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, H-2-TPR, XPS, and DTG. H-2-TPR shows that the exchanged Co ions in MFI are nonreducible even at 750degreesC, while the reduction temperature of incorporated Co is at 690degreesC. 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 418degreesC 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 481degreesC 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. (C) 2003 Elsevier Inc. All rights reserved.