Support Effects on de-NOx Catalytic Properties of Supported Tin Oxides

The influence of the carrier (i.e., SiO2, ZrO2, TiO2, γ-Al2O3, and MgO) on the reduction pattern, the acid–base properties, and the catalytic activity of supported tin dioxide catalysts has been investigated by temperature-programmed reduction/oxidation, adsorption calorimetry, and reduction of NOx by ethene in an oxygen-rich atmosphere. Two series of SnO2 catalysts of low (∼3 wt%) and high (∼20 wt%) Sn content were prepared by impregnation. The dramatic influence of the support on the activity and selectivity of the SnO2 surfaces in the NO reduction by C2H4 was evidenced. 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 h−1 were observed at 500°C on ZrO2, Al2O3, TiO2, SiO2, and MgO supports, respectively. The most active catalysts at low Sn loading were those based on ZrO2 and Al2O3. The integral N2 formation rates per mole of SnO2 ranged from 2 to 5×10−3 s−1 in the 350–500°C temperature domain. An increase of the Sn loading led to small positive or negative effects on the extent of the NO reduction depending on the support. A direct relationship between reducibility and catalytic activity has also been observed. Above monolayer coverage, the molecular structures of SnO2 play an important role. For the 20 Sn wt% series, the reducibility scale for SnIV→SnII, based on the temperature at the maximum of the reduction peak, is in the order SnSi-20>SnTi-20>SnAl-20>SnZr-20, while the competitiveness factor increases in the same order. Finally, it appears that a relatively strong acidity is necessary for good catalytic performance, but no direct correlation between the number of acid sites and the catalytic activity was observed.