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 NOx 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 NOx 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 NOx reduction by competing for adsorption sites with the propene and/or NOx 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.