Lean NO_x reduction with dodecane over cerium and palladium loaded mordenite

摘要

The selective catalytic reduction (SCR) of NO_x 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-HMOR, displayed a maximum of about 70% NO_x conversion to N_2 at 350 deg C and GHSV = 30,000h~(-1), while either Ce-HMOR or Pdd-HMOR exhibited low activity for the SCR reaction. The presence of both Ce and Pd in the zeolite was crucial for high deNO_x activity. The dodecane concentration is very important for the reaction. the catalyst is not active in the absence of the reducing agent and inhibition of NO_x reduction is observed at dodecane concentrations higher than 440-500 ppm. The presence of 40 ppm SO_2 in the gas feed suppresses the reaction. however, the coexistence of 15% H_2O and 40 ppm SO_2 has no appreciable effect on the catalyst activity. Enhanced activity and a broader temperature window was observed in the NO_2 + dodecane + O_2 reaction in the presence of 15% H_O and 100 ppm SO_2. In this reaction mixture, the catalyst was capable of retaining a stable NO_2 conversion to N_2 for a period of tem days. Characterization by XPS and UV-VIS-diffuse reflectance spectra (UV-VIS-DRS) of selected fresh and aged catalyst samples indicates that Pd exists, mainly as Pd~(2+) cations in Ce-Pd-HMOR. Cerium is mostly present as CeO_2 on the surface of the zeolite particles. However, part of the cerium in 6Ce-Pdd-HMOR, exists as sable Ce~(3+) species.