The incorporation of > 30% silica in alumina prior to platinum impregnation improves the NOx conversion efficiency in the 200-300 degrees C range from 45 to 57% and reduces light-off temperature. Further increase in the amount of silica to 50% is detrimental to NOx conversion efficiency. The H-1 and Si-29 NMR of the materials suggest that this trend is probably related to the surface acidity. The analyses of these materials by X-ray powder diffraction and electron microscopy do not reveal significant differences. Additional NOx conversion in the 350-450 degrees C range with a maximum of 30% at 400 degrees C can be achieved if a rhodium-impregnated 30% silica-alumina, Rh-30% SiO2-Al2O3, is placed upstream of Pt-30% SiO2-Al2O3. it is important to note that mixing Pt-30% SiO2-Al2O3 with Rh-30%SiO2-Al2O3 does not enhance conversion efficiency or effective temperature range.