Comparison of methanol, ethanol and iso-propanol oxidation on Pt and Pd electrodes in alkaline media studied by HPLC

doi:10.1016/j.elecom.2011.02.022

Electrochemistry Communications

Volume 13, Issue 5, May 2011, Pages 466-469

https://doi.org/10.1016/j.elecom.2011.02.022 Get rights and content

Abstract

The oxidation of methanol, ethanol and iso-propanol and their respective product formation on platinum and palladium electrodes in alkaline solution are studied by voltammetry combined with high performance liquid chromatography. The oxidation products observed at platinum are formaldehyde and formate for methanol, acetaldehyde and acetate for ethanol and acetone for iso-propanol oxidation. On palladium, the same products (except formaldehyde) are detected. Palladium appears to be a better catalyst for the selective oxidation of the alcohol group in alkaline media, but as soon as poisoning by adsorbed carbon monoxide plays a significant role, such as in methanol oxidation, platinum is the preferred catalyst.

Research highlights

► HPLC can be combined with voltammetry to obtain liquid reaction products. ► Pd is a selective catalyst for alcohol oxidation to corresponding acid in base media. ► Pt is a good catalyst for adsorbed CO also in alkaline media. ► Acetone is the only liquid product of 2-Propanol oxidation on Pt and Pd electrodes.

Introduction

For low power demand applications such as laptops and mobile phones, alcohols are interesting fuels for direct alcohol fuel cells (DAFC) due to their high energy density, easy storability and refuelling compared to gaseous hydrogen. However, real commercial breakthroughs have not been attained due to the expensive cell components, especially noble metal catalyst materials. To decrease the cost of these materials, the amount of noble metal in the catalyst has to be reduced. In alkaline media the reaction kinetics are enhanced compared to acidic media [1], and therefore, the same current density can be obtained with lower catalyst loadings. The interest in alkaline media has re-emerged after decades due to the introduction of new anion exchange membranes for direct alcohol fuel cells [2], [3]. Platinum is generally considered the best monometallic catalyst for the electro-oxidation of small organic molecules due to its capability to catalyse carbon hydrogen bond rupture as well as CO oxidation at comparatively low overpotential. Platinum and its alloys are therefore frequently studied for alcohol oxidation in acid media. Palladium has been studied less actively in acid media due its lower performance for alcohol oxidation. However, in alkaline media, palladium appears to be quite active for ethanol oxidation [4]. Palladium-based alloys in alkaline media have also received significant attention [5], even though the fundamental understanding of alcohol oxidation on a pure palladium electrode has not yet been fully obtained.

At present, methanol is the most widely studied fuel in acid media due to the favourable current densities obtained and the relatively well-known kinetics. However, alcohols of higher molar mass, such as ethanol and iso-propanol, are less toxic, have higher boiling points and energy densities, making them interesting options for DAFC. The oxidation mechanisms of methanol [6], [7], ethanol [1], [8], [9] and iso-propanol [7], [10] have been widely studied in alkaline media with cyclic voltammetry. However, the disadvantage of purely electrochemical methods is that no information on the adsorbates or the oxidation products is obtained. Therefore, several spectroscopic methods, such as Fourier Transform Infrared Spectroscopy (FTIRS) [11], [12], [13], Surface Enhanced Raman Spectroscopy [1] and Differential Electrochemical Mass Spectrometry [14], have been applied to probe reaction intermediates and mechanisms. With these methods (volatile) reaction products and/or adsorbed species can be probed. CO₂ is the main product of methanol oxidation as no further carbon cleavage is required, but for the oxidation of higher alcohols, incomplete oxidation products such as aldehydes, ketones and carboxylic acids (carboxylates in alkaline media) tend to dominate the product spectrum. High performance liquid chromatography (HPLC) is the method of choice to study liquid, stable reaction products of the oxidation of alcohols and the identification of these products is more straightforward than for instance with FTIRS. HPLC has been successfully combined with voltammetry recently to study the reaction products of glycerol [15]. Although the limitation of the HPLC method is the inability to detect gaseous products such as CO₂ and adsorbed intermediates, it is a good method to complement spectroscopic findings. In this communication, we compare the oxidation of three simple alcohols on platinum and palladium electrodes in alkaline solution, and probe product formation using HPLC.

Section snippets

Experimental

The experiments were performed in an electrochemical cell with 0.5 M KOH electrolyte with or without 1 M of the studied alcohol. A large platinum sheet was used as a counter electrode and a Hg/HgO/0.5 M KOH was used as a reference electrode; however, all potentials reported are referred to the RHE scale. Platinum or palladium disks (6.1 mm diameter) were used as working electrodes and cleaned mechanically both with alumina and ultrasonically, and electrochemically by cycling between 0.1 and 1.5 V

Oxidation and product analysis on platinum

Potential sweeps for alcohol oxidation on platinum (Fig. 1) are shown with (solid line) and without (dashed line) sample collection. Comparison of these curves demonstrates the formation of poisoning species and their solubility: if the current density is higher in the presence of sample collection, some (precursors to) poisoning species exist on/near the catalyst surface and they are removed by the sample collection [15]. This effect can be observed clearly with methanol oxidation (Fig. 1a)

Conclusions

This communication has applied a new HPLC method to study the dissolved reaction products of the oxidation of three simple alcohols. For methanol oxidation, platinum provides a lower onset potential and almost four times higher current density compared to palladium, which we ascribe to the higher ability of platinum to oxidize the CO_ads intermediate. In the case of ethanol, both electrodes produce about the same current density. However, on palladium ethanol is more selectively oxidized to

Acknowledgments

The Finnish Foundation for Technology Promotion is gratefully acknowledged for providing financial support to A. S-A to visit Leiden University, the Netherlands. The Academy of Finland is acknowledged for financial support. The work of Y. K. and M.T.M. K. has been performed within the framework of the CatchBio program. They gratefully acknowledge the support of the Smart Mix Program of the Netherlands Ministry of Economic Affairs and the Netherlands Ministry of Education, Culture and Science.

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Comparison of methanol, ethanol and iso-propanol oxidation on Pt and Pd electrodes in alkaline media studied by HPLC

Abstract

Research highlights

Introduction

Section snippets

Experimental

Oxidation and product analysis on platinum

Conclusions

Acknowledgments

J. Power Sources

Electrochim. Acta

Electrochim. Acta

Electrochim. Acta

J. Power Sources

J. Power Sources

J. Electroanal. Chem.

J. Power Sources

Catal. Today