Yufeng Yang,Shijing Zhang,Jiamei Ma,Jiating Shen,Jiajun Huang,Xianglan Xu,Xiuzhong Fang,Junwei Xu,Xiang Wang
Adopting effective methods to identify clearly the active surface sites is crucial to engineer high-performance catalysts. To develop cost-effective and applicable NH 3 -SCR catalysts, the distribution states of Ag on HZSM-5 support have been identified by XRD and XPS extrapolation methods. As quantified, Ag dispersion on HZSM-5 has a threshold of 0.12 mmol Ag/100 m 2 . Above the threshold, bigger Ag 0 particles with better capability to activate O 2 and NO x molecules can be formed, thus achieving Ag/ZSM-5 catalysts with much better reaction performance below 300 °C. In contrast, below the threshold, Ag is mainly present in the form of highly dispersed nano-clusters/micro-crystallites, and partly as Ag + ions, thus being unfavorable to the reaction at low temperature. DFT calculation has testified that the aggregated Ag 0 particles with bigger sizes (>5 nm) have weaker interfacial interaction with the support, thus showing better capability to activate the reactant molecules. The optimal NH 3 -SCR performance is achieved on 8% Ag/ZSM-5 with a mean Ag size of 8.6 nm and a Ag 0 /Ag + ratio of 1.72, on which 100% NO conversion with a wide window from 250 to 400 °C can be achieved with H 2 assistance. In conclusion, both the Ag particle size and Ag 0 /Ag + ratio are the two main factors determining NH 3 -SCR performance on Ag/ZSM-5 catalysts.
Urea is applied as an ammonia source for SCR in diesel vehicles, but incomplete urea decomposition may lead to insufficiency of NH3 and deposition of solid by-products. To improve the urea decomposition activity of Cu-SSZ-13 SCR catalysts, Cu-SSZ-13 was combined with TiO2 nanoparticles by a simple impregnation method. TiO2/Cu-SSZ-13 composites exhibited lowered decomposition temperatures of urea and related solid by-products. Thus, the composited catalysts showed higher NOx conversion and a lower amount of urea-related deposits than unmodified Cu-SSZ-13 in a simulated urea deposition scenario. The superiority of the urea decomposition activity of the composited catalysts was maintained upon 800 °C hydrothermal aging due to stabilized anatase over the catalysts according to XRD and Raman spectroscopy. H2-TPR, UV-vis, and STEM revealed the formation of Cu–Ti–O species due to Cu–Ti interactions, which was adverse to SCR activity, but no extra hydrothermal aging effect was observed on the composited catalysts compared with unmodified Cu-SSZ-13. This work showed the potential to improve the low-temperature urea-SCR performance of Cu-SSZ-13 via TiO2 modification.
This study investigated the effects of sequential acid treatment and phosphorus incorporation on the performance of Fe–Mn-Ce composite metal oxide catalysts. The FeMnCeO x catalyst was prepared using the sol–gel method and modified through two methods: treatment with phosphoric acid alone and a sequential treatment with phosphoric acid followed by sulfuric acid. The results show that the sequentially acid-treated catalyst exhibited over 90% NO x conversion and N 2 selectivity in the temperature range of 200–350 °C, outperforming other catalysts. Characterization analyses such as XRD and NH 3 -TPD revealed that acid modification enhanced the surface acidity and redox properties of catalysts. XPS analysis showed that sequential acid treatment increased the surface chemisorbed oxygen content of the catalyst and optimized its surface structure. In-situ DRIFTS spectroscopy further elucidated that the catalytic reaction follows both Langmuir–Hinshelwood and Eley–Rideal mechanisms concurrently.
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