Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/xBa/Support (Support = Al2O3, SiO2, Al2O3-5.5 wt.%SiO2 and Ce0.7Zr0.3O2, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NOx trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO2-TPD. At high temperature (400 °C) in the absence of CO2 and H2O, the NOx storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO2 decreased catalyst performances. The inhibiting effect of CO2 on the NOx uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NOx storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO2 and H2O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO2 was responsible for the loss of NOx storage capacity at 400 °C. Finally, under realistic conditions (H2O and CO2) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NOx uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO2 competition for the storage sites.