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 Gao2,*, Fudong Liu3,*, Lin Dong1,2

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)



Nitrogen oxides (NOx) are crucial components of air pollutants and seriously affecting eco-environmental and human life. Selective catalytic reduction of NOx with NH3 (NH3-SCR technology) has been widely used for eliminating NOx 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 CexZr1-xO2 materials were prepared and then sulfated at 200 oC. The sulfated Ce0.6Zr0.4O2 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 Ce0.6Zr0.4O2 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 Ce0.6Zr0.4O2 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 Ce0.6Zr0.4O2 with no NH3-SCR reactivity could hardly deposit on the sulfated samples. The sulfated Ce0.6Zr0.4O2 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: NOx, NH3-SCR, CexZr1-xO2, sulfate species, reaction mechanism