SOx uptake and release properties of TiO2/Al2O3 and BaO/TiO2/Al2O3 mixed oxide systems as NOx storage materials
Titania was used as a promoter to obtain novel materials in the form of TiO2/Al2O3 (Ti/Al) and BaO/TiO2/Al2O3 (Ba/Ti/Al, containing 8 wt% or 20 wt% BaO) that are relevant to NOx storage reduction (NSR) catalysis. Two different protocols (P1, P2) were utilized in the synthesis. Ti/Al(P1) manifests itself as crystallites of TiO2 on -Al2O3, while Ti/Al(P2) reveals an amorphous AlxTiyOz mixed oxide. The structures of the synthesized materials were investigated via TEM, EDX, BET analysis and XPS while the catalytic functionality/performance of these support materials upon SOx and subsequent NOx adsorption were investigated with TPD and in situ FTIR spectroscopy. Ti/Al(P1, P2) revealed a high affinity towards SOx. Overall thermal stabilities of the adsorbed SOx species and the total SOx uptake of the Ba-free samples increase in the following order: TiO2(anatase) -Al2O3 < Ti/Al(P1) < Ti/Al(P2). The superior SOx uptake of Ti/Al(P1, P2) support materials can be tentatively attributed to the increasing specific surface area upon TiO2 promotion and/or the changes in the surface acidity. Promotion of BaO/Al2O3 with TiO2 leads to the attenuation of the SOx uptake and a significant decrease in the thermal stability of the adsorbed SOx species. The relative SOx adsorption capacities of the investigated materials can be ranked as follows: 8Ba/Ti/Al(P1) < 8Ba/Ti/Al(P2) < 8Ba/Al ∼ 20Ba/Ti/Al(P1) < 20Ba/Al < 20Ba/Ti/Al(P2).