Adsorbate sulphur

Van Huong CN, Parsons R, Marcus P, Montes S, Oudar J (1981) Electrochemical behaviour of silver and gold single-crystal surfaces covered with a monolayer of adsorbed sulphur. J Electroanal Chem 119 137-148... 

Glass capillary column (no packing material) COS, HjS, CS2, CHjSH, CH3)2S, (CH,)2S2 Polar glass surface may adsorb sulphur compounds... 

Although sulphur doping has many advantages, it has the disadvantage that the amount of adsorbed sulphur depends on an equilibrium with the gas phase... 

Oxides of metals such as aluminium, calcium, chromium, iron, lead, or vanadium may also serve as catalysts for the heterogeneous oxidation of sulphur dioxide. These oxides may also adsorb sulphur dioxide (4]. 

The exhaust gases are cooled and steam saturated when they enter the adsorbers. The lower sections of the adsorbers are filled with activated carbon which is impregnated with potassium iodide (KI) to facilitate the conversion of H2S to elementary sulphur. The adsorbed sulphur is used for sulphuric acid production as described in Section 12.7.4.2 after being desorbed by CS2 and subsequent evaporation of CS2 and distillation. 

The progress in surface science has provided a basis for a detailed description of the phenomena (Rostrup-Nielsen, 1991a). The adsorbed sulphur atom results in a deactivation of the neighboring nickel atoms. The rate of carbon formation decreases more with sulphur coverage than does the reforming rate reflecting that the ensemble for the reforming reaction is smaller than that required for the nucleation of the carbon whisker. 

On the other hand and in relation with the role of manganese sulphide in pitting of stainless steels already mentioned, R.C Al-kire, performed SECM experiments to demonstrate the role of adsorbed sulphur species near sttlphide inclusions like a driving force for pitting of Nickel. 

Sulphur dioxide plays such an important role in the corrosion of metals in the atmospheres of industrialised countries that detailed consideration of its action seems justified. For all metals SO appears to be selectively adsorbed from the atmosphere, less so for aluminium than for other metals, and for rusty steel it is almost quantitatively adsorbed even from dry air at 0°C Under humid conditions sulphuric acid is formed, the oxidation of SOj to SOj being catalysed by metals and by metallic oxides. 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

In the glass-bottle industry the bottles can be cooled in a dilute SOj/SO, atmosphere to increase chemical resistance. A similar effect has been noted with vitreous enamel. It has been postulated that a thin layer of —OH groups or — OH—HjO (hydronium) ions is adsorbed on the surface of a fired enamel. These ions are transformed into — OSO, or —OSO3 in the presence of oxides of sulphur which are more resistant to further acid attack. It is known that the acid resistance of a recently fired enamel improves on ageing, probably due to the enamel reaction with SOj/SO, in the atmosphere and it is quite common for the grading to improve from Class A to Class AA (BS 1344). 

Blocking of reaction sites The interaction of adsorbed inhibitors with surface metal atoms may prevent these metal atoms from participating in either the anodic or cathodic reactions of corrosion. This simple blocking effect decreases the number of surface metal atoms at which these reactions can occur, and hence the rates of these reactions, in proportion to the extent of adsorption. The mechanisms of the reactions are not affected and the Tafel slopes of the polarisation curves remain unchanged. Behaviour of this type has been observed for iron in sulphuric acid solutions containing 2,6-dimethyl quinoline, /3-naphthoquinoline , or aliphatic sulphides . 

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

FeUu JM, Gomez R, Llorca MS, Aldaz A (1993) Electrochemical behavior of irreversibly adsorbed selenium dosed from solution on Pt(h,k,l) single crystal electrodes in sulphuric and perchloric acid media. Surf Sci 289 152-162... 

Gomez R, Orts JM, Feliu JM, Clavilier J, Klein LH. 1997. The role of surface crystalline heterogeneities in the electrooxidation of carbon monoxide adsorbed on Rh(lll) electrodes in sulphuric acid solutions. J Electroanal Chem 432 1 -5. 

Eeliu JM, Orts JM, Gomez R, Aldaz A, Claviher J. 1994. New information on the unusual adsorption states of Pt(l 11) in sulphuric acid solutions from potentiostatic adsorbate replacement by CO. J Electroanal Chem 372 265-268. 

Electroneutral substances that are less polar than the solvent and also those that exhibit a tendency to interact chemically with the electrode surface, e.g. substances containing sulphur (thiourea, etc.), are adsorbed on the electrode. During adsorption, solvent molecules in the compact layer are replaced by molecules of the adsorbed substance, called surface-active substance (surfactant).t The effect of adsorption on the individual electrocapillary terms can best be expressed in terms of the difference of these quantities for the original (base) electrolyte and for the same electrolyte in the presence of surfactants. Figure 4.7 schematically depicts this dependence for the interfacial tension, surface electrode charge and differential capacity and also the dependence of the surface excess on the potential. It can be seen that, at sufficiently positive or negative potentials, the surfactant is completely desorbed from the electrode. The strong electric field leads to replacement of the less polar particles of the surface-active substance by polar solvent molecules. The desorption potentials are characterized by sharp peaks on the differential capacity curves. 

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