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