Effect of CO2, H2O and SO2 in the ceria-catalyzed combustion of soot under simulated diesel exhaust conditions

Abstract

The effect of CO2, H2O and SO2 in the Ce0.73Zr0.27O2 and Ce0.64Zr0.27Nd0.09O2 catalyzed combustion of soot with NOx + O2 has been studied. Combustion experiments performed in a fix-bed reactor with soot-catalyst mixtures prepared in loose contact mode showed that CO2, H2O and SO2 lower the activity of both catalysts, and the inhibiting effect follows the trend SO2 > H2O > CO2. Regardless the gas mixture composition, the catalytic activity for soot combustion of Ce0.64Zr0.27Nd0.09O2 is equal or higher to that of Ce0.73Zr0.27O2 because Nd3+ doping seems to promote the participation of the active oxygen mechanism together with the NO2-assisted mechanism in the catalytic combustion of soot. The maximum soot combustion rate achieved during a Ce0.64Zr0.27Nd0.09O2-catalyzed reaction in NOx/O2/CO2/H2O/N2 is about three times higher than that of the uncatalyzed combustion, and this catalyst also improves the CO2 selectivity. In situ DRIFTS experiments showed that CO2, H2O and SO2 compete with NOx for the adsorption sites on the catalysts’ surface. CO2 partially impedes the catalytic oxidation of NO to NO2, affecting much more to the Nd3+-containing catalyst; however, the contribution of the active oxygen mechanism seems to remain relevant in this case. H2O also hinders the catalytic oxidation of NO to NO2 on both catalysts, and therefore the catalytic combustion of soot, because delays the formation of nitrogen reaction intermediates on the catalysts’ surface and favors the formation of more stable nitrogen surface species than in a H2O-free gas stream. For both catalysts, SO2 chemisorption (with sulfate formation) is even able to remove nitrogen surface groups previously formed by NOx chemisorption, which significantly inhibits the catalytic oxidation of NO to NO2 and the catalytic combustion of soot.