Catalytic selective reduction of NO with ethylene over a series of copper catalysts on amorphous silicas

Catalytic selective reduction of NO to N₂ was studied comparing a series of Cu-based catalysts (ca. 8 wt.%) supported over amorphous pure and modified silicas: SiO₂, SiO₂-Al₂O₃, SiO₂-TiO₂, SiO₂-ZrO₂. The catalysts were prepared by the chemisorption-hydrolysis method which ensured the formation of a unique copper phase well dispersed over all supports, as confirmed by scanning electron micrographs (SEMs). Temperature-programmed reduction (TPR) analyses confirmed the presence of dispersed copper species which underwent complete reduction at a temperature of about 220°C, independently of the support. It was found that the support affects the extent of NO reduction as well as the selectivity to N₂ formation. Maximum N₂ yield was found in the range 275–300°C. The catalyst prepared over SiO₂-Al₂O₃ was the most active and selective with respect to the other silicas. Competitiveness factors (c.f.’s) as high as 13–20% in the temperature range 200–250°C could be calculated. For all catalysts, the temperature of the N₂ peak maximum did not correspond to that of the maximum C₂H₄ oxidation to CO₂, suggesting the presence of two different sites for the oxidation and the reduction activity. On the catalyst prepared on SiO₂-Al₂O₃, a kinetic interpretation of catalytic data collected at different contact times and temperatures permitted evaluating the ratio between kinetic coefficients as well as the difference between activation energies of NO reduction by C₂H₄ and C₂H₄ oxidation by O₂.