Support effects on de-NOx catalytic properties of supported tin oxides

The effect of the carrier on the reduction pattern, the acid-base properties, and the catalytic activity of supported tin dioxide catalysts was studied using TPR, adsorption calorimetry, and reduction of NOx by ethylene in an oxygen-rich atmosphere. Through impregnation, two series of SnO2 catalysts of low (~ 3 wt%) and high (~ 20 wt%) Sn content were prepared. The dramatic effect of the support on the activity and selectivity of the SnO2 surfaces in the NO reduction by C2H4 was observed. For the 3 wt% Sn series, 39, 38, 29, 24, and 0% conversions of 5000 ppm NO to N2 in the presence of 90,000 ppm of O2 at a space velocity of 50,000/hr were noted at 500\ub0C on ZrO2, Al2O3, TiO2, SiO2, and MgO supports, respectively. The catalysts based on ZrO2 and Al2O3 at low Sn loading were the most active catalysts. An increase of the Sn loading resulted in small positive or negative effects on the extent of the NO reduction depending on the support. A direct relationship between reducibility and catalytic activity was also observed. The molecular structures of SnO2 played a significant role above monolayer coverage. For the 20 Sn wt% series, the reducibility scale for SnIV \u2192 SnII, based on the temperature at the maximum of the reduction peak, was in the order SnSi-20 > SnTi-20 > SnAl-20 > SnZr-20, while the competitiveness factor increased in the same order. To have a good catalytic performance, a strong acidity was required but no direct correlation between the number of acid sites and the catalytic activity was noted