Removal of NOx from lean exhaust gas by storage/reduction on H3PW12O40 x 6H2O supported on Ce(x)Zr4-xO8.

We have demonstrated the interest of using Zr-Ce mixed oxides as supports for the system H3PW12O40 x 6H2O (HPW) and Pt for the storage (absorption into HPW and adsorption on Zr-Ce support)-reduction of NOx. Zr-Ce oxides adsorb NOx (mainly NO2) as nitrates, which are desorbed thermally as NO2 and NO (12 mg of NOx x g(-1)). On (HPW-Pt)/Zr-Ce system, NO and NO2 are stored equimolarly by substitution of water molecules of the HPW structure. The Zr/Ce support molar ratio has an important influence on the quantity of NOx stored with a maximum for the system Zr/Ce = 0.5. The presence of Pt has no influence on storage capacity but rather on its efficiency, and it is essential when reducing. Pt strongly accelerates both NOx desorption and reduction processes. Reduction of NOx occurs with H2, CO, or an H2/CO mixture. With H2, NO2 is reduced to NO. With CO, it seems reasonable that the mechanism of NO2 reduction concerns its reaction with CO to produce CO2 and NO. Addition of H2 to CO accelerates the reduction process. However, the difference between NOx stored and desorbed implies also the presence of another reduction step. This could be related to an NO interaction with an active site which produces its decomposition with nitrogen as product. The catalytic system proposed is highly selective since no N2O formation was detected.