Characterization of Formic Acid Oxidation at Surface-Modified Pt/C Catalysts

Abstract

Over the last several decades, much attention has been given to low temperature fuel cells which provide energy based on electrochemical oxidation of fuels such as hydrogen and small organic molecules. Many researchers in the field of fuel cell technology have been focusing on the development of cost effective and durable catalysts. Among the small organic fuel molecules, electrochemical study of formic acid oxidation is being intensively investigated for two main reasons; formic acid can be used as a fuel in direct formic acid fuel cells and it can serve as a model reaction that can be used to understand more complex oxidation reactions, such as the oxidation of methanol and ethanol. Platinum is one of the most commonly used and effective catalysts for the oxidation of small organic molecules, despite the fact that it has significant disadvantages, such as high cost and loss of performance due to extreme susceptibility to poisoning by CO. To solve these problems several techniques have been applied, most of which are based on modifying the bulk and/or surface of Pt by a second metal. Heterogeneous bimetallic catalysts often show significantly enhanced catalytic performances because of their improved electronic and chemical properties that are different from their parent metals. Bimetallic catalysts provide opportunities to significantly improved catalytic selectivity, activity and durability. In order to improve the electrochemical oxidation of formic acid, addition of second metals such as Ru, Pb, Os, Sn, Cu, Pd, Rh, Bi has been widely applied. Among them, bismuth especially has received much attention as a Pt surface modifier. Various structures such as PtBi alloy, PtBi intermetallic, and surface modified systems have been reported. Co-deposited, carbon supported PtBi/C [1], under potential deposition [2] and irreversibly absorbed [3] Bi modified Pt surfaces were reported to be good catalysts for formic acid oxidation, in terms of onset potential, activity as well as stability and durability. The primary objective of this research is to obtain mechanistic information on the oxidation of formic acid, methanol and ethanol by comparing the effects of various modifying metals on voltammetric data at surface-modified Pt/C catalysts. For example, Bi was found to greatly enhance oxidation of formic acid through the direct pathway in which absorbed CO is not formed, while it strongly inhibits methanol oxidation. Data for Pb and Rh modified Pt will also be presented.