SCR of lean NOx with C3H8 over Co/MFI catalysts: dependence on synthesis condition of MFI and Co location

The effects of different synthesis conditions for MFI and the different locations of Co2+ on MFI on the catalyst properties for the selective catalytic reduction of NO by C3H8 [C3H8–SCR] in the presence of excess oxygen were investigated, and the nature of Co2+ was characterized by XRD, FTIR, H2-TPR, XPS, and DTG. H2-TPR shows that the exchanged Co ions in MFI are nonreducible even at 750 °C, while the reduction temperature of incorporated Co is at 690 °C. Three types of DTG peaks, which correspond to different temperatures for the thermal decomposition or disassociation of TPA, are observed in the differential thermogravimetric (DTG) analysis of the as-synthesized MFI prepared under different conditions, and are related well to C3H8–SCR of NO for three different types of Co sites in MFI: incorporated Co2+ site and two different exchanged Co2+ sites, respectively. The incorporated Co2+ in MFI with the DTG peak temperature at 418 °C is almost inactive for C3H8–SCR of NO. For the Co2+-exchanged MFI, the sample with a low Co2+-exchange degree has a higher TOF for NO reduction than that having a higher Co2+-exchange degree, indicating that at least two types of Co2+ sites exist in the Co2+-exchanged MFI sample. The corresponding DTG peak temperatures located at 455 and 481 °C show two types of exchanged Co ion sites having different surface energies for different activities for SCR of NO, with the Co2+ site with higher surface energy having higher specific catalytic activity for NO reduction. The exchange degree and position of Co2+ depend on the alkalinity of the hydrothermal synthesis condition and the sodium content of the resulting Co2+-exchanged MFI.