Selective reduction of NOx by liquid hydrocarbons with supported HPW–metal catalysts

The catalytic multifunctional system based on H3PW12O40·6H2O (HPW), Pt and support (Zr–Ce or Zr–Ti mixed oxides) has been investigated for lean NOx storage and reduction. It was applied in the NOx storage and reduction (NSR) concept by including a cyclic operation of short NOx storage (oxygen rich phase) and reduction (hydrocarbon rich phase: hexane). For NOx reduction, the assistance of hydrogen is one of the key parameters possibly as a result of the regeneration of metallic sites active for mild oxidation of the hydrocarbon (by forming CxHyOz). The difference in oxygen mobility of supports (higher in the case of Zr–Ce than for Zr–Ti) became another strategic parameter for catalyst selection since the possibility of total hydrocarbon oxidation. The influence of temperature was also considered for optimizing NOx storage and reduction.