Electrochemical Scanning Tunnelling Microscopy investigations of Fe@N-based macrocyclic molecules adsorbed on Au(111) and their implications in Oxygen Reduction Reaction

Fuel-cells can be a valuable alternative to fossil-fuel non-renewable energy sources since H2 and O2 are used as fuel and comburant, respectively, and H2O is the main product. O2 is entailed at the cathode of the cell and it undergoes a reduction reaction, which however reveals to be the bottle neck step in a common proton exchange membrane fuel cell. High costs and scarcity make Pt-based materials no longer desirable catalysts for ORR. Among many non-platinum group materials, metal-coordinating nitrogen atoms dispersed in mesoporous carbonaceous matrix (MNx) exhibit remarkable catalytic activity. In the Volcano-like plot describing the correlation between the number of d electrons and the reduction potential of MN4 metal centre, iron lays at the top in accordance with Sabatier’s principle of catalysis. There is however a general lack in understanding the O2 reduction mechanism occurring at FeNx sites so that collecting information about the Fe-N active sites holds a primary relevance in understanding the O2 reduction reaction pathway. On this regard, Metal-centred phthalocyanines and porphyrins are known to act as catalysts for ORR since 1960’s. They are indeed good model systems for mimicking a specific class of MNx sites, namely MN4. In this thesis project, FeN4 macrocycles were used to functionalise an Au(111) single crystal, which allowed to evaluate their performance towards ORR thanks to the well-known inertness of Au towards oxygen chemistry. Fe(II)-phthalocyanine, Fe(III)-phthalocyanine chloride and Fe(III)-tetramethoxyphenyl porphyrin chloride were examined in their ability to promote the O2 reduction reaction by means of electrochemical scanning tunnelling microscopy (EC-STM), as well as by cyclic voltammetry (CV). From CV investigations in Ar purged electrolyte, the redox behaviour of the Fe centres was pointed out as a basic step in the active site formation, which was assigned to the low-oxidation state Fe(II). By further registering cyclic voltammograms of FeN4/Au electrodes in O2 saturated electrolyte, a clear reduction peak was observed at potentials close to the previously found Fe(III)Fe(II) peak potentials. Therefore, a “redox-catalysis” mechanism holds, in which the reduction of the metal centre from (III) oxidation state to (II) is first required to then allow the O2 reduction. EC-STM revealed to be a powerful tool to explore the self-assembly processes at the solid/liquid interface characterising the studied systems. Moreover, the O2 adsorption step will be actually visualised, succeeding in distinguishing the end-on and/or the side-on adsorption geometry. Potentiodynamic imaging enabled to correlate the intense reductive current revealed by CV at a definite applied potential to the system response at single sites provided by EC-STM. The effect of chloride in altering the adsorption behaviour of FeN4 molecules, as well as in modifying the O2 catalytic activity, was encountered by comparing Fe(II)Pc and Fe(III)Pc-Cl, and dedicated CV investigation were carried out to gather further information.