Activated complex thermodynamic aspects

Except for the use of classical thermodynamics in the treatment of galvanic cells at zero current (including thermogalvanic cells, for which some aspects of the thermodynamics of irreversible processes have to be brought in—a, topic not treated here— see Agar s recent review where a complete bibliography will be found) and for the introduction of thermodynamic quantities pertaining to the formation of activated complexes from reactants in electrode processes, little systematic use of thermodynamics has been made in the various areas of electrochemistry concerned with nonzero currents. 

The kinetic scheme such as the one shown in figure 4 can be used for an analysis of the thermodynamic aspects of photoredox reactions. In the classical approach, the conversion of the precursor complex [S. ..(2] to the successor complex [S. ..Q ] is treated according to the activated complex formalism ... 

The thermodynamic stability of the binuclear site has been demonstrated by the spontaneous assembly of [Fe20(02CR)2L2] (13) from ferric salts in the presence of water, an alkyl carboxylate salt, and a tridentate nitrogen donor ligand L that can cap an octahedral face on iron (8). Suitable ligands include tris(pyrazolyl)borates and 1,4,7-triazacyclononanes. Structure (13) is in essence a portion of the basic ferric acetate structure. The complexes are excellent physical and structural models of the diiron sites and model some aspects of reactivity including redox activity and interconversion of the oxo and hydroxo bridge. 

The complex nature of the HDS and HDN problems requires a broad, transdisciplinary approach in order to try to answer the most varied questions related to these important classes of reactions. The key issues include the practical aspects related to process and product engineering, a precise knowledge of the nature and the composition of petroleum and of refinery fractions, and the thermodynamics and detailed kinetics of the different processes involved. Also, a number of more fundamental solid-state and surface chemistry considerations regarding the preparation, the characterization, and the resulting properties of HDS and HDN catalysts, as well as the complicated reaction mechanisms involved for the various important families of substrates, need to be understood in depth. Even though some very impressive achievements have been disclosed over the last 30-40 years, it seems that some of the major new discoveries desired today may have been held back by the lack of a better understanding of some key issues. Of particular importance are the nature and the structure of HDS-HDN active sites on metal sulfide catalysts, and the intimate details of the elementary reactions implicated in the commonly accepted catalytic schemes. 

---