The polymer electrolyte membrane fuel cell (PEMFC) has come to be regarded as one of the most promising candidates for utilizing hydrogen to produce heat and electricity, especially for electric vehicles or residential co-generation systems.[1] Pt and Pt-based alloys, which are generally used as the anode of PEMFCs, are known to be easily poisoned by even small amounts of CO in the hydrogen-rich stream that can be produced from various hydrocarbons by the reforming and water gas shift (WGS) reactions.[2] Due to the limited catalytic activities of the current WGS catalysts for complete CO conversion, which is thermodynamically favored at low temperatures, approximately 0.5-1 vol % of unconverted CO remains in the effluent, and this should be removed to a trace level of below 10 ppm before reaching the PEMFC. Among the methods proposed to remove this residual CO, preferential CO oxidation (PROX) has been accepted as one of the most promising.[2] Three main reactions take place in this system. The reaction that competes most effectively with CO oxidation [Eq. (1)] is H 2 oxidation [Eq. (2)] because of the H 2 -rich conditions in the gas stream in practical fuel cell applications. In addition, CO can consume additional H 2 by undergoing hydrogenation [Eq. (3)], a process also known asmethanation, which should be avoided unless the CO concentration in the reactant stream is quite low, because it consumes relatively large amounts of hydrogen (3 mol per mol CO). Thus, a highly active and selective catalyst is required to remove CO from the H 2 -rich stream before it reaches the PEMFC. Among a number of catalysts reported to be active for PROX, [3] supported platinum catalysts have been considered to be promising in view of their high catalytic performance. However, they usually only show noticeable activities under practical conditions above 423 K, [3a] where the reverse WGS reaction could occur, thereby hindering complete CO removal. Many researchers have therefore made efforts to enhance the PROX activity of supported platinum catalysts at low temperatures, for example by water vapor pre-treatment of the Pt catalyst, [3b] the addition of alkali metals, [3c] and the addition of other metals.[3d-n] In a previous study, we found that Pt-Co/g-Al 2 O 3 is one of the most active catalysts among the supported Pt catalysts tested under the same reaction conditions.[3k,l] Until now, most work has been conducted on Pt-based catalysts supported on g-Al 2 O 3 , and the effect of supports on Pt-based catalysts has been limited to TiO 2[3m] and CeO 2 .[3n] Furthermore, the Pt loading in some active Pt-based catalysts is relatively high, which can hinder their practical applications.[3b,e-g] Herein, we report that a Pt-Co bimetallic catalyst supported on yttria-stabilized zirconia (YSZ) is highly efficient for PROX in a H 2 -rich gas stream even with a small amount of Pt (0.5 wt %) at temperatures below 423 K. By optimizing the calcination and reduction pre-treatment conditions for Pt-Co/YSZ, the CO concentration can be decrease...
