Biomimetic O2 Reduction at MN4 Catalysts Adsorbed on Carbon Nanotubes and on Pyridine Grafted Carbon Nanotubes

Metallophthalocyanines and metalloporphyrins are well known electrocatalysts for the reduction of O₂(ORR) and have been extensively investigated in order o achieve a better understanding of the fundamentals of ORR at modified electrodes and with the final scope of replacing Pt in fuel cells. However, heat-treated MNx catalysts are more active and stable than intact MN4 complexes for ORR.  In previous studies, we have established that the formal redox potential is a good reactivity index for the electrocatalytic activity of surface-confined MN4 complexes (MN4 complexes) toward the reduction of molecular oxygen [1], and this is also valid for pyrolyzed materials [2]. Those experiments were performed with the MN4 macrocyclic adsorbed directly on the edge plane graphite electrode [1]. The results show a linear, maybe a semi-volcano correlation where the catalytic activity of the complex increases when the potential of the redox couple associated to the catalytic process increases [1]. According to those results the best strategy to increase the catalytic activity of the complexes is to tune the redox potential oft he complex or catalyst to more positive values by the chemical modification of the ligand or the environment around the the site [1,2]. In a recent study, Cho and co-workers [3] found higher catalytic activity than Pt for electrodes modified with Fe-phthalocyanine with an axial ligand anchored on single walled carbon nanotubes [2]. Fundamental studies of MN macrocyclic complexes adsorbed on carbon nanotubes (CNTs) and on pyridine modified CNTs as catalysts for the ORR are missing. Preliminary studies in basic media show that when MN4 macrocyclic complexes are adsorbed on CNTs (single walled and double walled) modified electrodes higher catalytic current can be obtained (almost two orders of magnitude increment) and similar half-volcano trend reported for MN4 macrocyclic complexes is found for these hybrid electrodes where the activity increases as the formal potential of the metal catalyst redox center is shifted to more positive values were obtained [3]. When CNTs are modified with 4-aminopyridine aryl diazonium salts and then with axially ligated phthalocyanines even higher activities are observed but volcano correlations are distorted by the effect of the axial ligand. We have observed this enhancement of he activity by the action of an axial ligand on phthalocyanines anchored on gold [4].  The axial ligand mimics the action of natural occurring ORR catalyst cytochrome c oxidase that contains an active site of a Fe(II)-porphyrin which has a 5-coordinated structure with an axial ligand on the back side [3].

Ackowledgements.  This work was funded by Fondecyt Projects 1140199, 3130538 and 3150271 and to Dicyt-USACH. 

References

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