Final stage CO removal by oxidation or hydrogenation using supported PGM catalysts for fuel cell applications.

Abstract

Hydrogen has recently become a promising alternative fuel for small scale energy generation with the aid of fuel cells. The most prefered method for on-board production of pure hydrogen from methane is through a series of catalytic reactions. However, prior to entering the fuel cell stack, the CO concentration in the reformate gas must not exceed 10 ppm. Concentrations of CO greater than 10 ppm poison the Pt anode which results in the loss of activity and, the power output. Post water-gas shift reaction, two methods show promise for the effective CO removal to the desired levels of less than 10 ppm. In the first method, known as preferential oxidation (PROX), CO is oxidized to CO2, whereas in the second method, known as selective methanation (SMET), CO is hydrogenated to CH4. The catalysts for these reactions must be highly active and selective for the specific reaction (CO oxidation and/or CO hydrogenation), since unwanted side reactions could result in the additional loss of hydrogen. This study presents the synthesis, characterization and testing of Pt, Ir and Ru supported on reducible oxides, TiO2 and ZrO2, for both the oxidation and hydrogenation of CO in H2 rich streams. The effect of synthesis methods (wet impregnation and deposition precipitation), controlling the isoelectric points of the supports, the nature of the active metals (metal dispersion, particle sizes, CO chemisorption capacities) and the metal support interactions were investigated. The catalysts were characterized by ICP-OES, BET, XRD, XPS, temperature programmed studies, FTIR-CO, CO chemisorption and HRTEM. Catalytic testing of these materials included CO oxidation, CO oxidation in the presence of H2 and the hydrogenation of CO in dry and realistic water-gas shift reformate feeds. All the catalysts showed appreciable activy for the total oxidation of CO below 200 °C, but in the presence of H2, the activity decreased significantly. The Pt and Ir catalysts, although showing low CO conversions, favoured the undesired oxidation of H2, which was due to the strong metal support interactions of these materials, resulting in higher H2 spillover on the supports, reducing them and thus forming H2O. The Ru systems showed slightly better activity but tend to simultaneously hydrogenate CO and oxidize it, which is not selective or desired since increased H2 consumption takes place.CO hydrogenation, on the other hand, showed promising results for all the catalysts in the dry reactions. However, the Pt and Ir systems tested with realistic water-gas shift feeds, which included CO2 and H2O, favoured the forward and reverse water-gas shift reaction, as well as CO2 hydrogenation. The Ru systems showed the best activity towards the selective methanation of CO with realistic feeds at a temperature 100 °C lower than the Pt and Ir systems, giving 99.9 % CO conversions and 99.9 % selectivity towards CH4. CO2 methanation was only observed once all CO in the feed was converted. The superior results of the Ru systems were attributed to the active metal which has a lower heat of CO adsorption and a higher CO dissociative adsorption energy compared to that of Pt and Ir. The CO content in the feed stream was effectively reduced to less than 10 ppm over the Ru catalysts which is crucial for fuel cell applications.