The preferential oxidation of CO nickel oxide catalysts and the doping effects of platinum in hydrogen rich streams.

Abstract

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 %.