Ce-Based Catalysts for the Selective Catalytic Reduction of NO_<i>x</i> in the Presence of Excess Oxygen and Simulated Diesel Engine Exhaust Conditions

A family of various cerium oxide-based catalysts were synthesized by adopting flame aerosol (FSP), coprecipitation, wet impregnation, and hydrothermal synthesis techniques. The resulting catalysts were explored for the selective catalytic reduction (SCR) of NO_<i>x</i> using NH₃ as reductant. In our studies, both the preparation method and the Ce/W ratios were found to be critical variables for successful catalyst promotion. For the industrial realization, we have scaled up the SCR activity tests. The microreactor catalytic formulations at simulated diesel engine exhaust conditions revealed that the Ce–W (1:1 atomic ratio) and Ce–W/TiO₂ catalysts showed high deNO_<i>x</i> activity, while the other catalysts’ activity was found to be rather low. Of interest is the finding that the Ce–W/TiO₂/cordierite and Ce–W (1:1 atomic ratio)/cordierite formulations show impressive deNO_<i>x</i> performance and high N₂ selectivity with respect to a commercial vanadia based reference currently used for mobile applications. To gain fundamental insights which may acquaint further improvements to the promoted Ce-based catalysts, X-ray photoelectron spectroscopy and other characterizations were executed to study the relationship between catalyst surface and NO_<i>x</i> reduction activity. Our XRD results indicate smaller lattice parameters of prepared catalysts compared to that of CeO₂ (0.540 nm). The crystal lattice contraction is attributed to the lesser ionic radius of relevant foreign metal ions (W⁶⁺ = 0.067 nm and Ti⁴⁺ = 0.074 nm) in relation to Ce⁴⁺ (0.092 nm) in the host lattice. This lattice shrinkage elucidates the formation of solid solutions. These results illustrate that the synthesis technique and various promoters could indeed influence the lattice structures and electronic state of the active components. The XPS results illustrate the higher atomic ratios of Ce³⁺/(Ce³⁺ + Ce⁴⁺) 0.30 and 0.29 in Ce–W/TiO₂ and Ce–W (1:1) coprecipitation catalysts, respectively, compared to other samples. The higher surface Ce³⁺/Ce⁴⁺ ratio in Ce–W (1:1) coprecipitation and Ce–W/TiO₂ samples indicate the enrichment in surface oxygen vacancies, which results in activation of reactive molecules and enhanced adsorption of oxygen species in SCR reaction. Interestingly, the surface atomic ratio of Ce³⁺/Ce⁴⁺ and Ce³⁺/Ce^<i>n</i>+ are interrelated to the SCR activity of the individual catalysts