boiler metal corrosion products build up at the anodic sites and a film of monoatomic adsorbed hydrogen develops at the cathodic sites, so the difference in potential lessens. This voltage change is called polarization. 

The metals of the Pt-group constitute a particular case. Their catalytic activity has long frustrated the determination of the pzc because of interference from adsorbed hydrogen and oxygen. Nevertheless, estimated values of pzc for polycrystalline Pt are included in all compilations in Table 26. However, after the publication by Frumkin and Petrii14 of a summary of pzc values forPt, Rh, Ir, and Pd, no further progress was made for about 20 years until recently UHV techniques of surface preparation have enabled pzc determinations using methods other than the traditional... 

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... 

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