Novel Catalytic Materials for Selective Oxidation of Carbon Monoxide for Use in Fuel Cells

Abstract

The present work includes the preparation, characterisation and testing of alumina supported Pt-based bimetallic catalysts for the selective removal of carbon monoxide in the presence of excess hydrogen, to be used as the feed stock for fuel cells. In this study, an attempt has been made to modify the Pt/Al203 (low- and high-dispersed) catalyst with Sn, Fe, Co, Cr, Mn, Ni and Cu as promoters using the surface organometallic chemistry (SOMC) route. Various techniques have been employed to characterise the Pt/Al2O3 and the M-Pt/Al2O3 bimetallic catalysts. Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES) technique has been used to determine the composition of the promoted catalysts. The presence of the expected amounts of the second metal in the bimetallic catalysts indicated success of the SOMC route for the preparation of the bimetallic catalysts. The CO uptake by bimetallic catalysts has been found to be lower than that by Pt/AEO] catalyst. Decrease in the CO uptake during CO chemisorption experiments relative to the basic Pt/Al203 catalyst indicates the physical presence of the second metal on the Pt sites suggesting a selective reaction between the reduced surface Pt of the Pt/Al203 catalyst and the organometallic precursor of the second metal had occurred. The shift in the reduction peaks for the bimetallic catalysts in the Temperature Programmed Reduction (TPR) profile compared with that of unpromoted catalyst suggested physical proximity of the two metal species in the catalyst. The presence of the two metals together with no observable change in the size of the particles in the bimetallic catalysts has been confirmed by using Transmission Electron Microscopy (TEM/STEM) together with Energy Dispersive X-Ray (EDX) analysis. This was yet another proof of successful deposition of second metal on Pt by the SOMC technique. All the M-Pt/Al203 bimetallic catalysts (low-dispersed) prepared by the SOMC route were screened towards CO oxidation in the presence of excess H2. Out of all the bimetallic catalysts screened, the best performing catalysts were chosen for the detailed study using two different gas mixtures, containing 02:C0 ratios of 2.5 and 1. The most interesting feature of this study was the performance of Fe- and Co-promoted catalysts under different O2:CO ratios. Both the systems, FePt and CoPt, brought the temperature for the complete CO oxidation down to the temperature range of 80 - 100 °C which is highly preferred for the commercial applications, while maintaining the high value of CO selectivity. The enhanced activity of the promoted Pt catalysts was suggested to be due to the presence of a metal oxide, MOx (M = Fe and Co) which provides the site for O2 absorption and dissociation and therefore eliminates the normal inhibition observed in the oxidation of CO on Pt catalysts