Selective Reduction Of NO Research Articles

Selective catalytic reduction of NO with NH3 over HZSM-5/CeO2 hybrid catalysts: Relationship between acid structure and reaction mechanism

CeO2 is active in the NH3 selective catalytic reduction of NO (NH3-SCR) reaction due to its excellent redox properties. However, the low surface acidity of CeO2 limits its NH3-SCR activity. In this study, highly active HZSM-5-modified HZSM-5/CeO2 (Z/Ce) mixed oxide catalysts were prepared by a simple physical milling method. The HZSM-5 modification stimulated many thermally stable Brønsted-acid structures and high oxygen mobility on the surface of the Z/Ce catalysts, promoting the acid cycling pathways driven by redox macrocycles, and accelerating NO removal. The NO conversion of 25Z/Ce at 200–400 °C is about 34.8 %-62.3 % higher than that of CeO2. A series of physicochemical and in situ interfacial reactions were analyzed to explore the medium- and high-temperature reaction mechanism on the CeO2 surface. The Eley-Rideal reaction mechanism on the Lewis-acid structure with NO(g) as the reactant is deduced. When HZSM-5 modified CeO2, more NO was activated to NO2(g) on the surface of the Z/Ce catalyst at medium and high temperatures (300 °C). This significantly enhanced the Eley-Rideal reaction mechanism with NO2(g) as the main reactant on the Brønsted-acid structure as well as the occurrence of the “Fast-SCR” reaction. This work provides a simple method to improve the performance of CeO2 in the NH3-SCR reaction and elucidates the reaction mechanism.

Modulating charge transfer and constructing synergistic bimetallic active sites for CO-SCR reactions in inverse spinel

The selective catalytic reduction of NO by CO (CO-SCR) are frequently catalyzed by spinel due to its exceptional stability and redox capabilities. In this research, a ternary metal-based inverse spinel catalyst Cu0.67Mn0.33Fe2O4 was synthesized utilizing the glycerol self-templating technique. Some Fe3+ (Fe3+Td) sites are transferred to vacant octahedral sites through the occupancy of tetrahedral sites by Mn species. Mn3+ (Mn3+Td) sites synergistically interact with Cu2+ and Fe3+ ions on the octahedral sites to promote charge transfer, producing numerous oxygen-defective and highly mobile oxygen species and constructing Cu+-Ov-Fe2+ active sites on the surface. Various characterizations demonstrated that Cu+-Ov-Fe2+ has greater catalytic activity, resistance to alkali metals, and water toxicity than the Cu2+-Ov-Fe2+ active site of CuFe2O4. In situ DRIFTS further revealed the reaction mechanism of the ternary metal-based inverse spinel catalyst, and this study provides novel perspectives on the development of Cu-based catalysts for CO-SCR.

The Catalytic Mechanisms and Design Strategies of Noble Metal Catalysts for Selective Reduction of NOx with CO

AbstractThe selective catalytic reduction of NOx by CO (CO‐SCR) is a viable method for simultaneously mitigating NOx and CO emissions in motor vehicle exhaust and industrial flue gases. Extensive research has been conducted on noble metal catalysts because of their superior catalytic activity and stability compared to non‐noble metals. Noble metal catalysts demonstrate efficient NOx conversion and high selectivity for N2. However, achieving high conversion, selectivity, and stability over a wide temperature range in the presence of excess O2, H2O, and SO2 remains a significant challenge. This review summarizes recent advancements in CO‐SCR over noble metal catalysts, providing insights for the development of CO‐SCR catalysts. This paper first examines the reaction mechanisms of CO‐SCR, followed by a discussion on the design and optimization strategies of noble metal catalysts, with a focus on composition and structural effects. This includes creating active metal sites, selecting appropriate supports, integrating catalytic additives, choosing monometallic nanoparticles, alloying, and using single‐atom catalysts. Finally, remaining challenges and potential avenues for future research have been suggested. The discovery of negatively charged noble metal single‐atom‐site catalysts presents new research opportunities.

Strikingly Facile Cleavage of N-H/N-O Bonds Induced by Surface Frustrated Lewis Pair on CeO2(110) to Boost NO Reduction by NH3.

Ceria with surface solid frustrated Lewis pairs (FLPs), formed by regulating oxygen vacancies, demonstrate remarkable ability in activating small molecules. In this work, we extended the application of FLPs on CeO2(110) to the selective catalytic reduction of NO by NH3 (NH3-SCR), finding a notable enhancement in performance compared to ordinary CeO2(110). Additionally, an innovative approach involving H2 treatment was discovered to increase the number of FLPs, thereby further boosting the NH3-SCR efficiency. Typically, NH3-SCR on regular CeO2 follows the Eley-Rideal (E-R) mechanism. However, density functional theory (DFT) calculations revealed a significant reduction in the energy barriers for the activation of N-O and N-H bonds under the Langmuir-Hinshelwood (L-H) mechanism with FLPs present. This transition shifted the reaction mechanism from the E-R pathway on regular R-CeO2 to the L-H pathway on FLP-rich FR-CeO2, as corroborated by the experimental findings. The practical application of FLPs was realized by loading MoO3 onto FLP-rich FR-CeO2, leveraging the synergistic effects of acidic sites and FLPs. This study provides profound insights into how FLPs facilitate N-H/N-O bond activation in small molecules, such as NH3 and NO, offering a new paradigm for catalyst design based on catalytic mechanism research.

Catalytic functionalization of MCM-41 and MCM-48 with copper by modified template ion-exchange method for low-temperature NH3-SCR process: the role of complexing agents for copper dispersion

Mesoporous silicas of MCM-41 and MCM-48 types were synthesized and modified with copper by template ion-exchange (TIE) technique. A high dispersion of deposited copper species was managed by subsequent treatment of the samples with ammonia, urea, or acetonitrile solution. Copper-modified silicas were analysed in terms of their chemical composition (ICP-OES), ordering of porous structure (XRD), textural parameters (low-temperature N2 sorption), dispersion and form of introduced copper species (UV–vis-DR, XRD, H2-TPR, NH3-TPD). Mesoporous silicas modified with copper exhibited promising catalytic efficiency in selective catalytic reduction of NO with ammonia (NH3-SCR). The samples containing dispersed copper species (predominantly monomeric copper cations) presented enhanced catalytic activity compared to catalysts containing CuO aggregates. On the other hand, CuO aggregates showed greater catalytic activity in the side process of direct ammonia oxidation by oxygen present in the reaction mixture. It was demonstrated that TIE post-treatment of the samples with urea resulted in most effective improving dispersion of deposited copper and is less destructive for ordered porous structures of mesoporous silica comparing to treatment with ammonia solution.Graphical abstract

Selective Reduction of NO into N2 Catalyzed by the RhM2O3- Clusters (M = Ta, V, and Al): Importance of the Triatomic Lewis Acid-Base-Acid Mδ+-Rhδ--Mδ+ Site.

Catalytic NO reduction by CO into N2 and CO2 is imperative owing to the increasingly rigorous emission regulation. Identifying the nature of active sites that govern the reactivity and selectivity of NO reduction is pivotal to tailor catalysts, while it is extremely challenging because of the complexity of real-life systems. Guided by our newly discovered triatomic Lewis acid-base-acid (LABA, Ceδ+-Rhδ--Ceδ+) site that accounts for the selective reduction of NO into N2 catalyzed by the RhCe2O3- cluster in gas-phase experiments, the reactivity of the RhM2O3- (M = Ta, V, and Al) clusters in catalytic NO reduction by CO was explored. We determined theoretically that the LABA site still prevails to reduce NO to N2 mediated by RhTa2O3- and RhV2O3-, and the strong M-oxygen affinity was emphasized to construct the LABA site. An overall assessment highlights that RhV2O3- functions as a more promising catalyst because the well-fitting V-O bonding strength facilitates both elementary reactions of NO reduction and CO oxidation.

Calcium-Poison-Resistant Cu/YCeO2-TiO2 Catalyst for the Selective Catalytic Reduction of NO with CO and Naphthalene in the Presence of Oxygen.

The pollution from industrial processes based on biomass combustion is still an ongoing problem. In the present contribution, the selective catalytic reduction of NO with CO and naphthalene is carried out in the presence of 10% oxygen. The accumulation of alkaline and alkaline earth metals during biomass combustion is here simulated by the addition of calcium to a Cu-impregnated YCeO2-TiO2 support. The results show that a high dispersion of copper is obtained, which is resistant to the accumulation of calcium. Full conversion of CO and naphthalene is achieved above 200 °C, whereas NO conversions of 80, 90, and 87% are obtained for the catalysts with Ca loadings of 2.6, 5.2, and 13%, respectively, at 350 °C. It is proposed that the high activity of the catalysts is ascribed to the formation of Cu-Ox-Ce species and that the accumulation of Ca acts as a barrier to avoid copper sintering. It was found that different forms of carbonate and nitrite/nitrate species form during reaction, coexisting as adsorbed species during the SCR reaction. The selectivity to N2 was almost 100% in all cases, due to the small presence of NO2 in the reactor outlet (no N2O was detected in any conditions).

Theoretical study on borophene as metal-free catalyst for selective conversion of NO

The electrocatalytic reduction of NO (NORR) and selective catalytic reduction of NO by CO (CO-SCR) are the two most attractive approaches for selective conversion of NO. Herein, a bifunctional metal-free catalyst 2-Pmmn borophene is reported that is effective for both NORR and CO-SCR. NO can form NH3 and N2 through NORR and CO-SCR respectively. The results show that NO chemically adsorbed on the surface of borophene through the NO terminal can be electrocatalytically reduced to NH3. The optimal reaction path for NO to generate NH3 is through the protonation process of *NHO instead of *NOH. The rate-determining step is the hydrogenation of *NH2O to *NH2OH, and the free energy increases by 0.41 eV. At the same time, NO can also react with CO on the surface of borophene to form N2 and CO2. First, NO can form chemically adsorbed ONNO intermediate through NN coupling, then ONNO can be denitrified to form N2 and residual oxygen, and finally residual oxygen and CO can generate CO2 through the LH mechanism. The rate-determining step of the reaction is the NN coupling process of NO, and activation energy barrier is 1.27 eV. The present work provides theoretical insights for the effective conversion of NO.

Reaction mechanism of 1,2-dichlorobenzene oxidation over MnOX-CeO2 and the effect of simultaneous NO selective reduction

MnOX-CeO2 formulation has been studied for the catalytic oxidation of 1,2-dichlorobenzene. Among the samples with different Mn and Ce content, the most active was 85%mol Mn and 15%mol Ce, due to its better morphological properties and the synergy achieved between the two phases composing it (mixed oxide phase and segregated Mn oxide). Deactivation was monitored at low temperature and a transient change in the oxidative capability at high temperature because of active sites with different oxidative capability. Based on in-situ FTIR results, oxidation reaction pathway is proposed. Active sites with different oxidative capability causes some reaction steps to occur faster than others as a function of temperature, which modifies the distribution of intermediate species. In the reduction of NO, its adsorption leads to nitrate species contributing to a faster re-oxidation of active sites and the presence of NH3 promotes the removal of adsorbed Cl.

Influence of Catalyst Structural Remodelling on The Performance of NH3‐SCR Reactions: A Mini Review

AbstractWith the continuous expansion of industrial activities worldwide, industrial emissions of nitrogen oxides (NOx) pose a serious threat to both ecosystems and human health. Ammonia selective catalytic reduction of NO (NH3‐SCR) technology has emerged as the most effective means to reduce NOx emissions, and the development of catalysts is crucial for the successful implementation of this technology. In gas‐solid multiphase catalytic systems, the performance limitations of conventional catalysts can be effectively overcome by meticulously designing the nanostructures of the catalysts to achieve improved catalytic efficiency. In this review, the unique structural features of core‐shell structures, layered double hydroxide (LDHs), hollow nano spheres and nanotubes, along with their preparation processes, are systematically examined, beginning with the effects of structural changes on catalytic performance. Based on this review, the impact of structural modifications on the catalytic efficiency of the catalysts for the NH3‐SCR reaction and their anti‐poisoning performance were investigated. Ultimately, the influence of catalyst structural changes on the future design of catalysts is anticipated. This provides a robust scientific foundation for the conception of higher performance catalysts, paving the way for technological innovation and advancements in NOx emission reduction practices.

Crystal facet-modulated CuO/CeO2 catalysts for highly selective catalytic reduction of NO by CO

The selective catalytic reduction of NO by CO (CO-SCR) has a significant impact on the sustainable development of the environment. It is crucial to design efficient low-temperature catalysts to overcome the challenges associated with the industrialization of CO-SCR. Herein, we present CuO/CeO2 catalysts with three different exposed crystal facets of CeO2 ((100), (110), and (111)) synthesized by a hydrothermal method followed by impregnation for CO-SCR. Among the three CuO/CeO2 catalysts, CuO loaded on rod-like CeO2 catalyst (CuO/CeO2-R) with exposed CeO2 (110) crystal facet showed the highest catalytic performance in the CO-SCR reaction, achieving complete conversion of NO and 99 % conversion of CO at 250 °C. A series of characterizations reveal that the interaction between CuO and CeO2 with different crystal facets is crucial to regulating the interface structure and oxygen vacancy (Ov) content. During in situ DRIFTS analysis, it was found that the primary adsorption sites for CO are the Cu+-Ov-Ce3+ interface sites. The CuO/CeO2-R catalyst had the highest number of these sites. Furthermore, CuO/CeO2-R contained many Ov sites, which promote NO dissociation and enhance its catalytic efficiency. This study offers insights into CO-SCR catalysis and the design of efficient catalysts by manipulating the exposed crystal facets of support to enhance CO-SCR activity.

Tailoring the proceeding of the NH3-SCO and NH3-SCR reactions over FeOx catalysts by modifying with NbOx

This study presents the synthesis of a series of FeOx-NbOx mixed oxide catalysts for the selective catalytic reduction (SCR) of NO by NH3. By meticulously controlling the Fe/Nb molar ratio, we have rationally tailored the proceeding of the main reaction of NO reduction and the side reaction of NH3 oxidation to NOx. The incorporation of NbOx introduced a significant number of acid sites, which enhanced the adsorption of NH3 on the catalyst surface, particularly at elevated temperatures. Additionally, the oxidative capacity of the catalyst was moderated by the addition of NbOx, hindering the over-oxidation of NH3 to NO or NO2, thus preserving more NH3 to act as a reductant for NO reduction. Consequently, the NbOx-enriched samples exhibited improved deNOx performance. However, an excessive amount of NbOx led to a notably weakened oxidative ability, which negatively impacted the activation of reactants and resulted in decreased NO conversion at lower temperatures. The optimized catalyst presented >80% NO conversion and >95% N2 selectivity within a temperature range of 250-400 °C. These findings offer valuable insights for the development of new catalysts with an extended operational temperature window.

Competitive Coordination-Induced Formation of Fe-O-Ce Structure in Beta Zeolite to Enhance Moderate-to-High Temperature Selective Catalytic Reduction of NOx with NH3

Competitive Coordination-Induced Formation of Fe-O-Ce Structure in Beta Zeolite to Enhance Moderate-to-High Temperature Selective Catalytic Reduction of NO_x with NH₃

A comparative study on the activity and adsorption of bimetallic MOF-derived Me-Cu/TiO2 (Me=Fe, Co, Ni) for low-temperature selective catalytic reduction of NO by CO

The catalytic activity, adsorption and synergistic effect over bimetallic MOF derivatives with different dopant metal were systemically compared. A series of Me-Cu/TiO2 (Me=Fe, Co, Ni) derived from bimetallic Me/Cu-BTC MOFs were fabricated via a sacrificing template method and applied for CO-SCR. Among the bimetallic catalysts, Fe-Cu/TiO2 presented the best low-temperature of nearly 100% NO conversion from 150 °C to 350 °C with good NO selectivity. For Me-Cu/TiO2 (Me=Fe, Co, Ni), the productions of crucial Cu+ intermediate were determined to be 51.17%, 50.45%,49.4% by XPS, which were consistent with their catalytic activity results. The superior redox behavior of Fe-Cu/TiO2 can be attributed to its higher Oβ / (Oα +Oβ) ratio of 32.90% and the higher Me2+ / Me3+ ratio of 1.79. In situ infrared analysis revealed that the Fe doping not only improved the formation and stability of nitrate species, but also enhanced the quantity of Lewis acid sites by forming Cu+-CO. Furthermore, a possible low-temperature Langmuir-Hinshelwood (L-H) mechanism over Me-Cu /TiO2 (Me=Fe, Co, Ni) was proposed.

The nature of synergy effects between VOx and TiO2 in low temperature NH3-SCR reaction

In this study, we investigated the low-temperature activity of selective catalytic reduction of NO by NH3 (NH3-SCR) with the effect of different vanadium oxide (VOx) loading in the VOx/TiO2 catalyst. By calculating the specific rate of activity, it was found that the fastest specific rate of activity was achieved when the loading of V2O5 reached 10 wt%, and the nitrogen oxides (NOx) conversion of the above optimal catalyst reached over 80 % in the temperature range of 180–465 °C. The effects of preparation method, calcination temperature, and support on the aggregation state of the vanadium species and the SCR activity of the catalyst were also investigated on the basis of the ideal ratio. Combining the activity and characterization results, it is found that the impregnation method and calcination at 410 °C are more favorable for the moderate dispersion of the vanadium species on the TiO2 surface, producing more highly reactive polymorphic VOx, which is more readily dispersed on the surface of the TiO2 support and interacts more strongly with the TiO2. The interaction between the two can promote the transfer of electrons, resulting in the production of more active O species and low-valent vanadium. The aforementioned species can enhance the catalyst's low- temperature denitrification activity.

3DOM Cu-Ce binary composite oxides for selective catalytic reduction of NO with CO

A series of 3DOM Cu-Ce catalysts for CO-SCR were prepared by a colloidal template method. Among these catalysts, 3DOM Cu1Ce1 catalyst exhibited the best CO-SCR performance (about 100 %) in the widest temperature window (450–700 ℃). Notably, this catalyst still showed excellent stability in the presence of H2O and SO2. It was found that the appropriate molar ratio of Cu and Ce could optimize the pores of Cu-Ce catalysts, and increase their specific surface area. This could contribute to the contact between reaction gas and active sites. The stronger synergistic effect between Cu and Ce could be conducive to the increase of chemisorbed oxygen, and then enhance the redox properties. These might be the key factors for the improvement of 3DOM Cu1Ce1 catalytic activity. In addition, the molar ratio of Cu to Ce had a significant influence on the adsorption of CO, which might cause the difference in the consumption of active oxygen and the formation of oxygen vacancies, thus affecting the CO-SCR performance of 3DOM Cu-Ce catalysts.

A High-Performance Mn/TiO2 Catalyst with a High Solid Content for Selective Catalytic Reduction of NO at Low-Temperatures.

Mn/TiO2 catalysts with varying solid contents were innovatively prepared by the sol-gel method and were used for selective catalytic reduction of NO at low temperatures using NH3 (NH3-SCR) as the reducing agent. Surprisingly, it was found that as the solid content of the sol increased, the catalytic activity of the developed Mn/TiO2 catalyst gradually increased, showing excellent catalytic performance. Notably, the Mn/TiO2 (50%) catalyst demonstrates outstanding denitration performance, achieving a 96% NO conversion rate at 100 °C under a volume hourly space velocity (VHSV) of 24,000 h-1, while maintaining high N2 selectivity and stability. It was discovered that as the solid content increased, the catalyst's specific surface area (SSA), surface Mn4+ concentration, chemisorbed oxygen, chemisorption of NH3, and catalytic reducibility all improved, thereby enhancing the catalytic efficiency of NH3-SCR in degrading NO. Moreover, NH3 at the Lewis acidic sites and NH4+ at the Bronsted acidic sites of the catalyst were capable of reacting with NO. Conversely, NO and NO2 adsorbed on the catalyst, along with bidentate and monodentate nitrates, were unable to react with NH3 at low temperatures. Consequently, the developed catalyst's low-temperature catalytic reaction mechanism aligns with the E-R mechanism.

Zr‐doped β‐MnO2 for Low‐temperature Selective Catalytic Reduction of NO by NH3: Preparation, Characterization and Performance

Nanowire‐MnO2 and a series of Zr‐doped MnO2 catalysts with different Zr/Mn molar ratios were successively prepared by hydrothermal and impregnation methods. The Zr‐doped MnO2 catalyst with Zr/Mn molar ratio of 0.04 and calcination temperature of 400°C, proved to be the optimal that possessed the highest low‐temperature denitration efficiency. It showed the NO conversion of ~92% and N2 selectivity of ~80% at 150°C. Characterization results demonstrated that the main active phase of the catalyst was β‐MnO2, Zr atoms interacted with Mn atoms, and Zr doping increased the structural defects, oxygen vacancies and weak acid sites, which effectively enhanced the low‐temperature denitration activity and N2 selectivity of β‐MnO2, also improved the water and sulphur resistance to some extent.

Insight into the Alkali Resistance Mechanism of CoMnHPMo Catalyst for NH3 Selective Catalytic Reduction of NO

Insight into the Alkali Resistance Mechanism of CoMnHPMo Catalyst for NH3 Selective Catalytic Reduction of NO

Unveiling the Promoting Effect of Niobium on Cu-KIT-6 Nanoporous Catalysts for the Selective Catalytic Reduction of NOx with High Resistance to Ammonium Bisulfate Poisoning

Unveiling the Promoting Effect of Niobium on Cu-KIT-6 Nanoporous Catalysts for the Selective Catalytic Reduction of NO_<i>x</i> with High Resistance to Ammonium Bisulfate Poisoning