Proton exchange membrane fuel cells (PEMFCs) using H₂ fuel are a promising source of portable power for electronic devices and transportation. However, as CO is a common poison for PEMFC electrodes and also a common contaminant in H₂, it is critical to be able to detect and quantify CO contamination at low levels in concentrated H₂ fuel. On-line CO detection in portable devices requires sensors with small size, low cost, low power consumption, and high sensitivity. These requirements have been met by microelectromechanical system (MEMS) signal transducers. However, all current microsensor technologies use materials that are inherently unselective to CO, give rise to detrimental false positive responses in the presence of H₂, and require arrays of sensors for gas identification. A new sensor paradigm will be presented utilizing an intrinsically selective catalytic reaction coupled with micro thermal transduction. In this sensing mechanism, a substrate selectively and exothermically oxidizes CO to CO₂, causing a measurable temperature rise that can be related to CO concentration.

CuO_x-CeO₂ catalysts have been shown to oxidize CO to CO₂ with up to 100% CO oxidation selectivity in process streams containing ~10,000 ppm CO or more in H₂ excess. It is critical to maximize selectivity towards CO in order to avoid false positive responses resulting from H₂ oxidation. CO oxidation selectivity (relative to H₂ oxidation) must be nearly 100%; the potential sensing range of the catalyst at a particular operating temperature is determined by the concentration at which selectivity drops from this value. The lower limits at which CuO_x-CeO₂ maintains this high selectivity were found to be 200 and 500 ppm at operating temperatures of 333 and 353 K, respectively. Thus, CuO_x-CeO₂ catalysts are a promising sensing substrate for ppm level detection of CO in H₂ fuels by reaction calorimetry. The high selectivity of this catalyst is shown to be a product of both high kinetic selectivity and high surface coverage of CO. The loss of 100% selectivity at decreasing CO concentration is attributed to a lower driving force for CO adsorption. Integration of this catalyst with MEMS sensor technologies will provide new capabilities in portable sensors for the selective quantitative detection of CO in H₂ while avoiding false positive responses.