Selective Catalytic Reduction (SCR) is a method of converting nitrogen oxides, which are harmful pollutants, into nitrogen (N2) and water (H2O) by reacting them with ammonia. This reaction takes place in the presence of a catalyst, typically at high temperatures. The commercial catalysts are usually made of vanadium, tungsten (or molybdenum), and titanium composite metal oxides. These materials are chosen for their ability to facilitate the reaction without being consumed in the process.
The primary reaction is: 4NO + 4NH3 + O2 → 4N2 + 6H2O. Secondary reactions may also occur, depending on the operating conditions and the catalyst used. SCR systems are installed in the exhaust streams of power plants, industrial boilers, and even in vehicles. In these systems, an ammonia-based reagent (like anhydrous ammonia, aqueous ammonia, or urea) is injected into the exhaust stream before it passes over the catalyst. The primary importance of SCR is its role in reducing air pollution. NOx gases are significant contributors to the formation of haze and acid rain, all of which have serious environmental and health impacts.
In NH3-SCR, the primary challenge is achieving the desired selectivity towards N2 while minimizing the formation of N2O, a potent greenhouse gas and ozone-depleting substance. In a new paper published in the peer-reviewed journal Environmental Science & Technology led by Professor Guangzhi He and Professor Hong He and conducted by Meng Gao, Zhuocan Li, Yulong Shan, and Yu Sun, the authors highlighted that various oxide catalysts, such as Mn-based, Fe-based, and V-based catalysts, exhibit distinct characteristics of activity and selectivity. Notably, Mn-based catalysts demonstrate exceptional low-temperature activity but poor N2 selectivity, primarily due to N2O formation. Conversely, Fe- and V-based catalysts showed better N2 selectivity but lower low-temperature activity. Indeed, the new research work addressed the long-standing puzzle of the trade-off between activity and selectivity in NH3-SCR and provided critical insights for designing high-performance catalysts with balanced activity and selectivity, a crucial aspect in addressing environmental concerns related to NOx emissions.
The authors combined experimental measurements and density functional theory (DFT) calculations to unravel the selectivity mechanism of these catalysts. The focus is on understanding the energy barriers involved in the formation of N2 and N2O from the key intermediate NH2NO. The study suggests that these energy barriers dictate the selectivity of the catalysts. The authors prepared nanocrystals of α-MnO2 and α-Fe2O3 through hydrothermal methods, and a V2O5/TiO2 catalyst via an impregnation method. N2O formation tests under NH3-SCR conditions and temperature-programmed surface reaction (TPSR) experiments with isotopic labeling are conducted to understand the reaction pathways and mechanisms. They employed DFT calculations to evaluate the thermodynamics and kinetics of the reactions on the catalyst surfaces. These calculations help identify the energy barriers for different reaction pathways and the transition states involved. They found that the gap in energy barriers for N2 and N2O formation from NH2NO is a critical determinant of the selectivity of oxide catalysts. Mn-based catalysts have a smaller energy barrier gap, leading to lower N2 selectivity due to the easier formation of N2O. In contrast, Fe- and V-based catalysts have larger energy barrier gaps, favoring N2 formation and thus showing better N2 selectivity. In conclusion, the study conducted by Professor Guangzhi He and Professor Hong He with their colleagues opens avenues for further research in catalyst design, particularly in fine-tuning the properties of catalysts to achieve desired reaction outcomes. Understanding the atomic-level mechanisms of catalysis can lead to the development of more efficient and environmentally friendly catalytic processes, not only in NH3-SCR but also in other catalytic systems where selectivity is a key concern.
Meng Gao, Zhuocan Li, Guangzhi He,* Yulong Shan, Yu Sun, and Hong He*. Unveiling the Origin of Selectivity in the Selective Catalytic Reduction of NO with NH3 over Oxide Catalysts. Environ. Sci. Technol. 2023, 57, 8426−8434.