Unraveling Reactivity Descriptors and Structure Sensitivity in Low-Temperature NH<sub>3</sub>-SCR Reaction over CeTiO<i><sub>x</sub></i> Catalysts: A Combined Computational and Experimental Study
Bing Liu, Jie Liu, Xin Lei, Tao Zhang, Yuebing Xu, Feng Jiang, Xiaohao Liu
Abstract
Developing ceria-based NH3-SCR catalysts possessing excellent NO conversion, N2 selectivity, and SO2-tolerance at low-temperatures remains a great challenge. Precisely regulating the surface structure of ceria-based NH3-SCR catalysts at the atomic scale is paramount to boosting catalytic performance. Herein, we carried out a combined computational and experimental study to rationally engineer the surface structure of CeTiOx NH3-SCR catalysts and to unravel the reactivity descriptors and structure sensitivity. DFT calculations indicate that the Ti-doped CeO2 solid solution structure not only displays a lower activation barrier for the rate-determining step but also separates the SO2 binding site and the catalytic active site, where the Ti dopant serves as a SO2-trapping site while Ce site neighboring Ti and O vacancy next-neighboring Ti act as the dominant active sites for NH3-SCR. These DFT calculation results successfully guided the design and synthesis of highly effective Ti-doped CeO2 solid solution catalysts, which exhibit superior intrinsic activity and excellent SO2-resistant ability for low-temperature NH3-SCR reaction in comparison with the Ce-Ti catalysts containing an amorphous Ce-O-Ti structure and CeO2-TiO2 interface structure. Both DFT calculations and in situ DRIFTS results validate that the NH3-SCR reaction on Ti-doped CeO2 solid solution dominantly follows the Eley–Rideal mechanism. The combined DFT calculations, H2-TPR, NH3-TPD, and catalyst evaluation results reveal that EH/ENH3 and Evac are the two key reactivity descriptors that determine the overall NH3-SCR reaction on Ce-Ti-based catalysts. A volcano-type relationship between NH3 dissociation activity and EH/ENH3 and a strong linear correlation between N–O bond breaking activity and Evac were established. The superior intrinsic NH3-SCR activity of Ti-doped CeO2 solid solution originates from its moderate EH/ENH3 and Evac values. The role of the Ti dopant in Ti-doped CeO2 solid solution as a SO2-trapping site to protect the catalytic active sites from sulfation was unraveled based on TG, TPDC, and in situ DRIFTS characterizations and DFT calculations. The structure sensitivity of Ce-Ti catalysts in NH3-SCR reaction was analyzed based on the atomic coordination structure, and it was found that the four-coordinated tetrahedron-type Ti configuration in Ti-doped CeO2 solid solution assures its superior catalytic performance.