Impact of Water Vapor on the Performance of a Cu-SSZ-13 Catalyst under Simulated Diesel Exhaust Conditions

Cu-SSZ-13 selective catalytic reduction (SCR) catalysts are broadly applied in diesel aftertreatment systems for the catalytic conversion of oxides of nitrogen (NO + NO₂). Diesel exhaust contains a wide range of water vapor concentrations depending on the operating condition. In this study, we evaluate the impact of water vapor on the relevant SCR catalytic functions including NO_x conversion, NO oxidation, NH₃ oxidation, and N₂O formation under both standard and fast SCR conditions. Reactor-based experiments are conducted in the presence and absence of water vapor. Results indicate that water vapor can have both a positive and negative impact on low temperature NO_x conversion for standard SCR reaction. At low inlet NO_x concentrations, the presence of water vapor has a negative effect on NO_x conversion, whereas, at high inlet NO concentrations, water vapor improves NO_x conversion. At high temperatures, water vapor has a beneficial effect on NO_x conversion at all NO_x inlet concentrations evaluated. Experiments under fast SCR conditions reveal that water vapor improves low temperature NO_x conversion mainly by minimizing the accumulation of ammonium nitrate, as evidenced with temperature programmed desorption (TPD) results. Furthermore, NO oxidation by ammonium nitrate formed under fast SCR conditions is shown to be significantly faster in the presence of water vapor.