THE CATALYTIC REMOVAL OF CO AND NO OVER CO-PT(PD,RH) GAMMA-AL2O3 CATALYSTS AND THEIR STRUCTURAL CHARACTERIZATIONS/

A series of Co-Pt(Pd, Rh)/gamma-Al2O3 catalysts were prepared by succe ssive wetness impregnation. The catalytic activities for CO oxidation, NO decomposition and NO selective catalytic reduction (SCR) by C2H4 o ver the samples calcined at 500 degrees C and reduced at 450 degrees C were determined. The activities of the samples calcined at 750 degree s 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 o f 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 r eaction, but with the increase of reaction time, the activities decrea se obviously because of the oxidation of surface cobalt phase. For NO selective reduction by C2H4, the reduced samples are oxidized more qui ckly by the excess oxygen in reaction gas. The oxidized samples posses s 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-Al2O 3 has been reduced to zero valence during reduction by H-2 at 450 degr ees C, but in Co/gamma-Al2O3 only a part of the cobalt has been reduce d to zero valence, the rest exists as CoAl2O4-like spinel which is dif ficult to reduce. For the samples calcined at 750 degrees C, the cobal t exists as CoAl2O4 which cannot be reduced by H-2 at 450 degrees C an d possesses better activities for NO selective reduction. The results of XANES spectra show that the cobalt in Co/gamma-Al2O3 has lower coor dination symmetry than that in Co-Pt(Pd, Rh)/gamma-Al2O3. This differe nce mainly results from the distorting tetrahedrally-coordinated Co2ions which have lower coordination symmetry than Co-0 in the catalysts . The coordination number for the Go-Co shell from EXAFS has shown tha t the cobalt phase is highly dispersed on Co-Pt(Pd, Rh)/gamma-Al2O3 ca talysts. The TPR results indicate that the addition of noble metals to Coly-Al2O3 makes the TPR peaks shift to lower temperatures, which imp lies the spillover of hydrogen species from noble metals to cobalt oxi des. The oxygen spillover from noble metals to cobalt is also inferred from the shift of TPO peaks to lower temperatures and the increased a mount of desorbed oxygen from TPD. For CO oxidation, the Co-0 is the m ain active phase. For NO decomposition and selective reduction, Co-0 i s also catalytically active, but it can be oxidized into Co3O4 by oxyg en at high reaction temperature.