Reaction involving metal ions adsorbed

Reactions Involving metal Ions adsorbed on minerals... 

Such a transfer reaction of metallic ions in a series connection of two elementary steps involving an intermediate of an adsorbed metallic ion complex may be illustrated by the anodic dissolution of metallic iron in acidic solution [Despic, 1983] as shown in Eqn. 9-21 ... 

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

Hydrolysis and Adsorption. Some years ago, a theory was advanced, that hydrolyzed metal species, rather than free metal ions, are adsorbed to hydrous oxides. The pH-dependence of adsorption (the pH edge for adsorption is often close to the pH for hydrolysis) was involved to account for this hypothesis. As Figs. 2.7b and c illustrate, there is a correlation between adsorption and hydrolysis but this correlation is caused by the tendency of metal ions to interact chemically with the oxygen donor atoms with OH, and with S-OH. The kinetic work of Hachiya et al. (1984) and spectroscopic information are in accord with the reaction of (free) metal ions with the surface. 

Chemical reactions of adsorbed species are of importance in vast areas of science, the involvement of adsorbed metal ions in catalysis being one example of great economic value. In addition reactions involving adsorbed species can sometimes produce products that may be either difficult or impossible to prepare away from the mineral surface. Therefore, an understanding of the chemical processes that occur in such systems is of potential economic benefit to industrial operations. Such knowledge is also of much wider significance, however, because the movement of ions in most environmental situations is controlled by sorption processes, and aluminosilicate minerals play a major role in many situations. 

For some metals the transfer of metallic ions involves a reaction intermediate of an adsorbed metallic ion complex which is coordinated with anionic ligands hence, the overall reaction occurs in a series of two elementaiy steps rather than one. Such a multistep transfer of ions can result, in the course of metallic ion transfer, from the reduction of the activation energy for ion transfer due to the formation of adsorbed intermediates. We examine a transfer reaction of divalent metallic ions via an adsorbed complex ion according to the steps in Eqn. 9-13 ... 

This, together with the known tendency of metal ions to form mixed hydroxy-am-mine complexes, snggested to them that two ammonia molecnles were involved in the first step and that the adsorbed species in Reaction (3.58) was a hy-droxy-ammine species, viz. Cd(OH)2(NH3)2. Decomposition of the hydroxide-ammine-thiourea complex was then assumed to occur by nucleophilic attack of an ammonia species on the S=C bond of the thiourea. 

Neither C5- nor C6-cyclization involve carbonium-ion intermediates over platinum metal. The rates of the -propylbenzene - indan reaction (where the new bond is formed between a primary carbon atom and the aromatic ring) and the n-butylbenzene- 1-methylindan reaction (which involves a secondary carbon atom) are quite similar (13). Furthermore, comparison of the C6-cyclization rates of -butylbenzene and n-pentylbenzene (forming naphthalene and methylnaphthalene, respectively) over platinum-on-silica catalyst shows that in this reaction a primary carbon has higher reactivity than a secondary carbon (Table IV) (29). Lester postulated that platinum acts as a weak Lewis acid for adsorbed cyclopentenes, creating electron-deficient species that can rearrange like carbonium ions (55). The relative cyclization rates discussed above strongly contradict Lester s cyclization mechanism for platinum metal. 

In the many studies that have been done using metallic catalysts, it has been shown that the adsorbed species is a formate ion So the chemisorbtion step in the dehydrogenation reaction involves a heterolytic splitting of the OH-bond ... 

For quite some time, it was not clear whether the catalytic cracking process of these aluminosilicates involves the concerted action of both Lewis and Bronstedt acid sides, or only Lewis sites, or even the dissociation of the adsorbed hydrocarbons by surface metal ions (impurities).16,17,18 19 The need for oxide systems that allow evidence concerning the above catalytic theories was one of the driving forces for the efforts that have been done to coat a silica surface with aluminium compounds. One of the earlier studies, concerning both the reaction mechanisms of the aluminium... 

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... 

Longer times described by the dashed portion of the adsorption-desorption line represent the slower rates of metal and ligand adsorption found in some laboratory experiments and observed in most soil and groundwater systems (cf. Jenne 1995). Adsorption of metal ions generally involves concurrent desorption of protons or other metals. When adsorbed metals are strongly surface-bound (see Chap. 10) both adsorption and desorption reactions may be slow and desorption partially irreversible, requiring adsorption or desorption equilibration times that may approach 10 to 100 hours. Based on an analysis of published laboratory and field studies, Jenne (1995) concludes that metal adsorption and desorption reactions have initial fast and then slower reaction steps and are usually diffusion-rate limited. 

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