Preparation and voltammetric characterisation of bismuth-modified mesoporous platinum microelectrodes. Application to the electrooxidation of formic acid

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Abstract

In this paper mesoporous platinum microeletrodes (Pt-ME) modified with submonolayers of adsorbed bismuth (Bi-PtME) were prepared and characterised by cyclic voltammetry (CV). The mesoporous platinum films were electrodeposited from hexachloroplatinic acid dissolved in the aqueous domain of the lyotropic liquid crystalline phase of Brij 78®, to form metal films with hexagonal arrays of nanometer-sized channels. Bismuth deposition was performed by different procedures involving either the spontaneous adsorption of bismuth onto the Pt surface from Bi3+ solutions, or by under potential deposition (UPD) of bismuth from Bi3+ solutions, by cycling the potential over an useful range, or applying a constant potential for a given time. The latter procedures provided high bismuth coverage (θBi), whereas only small amounts of bismuth could be adsorbed from the simple immersion of the Pt-ME at open circuit. The coverage by irreversibly adsorbed bismuth was checked in a 0.5 M H2SO4 solution free of Bi3+ ions and exploiting the charge involved in the hydrogen adsorption/desorption, which decreased in proportion to the amount of platinum sites covered by bismuth. The ability of the prepared Bi-PtME towards the oxidation of formic acid was also investigated. It was found that Bi-PtME with θBi = 0.6 provided stationary voltammograms characterised by a low hysteresis between the anodic and cathodic scans. The onset of the waves resulted shifted by about 150 mV towards less positive potentials with respect to that of the corresponding Pt-ME. At 0.1 M HCOOH current densities of about 70 mA cm−2 were achieved. These results were discussed in terms of high tolerance towards the intermediate poisons of the Bi-PtME investigated here. Bi-PtME with much lower real surface area and bismuth coverage displayed both lower catalytic activity and tolerance to poisons.

Introduction

In recent years, there has been a drive to study the direct electrocatalytic oxidation of small organic molecules such as formic acid and methanol, for their potential use as fuels in direct fuel cells [1], [2], [3], [4], [5], [6], [7]. Platinum is considered one of the most efficient materials for the oxidation of small organic molecules. However, it is susceptible to poisoning effects, due to strongly adsorbing intermediates, which are formed during the electrode processes [5], [8], [9], [10], [11], [12], [13], [14]. Recently, it has been shown that mesoporous platinum electrodes, because of their large specific surface area with a well defined periodic nanostructure, under given conditions, may provide high activity towards the oxidation of the above species and tolerance to poisons [15], [16], [17]. It has also been shown that the electrocatalytic activity of Pt towards small molecule oxidation, by minimising the poisoning effects, can be improved by modification of the Pt surface by foreign metal adatoms [18], [19], [20], [21], [22]. In particular, bismuth has received significant attention as Pt modifiers [4], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], and PtBi intermetallic systems have been proposed as powerful catalysts for formic acid oxidation [32]. Thus, it seemed us advantageous to combine the intrinsic activity of mesoporous platinum and the beneficial effect of irreversibly adsorbed of submonolayers of bismuth to further enhance the platinum electrocatalytic activity. Underpotential deposition of metal ions onto the mesoporous platinum surface has previously been reported [33], [34] and also exploited for anodic stripping voltammetric analysis of Ag+, Pb2+ and Cu2+ ions [34]. To the best of our knowledge, however, no report exists on the preparation and use of bismuth modified mesoporous platinum electrodes for small molecule oxidation. This paper focuses on the preparation and characterisation of nanostructured platinum microelectrodes modified by bismuth adatoms. The performance of these modified electrodes towards the oxidation of formic acid are also preliminarily investigated. A 25 μm diameter Pt disk is used as the substrate electrode for the deposition of the mesoporous platinum films. These systems combine the unique characteristics of microelectrodes [35] and the high real surface area of the nanostructure [33], [34], [36], [37], [38], [39], [40], [41], and make them very attractive not only for fundamental investigation in electrocatalysis [15], [16], [17], but also for their potential use as sensors in electroanalysis [34], [40], [41].

Section snippets

Chemicals and reagents

All chemicals employed were of analytical reagent grade. Hexachloroplatinic acid (HCPA), and sulphuric acid were from Carlo Erba reagents; potassium chloride, the surfactant Brij 78®, and Bi(NO3)3 standard (1005 μg/L + 5 wt% HNO3) were purchased from Aldrich; formic acid was supplied by Fluka, and Ruthenium(III) hexaammine trichloride was supplied by J. Matthey. All chemicals were used as received, and all aqueous solutions were prepared with deionised water purified via a Milli-Q unit (Millipore,

Mesoporous platinum films and their modification with Bi

Fig. 1 shows typical cyclic voltammograms obtained in 0.5 M H2SO4 at 200 mV s−1 over the potential range from −0.250 to 1.45 V, with a Pt-ME (RF = 142) (dashed line) and the corresponding smooth Pt microdisk (dotted line). At both types of microelectrodes the typical voltammetric pattern expected for polycrystalline platinum was observed [46]. The larger current involved at the Pt-ME, with respect to the smooth Pt, was clearly due to the enhanced effective surface area, upon Pt deposition. Cyclic

Conclusions

In this paper mesoporous platinum microdisk electrodes have been prepared by using a lyotropic liquid crystalline mixture made by the inexpensive non-ionic surfactant Brij® 78, and then modified by submonolayers of bismuth. The voltammetric characterisation of the so prepared electrodes in H2SO4 solutions has shown that, sufficiently high coverage of the Pt surface by irreversibly adsorbed bismuth could be achieved, provided that the upper potential limits in the measurements are properly

Acknowledgement

Financial support of the Ministry of University and Scientific Research (MIUR) is gratefully acknowledged.

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