Macrocycles catalysts

Dloxygen reduction electrocatalysis by metal macrocycles adsorbed on or bound to electrodes has been an Important area of Investigation (23 ) and has achieved a substantial molecular sophistication in terms of structured design of the macrocyclic catalysts (2A). Since there have been few other electrochemical studies of polymeric porphyrin films, we elected to inspect the dloxygen electrocatalytic efficacy of films of electropolymerized cobalt tetraphenylporphyrins. All the films exhibited some activity, to differing extents, with films of the cobalt tetra(o-aminophenylporphyrin) being the most active (2-4). Curiously, this compound, both as a monomer In solution and as an electropolymerized film, also exhibited two electrochemical waves... 

Iliev I., Gamburzev S., Kaisheva A., Gas-diffusion electrodes with transition metal macrocyclic catalysts for electrochemical reduction of oxygen, Proceedings of the 31 ISE Meeting 1980 Sept. 22 - 26, Venice, Italy, Vol. I, p. 286-288. 

CO2 molecule, or Mg + and CO2 play the role of oxide acceptor to form water, carbonate, and MgC03, respectively [38]. The reactions of the iron carboxylate with these Lewis acids are thought to be fast and not rate determining. For the cobalt and nickel macrocyclic catalysts, CO2 is the ultimate oxide acceptor with formation of bicarbonate salts in addition to CO, but it is not clear what the precise pathway is for decomposition of the carboxylate to CO [33]. The influence of alkali metal ions on CO2 binding for these complexes was discussed earlier [15]. It appears the interactions between bound CO2 and these ions are fast and reversible, and one would presume that reactions between protons and bound CO2 are rapid as well. 

Figure 6.16 Chemical formulae of the macrocyclic catalyst 16+, ligand 17 and three-component rotaxane 186+ which mimics the operation of topologically linked enzymes.

The zeolite-encapsulated iron phthalocyanine macrocycle catalyst was also used for the oxidation of hydroquinone (Fig. 2). When the oxygen uptake in the first run had ceased, 250 mg of hydroquinone was injected into the reactor (second run), and the oxygen uptake was measured again. 

Rotating-copper-disk electrode techniques have been used to evaluate the efficiency of the nickel macrocycle catalyst for the reduction of CO2 to Studies have been performed using Ni(diazacyclam) + (diazacyclam = 3,10-dimethyl-1,3,5,8,10,12-hexaazacyclophane) (5.6), a complex derived from cy-clam, which appears to be more active than [Ni(cyclam)2] " under the same conditions (see Figure 5.70) . These results are consistent with a mechanism proposed by other authors - . ... 

Smith, C.I., J.A. Crayston, and R.W. Hay (1993). Reduction of carbon dioxide by nickel macrocyclic catalysts adsorbed on a mercury electrode or a copper rotating disc electrode. J. Chem. Soc. Dalton Trans. 21, 3267-3269. 

Normally, Fe-based macrocycle catalyst such as Fe TPFPP could catalyze the ORR with a 4-electron-transfer pathway from O2 to H2O, while those of Co-based could only catalyze the process with a 2-electron-transfer pathway to produce peroxide. For example, a monolayer of cobalt(II) 1,2,3,4, 8,9,10,11, 15,16,17,18, 22,23,24,25-hexadecafluoro-29H,3 IH-phthalocyanine (abbreviated as Co HFPC) was coated on a graphite electrode by spontaneously adsorption displayed a strong electrocatalytic activity... 

Masllorens, J., Moreno-Manas, M., Pla-Quintana, A., Roglans, A. (2003) First Heck Reaction with Arenediazonium Cations with Recovery of Pd-triolefinic Macrocyclic Catalyst. Org. Lett. 5 1559-1561. 

In spite of a very significant progress achieved with heat-treated macrocyclic compounds as ORR catalysts since the early 1970s, the activity and durability of that family of catalysts are stiU insufficient for replacing platinum at the fuel cell cathode and in other applications. Furthermore, the complex structure of macrocyclic compounds makes their synthesis expensive and potentially noncompetitive with precious-metal-based catalysts also from the materials cost point of view. For those reasons, much effort has been invested by the electrocatalysis research community in recent years into finding less expensive and catalytically more active non-precious metal ORR catalysts that would not rely on macrocylic compounds as either catalysts or catalyst precursors. In the past decade, there has been a significant improvement both in the activity and of non-macrocyclic catalysts, expected to be manufactured at a fraction of the cost of their macrocyclic counterparts. In this section, we review the precursors, synthesis routes, and applications of this relatively new family of catalysts. 

Another insight provided by their study is that the side-on oxygen adsorption requires more space than the end-on adsorption configuration. This may be one of the reasons why cobalt porphyrin and phthalocyanine systems cannot form stable side-on adducts which generally lead to 4e transfer products. The destruction of the ordered structure of such a macrocyclic catalyst during the heat treatment is likely to increase the number of sites that facilitate the side-on oxygen adsorption, and this leads to an increase in the number of electrons transferred in the ORR. 

The studies of O2 reduction on macrocyclic catalysts have usually been carried out on thin films of the macrocyclic on a carbon, graphite, or metal substrate. These experimental studies can be divided into three... 

Figure 33. pH dependence of half-wave potentials for O2 reduction on carbon black with and without macrocyclic catalysts. Rotating disk technique., Carbon black alone A, H2PC or CoPc +, (CoPc) O, CoTPhP. ... 

Gutierrez CA, Silva JF, Redo FJ, Griveau S, Bedioui F, Caro CA, Zagal JH. In search of the best iron N4-macrocyclic catalysts adsorbed on graphite electrodes and on multi-walled carbon nanotubes for the oxidation of 1-cysteine by adjusting the Fe(II)/(I) formal potential of the complex. Electrocatal 2014 5 426-37. 

Pyrolysis of macrocyclic precursors has been shown in some cases to yield more active and stable catalysts for instance, pyrolysis of a mixture of Fe and Cu phthalocyanine-based complexes on carbon black produces a catalyst with 4-electron ORR activity higher than Pt/C at potentials lower than 0.84 V (RHE) in RDE experiments [13]. Such pyrolyzed macrocycle catalysts contain nitrogen coordinated to metal atoms, as well as nitrogen in CxNy compounds in which nitrogen is not coordinated to any metal. While the central importance of nitrogen to such catalysts is widely attested, the complex heterogeneous nature of these catalysts has thus far prevented any consensus on the identity of the ORR active site(s). Similarly, pyrolysis of Fe... 

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