Metal complexes, adsorption reactions involving

The biosorption reactions of heavy metals and metal-cyanide complexes were endothermic, and the heats of adsorption were in the range of 3.3-5.3 kcal-mol", which imply that both physical and chemical adsorptions are involved. 

Methane reforming with carbon dioxide proceeds in a complex sequence of reaction steps involving the dissociative adsorption/reaction of methane and COj at metal sites. Hydrogen is generated during methane dissociation In the second set of reactions CO2 dissociates into CO and adsorbed oxygen. The reaction between the surface bound carbon (from methane dissociation) and the adsorbed oxygen (from CO2 dissociation ) yields carbon monoxide. A stable catalyst can only be achieved if the two sets of reactions are balanced. 

Among the main goals of electrochemical research are the design, characterization and understanding of electrocatalytic systems, (1-2) both in solution and on electrode surfaces. (3.) Of particular importance are the nature and structure of reactive intermediates involved in the electrocatalytic reactions.(A) The nature of an electrocatalytic system can be quite varied and can include activation of the electrode surface by specific pretreatments (5-9) to generate active sites, deposition or adsorption of metallic adlayers (10-111 or transition metal complexes. (12-161 In addition the electrode can act as a simple electron shuttle to an active species in solution such as a metallo-porphyrin or phthalocyanine. 

It has been shown that the interpretation of catalytic reactions involving group VIII transition metals in terms of n complex adsorption possesses considerable advantages over classical theories by providing a link between theoretical parameters and chemical properties of aromatic reagents and catalysts. The concept has led to the formulation of a number of reaction mechanisms. In heavy water exchange the dissociative tt complex substitution mechanism appears to predominate it could also play a major role when deuterium gas is used as the second reagent. The dissociative mechanism resolves the main difficulties of the classical associative and dissociative theories, in particular the occurrence... 

In order to obtain quantitative measurements of hydrogenation activity and acidity, various schemes are employed. For example, metal surface area has been related to hydrogenation activity and the adsorption of bases such as pyridine and ammonia have been correlated with acidity ((3). Some authors have used certain key reactions involving pure compounds as an indication of catalytic properties (16). Each of these methods is useful however, because of the complex interdependence of the catalytic functions of the hydrocracking catalysts and changes in these functions with catalyst aging, results from each method must be interpreted with caution. 

An example of a speciation calculation involving metals and ligands that adsorb to form only inner-sphere surface complexes is shown in Table 9.10 for a soil solution at pH 7.5. The adsorption reactions for these metals and ligands are exemplified by the first and eighth rows in Table 9.7 ... 

The adsorption reaction that occurs between metallic ions and the charged surfaces of clay-organics may involve formation of either relatively weak outer-sphere complexes, or strong inner-sphere complexes. 

Inner-sphere complexes are relatively stable in comparison to outer-sphere complexes under equivalent solution conditions (i.e. pH, ionic strength), and in a competitive situation will tend to displace less stable adsorbates. This is a fundamental property of coordination reactions, and explains the observed trends in metal uptake preference observed in lichen studies (Puckett et al., 1973). Metal sorption results previously attributed to ion exchange reactions are more precisely described as resulting from competitive surface complexation reactions involving multiple cation types. Strictly speaking, each metal adsorption reaction can be described using a discrete mass law relation, such as... 

Complex Ionic Adsorption from Solution. - Much of the metal adsorption that normally is of importance in catalyst preparative chemistry concerns the interaction with complex ions. Obviously, for example, all metal anionic adsorption must of necessity involve the metal as a complex ion. This tendency to form complex ions can cause substantial deviations from what one might normally expect from considerations of simple ion-exchange equilibria. This is true in the case of ferric chloride, for example reactions (12)-(15). 

Thus it appears from the knowledge gained from the chemistry of metal complexes of n-bonded thiophenes that n adsorption on highly (triply) unsaturated metal sites, which has been frequently invoked in the heterogeneous literature, may in itself be a peripheral situation, rather than a crucial phenomenon directly related to the actual HDS reactions taking place, since a number of other pathways involving the more abundant singly or doubly CUS are available under desulfurization conditions. 

Generally, sorption of metals seems to be more nearly reversible at low than at high pH. This may arise from the fact that the monodentate complexation reaction (4.11) should give way to the bidentate reaction (4.12) at higher pH. The latter reaction, involving two metal-surface bonds, is expected to have a very slow rate in the reverse direction (desorption). Studies of heavy metal bonding on pure oxides have indicated that the adsorption reaction step is fast and probably diffusion controlled kf 10 moles liter sec ), whereas the desorption reaction step has a rate constant that may be as much as three orders of magnitude slower. 

Removal of color from dye solutions is a complex process and involves phys-iochemical mechanisms of coagulation and/or chelation-complexation type reactions. The dyes, due to the chelation/complex formation reaction with chemical coagulants, lead to the formation of insoluble metal dye complexes which may either precipitate from solution or may be removed by adsorption onto metal hydroxyl species [86]. 

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