Enhanced catalytic activity of Fe phthalocyanines linked to Au(111) via conjugated self-assembled monolayers of aromatic thiols for O2 reduction

doi:10.1016/j.elecom.2011.08.050

Electrochemistry Communications

Volume 13, Issue 11, November 2011, Pages 1182-1185

https://doi.org/10.1016/j.elecom.2011.08.050 Get rights and content

Abstract

We have obtained and characterized self assembled monolayers (SAMs) of 4-aminothiophenol (4-ATP) and 1-(4-mercaptophenyl)-2,6-diphenyl-4-(4-pyridyl)pyridinium tetrafluoroborate (MDPP) functionalized with iron phthalocyanine (FePc) adsorbed on gold (111) electrodes. The catalytic activity of these SAMs/FePc was examined for the reduction of O₂ in aqueous media (pH = 4) and compared with that of bare gold and of gold coated directly with FePc molecules. Scanning tunneling microscopy (STM) studies confirm the functionalization of the 4-ATP by FePc. The electrocatalytic studies carried out with Au/FePc, Au/4-ATP/FePc and Au/MDPP/FePc electrodes show that the O₂ reduction takes place by a 4-electron transfer to give water in contrast to a 2-electron-transfer process observed on the bare gold. The activity of the electrodes increases as follows: Au < Au/FePc < Au/4-ATP/FePc < Au/MDPP/FePc.

Graphical abstract

Highlights

► Fe phthalocyanine (FePc) deposited on gold surfaces catalyses the O₂ reduction reaction. ► Catalytic activity of FePc for ORR is enhanced when linked to Au via self-assembled monolayers of aromatic thiols. ► The enhancement is greater for bulkier and longer aromatic spacers. ► A 4-electron pathway for ORR predominates for bulkier aromatic thiol spacers linked to FePc. ► The rate-determining step in all cases is the same, irrespective of the FePc distance from the gold surface.

Introduction

Metallophthalocyanines are well known electrocatalysts for the reduction of O₂ (ORR) [1], [2], [3], [4], [5]. Fe phthalocyanines catalyze the reduction of O₂ directly to water via 4-electrons, with rupture of the O―O bond in contrast to Co phthalocyanines [1], [2], [3], [4], [5]. Metal macrocyclics can be confined to electrode surfaces by simple adsorption [1], [2], [3], [4], [5]. More recently, preformed self-assembled thiol monolayers (SAMs) have been used as molecular anchors for these metal complexes [6], [7], [8], [9], [10]. When using SAMs, the macrocyclic complex can be immobilized onto a gold substrate by two methods: i) by using the thiol functionality (―SH), incorporated on the macrocyclic ligand that serves as anchor to form the SAM [11] and ii) by preforming a SAM with a thiol group and then binding to it macrocyclic molecules via a nitrogen functionality present on the other end of the thiol molecule [7], [8], [9], [10]. The second approach has some advantages because it avoids the time-consuming preparation of macrocyclic molecules with thiol functionalities. There are several reports in the literature that use a preformed SAMs to immobilize Co porphyrins [6], [7] and Fe phthalocyanines [8], [9], [10], [12]. In this work we have investigated the catalytic activity of Fe-phthalocyanine for ORR adsorbed directly on Au (111) and also anchored on SAMs pre-formed on Au (111).

Section snippets

Experimental

Electrochemical measurements were performed using water purified with a MilliporeMilli-QBiocel Ultrapurewater system. Solutions were purged with ultrapure nitrogen or ultrapure O₂ for 45 min prior to each electrochemical measurement. 4-Aminothiophenol (4-ATP, 97%) and Fe phthalocyanine were from Aldrich. 1-(4-Mercaptophenyl)-2,6-diphenyl-4-(4-pyridyl)pyridinium tetrafluoroborate (MDPP) was synthesized according to ref. [13]. Tetrahydrofuran (THF) was from JT Baker. All other reagents were

Results and discussion

Fig. 1a shows the STM image of the Au(111) surface modified with a Fe-phthalocyanine anchored via a 4-ATP SAM. Bright spots stand out showing evident regular structures on the SAM surface. These local bright regions, with diameters between 2 and 3 nm and about 0.5 nm height, are associated with the presence of the FePc molecules anchored to the 4-ATP layer (see Fig. 1b). The sectional analysis of surface shown in Fig. 1b, passing through the top of bright regions, reveals sub-nanometric height

Acknowledgements

This work was funded by Fondecyt Projects1100773 and 1090627 and Núcleo Milenio de Ingeniería Molecular P07-006.

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We have examined the electrocatalytic activity of iron phthalocyanine (FePc) and perchlorinated iron phthalocyanine 16(Cl)FePc for the oxygen reduction reaction (ORR) in alkaline medium with the two molecules either adsorbed on the external surface of single-wall carbon nanotubes (SWCNT) or covalently anchored via an axial pyridine ligand on pyridine-functionalized single-wall carbon nanotubes (py-SWCNT). Regardless of the particular phthalocyanine type, the ORR activity is higher when the substrate is py-SWCNT rather than SWCNT. The Tafel slopes for ORR are very similar for the two Fe macrocyclic complexes attached to SWCNTs in the two different configurations, suggesting a common rate-determining step for the ORR for all four catalysts. It is also observed that, for both the SWCNT and py-SWCNT supports, the ORR activity is higher for 16(Cl)FePc than for FePc. This is attributed to the electron-withdrawing effect of the peripheral and non-peripheral chlorine atoms in the macrocyclic ligand. While FeN4 macrocycles are known to be located on the strong binding side of a volcano correlation including several MN4 species, the chlorine substituents decrease the binding energy of O₂ on the central Fe cation, thereby moving up the macrocycle catalyst towards the apex of the volcano correlation. Both the carbon surface and macrocyclic ligand effects were optimized with 16(Cl)FePc attached to py-SWCNT, which is more active than both FePc on SWCNT and FePc on py-SWCNT. Here we show that both the axial ligand and the electron withdrawing groups (-Cl) have a combined collaborative effect in increasing the catalytic activity for ORR of Fe-phthalocyanines confined on the external walls of single-wall carbon nanotubes.
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Self-assembled metallophthalocyanines (MPcs) systems on electrode surfaces provide an interesting platform for carrying out fundamental studies on the electrocatalytic activities of transition metal MPc complexes for many reactions of interest for biological and energy conversion, due to their relatively high catalytic activity, stability, sensitivity and easy of construction, among other features [1,2]. Self-assembled MPc systems have been previously developed via axial ligation, using self-assembled monolayers (SAMs) on gold surfaces with the outermost part of the SAM molecules functionalized with MPc molecules of different transition metals (FePc, MnPc and CoPc), forming an “umbrella” configuration (Fig. 1a) [1,3–6]. In particular, assembled systems of iron phthalocyanines (FePcs) exhibit electrocatalytic properties especially for the oxygen reduction reaction (ORR) in a variety of systems such as FePc-py-carbonanotubes [7,8].
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MN₄ macrocyclic metal complexes have been studied for many years as potential catalysts for the O₂ reduction reaction (ORR) in order to replace costly Pt and Pt group metals from the cathode of fuel cells. These macrocyclic complexes lack the necessary stability over long periods of time required for fuel cell operation especially in acidic electrolytes. However, they have provided very good models for establishing reactivity descriptors. The activity plotted with the M–O binding energy describes a volcano correlation. For Mn and Fe complexes, the onset for ORR occurs at potentials very close to the M(III)/(II) redox potential of the complexes in contrast to Co complexes. In general, the more positive the M(III)/(II) redox potential, the highest the activity. This is also true for MN₄ catalysts dispersed on carbon nanotubes using different strategies. MN_x catalysts obtained by pyrolysis of MN₄ complexes show higher activity and stability and are more promising for fuel cell applications. The M(III)/(II) transition is a reactivity descriptor for these materials as observed for intact MN₄ complexes.
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Enhanced catalytic activity of Fe phthalocyanines linked to Au(111) via conjugated self-assembled monolayers of aromatic thiols for O2 reduction

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Experimental

Results and discussion

Acknowledgements

Coordination Chemistry Reviews

Coordination Chemistry Reviews

Electrochimica Acta

Talanta

Journal of Electroanalytical Chemistry

Electrochimica Acta

Journal of Electroanalytical Chemistry

Chemical Physics Letters

Langmuir

Langmuir

Enhanced catalytic activity of Fe phthalocyanines linked to Au(111) via conjugated self-assembled monolayers of aromatic thiols for O₂ reduction