The Absence of Oxygen in Sulfation Promotes the Performance of the Sulfated CeO2 Catalyst for Low-Temperature Selective Catalytic Reduction of NOx by NH3: Redox Property versus Acidity

Tuning surface acidity with SO2 sulfation is well established to enhance the catalytic performance of CeO2 for selective catalytic reduction of NOx by NH3 (NH3-SCR). However, the role of oxygen, which strongly affects the chemical potential of SO2 in sulfation, remains unclear. Herein, we found that anaerobic sulfation promotes the SCR rate of the sulfated CeO2 catalyst by 2.8-fold at 275 degrees C without detriment to selectivity and stability. More importantly, spectroscopies using different probes indicate that the anaerobic SO2 treatment hardly modifies the redox property of the sulfated CeO2 catalyst, but strongly enhances the number and strength of Lewis acid sites. Furthermore, sulfation followed by in situ infrared and density functional theory calculations suggest that the formed chelated bidentate sulfate species could contribute to the increased Lewis acidity through the electrophilic effect. This work provides a direct evidence of how the acidity rather than the redox property determines the performance of Ce-based SCR catalysts.