Preparation Of Macroporous Catalysts And Their Application To Preferential Oxidation Of CO In H₂-Rich Gases

Preferential oxidation of CO (CO-PROX) is the most promising way for removing CO from H₂-rich gases in fuel cells. It is difficult to reduce the CO content to below the required ppm level due to the diffusive limitation of CO in its oxidation reaction. Furthermore, for CO-PROX, an important part of fuel cell oriented hydrogen production from hydrocarbons, miniaturization is another key challenge. For these two crucial issues, three-dimensionally ordered macroporous (3DOM) catalysts and macroporous monolithic catalysts were prepared via templating method in this work. Properly adjusting the porous structure of the catalysts is favor of CO diffusion and mass transfer. At the same time, the macroporous monolith supports possess monolithic shape and high specific surface area to volume ratio, which are beneficial to compacting reactor volume. Therefore, it is a potential alternative for the miniaturization. The main experimental results and conclusions are as follows:3DOM CuO-CeO₂ catalysts were prepared by using polystyrene (PS) colloidal crystal as the template, nitrate as the precursor and citric acid as the chelator. For CO-PROX reaction, at a higher space velocity of 160,000 mL.gcat^-1.h^-1, complete CO conversion has been obtained between 150 oC and 175 oC. As the macropores of 3DOM are destroyed, complete CO conversion cannot be obtained even at a lower space velocity of 80,000 mL.gcat^-1.h^-1, and likewise over particulate CuO-CeO₂ catalysts. It is proposed that the ordered macropores in the 3DOM catalyst favor mass transfer of CO, resulting in the excellent catalytic performance.Macroporous silica and alumina monoliths have been prepared for the first time by filling polystyrene foam templates with the corresponding hydrosols. The macroscopic shape and macroporous size of the monoliths can be adjusted by the moldable polymer template. Besides, the obtaining macroporous monoliths have the advantages such as cheap and facile raw materials, high porosity, small volume shrinkage and inter-connected macropores, which make them very suitable for using as catalyst supports. The effects of the precursor and the filling times of the alumina hydrosols, the mesopore surfactant addition in the hydrosols and the calcination temperature on the properties of the alumina monolith have been investigated. The characterization results show that the calcination temperature has the most remarkable influence to the porous structure, crystalline phase and compressive strength of the alumina monoliths.CuO-CeO₂ catalysts and Pt-based catalysts were loaded on the macroporous alumina monoliths mentioned above. The influences of the preparation methods and conditions to the catalytic performance were investigated. For macroporous CuO-CeO₂/α-Al₂O₃ monolithic catalysts, the loading amount of the CuO-CeO₂ active component is very important. The monolithic catalysts with higher loading amounts give better catalytic performance for CO-PROX reaction. Among the macroporous Pt-Ni/α-Al₂O₃ monolithic catalysts, excellent performance was observed over the sample prepared via co-impregnating of Pt(NH₃)₂(NO₂)2 and Ni(NO₃)2.6H₂O under pH = 7. At 160 oC and a space velocity of 20,000 h-1, the exit CO content could be reduced to 40 ppm under the reactant feed gas of 1vol.% CO, 1vol.% O₂, 50vol.% H₂,12.5vol.% CO₂, 15vol.% H₂O in N2. Compared to honeycomb monolith and micro-channel catalytic reactor, the volume of the macroporous monolith should be much smaller, indicating that macroporous monolithic catalyst is a potential alternative for CO-PROX compact reactor.