Browsing by Author "Mohamed, Ziyaad."

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Final stage CO removal by oxidation or hydrogenation using supported PGM catalysts for fuel cell applications.
(2015) Mohamed, Ziyaad.; Friedrich, Holger Bernhard.; Singh, Sooboo.
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.
The preferential oxidation of CO nickel oxide catalysts and the doping effects of platinum in hydrogen rich streams.
(2012) Mohamed, Ziyaad.; Friedrich, Holger Bernhard.; Singh, Sooboo.
Hydrogen has now become a suitable candidate for alternative energy generation for small scale applications with the aid of fuel cells. On-board production of hydrogen from methane is the most preferred method via a series of catalytic reactions. However, the carbon monoxide (CO) concentrations following these reforming steps is still too high (±1 %) and is detrimental to the anode of the fuel cell. For maximum output and efficiency of the fuel cell CO concentrations must be reduced to less than 10 ppm. Preferential oxidation (PROX) following the water-gas shift reaction is a promising method that could be employed to reduce the CO content in the reformate gas. This project entails the synthesis, characterization and testing of nickel based catalysts for the oxidation of CO in H₂ rich streams, and to dope with Pt to determine the effects of the platinum group metal on the catalyst for this reaction. A series of NiO/Al₂O₃, Pt/Al₂O₃ and Pt/NiO/Al₂O₃ catalysts were prepared by incipient wetness technique. These catalysts were characterized by TGA, ICP-OES, XRD, BET, TPR, TPD, N₂ adsorption desorption isotherms, CO chemisorptions, SEM-EDX and TEM. The catalysts were then tested for the oxidation of CO in H₂ rich streams. XRD patterns of the catalysts indicated the presence of NiO and PtO phases on the respective supports and in situ redox reactions showed catalysts had reversible phase changes (oxide and metallic) that were stable. N2 adsorption-desorption isotherms indicated the presence of mesoporous materials for all catalysts studied. Impregnation of Pt on the NiO/Al₂O₃ catalysts promoted the reduction of the catalyst to lower temperatures. All catalysts were stable for long periods of time in the presence of H₂ at 150 °C. NiO/Al₂O₃ catalysts were not very active for the preferential oxidation of CO within its stipulated temperature ranges giving the highest CO conversion at 290 °C of 11 % with the selectivity towards CO₂ of ± 25 %. The Pt/Al₂O₃ showed much better activity at higher PROX temperatures compared to the NiO/Al₂O₃ with regards to CO conversion and selectivity towards CO₂. The highest CO conversion obtained within the PROX range was ±56 % with a selectivity towards CO₂ of 68 % at 200 °C. The Pt/NiO/Al₂O₃ showed a synergistic effect, with much higher CO₂ selectivity and CO conversion within the PROX temperature ranges compared to both mono-metallic catalysts studied. The highest CO conversion obtained for this catalyst was at 180 °C of 99.9 % with a selectivity towards CO₂ of 74 %.