Effect of the Carbon/Nitrogen Ratio on the NO H2-SCR and H2–C3H8-SCR Catalyzed by Ag/γ-Al2O3

Abstract

We report the effect of the carbon/nitrogen ratio (C/N), from 0 to 26.2, on the activity and selectivity of the NO Selective Catalytic Reduction with propane and hydrogen (H₂-C₃H₈-SCR) over 2 wt% Ag/γ-Al₂O₃ between 25 and 500 °C. The gas composition included 3% (v/v) O₂ and 6% (v/v) H₂O at GHSV = 70,651 h⁻¹. There are two NO conversion zones as a function of temperature, previously reported. The low temperature ranged from 60 to 180 °C and reached 100% conversion of NO to N₂ at 140 °C. The addition of C₃H₈ had a small effect, decreasing NO conversion to 90–95%, and the temperature window about 10 °C. The H₂ conversion increased as a sigmoidal curve with temperature in all cases, and it shifted to low temperature too. This NO reduction range corresponds to the H₂-SCR. The high temperature activity region involves competition for O₂ between NO and C₃H₈. When C/N = 0 there was only oxidation to NO₂. At C/N = 3.2 and 6.5 there was emission of NO₂ between 160 and 440 °C. At higher temperatures the selectivity to N₂ was > 88%. Above C/N = 13.1 the NO conversion reached 90 to 98%. The N₂O concentration was negligible. The optimum C/N value was 13.1, with high NO conversion at low and high temperatures, and nearly 100% selectivity to N₂. Analysis by XPS and UV–Vis–NIR showed the coexistence of Ag⁺ and Ag⁰ moieties that helps explain the variations and competition of the oxidation–reduction reactions of the NO–H₂–C₃H₈–O₂ system with temperature and with the C/N feed values.