Ce-Based Catalysts for the Selective Catalytic Reduction of NO_x in the Presence of Excess Oxygen and Simulated Diesel Engine Exhaust Conditions

Abstract

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 x 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 x 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 x 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 x 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 n+ are interrelated to the SCR activity of the individual catalysts.

Enhancement of SCR performance of monolithic Mn–Ce/Al2O3/cordierite catalysts by using modified deposition precipitation method

“…It could be resolved into eight components (labeled as v and u, respectively). The bands labeled u′ and v′ represented the 3d 10 4f 1 initial electronic state corresponding to Ce 3+ , whereas the peaks labeled u, u″, u‴, v, v″, and v‴ represented the 3d 10 4f 0 state of Ce 4+ ions . It is a consensus that the presence of more Ce 3+ is beneficial to the SCR reaction because Ce 3+ could create charge imbalance and would lead to vacancies and unsaturated chemical bonds on the catalyst surface, which facilitated in the increase of surface‐absorbed oxygen .…”

High-Efficiency Catalytic Conversion of NOx by the Synergy of Nanocatalyst and Plasma: Effect of Mn-Based Bimetallic Active Species

“…According to the binding energies of different peaks, the Ce 3d spectra could be elaborately separated into eight peaks, labeled as u, u , u , u and v, v , v , v , respectively [54]. The u and v peaks were matched with the 3d 10 4f 1 electronic state of Ce 3+ , and the u, u , u , v, v , and v peaks were ascribed to the 3d 10 4f 0 electronic state of Ce 4+ [55]. These distinctive peaks verified the coexistence of of Ce 3+ and Ce 4+ species on Mn−Co/TiO 2 nanocatalyst surface.…”

Effects of WO3 and SiO2 doping on CeO2–TiO2 catalysts for selective catalytic reduction of NO with ammonia

“…Studies in recent years revealed that the presence of rare-earth components can effectively regulate the surface acidity and alkalinity, modify the structure of catalytic active centers, improve the oxygen storage/release capacities, and enhance the structural stability and active species dispersion of catalysts. 13,14 As an important component of rare-earth catalytic materials, CeO 2 is a potential substitute for V 2 O 5 due to its high oxygen storage capacity and redox ability when Ce species shi between Ce 4+ and Ce 3+ . 15 28 Furthermore, SiO 2 is widely used as catalyst supports due to their high mechanical strength and excellent thermal stability.…”