| Palladium-based catalysts supported on titania were studied for the catalytic reduction of nitrogen oxides (NO, NO2, and N2O) by CH 4. The catalysts were synthesized by wet-impregnation and a modified sol-gel method. Characterization of the catalysts were performed using various techniques such as atmosphere-controlled X-ray photoelectron spectroscopy (XPS), in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), temperature-programmed desorption/reduction (TPD/TPR), BET surface measurement, and x-ray diffraction (XRD).; The role of lanthanide elements (Ce, Gd, La, and Yb) on Pd/TiO2 catalysts was investigated for the NO-CH4-O2 reaction. Steady-state reaction experiments in the presence of oxygen showed that the addition of lanthanide elements increases the oxygen resistance of the catalyst. The post-reaction XPS characterization results revealed that majority of the Pd sites remained in the zero oxidation state in the presence of Ce or Gd. The effect of SO2 and H2O in the NO-CH 4-O2 reaction over supported Pd and Gd-Pd catalysts was also investigated. Among the lanthanide elements tested, Gd is the most effective, allowing the use of above stoichiometric oxygen concentration.; In-situ DRIFTS was used to identify adsorbed species under NO, NO+CH4, and NO+CH4+O2 flows. NO adsorption resulted in formation of various nitrogen-oxo adspecies such as bridged/bidentate nitrate, monodentate nitrate, nitro, and linear NO. The thermal stability of these species is examined and related to the NO desorption features observed in temperature-programmed desorption profiles. CH4 was found to adsorb on the surface even at room temperature and hinder the NO adsorption. The Pd0 sites were essential for dissociative adsorption of methane. Under conditions where the catalyst is fully active for NO reduction, NH3 was seen to form as molecularly adsorbed is a prerequisite for the formation of NHx species, which are believed to act as a reducing agent for adsorbed NO, although direct interaction of nitro/nitroso species with surface CHx remains as a possibility. The Gd addition was found to change the catalytic chemistry significantly, possibly due to its high electropositivity, which helps stabilize the electron density at the Pd sites. |
