Preparation Of Meso-macroporous Monolithic Catalysts And Their Application To The Preferential Oxidation Of CO

The preferential oxidation of CO (CO-PROX) is a requisite step of hydrogen generator process for proton exchange membrane fuel cells. Recently, the miniaturization of the CO-PROX reactor has been the focus of widespread research. This work is to develop a new catalyst with high catalytic performance for CO-PROX, as well as to meet the requirements of the miniaturization.The macroporous monolithicα-Al₂O₃ (referred to M-α-Al₂O₃) was prepared by imbibing macroporous polystyrene foams with alumina hydrosols. Addingγ-Al₂O₃ to the macroporous walls could increase the specific surface area of M-α-Al₂O₃. Using M-γ/α-Al₂O₃ as supports loaded Pt-Ni catalyst for the CO-PROX reaction. This catalyst could purify the exit concentration of CO to less than 100 ppm in the temperature range of 140-180oC in 1 vol. % CO, 1 vol. % O₂, 50 vol. % H₂, 12.5 vol.% CO₂, 15 vol.% H₂O and N2 gases with a volume space velocity of 16,000 h-1. The results show that preparing catalysts to macroporous monolithic structure is a promising way for the miniaturization of CO removing reator.The meso-macroporous monolithic alumina was fabricated via using aluminum iso-propoxide as a alumina precursor, nonionic surfactant triblock copolymer P123 as a soft template for the meso-structure and PS as a hard template for macro-structure. The prepared samples had interconnected macropores and wormhole-like mesopores. Mesoporous alumina could assemble on the macroporous walls of M-α-Al₂O₃. The specific surface area and meso-structure of sample were affected by the loading amount of mesoporous alumina. The sample with 4.4% amount of mesoporous alumina was used as support to load Pt-Ni catalyst for the CO-PROX reaction. The experimental results indicated that the prepared catalyst was well tolerant to CO₂ and H₂O.Carbon nanotubes oxidized with H₂SO4/HNO3 solution supported Pt-Ni catalysts were prepared and used for CO-PROX. The results of catalytic preformance tests showed that the prepared catalysts were very active and highly selective at low temperature in 1 vol. % CO, 1 vol. % O₂, 50 vol. % H₂ and N2 gases. Adding 12.5 vol. % of CO₂ into the feed gases had slight negative influence on CO conversion. Adding 15 vol. % of H₂O led to a little decrease of CO conversion at the temperature range of 100 to 120 oC, which was proposed to be caused by capillary wetting of water in the micro-pores of carbon nanotubes. As the reaction temperature was higher, adding water could improve CO conversion.A series of carbon nanotube (CNT)-alumina composite monoliths with meso-macroporous structures were successfully synthesized by imbibing macroporous monolithic polystyrene foams with carbon nanotube-alumina hydrosols. These composite monoliths possessed interconnected spherical macropores and adjustable mesopores of several nanometers. CNTs were uniformly dispersed throughout the alumina matrix. The mechanical strength and thermal conductivity of composite monoliths can be improved with adding appropriate amounts of CNTs and with suitable calcination temperature. The sample of 5 wt.% CNT-Al₂O₃-1300-M had good mechnical strenghth and high thermal conductivity. The Pt-Ni/CNT-Al₂O₃ monoliths exhibited high activity and selectivity. The residual concentration of CO was purified to less than 100 ppm in the temperature range of 120-180oC in 1 vol. % CO, 1 vol. % O₂, 50 vol. % H₂, 12.5 vol.% CO₂, 15 vol.% H₂O and N2 gases with a volume space velocity of 10,400 h-1. This catalyst exhibited good stability.