Electrocatalytic and adsorption properties of platinum microparticles electrodeposited into polyaniline films
Introduction
There are numerous indications of catalytic effects for the oxidation of methanol, formic acid, formaldehyde, carbon oxide, hydrogen and glucose in systems containing microdeposits of platinum metals incorporated into conducting polymers (polyaniline, polythiophene, polypyrrole and their derivatives) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. In most of these papers the main reason for these catalytic effects is assumed to be a lowering of poisoning of the dispersed catalyst particles by strongly adsorbed organic intermediates. For a more profound analysis of the catalytic effects it is necessary to compare the specific oxidation currents (referred to the unit of the catalyst's true surface) to those measured under similar conditions on smooth and platinized platinum (Pt/Pt) in the absence of the polymeric matrix. However, because of difficulties in measuring the true surface area of platinum particles in most of the papers mentioned, the currents are referred to the unit of the electrode geometric surface area. The most commonly used method for determining the true surface area of platinum particles by measuring the amount of adsorbed hydrogen cannot be used for platinum incorporated into thick (500 nm) polymeric films because of the absence of well pronounced adsorption maxima on the voltammetric curves [1], [8], [10]. For determining the true surface area of platinum deposits incorporated into polyaniline in recent years some authors have used the methods of CO adsorption [7] and of Cu-atom adsorption [11]. In addition to the knowledge of the true surface area it is also necessary to know the surface coverages of the catalyst with strongly chemisorbed organic species.
In our previous paper [12] the results of determining methanol oxidation currents on platinum deposits, both incorporated into Nafion® films and deposited on a bare glassy carbon (GC) surface, with simultaneous measurement of the surface coverage of the platinum particles were described. It was shown that, starting from some intermediate values of platinum loadings (mPt=60–100 μg cm−2) platinum microparticles incorporated into Nafion® or deposited on the bare GC surface show a similar catalytic activity which exceeds by far that of Pt/Pt and approaches the activity of smooth platinum. For these deposits the surface coverages with organic species remain the same as on smooth platinum and on Pt/Pt. Hence it was concluded that the catalytic effect is due to structural peculiarities of the deposits. In the presence of a Nafion® film, the specific surface of platinum particles was higher than that of deposits on GC with a corresponding increase of the overall current. Low values of the surface coverage were found only for low platinum loadings (mPt=10–30 μg cm−2) which practically showed no catalytic activity.
In the present investigation we studied the behavior of a catalytic system including polyaniline (PANI) films in order to elucidate the influence of the nature of the polymeric film on the mechanism of these catalytic effects.
Section snippets
Experimental
The working electrodes were in the form of rotating GC discs with diameters of 3 and 4 mm in Teflon holders, onto which a PANI film and subsequently small amounts of platinum were deposited. Platinum gauze was used as a counter electrode. All potentials (Er) are presented on the RHE scale.
The PANI films on the GC surface were obtained using electropolymerization of freshly distilled aniline from a solution of 0.1 M C6H5NH2+0.5 M H2SO4 by cycling the electrode in the potential range 0.05–1.1 V
Results
The platinum microdeposits were characterized by their specific surface area (Ssp in m2 g−1) and the average diameter of the platinum particles (dPt in nm) calculated as described in Ref. [12] for platinum microdeposits in Nafion® films. In Table 1 the microdeposits in Nafion® and in PANI films are compared. It can be seen that, in both cases, with increasing platinum loadings, the particle size increases and the specific surface decreases. The smallest loading investigated in PANI films was 26
Discussion
The most interesting result seems to be the well pronounced difference in the catalytic behavior and in the reaction mechanism for Pt/PANI/GC electrodes with different amounts of platinum microdeposits. The increase of platinum loading leads to an increase of the platinum particle size and to a change of its distribution into and onto the polymer layer. A strong dependence of the platinum metal catalytic activity (and even the composition of the reaction products) on the catalyst particle
Conclusions
Platinum microdeposits incorporated into thin PANI films show a higher catalytic activity towards methanol oxidation than platinized platinum. The catalytic effect depends on the amount of microdeposits and their distribution in the polymer layer. At moderate and high platinum loadings (60–750 μg cm−2) when the catalyst particles are located on the PANI layer, the reaction mechanism remains the same as on smooth platinum. The catalytic activity increases with increasing platinum loadings,
Acknowledgements
The authors gratefully acknowledge the support of the Russian Foundation for Basic Research (Project # 98-03-32249). Special thanks are due to Dr N.M. Alpatova (A.N. Frumkin Institute of Electrochemistry) for helpful and methodical advice and to Dr A.N. Skundin (A.N. Frumkin Institute of Electrochemistry) for his participation in the discussions.
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