Effect of sulfation treatment on NH3-SCR mechanism over ceria-zirconia solid solutions

Fudong  Liu, University of Central Florida

Effect of sulfation treatment on NH₃-SCR mechanism over ceria-zirconia solid solutions

Wei Tan¹, Fei Gao^2,*, Fudong Liu^3,*, Lin Dong^1,2

¹ Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China

² Jiangsu Key Laboratory of Vehicle Emissions Control, School of Environment, Center of Modern Analysis, Nanjing University, 210093 Nanjing, P. R. China

³ Department of Civil, Environmental, and Construction Engineering, Catalysis Cluster for Renewable Energy and Chemical Transformations (REACT), University of Central Florida, Orlando, FL 32816, United States

*Corresponding E-mail address: gaofei@nju.edu.cn (F. Gao); fudong.liu@ucf.edu (F. Liu)

Phone: +86 25 83596545 (F. Gao); +1-407-823-6219 (F. Liu)

 

ABSTRACT

Nitrogen oxides (NO_x) are crucial components of air pollutants and seriously affecting eco-environmental and human life. Selective catalytic reduction of NO_x with NH₃ (NH₃-SCR technology) has been widely used for eliminating NO_x emissions. In recent years, researchers have been developing new environmental-friendly catalysts, and fruitful progress has been made on Ce-based oxide catalysts.

It has been reported that pretreating CeO₂ with SO₂ + O₂ at suitable temperature could promote the NH₃-SCR activity markedly. However, pure CeO₂ has some drawbacks such as limited specific surface area and poor thermal stability restricting their wide application. CeO₂-ZrO₂ solid solution has been widely used as OSC materials other than pure CeO₂ in three-way catalysts (TWC) due to enhanced redox capacity, larger surface area and improved hydrothermal stability. Consequently, CeO₂-ZrO₂ solid solutions have high potent to substitute pure CeO₂ in NH₃-SCR reaction and may exhibit significant enhancement after sulfation treatment.

In this work, a series of Ce_xZr_1-xO₂ materials were prepared and then sulfated at 200 ^oC. The sulfated Ce_0.6Zr_0.4O₂ showed the highest NH₃-SCR activity among all samples as well as improved SO₂ tolerance. The results of a series of characterization methods demonstrated that Ce_0.6Zr_0.4O₂ could adsorb more surface sulfate species and then produce more acid sites with better stability than pure CeO₂ at 200 ^oC. Redox ability was also enhanced with more Ce³⁺ species and oxygen vacancies formed on the surface of Ce_0.6Zr_0.4O₂ after sulfation treatment. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) experiments suggested that the nitrate species deposited on the surface of pristine Ce_0.6Zr_0.4O₂ with no NH₃-SCR reactivity could hardly deposit on the sulfated samples. The sulfated Ce_0.6Zr_0.4O₂ had more active sites to participate in the NH₃-SCR, and the reaction proceeded via an Eley-Rideal mechanism. This work proved that sulfation treatment could be used in designing an efficient cerium-zirconium based NH₃-SCR catalyst with practical application prospect.

Keywords: NO_x, NH₃-SCR, Ce_xZr_1-xO₂, sulfate species, reaction mechanism