The Preparation Of Macroporous Monolithic Pt/Al₂O₃ And Its Application For Preferential Oxidation Of CO

Among the various possible methods for CO removal from hydrogen-rich gases,the preferential oxidation of CO (CO-PROX) has been regarded as the most straighforward,simple and promising process. The key problem of the CO-PROX catalysts is: the concentration of CO is hard to be eliminated to 10 ppm while it is between 50-100 ppm. And the main reason of this is the CO diffusion limitation. So in this work, monolithicγ-Al₂O₃ supports with macroporous were prepared so that CO will be easy to diffuse and transfer through the catalysts by enlarging the pore size and accommodating the pore distribution properly. This kind of pore structure is also good for heat transfer.Based on the ideas stated above, in this work, the monolithic catalysts with macro-porous is prepared by a wet impregnation method from an aqueous solution of Pt precursor. Four schemes were designed to prepare the catalysts with good activity and selectivity in CO-PROX. By different tests and characterization techniques, the main results are as follows:1. The monolithic Pt/γ-Al₂O₃ catalysts with macroporous show higher activity and selectivity than the particle catalysts at a higher temperature while the activity and selectivity are still low at a lower temperature. The catalysts with uniform distribution could be made by using of competitive adsorbate. But most of the active components are formed as dispersived phase which has a strong interaction with the supportγ-Al₂O₃.2. As to the catalysts prepared by a wet impregnation method from an aqueous solution of Pt precursor Pt(NH3)2(NO2)2, the pH value has great effect to the activity and selectivity of the CO-PROX. The catalysts show the highest activity and selectivity while the pH value is proper (pH=7).3. The 40-60 mesh particle catalysts show similar activity and selectivity compare with the monolithic catalysts. The active components are well dispersed on the particle support while the aggregation of Pt particles happened on the monolithic support.4. The monolithic Pt/γ-Al₂O₃ catalysts with potassium promoter with macroporous showed higher activity and selectivity than the one without potassium. The conversion of CO reached above 99.8% at 225℃while the yield of CH4 was quite low. The O₂ selectivity to CO was maintained around 43% at high temperature. The mechanism was investigated by comparing the catalysts used under different reaction conditions (PROX in H₂-rich gas and CO oxidation without H₂). The OH group is increased, which could be due to the adding of potassium promoter and the H₂-rich atmosphere. The increased OH group enhanced the CO conversion.5. The result of Temperature Programmed Reduction (TPR) results indicates that the states of Pt species are different when the Pt precursor is different. The states of Pt are possibly PtOx and PtO_xCl_y with H₂PtCl₆ precursor, while the one may be PtOx and PtOx which has strong interaction with the support with Pt(NH₃)₂(NO₂)₂. For the K₂O-Pt/Al₂O₃ catalyst, only a small amount of H₂ is consumed by Pt oxide and most of H2 are comsumpted by the surface oxygen shared with potassium.