Fuel processing for fuel cells: Preferential oxidation (PROX) of carbon monoxide from practical reformates for PEM hydrogen-oxygen fuel cells using high contacting efficiency microfibrous entrapped catalysts.

摘要

Preferential oxidation (PROX) of CO in H2 is believed to be the most efficient way to remove CO from practical reformate streams for Polymer Electrolyte Membrane (PEM) fuel cells. Pt/Al2O3 has long been known as a suitable catalyst for this purpose. Over a conventional Pt/Al2O3 catalyst, however, preferential oxidation of CO in H2 is known to occur at temperatures above 150°C, and the maximum CO conversion usually takes place at around 200°C. In this study, modification of Pt/Al2O3 with a transition metal resulted in significantly enhanced catalytic performance for preferential CO oxidation from practical reformates in the temperature range of 25 to 150°C. The active reaction temperature window was enlarged to 25--200°C, compared to the narrow window at around 200°C using the conventional Pt/Al 2O3. Differential reactor studies, hydrogen and oxygen chemisorption, X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDS), in-situ Fourier transform infrared spectroscopy (FTIR) and temperature programmed reduction (TPR) studies were performed to characterize the newly developed promoted catalysts. Microstructured materials have potential for enhanced mass and heat transfer compared to the typical catalyst particulates used in industrial processes. Microfiber composite materials made by a wet lay paper-making/sintering process can incorporate particles as small as 10mum into a micrometal fiber matrix. Traditional high speed and low cost paper making equipment and techniques were used in this study to prepare composite materials and Pt-Co/Al2O3 catalysts that were identified, by both integral and differential reactor studies, to be superior candidates for preferential oxidation of CO and entrapped into the microfibrous materials. The microfibrous entrapped catalyst bed was then compared with conventional packed beds of same and larger catalyst particulates. A microfibrous entrapped H2S sorbent layer was placed upstream of the microfibrous entrapped PROX catalyst layer to remove both H2S and CO from a sulfur-contaminated model reformate stream. The outermost H2S sorbent layer thus promotes the activity maintenance of a secondary non-poison tolerant PROX CO catalyst, which ultimately serves to maintain the activity of a CO-intolerant precious metal based MEA assembly. Finally, mathematical modeling utilizing plug flow reactor equations was used to construct a PROX reaction map by which two competing reaction rate constants for CO and H2 oxidations can be determined by collecting a pair of CO conversion and O2 selectivity values with minimal experimental effort.