Unexpected Promotion Effect of H2O on the Selective Catalytic Reduction of NOx with NH3 over Cu-SSZ-39 Catalysts

Water molecules commonly inhibit the selective catalytic reduction (SCR) of NOx with NH3 on most catalysts, and water resistance is a long-standing challenge for SCR technology. Herein, by combining experimental measurements and density functional theory (DFT) calculations, we found that water molecules do not inhibit and even promote the NOx conversion to some extent over the Cu-SSZ-39 zeolites, a promising SCR catalyst. Water acting as a ligand on active Cu sites and as a reactant in the SCR reaction significantly improves the O2 activation performance and reduces the overall energy barrier of the catalytic cycle. This work unveils the mechanism of the unexpected promotion effect of water on the NH3-SCR reaction over Cu-SSZ-39 and provides fundamental insight into the development of zeolite-based SCR catalysts with excellent activity and water resistance.

Potential Risk of Significant N2O Emission without Changing NOx Conversion on Commercial V2O5/TiO2 Catalyst under Working Conditions

Vanadium-based catalysts play a pivotal role in the emission control of industrial NOx via selective catalytic reduction (SCR) technology. However, little attention has been paid to the potential emission of greenhouse gas N2O under complex working conditions. This work reports that a commercial V2O5/TiO2 catalyst may lead to significant N2O emission without greatly changing the outlet NOx concentration after chromium (Cr) deposition. With a Cr loading of 2 wt %, N2O concentration increased from 27.8 to 199.2 ppm at 350 °C with the value of outlet N2O/(N2O+N2) from 2.5% to 19.4%. Experimental results combined with DFT+U calculations suggest that nonselective catalytic reduction (NSCR) is the main route for N2O formation in a wide temperature range of 250 ∼ 400 °C. It is stemmed from the fact that the covalent interaction between Cr and V species on the V2O5/TiO2 surface accelerates the conversion of V4+ + Cr6+ → V5+ + Cr3+, leading to a larger proportion of surface V5+. More importantly, surface V5+ is highly related to the redox property of the V2O5/TiO2 catalyst, which is beneficial to NSCR reaction rather than the standard SCR process. The work suggests that to better inhibit the emission of greenhouse gases during the NH3-SCR process, monitoring N2O emission should be included along with the NOx concentrations, especially in complex flue gases.

Mechanistic Insight into the Promotion of the Low-Temperature NH3-SCR Activity over NiMnFeOx LDO Catalysts: A Combined Experimental and DFT Study

Mn-based catalysts have attracted much attention in the field of the low-temperature NH3 selective catalytic reduction (NH3-SCR) of NO. However, their poor SO2 resistance, low N2 selectivity, and narrow operation window limit the industrial application of Mn-based oxide catalysts. In this work, NiMnFeOx catalysts were prepared by the layered double hydroxide (LDH)-derived oxide method, and the optimized Ni0.5Mn0.5Fe0.5Ox catalyst had the best denitration activity, excellent N2 selectivity, a wider active temperature range (100-250 °C), higher thermal stability, and better H2O and/or SO2 resistance. A transient reaction revealed that Ni0.5Mn0.5Fe0.5Ox inhibited the NH3 + O2 + NOx pathway to generate N2O, which may be the main reason for its improved N2 selectivity. Combining experimental measurements and density functional theory (DFT) calculations, we elucidated at the atomic level that sulfated NiMnFeOx (111) induces the adjustment of the acidity/basicity of up and down spins and the ligand field reconfiguration of the Mn sites, which improves the overall reactivity of NiMnFeOx catalysts. This work provides atomic-level insights into the promotion of NH3-SCR activity by NiMnFeOx composite oxides, which are important for the practical design of future low-temperature SCR technologies.

Design of Bifunctional Cu-SSZ-13@Mn2Cu1Al1Ox Core-Shell Catalyst with Superior Activity for the Simultaneous Removal of VOCs and NOx

Synchronous control of volatile organic compounds (VOCs) and nitrogen oxides (NOx) is of great importance for ozone and PM2.5 pollution control. Balancing VOC oxidation and the NH3-SCR reaction is the key to achieving the simultaneous removal of these two pollutants. In this work, a vertically oriented Mn2Cu1Al1Ox nanosheet is grown in situ on the surface of Cu-SSZ-13 to synthesize a core-shell bifunctional catalyst (Cu-SSZ-13@Mn2Cu1Al1Ox) with multiple active sites. The optimized Cu-SSZ-13@Mn2Cu1Al1Ox catalyst delivered excellent performance for the simultaneous removal of VOCs and NOx with both 100% conversion at 300 °C in the presence of 5% water vapor. Physicochemical characterization and density functional theory (DFT) calculations revealed that Cu-SSZ-13@Mn2Cu1Al1Ox possesses more surface acidity and oxygen vacancies. The charge transfer between the core and shell is the intrinsic reason for the improved activity for both VOC and NOx removal. The molecular orbital theory is used to explain the different adsorption energies due to the different bonding modes between the core-shell and mixed individual catalysts. This work provides a novel strategy for designing efficient catalysts for the simultaneous removal of VOCs and NOx or other multiple pollutants.

Potential Risk of NH3 Slip Arisen from Catalytic Inactive Site in Selective Catalytic Reduction of NOx with Metal-Free Carbon Catalysts

Ammonia emissions from industrial processes have rapidly increased in the past years. Recent advances have used carbon-based selective catalytic reduction (SCR) technology combined with a reaction-regeneration process to reduce NO_x from sintering flue gas, while NH₃ slip is seldom accounted for in this process. This study demonstrates that although the electrophilic carboxyl groups (-COOH) on metal-free carbon catalysts exhibit strong adsorption toward NH₃, they do not participate in the SCR reaction. As a result of the competitive adsorption of NH₃ in the reaction step, these catalytic inactive carboxyl groups not only prolong the time to the SCR steady state, but also result in the potential risk of NH₃ slip. A linear relationship with the equimolar ratio between carboxyl groups and slipped NH₃ was established in the regeneration steps. The slip of NH₃ could be alleviated by the decomposition of carboxyl groups, and special attention should be paid to the presence of inactive sites with strong NH₃ adsorption on industrial-employed carbon catalysts. In addition to advancing the understanding of the NH₃-SCR mechanism, this work also provides valuable opportunities for the control of ammonia emissions from industrial processes.

Ce-Si Mixed Oxide: A High Sulfur Resistant Catalyst in the NH3-SCR Reaction through the Mechanism-Enhanced Process

Investigating catalytic reaction mechanisms could help guide the design of catalysts. Here, aimed at improving both the catalytic performance and SO₂ resistance ability of catalysts in the selective reduction of NO by NH₃ (NH₃-SCR), an innovative CeO₂-SiO₂ mixed oxide catalyst (CeSi2) was developed based on our understanding of both the sulfur poisoning and reaction mechanisms, which exhibited excellent SO₂/H₂O resistance ability even in the harsh working conditions (containing 500 ppm of SO₂ and 5% H₂O). The strong interaction between Ce and Si (Ce-O-Si) and the abundant surface hydroxyl groups on CeSi2 not only provided fruitful surface acid sites but also significantly inhibited SO₂ adsorption. The NH₃-SCR performance of CeSi2 was promoted by an enhanced Eley-Rideal (E-R) mechanism in which more active acid sites were preserved under the reaction conditions and gaseous NO could directly react with adsorbed NH₃. This mechanism-enhanced process was even further promoted on sulfated CeSi2. This work provides a reaction mechanism-enhanced strategy to develop an environmentally friendly NH₃-SCR catalyst with superior SO₂ resistance.

Scale-Activity Relationship of MnOx-FeOy Nanocage Catalysts Derived from Prussian Blue Analogues for Low-Temperature NO Reduction: Experimental and DFT Studies

Size effects have been recognized to promote the catalytic activity and selectivity of metal oxide particles. So far, limited works and studies are conducted to investigate the size effect of metal oxide with the tailored shape in the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). Herein, the MnO_x-FeO_y nanocage catalysts with varied scales (0.25, 0.5, 1, and 2 μm) were synthesized via a Prussian blue analogue (PBA)-derived method and used for NH₃-SCR of NO. By preforming a series of the activity tests over the nanocages with different scales, the NH₃-SCR activity of 0.5 μm MnO_x-FeO_y nanocage catalysts exhibits the highest deNO_x activity in the temperature range of 80-200 °C owing to more preferable physical and chemical properties. It has been demonstrated that there is a strong interaction among Mn and Fe cations in the 0.5 μm MnO_x-FeO_y nanocages. Moreover, the H₂-TPR and XPS analysis prove 0.5 μm nanocages exhibit excellent redox properties, which contribute to the higher conservation of NO_x. Through the DFT studies, it is also demonstrated that the 0.5 μm MnO_x-FeO_y nanocage catalysts could provide more preferable electronic charge, which gives rise to the varied adsorption behavior of the NH₃ species and NO_x species compared to the nanocages with other scales. The in situ DRIFTs were also employed to evaluate the adsorption status of NH₃ with NO_x species over MnO_x-FeO_y nanocage catalysts with varied scales. Finally, the scale-activity relationship of the MnO_x-FeO_y nanocage catalysts and their corresponding activities are also established. The deep insight into the scale-activity relationship of the PBA-derived MnO_x-FeO_y nanocage catalyst paves the way for developing and designing highly efficient Mn-based catalyst at lower temperature.

Interaction of NH3 with Cu-SSZ-13 Catalyst: A Complementary FTIR, XANES, and XES Study

In the typical NH3-SCR temperature range (100-500 °C), ammonia is one of the main adsorbed species on acidic sites of Cu-SSZ-13 catalyst. Therefore, the study of adsorbed ammonia at high temperature is a key step for the understanding of its role in the NH3-SCR catalytic cycle. We employed different spectroscopic techniques to investigate the nature of the different complexes occurring upon NH3 interaction. In particular, FTIR spectroscopy revealed the formation of different NH3 species, that is, (i) NH3 bonded to copper centers, (ii) NH3 bonded to Brønsted sites, and (iii) NH4(+)·nNH3 associations. XANES and XES spectroscopy allowed us to get an insight into the geometry and electronic structure of Cu centers upon NH3 adsorption, revealing for the first time in Cu-SSZ-13 the presence of linear Cu(+) species in Ofw-Cu-NH3 or H3N-Cu-NH3 configuration.