The catalytic removal of CO and NO over Co-Pt(Pd,Rh)/γ-Al2O3 catalysts and their structural characterizations

A series of Co-Pt(Pd, Rh)/gamma-Al2O3 catalysts were prepared by successive wetness impregnation. The catalytic activities for CO oxidation, NO decomposition and NO selective catalytic reduction (SCR) by C2H4 over the samples calcined at 500 degrees C and reduced at 450 degrees C were determined. The activities of the samples calcined at 750 degrees C and reduced at 450 degrees C for NO selective catalytic reduction (SCR) by C2H4 were also determined. All the samples were characterized by XRD, XPS, XANES, EXAFS, TPR, TPO and TPD techniques. The results of activity measurements show that the presence of noble metals greatly enhances the activity of Co/gamma-Al2O3 for CO or C2H4 oxidation. For NO decomposition, the H-2-reduced Co-Pt(Pd, Rh)/gamma-Al2O3 catalysts exhibit very high activities during the initial period of catalytic reaction, but with the increase of reaction time, the activities decrease obviously because of the oxidation of surface cobalt phase. For NO selective reduction by C2H4, the reduced samples are oxidized more quickly by the excess oxygen in reaction gas. The oxidized samples possess very low activities for NO selective reduction. The results of XRD, XPS and EXAFS indicate that all the cobalt in Co-Pt(Pd, Rh)/gamma-Al2O3 has been reduced to zero valence during reduction by H-2 at 450 degrees C, but in Co/gamma-Al2O3 only a part of the cobalt has been reduced to zero valence, the rest exists as CoAl2O4-like spinel which is difficult to reduce. For the samples calcined at 750 degrees C, the cobalt exists as CoAl2O4 which cannot be reduced by H-2 at 450 degrees C and possesses better activities for NO selective reduction. The results of XANES spectra show that the cobalt in Co/gamma-Al2O3 has lower coordination symmetry than that in Co-Pt(Pd, Rh)/gamma-Al2O3. This difference mainly results from the distorting tetrahedrally-coordinated Co2+ ions which have lower coordination symmetry than Co-0 in the catalysts. The coordination number for the Go-Co shell from EXAFS has shown that the cobalt phase is highly dispersed on Co-Pt(Pd, Rh)/gamma-Al2O3 catalysts. The TPR results indicate that the addition of noble metals to Coly-Al2O3 makes the TPR peaks shift to lower temperatures, which implies the spillover of hydrogen species from noble metals to cobalt oxides. The oxygen spillover from noble metals to cobalt is also inferred from the shift of TPO peaks to lower temperatures and the increased amount of desorbed oxygen from TPD. For CO oxidation, the Co-0 is the main active phase. For NO decomposition and selective reduction, Co-0 is also catalytically active, but it can be oxidized into Co3O4 by oxygen at high reaction temperature.