Promotional effects of phosphotungstic acid on CePO4 catalyst for the selective catalytic reduction of NOx with NH3

Abstract

CePO4 was prepared using the co-precipitation method. Recognizing the bifunctional nature of hetero-polyacid (salt), with both acidic and redox properties, it was modified by using phosphotungstic acid (HPW), and the HPW/CePO4 catalysts were synthesized using the impregnation technique. Various advanced techniques were employed to investigate aspects such as catalytic activity, microstructure, surface properties, and reaction mechanisms. These methods included NH3-SCR, XRD, XPS, SEM, BET, NH3-TPD, H2-TPR, and In-situ DRIFTs. Experimental findings indicated that the HPW-modified CePO4 catalysts exhibited enhanced SCR activity. Among them, the reaction activity window for NOx conversion of the W: Ce 0.05 catalyst was found to be 300∼400 °C, which represented the best activity temperature window. Following the incorporation of HPW, the HPW/CePO4 catalysts demonstrated an increased presence of Brønsted acidic sites, which concurrently led to a significant reduction in SO2 adsorption. The W: Ce 0.05 catalyst possessed the highest ratios of Oα/(Oα+Oβ+Oγ) and Ce4+/(Ce3++Ce4+). These ratios effectively regulated the sequestration and release of reactive oxygen species, thereby enhancing the activation of NH3. Consequently, this promoted the catalyst's low-temperature NH3-SCR reactivity. Lastly, the reaction process was examined using in-situ DRIFTs. The study revealed the generation of -NH2 intermediates at the Lewis acid sites of the W: Ce 0.05 catalyst. These intermediates subsequently underwent reactions with gaseous NO via the Eley-Rideal mechanism. This approach not only amplified the low-temperature catalytic efficacy of the catalysts but also bolstered their resilience to SO2, thereby fostering the long-term stability of the NH3-SCR reaction.