Reversible chemical fixation gases

In natural processes, metal ions are often in high oxidation states (2 or 3), whereas in chemical systems the metals are in low oxidation states (0 or 1). This fact inverts the role of the metal center, such that it acts as a one-electron sink in a natural system, but as a nucleophile in an artificial ones (see other chapters of this book and the review by Aresta et al. [109]). Nevertheless, important biochemical processes such as the reversible enzymatic hydration of C02, or the formation of metal carbamates, may serve as natural models for many synthetic purposes. Starting from the properties of carbonic anhydrase (a zinc metalloenzyme that performs the activation of C02), Schenk et al. proposed a review [110] of perspectives to build biomimetic chemical catalysts by means of high-level DFT or ah initio calculations for both the gas phase and in the condensed state. The fixation of C02 by Zn(II) complexes to undergo the hydration of C02 (Figure 4.17) the use of Cr, Co, or Zn complexes as catalysts for the coordination-insertion reaction of C02 with epoxides and the theoretical aspects of carbamate synthesis, especially for the formation of Mg2+ and Li+ carbamates, are discussed in the review of Schenk... 

Adsorption phenomena are anofher aspecf of colloid activify. Adsorption is the reversible fixation on a solid surface of a body in solution (liquid or gas). This fixation does not involve any chemical reactions and is governed by the thermodynamic equilibrium. Adsorption phenomena occur during winemaking (Ribereau-Gayon et al, 1976) and have an effect on colloidal chemistry. These phenomena are more extensive when the adsorbent body is divided into smaller units, as the interface is proportionally larger. 

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