(296f) Kinetic Experiments of Preferential Oxidation of Carbon Monoxide Over Platinum for Hydrogen Purification

There is significant interest in purifying H2 from CO for use in highly efficient fuel cells (1). To power a proton-exchange membrane fuel cell (PEMFC), one must provide a H2 stream with CO impurity levels below 10 ppm to prevent poisoning of the Platinum anode. Hydrogen is typically produced from hydrocarbons via the steam reforming and water gas shift reactions. However, a typical hydrogen stream from this process will contain approximately 1% (molar basis) of CO (2), much higher than allowed for PEMFC operation. One approach to removing the impurity is to oxidize the CO into CO2 using a Pt catalyst. However, the kinetics of this multi-component process is not completely understood. Specifically, it is unclear whether or not the water gas shift reaction pathways play an important role.

In this work, kinetic experiments for the catalytic oxidation of carbon monoxide over a highly diluted Pt/alumina catalyst were performed with excess hydrogen. The ratios of CO, O2, H2, and H2O were varied to identify the most important reaction pathways at each set of conditions. It was observed that carbon monoxide conversion increased in the presence of H2 at low temperatures but was inhibited at high temperatures. CO conversion increased by introducing water as a reactant at low temperatures, suggesting that the water formed via hydrogen oxidation can react with CO to form CO2. By increasing the O2 to CO ratio, the CO conversion was observed to go above the equilibrium value. Using a previously published microkinetic model (3), we found that this behavior is caused by the reaction zone being too short to allow for the system to reach equilibrium via reverse water gas shift. Therefore, by changing O2 to CO ratio and the residence time of the reactor, it is possible to remove CO above its equilibrium limit.

References:

1. E. D. Park, D. Lee, H. C. Lee, Catalysis Today 139, 280 (2009).

2. H. P. Dhar, L. G. Christner, A. K. Kush, Journal of the Electrochemical Society 134, 3021 (Dec, 1987).

3. A. B. Mhadeshwar, D. G. Vlachos, Journal of Physical Chemistry B 108, 15246 (2004).