Bi₂O₃/Al₂O₃ catalysts for the selective reduction of NO with hydrocarbons in lean conditions

Bi₂O₃/Al₂O₃ catalysts were prepared by sol-gel and impregnation methods, and their properties in the selective catalytic reduction of NO_x with propene and propane investigated. Under simulated diesel exhaust gas conditions (500 ppm NO, 10% O₂, 6% H₂O, GHSV of ca. 30,000 h^-1) with 525 ppm propene as reductant, maximum conversion levels to N₂ reached 62% for NO and 47% for NO₂, while maximum NO_x reduction was observed at 550 °C. Increasing the propene concentration to 1315 ppm (corresponding to a [C₁]/[NO] ratio of 8) resulted in a maximum NO reduction level of 96%, while the temperature corresponding to the maximum shifted to 525 °C. Propane was also found to function as an efficient reductant; maximum NO conversion levels were only slightly lower than for propene, although the temperature window for NO conversion was shifted by ∼25 °C to higher temperature relative to propene. While the presence of oxygen was found to be essential for SCR activity, water significantly inhibited NO_x reduction by competing for adsorption sites with the propene and/or NO_x reactants. Optimum SCR activity was observed for sol-gel catalysts at bismuth loadings of 3-7 wt%, XRD and XPS measurements showing that the bismuth oxide in these catalysts was present as a highly dispersed phase. The results of steady-state and transient DRIFTS measurements suggested a reaction scheme in which surface nitrate and acetate groups react to afford N-containing intermediates, including organo-nitrite and/or nitrate species, as well as isocyanate, which react further to form ultimately N₂.