Macrocycles

Abstract

The catalysis of the reduction of oxygen on supported metal macrocyclics is discussed in terms of electronic interactions of the dioxygen molecules with metal centers that lead to the weakening of the OO bond and to a lowering of activation energy. Correlations between electronic properties of the macrocyclic complexes and activity are also discussed, such as the influence of the redox potential of the metal center in the electrocatalysis. The overall 2‐ versus 4‐electron reduction mechanisms on metallo‐macrocyclics are discussed including the different approaches used in the literature to achieve 4‐electron reduction mechanisms by designing face‐to‐face binuclear complexes, polymerized metal complexes or macrocycles substituted with peripheral redox groups. The systems that are more promising for practical applications, such as the heat‐treated carbon supported catalysts, are discussed in detail and with explanations on how these pyrolyzed catalysts have higher activity and better stability compared with other supported macrocyclic systems. The requirement for the existence of the “burned‐in” MN 4 moiety also known as the “super‐site” in heat‐treated catalysts is emphasized. Heat‐treated carbon supported macrocyclics catalysts are not poisoned by methanol and in principle they can be suitable for low‐temperature acid electrolyte fuel cells. The methods of preparation of supported catalysts include: (a) modified electrodes by adsorption or surface modification; (b) heat‐treated carbon supported catalysts; (c) influence of carbon surface modification and electrodes prepared by polymerized macrocyclics; and (d) methanol tolerance and comparison of the activities with platinum‐based electrocatalysts.

Enhancement of the Catalytic Activity of Fe Phthalocyanine for the Reduction of O2 Anchored to Au(111) via Conjugated Self-Assembled Monolayers of Aromatic Thiols As Compared to Cu Phthalocyanine

“…If step 7 is the rate controlling, with a symmetrical energy barrier, the Tafel slope should be 0.039 V. Many authors have reported Tafel slopes in the range 0.030–0.040 V for O 2 reduction in alkaline media catalyzed by Fe phthalocyanines or similar Fe macrocyclic complexes using graphite or carbon substrates. ,,,,,, Figure shows Tafel lines for FePc adsorbed directly on Au(111) and via molecular spacers, and in all cases the slopes are close to 0.030 V, so essentially the kinetics parameters as far as Tafel slopes are concerned do not seem to depend on the way of surface confinement of the FePc catalyst on the electrode. They give practically the same values no matter whether the substrate is graphite, carbon, gold, or gold coated with SAMs.…”

“…A generally accepted mechanism for O 2 reduction by Fe phthalocyanines confined to graphite or carbon surfaces ,,,,,, that seems to operate for FePc anchored to Au(111) is the following: false[ Fe ( III ) PcOH false] ads + normale − ⇆ false[ Fe ( II ) Pc false] ads + OH − goodbreak0em1em⁣ fast false[ Fe ( II ) Pc false] ads + normalO 2 ⇆ false[ Fe III ‐ ‐ normalO 2 − false] ads goodbreak0em1em⁣ fast [ Fe III ‐ ‐ O 2 − ] ads + e − → intermediates ⁣ rds However, according to the calculated energy diagram of Figure , the formation of the adduct in step 5 will slow the reaction instead of catalyzing it because the energy barrier is larger as compared to that where the initial reactants proceed directly to the formation of the adduct, concerted with the transfer of one-electron. S...…”

“…O 2 reduction in aqueous media can undergo a 2-electron reduction to give peroxide, or 4-electron reduction to give water. − ,,, However, the 4-electron reduction releases more energy than does the 2-e reduction process . In this context, Fe phthalocyanines catalyze the reduction of O 2 directly to water via 4-electrons, with cleavage of the O–O bond in contrast to what is observed with Co phthalocyanines. − ,, Some authors have suggested that FePc catalyzes peroxide decomposition, − whereas other have suggested that FePc phthalocyanines form Fe–O–O–Fe bridges promoting the cleavage of the O–O bond . It is still not clear how these molecules adsorb on graphite, so the formation of Fe–O–O–Fe bridges is highly speculative .…”

“…Metallophthalocyanines have been extensively investigated as electrocatalysts for the O 2 reduction (ORR). − Together with other similar molecules, they belong to the “non-precious metal catalysts” class and are studied with the aim of replacing expensive Pt-based catalysts in the cathode of fuel cells. Such MN 4 chelates, when present on electrode surfaces, catalyze the ORR, but they lack long-term stability in the aggressive environment of a fuel cell, and new heat-treated MN 4 materials have been developed to solve this problem .…”

“…Such MN 4 chelates, when present on electrode surfaces, catalyze the ORR, but they lack long-term stability in the aggressive environment of a fuel cell, and new heat-treated MN 4 materials have been developed to solve this problem . O 2 reduction in aqueous media can undergo a 2-electron reduction to give peroxide, or 4-electron reduction to give water. − ,,, However, the 4-electron reduction releases more energy than does the 2-e reduction process . In this context, Fe phthalocyanines catalyze the reduction of O 2 directly to water via 4-electrons, with cleavage of the O–O bond in contrast to what is observed with Co phthalocyanines. − ,, Some authors have suggested that FePc catalyzes peroxide decomposition, − whereas other have suggested that FePc phthalocyanines form Fe–O–O–Fe bridges promoting the cleavage of the O–O bond .…”