The influence of CO2 and H2O on the storage properties of Pt-Ba/Al2O3 LNT catalyst studied by FT-IR spectroscopy and transient microreactor experiments

The effect of CO2 and H2O on the NO/O2 storage over a Pt-Ba/Al2O3 LNT catalyst was investigated in the temperature range 150-350 °C by combining FT-IR spectroscopy and microreactor flow experiments. It is found that the presence of water shows negligible effect on the NOx storage at all the temperatures studied. Otherwise, the presence of CO2 inhibits the "nitrite route" (initial formation of surface nitrites and their subsequent evolution to nitrates), whereas the "nitrate route" (involving NO oxidation to NO2 over Pt and its subsequent adsorption on Ba in the form of nitrates) proceeds as in the absence of CO2. The inhibition effect of CO2 on nitrite formation increases on increasing the temperature. This behavior is likely related to the different thermal stability of nitrites and carbonates and, in particular, to the decreasing ability of nitrites to displace carbonates on increasing temperature. Simultaneously, the presence of CO2 in the feed significantly shortens or eliminates the dead time in the NOx breakthrough at the reactor outlet. As a consequence, the presence of dead time in the absence of CO2 has been correlated to the occurrence of the nitrite route.