Lean NOx reduction with dodecane over cerium and palladium loaded mordenite

The selective catalytic reduction (SCR) of NOx by dodecane in excess oxygen -containing gas mixtures was studied on Ce and/or Pd-loaded HMOR. Under dry conditions the best fresh bimetallic catalyst, 6.2 wt.% Ce-0.08 wt.% Pd-HM OR, displayed a maximum of about 70% NOx conversion to N-2 at 350 degreesC and GHSV = 30,000 h(-1), while either Ce-HMOR or Pd-HMOR exhibited low acti vity for the SCR reaction. The presence of both Ce and Pd in the zeolite wa s crucial for high deNO(x) activity. The dodecane concentration is very imp ortant for the reaction. The catalyst is not active in the absence of the r educing agent and inhibition of NOx reduction is observed at dodecane conce ntrations higher than 440-500 ppm. The presence of 40 ppm SO2 in the gas fe ed suppresses the reaction. However, the coexistence of 15% H2O and 40 ppm SO2 has no appreciable effect on the catalyst activity. Enhanced activity a nd a broader temperature window was observed in the NO2 + dodecane + O-2 re action in the presence of 15% H2O and 100 ppm SO2. In this reaction mixture , the catalyst was capable of retaining a stable NO2 conversion to N-2 for a period of ten days. Characterization by XPS and UV-VIS-diffuse reflectanc e spectra (UV-VIS-DRS) of selected fresh and aged catalyst samples indicate s that Pd exists mainly as Pd2+ cations in Ce-Pd-HMOR. Cerium is mostly pre sent as CeO2 on the surface of the zeolite particles. However, part of the cerium in 6Ce-Pd-HMOR, exists as stable Ce3+ species. (C) 2001 Elsevier Sci ence B.V. All rights reserved.