Multifunctional catalysts for de-NOx processes: The case of H3PW12O40·6H2O-metal supported on mixed oxides

Abstract

The role of a multifunctional catalyst for de-NOx process has been investigated. The NOx storage capacity of H3PW12O40·6H2O (HPW) was improved by the presence of a noble metal (Pt, Rh or Pd). Both HPW and noble metal were deposited on a specific support (based on Zr–Ce or Zr–Ti). The presence of noble metal in several oxidation states, as evidenced by TPR and IR, involves the possibility of forming different catalytic sites: (i) M0 (zero-valent metal) and perhaps (ii) (metal–H)δ+ from specific interactions between noble metal and the HPW proton. Supports were also able to adsorb and activate NOx and to generate cationic catalytic sites (Mx+). These cationic sites seem to be the clue for their important activity toward NOx reduction. This catalyst presents an outstanding resistance to SO2 poisoning which can be related to NO and NO2 absorption mechanism in HPW. The use of alternating short cycles of lean/rich mixtures allows us optimising the performance of this catalytic system in terms of both NOx reduction capacity and NOx storage efficiency: up to 48 and 84%, respectively (with a 2% CO+1% H2 mixture for reducing). Experimental results sustain two hypotheses: first, HPW-metal-support catalyst includes several (independent) catalytic functions required for a de-NOx process to occur and second, the formation of oxygenate active species must be indispensable for NOx reduction into nitrogen.