NH3-SCR Activity Research Articles

MFNT catalysts enhanced by the radio-frequency cold plasma preparation with wide temperature NH3-SCR activity: A combined experimental and DFT study

MFNT catalysts enhanced by the radio-frequency cold plasma preparation with wide temperature NH3-SCR activity: A combined experimental and DFT study

Iron-containing tailings catalyst modified by Mn for SCR of NO with NH3 at low temperature: Study of carrier treatment and calcination temperature

The development of cost-effective low temperature selective catalytic reduction (NH3-SCR) catalysts for nitrogen oxides is a major challenge. To this end, a low-cost rare earth tailings catalyst modified by Mn was successfully synthesized by the impregnation method using rare earth tailings as raw materials. The iron-containing tailings catalyst modified by Mn exhibited high NH3 SCR activity with high N2 selectivity at low temperature range (150–200 ℃). The H2SO4 modification resulted in the weakening of the Fe-O bonds on the catalyst, thereby producing the active component, iron sulfate salt. The coordination occupancy of Brønsted acid sites is increased, thereby facilitating electron transfer and redox cycling of SCR. Furthermore, by modulating the calcination temperature, the synergistic effect of Fe and Mn is augmented, resulting in the generation of a substantial number of Mn-Fe composite oxides, which is conducive to the enhancement of NH3 adsorption and activation on the catalyst. The formation of additional Brønsted acids was identified as a primary factor influencing the enhancement of N2 selectivity in the catalyst. The catalytic process followed both the E-R and L-H mechanisms on the catalyst. The Iron-containing tailings catalyst modified by Mn represents a cost-effective and resource-conserving alternative. The provision of high denitrification efficiency, coupled with the capacity to address the issue of low N2 selectivity, offers a sustainable and low-carbon solution for the industrial reduction of NOx.

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.

Redox ability dependent SO2 tolerance for low-temperature NH3-SCR of MnO2@MeOX (Me = Ti, Ce, Cu) catalysts

Elucidating the deposition behavior of sulfates on catalysts of low-temperature ammonia-assisted selective catalytic reduction (NH3-SCR) helps improve their SO2 tolerance and applicability. Therefore, this study aimed to explore the impact of the redox properties of low-temperature NH3-SCR catalysts on their catalytic activity and SO2 tolerance. Oxidizing Mn@Ce, neutral Mn@Ti, and reducing Mn@Cu catalysts were prepared, and their NH3-SCR activities and SO2 tolerances were evaluated. Subsequently, the characteristics of the catalyst poisoned by SO2 over time and the deposition behavior of sulfates on the catalyst were analyzed to determine the reaction pathway between SO2 and the NH3-SCR catalyst. The SO2 tolerance of the catalysts decreased in the following order: oxidizing Mn@Ce > neutral Mn@Ti > reducing Mn@Cu. Combined with the characteristic results, we found that the amount of shell-metal sulfate increased with SO2 poisoning time, indicating it was the dominant factor in catalyst deactivation. Specifically, the amount of shell-metal sulfate followed the order Mn@Cu-S12 (31.4 µmol) > Mn@Ti-S12 (24.9 µmol) > Mn@Ce-S12 (9.5 µmol). Additionally, the increasing SO42− ratio from Mn@Me-S4 to Mn@Me-S12 followed the order Mn@Cu (6.7%) > Mn@Ti (4.4%) > Mn@Ce (1.7%), implying that the reduction of Mn@Cu favors chemical SO2 poisoning pathway and facilitates the deposition of copper sulfate. In contrast, Mn@Ce with its strong oxidation properties, is beneficial for inhibiting the chemical poisoning pathway of SO2 and alleviating the deposition of cerium sulfate. Consequently, this study demonstrates that the redox ability of catalyst regulates the SO2 poisoning pathways, aiding the development of low-temperature NH3-SCR catalysts with high SO2 tolerance.

A superior Mn5CeTi5Ox catalysts for synergistic catalytic removal of chlorobenzene and NOx: Performance enhancement and mechanism studies

The transition metal titanium-doped Mn-Ce-Ox catalysts catalyst were employed to achieve synergetic removal of NO and CB at 180–220 °C. The Mn5CeTi5Ox catalyst with a molar ratio of Mn/Ce/Ti = 5:1:5 exhibits excellent activity, and the NOx and CB removal efficiencies reach 96 % and 89 % at 160–220 °C, respectively. The selectivity for N2 and CO2 are 93 % and 78 %, respectively. The N2-physisorption, NH3-TPD, H2-TPR and XPS results show that Ti doping makes the catalyst possess a mesoporous structure, suitable particle sizes, and excellent redox and Lewis site properties. All of these features contribute to the observed high NO and CB removal efficiency. The synergetic removal of CB and NO over Mn5CeTi5Ox results from the synergistic catalysis between the redox and the solid acid. On the one hand, in the synergistic removal process, CB and NH3 are competitively adsorbed on the catalyst surface, resulting in a decrease in the NH3-SCR activity. On the other hand, the removal of NO and CB has a synergetic effect. The byproduct NO2 produced by the NH3-SCR reaction promotes the oxidation of CB, which is beneficial for CB removal. Moreover, the consumption of NO2 indirectly promotes the NH3-SCR reaction, which partially compensates for the decrease in the NO removal efficiency caused by competitive adsorption between NH3 and CB. Ti doping promotes the participation of the SCR byproduct NO2 in the CBCO reaction and promotes the formation of maleic acid, an intermediate product of CB oxidation. In summary, the Mn5CeTi5Ox catalyst exhibits good activity for the synergistic removal of NOx and CB and is a promising candidate for the effective and economical removal of NO and CB during waste incineration.

Catalytic hydrolysis of HCN on the coupled sites of Lewis acid and surface hydroxyl of Fe-BEA catalyst leading to the presence of HCHO positively affects NH3-SCR activity

Formaldehyde (HCHO) in the exhaust gas of diesel or natural gas engines is usually considered to be a serious hindrance to the NH3-SCR process. However, the promotion effects of HCHO on the NH3-SCR activity and its temperature dependence were discovered in this work for the first time. For the Fe-BEA catalysts, below 300 °C, a notable decrease in activity observed, up to 40 % at 300 °C, and HCN selectivity reached 35 %. However, with the increase in temperature, a significant promotion effect was produced. Not only for the increase of NOx conversion, but also the diminish of the by-product HCN. The disparity in the number and activity of Lewis acid sites (Fe3+) and surface hydroxyls was the primary reason why Fe-BEA catalysts with varying Fe contents exhibited different influence. The presence of HCHO resulted in a modification of the oxidation pathway of NH3 at high temperatures. Surface hydroxyls facilitated the hydrolysis reaction of the as-formed HCN to produce NH3, with NH3 on Lewis acid sites subsequently re-participating in the SCR process. This enhanced the NOx conversion and the selectivity of carbon-containing products. Furthermore, it was demonstrated that HCHO completely converted to hexamethylenetetramine (HMTA) by reacting with NH3 prior to passing through the SCR catalyst. The thermal decomposition of HMTA on the catalyst was identified as a crucial step for the subsequent generation of HCN, and the detailed mechanism of how to effectively inhibit the decrease of de-NOx activity and the formation of HCN in the HCHO-containing NH3-SCR conditions was elucidated. This study paves the way for the development of effective SCR catalysts for the synergistic control of HCN from mobile and stationary sources.

Recent Progress on Low-Temperature Selective Catalytic Reduction of NOx with Ammonia.

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) has been implemented in response to the regulation of NOx emissions from stationary and mobile sources above 300 °C. However, the development of NH3-SCR catalysts active at low temperatures below 200 °C is still needed to improve the energy efficiency and to cope with various fuels. In this review article, recent reports on low-temperature NH3-SCR catalysts are systematically summarized. The redox property as well as the surface acidity are two main factors that affect the catalytic activity. The strong redox property is beneficial for the low-temperature NH3-SCR activity but is responsible for N2O formation. The multiple electron transfer system is more plausible for controlling redox properties. H2O and SOx, which are often found with NOx in flue gas, have a detrimental effect on NH3-SCR activity, especially at low temperatures. The competitive adsorption of H2O can be minimized by enhancing the hydrophobic property of the catalyst. Various strategies to improve the resistance to SOx poisoning are also discussed.

A key and facile strategy for improving sulfur resistance of Ce-based SCR catalysts: Enhancing the dispersion of Fe species

Enhancing the sulfur resistance of ceria-based catalysts for the selective catalytic reduction of NOx with NH3 (NH3-SCR) has always been a major challenge. Herein, the NH3-SCR activity and sulfur tolerance of CeFeOx loaded with WO3 were improved by adjusting the precipitation sequence for Ce and Fe. W/Fe(f)-Ce catalysts (namely, first precipitate Fe and then Ce) presented the best sulfur tolerance due to their more uniformly dispersed Fe species, thereby giving rise to superior physicochemical properties, including smaller particle size, larger surface area, and stronger low-temperature redox capacity. Because of these factors, the contact between the catalyst and reactants was more thorough, the CeO2 sites were better protected, and the low-temperature reactivity for consuming NH4+ from ammonium bisulfate (ABS) was higher. This study suggests that doping with Fe can enhance the SO2 tolerance of Ce-based catalysts, and that the dispersion of Fe species is crucial, which provides guidance for preparing efficient and sulfur-tolerant NH3-SCR catalysts.

Synergistic effect of Cerium and Niobium enhances the low-temperature NH3-SCR activity of Fe-containing solid waste

To develop environment-friendly low temperature NH3-SCR catalyst and realize the resource utilization of industrial solid waste, this research prepared a series NH3-SCR catalysts by using Fe-containing solid waste loading CeO2 and Nb2O5. It was found that the Ce2Nb1/Fe-Si catalyst exhibited the highest NH3-SCR activity, reaching about 93 % NOx conversion and 99 % N2 selectivity at 300 °C due to the synergistic effect between Ce and Nb species. The characterization results showed that the interaction between Ce and Nb was stronger than that between Nb and Fe species, promoting the NH3 adsorption and redox cycle of Ce4+ + Nb4+ ↔ Ce3+ + Nb5+, Ce3+ + Fe3+ ↔ Fe2+ + Ce4+, which increased the contents of Fe2+ and surface adsorbed oxygen. Moreover, the co-existence of Ce and Nb species would increase the NOx adsorption ability and promote the oxidation of NO to NO2, which was beneficial for “Fast SCR” reaction. Finally, the introduction of CeO2 and Nb2O5 did not changed the reaction mechanism, followed by a Langmuir-Hinshelwood mechanism.

Uncovering the roles of Ce and Mn for NH3-SCR over Mn-Ce/IM-5 with extraordinarily superior wide-temperature performance

The development of wide-temperature catalyst with glorious activity and the resistance of SO2 for NH3-SCR reaction are of great importance. However, the preparation of wide-temperature catalyst with outstanding NH3-SCR performance and the resistance of SO2 is still challenging. In this work, Mn and Ce modified IM-5 molecular sieve catalyst (MC-5) was synthesized by incipient wetness impregnation method and the relationship of structure-activity was studied. MC-5 exhibited superior NOx conversion and it was more than 87% in the temperature range of 150-500 °C and ca. 100% at 225 °C due to the high concentration of Mn4+ and chemisorbed oxygen, the excellent redox ability and the synergistic effect of Mn and Ce. Meanwhile, the redox cycle of Mn4++Ce3+↔Mn3++Ce4+ promoted the electron transfer. 27Al NMR spectra and NH3-TPD confirmed Mn ion was mainly located on the bridge hydroxyl group (Brønsted acid sites). The localization energies ΔE of (Al, Mn) for IM-5 was determined by theoretical calculation and the reaction mechanism of M-5 catalyst on Brønsted acid sites was proposed. The reaction mechanism of M-5 catalyst on Brønsted acid sites followed L-H mechanism. And interestingly enough, the NH3-SCR activity of MC-5 significantly increased in the presence of SO2 at 200 °C. One of the reasons was that Ce binded more easily to SO42- and thus protected the active site Mn from sulfation. The other reason was the generation of more acid sites.

Improving activity and SO2 resistance for NH3-SCR of NOx over V-Ce-Ox catalysts by phosphorylation treatment to regulate both oxidation property and acid sites

SO2 poisoning remains an unresolved issue for NH3-SCR of NOx catalysts, particularly under low temperature conditions. In this study, a phosphorylation approach was used in preparation of V-Ce-Ox (VC) catalysts by regulating both oxidation property and acid sites to improve SO2 tolerance at low temperature in the NH3-SCR process. Among all the as-prepared catalysts, 2.4 wt%P-VC exhibited the widest temperature window (>90 % NO conversion, 240 ∼ 420 °C) and good resistance to SO2. A variety of characterization approaches were used to investigate the effects of the phosphorylation approach on the catalytic activity and microstructure of V-Ce-Ox catalyst. According to the characterization results, the improvement role of 2.4 wt%P-VC is connected to the balance between acidity and oxidation ability. This originates from the largest amount of Brønsted acid sites to adsorb NH3 and surface adsorbed oxygen species. The results also showed that different H3PO4 loading contents can promote or inhibit the NH3-SCR reaction, it is possible that this is because a lower H3PO4 content (≤2.4 wt%) can result in more Brønsted acid sites (P-O-NH4+), whereas the higher H3PO4 content (≥4.2 wt%) can cause a greater reduction in Lewis acid sites and the total surface acidity. Furthermore, the in-situ DRIFTS and SO2-TPD results reveal that the oxidation of SO2 is weakened by the formation of amorphous VOPO4 and CePO4 on xwt%P-VC catalysts, which reduces the formation of VOSO4 and Ce2(SO4)3 by reducing the oxidation of SO2 on the catalyst. This work eventually achieves the highest catalytic performance and the superior SO2 resistance of the V2O5-CeO2-based catalyst by presenting a viable technique for changing the acid and redox sites on the VC catalyst.

Enhanced NH3-SCR performance using hydrothermally synthesized cell-like MnOx loaded rectorite catalysts

This study evaluates the catalytic performance of hydrothermally synthesized MnOx loaded rectorite catalysts (MnOx/REC-HP) in the selective catalytic reduction of NH3-SCR. Comparative analyses were conducted with catalysts synthesized via pore-volume impregnation, coprecipitation, and in-situ deposition methods, all yielding comparable manganese mass fractions. Results reveal that catalysts derived from hydrothermal synthesis outperform others in NH3-SCR activity across the 100–300 °C temperature spectrum. MnOx/REC-HP exhibits a distinctive cell-like structure of supported MnOx atop the rectorite substrate, diverging from the particulate patterns observed in other synthesis methods. Hydrothermal synthesis of metal precursors into their respective metal oxides effectively meets the fundamental requirements for SCR composites. This method enables the production of composites that are uniformly attached to nanostructures with high purity and controlled crystallinity. The rectorite surface is embellished with a meticulously arranged honeycomb structure of MnOx, augmenting the BET surface area, intensifying redox capability, enhancing NH3 and NOx adsorption, and amplifying the Mn4+ molar ratio on the catalyst surface. Consequently, MnOx/REC-HP showcases remarkable low-temperature NH3-SCR performance.

Interface Induced by Hydrothermal Aging Boosts the Low-Temperature Activity of Cu-SSZ-13 for Selective Catalytic Reduction of NOx.

Hitherto, sulfur poisoning and hydrothermal aging have still been the challenges faced in practical applications of the Cu-SSZ-13 catalyst for the selective catalytic reduction (SCR) of NOx from diesel engine exhaust. Here, we elaborately design and conduct an in-depth investigation of the synthetic effects of hydrothermal aging and SO2 poisoning on pristine Cu-SSZ-13 and Cu-SSZ-13@Ce0.75Zr0.25O2 core@shell structure catalysts (Cu@CZ). It has been discovered that Cu@CZ susceptible to 750 °C with 5 vol % H2O followed by 200 ppm SO2 with 5 vol % H2O (Cu@CZ-A-S) could still maintain nearly 100% NOx conversion across the significantly wider temperature region of 200-425 °C, which is remarkably broader than that of the Cu-SSZ-13-A-S (300-400 °C) counterpart. The experimental results show that the hydrothermal aging process results in the migration of highly active Cu species within the cage of Cu-SSZ-13 to the CZ surface, forming CuO/CZ with abundant interfaces, which significantly enhances the adsorption and subsequent activation of NO, leading to the generation of reactive N2O3 and HONO intermediates. Moreover, density functional theory (DFT) calculations reveal that the H of the HONO* species can function as Brønsted acid sites, effectively adsorbing NH3 to generate the active NH4NO2* intermediate, which readily decomposes into N2 and H2O. Furthermore, this pathway is the rate-determining step with an energy barrier of 0.93 eV, notably lower than that of the "standard SCR" pathway (1.42 eV). Therefore, the formation of the new CuO/CZ interface profoundly boosts the low-temperature NH3-SCR activity and improves the coresistance of the Cu@CZ catalyst to sulfur poisoning and hydrothermal aging.

Essential features of weak current for excellent enhancement of NOx reduction over monoatomic V-based catalyst

Human society is facing increasingly serious problems of environmental pollution and energy shortage, and up to now, achieving high NH3-SCR activity at ultra-low temperatures (<150 °C) remains challenging for the V-based catalysts with V content below 2%. In this study, the monoatomic V-based catalyst under the weak current-assisted strategy can completely convert NOx into N2 at ultra-low temperature with V content of 1.36%, which shows the preeminent turnover frequencies (TOF145 °C = 1.97×10−3 s−1). The improvement of catalytic performance is mainly attributed to the enhancement catalysis of weak current (ECWC) rather than electric field, which significantly reduce the energy consumption of the catalytic system by more than 90%. The further mechanism research for the ECWC based on a series of weak current-assisted characterization means and DFT calculations confirms that migrated electrons mainly concentrate around the V single atoms and increase the proportion of antibonding orbitals, which make the V-O chemical bond weaker (electron scissors effect) and thus accelerate oxygen circulation. The novel current-assisted catalysis in the present work can potentially apply to other environmental and energy fields.

Synthesis of Cu-SSZ-13 with different Si/Al ratios by zeolite Y conversion and its NH3-SCR activity and hydrothermal stability

Interzeolite conversion is a convenient and low-cost method to get Cu-SSZ-13 zeolite for ammonia selective catalytic reduction (NH3-SCR) of NO, but it is usually difficult to regulate the Si/Al ratio of the SSZ-13 in a wide range by interzeolite conversion. In this work, Cu-SSZ-13 catalysts with Si/Al ratios of 5, 10 and 20 were accurately synthesized by zeolite Y conversion with additional Si source. Specifically, the physical properties of Cu-SSZ-13 were characterized by XRD, SEM, ICP-AES and BET. The Al distribution and Cu2+ locations in Cu-SSZ-13 were investigated by 29Si and 27Al MAS NMR, DR UV–vis, H2-TPR and EPR. The results show that Cu-SSZ-13 with Si/Al ratio of 10 shows the considerable NH3-SCR activity and hydrothermal stability simultaneously. Moreover, different types of relationships between the reaction rate constant k and the content of α-Cu2+, β-Cu2+ species were found, indicating the diverse catalytic properties of the two active copper sites.

Facile one-pot synthesis of Cu-SSZ-39 catalysts with excellent catalytic performance in NH3-SCR reaction

Nitrogen oxide (NOx) emission from diesel vehicles has caused severe pollution to the atmospheric environment. Selective catalytic reduction of NOx with NH3 (NH3-SCR) technology has been widely used to remove NOx from diesel vehicle exhaust. Cu-SSZ-39 zeolites have exhibited excellent NH3-SCR catalytic activity and hydrothermal stability, giving them promise for practical application. A facile one-pot synthesis method to prepare Cu-SSZ-39 zeolites was developed, with complete crystallization achieved in 24 h. Moreover, the Cu content could be controlled by adjusting the amount of Cu-TEPA added without the need for complex post-treatment processes. Cu-SSZ-39 catalysts synthesized through the one-pot synthesis method exhibited excellent NH3-SCR activity and hydrothermal stability. The one-pot-synthesized Cu-SSZ-39 catalysts contained in situ generated CuO species, which can act as sacrificial sites to protect Cu2+ under sulfidation conditions. Considering their efficient synthesis process, excellent NH3-SCR activity, hydrothermal stability and sulfur resistance, one-pot-synthesized Cu-SSZ-39 catalysts hold great promise for future applications.

Unraveling NH3-SCR performance and the role of Fe and Cu over the supported TNU-9 catalysts

Enlarging reaction temperature and raising the resistance of SO2 and H2O for NH3-SCR is still a challenge. Meanwhile, the role of Fe and Cu over the supported TNU-9 catalysts is not so clear either for NH3-SCR reaction. Aiming at the above-mentioned problems, Fe and Cu bimetallic co-doped TNU-9 catalysts were prepared by solution ion-exchanged method and the effect of the exchanged order of Fe and Cu on NH3-SCR activity was conducted. What's more, the resistance of SO2/H2O and DFT calculations are also investigated. As a comparison, the activity Fe/T9 and Cu/T9 were examined. The reason that the different activity of Fe and Cu was also explained through DFT calculations. The glorious catalyst, Cu-2Fe/T9 synthesized via exchanging Cu first for twice and then Fe for once, demonstrated outstanding catalytic performance with the NOx conversion ca. 100 % at 225–450 °C and exceeding 80 % in the temperature range of 200–500 °C. It was attributed to minimizing the oxidation of Fe2+. And the most amazing thing was that Cu-2Fe/T9 had better the resistance of SO2 and worse the resistance of H2O than Cu/T9 at 225 °C. It was because that SO2 was more easily adsorbed at the Fe site, resulting in Fe protecting the active site Cu and the sulfation also provided more strong acidic sites, which were confirmed by DFT calculations and NH3-TPD. In addition, monomeric iron was more susceptible to water and the migration of ion-exchanged iron aggregated to form metal oxide clusters.

Promotional effects of Fe2O3 additive on the NH3-SCR activity and alkali resistance of MoTiOx catalysts

Catalyst is the core of selective catalytic reduction (SCR) technology, and an ideal SCR technology requires that the catalyst should have high activity and resistance to alkali metal poisoning. Therefore, in this paper, the influence of Na poisoning on the catalytic performance of Fe2O3-MoTiOx catalysts in the selective catalytic reduction (SCR) reaction was investigated, and the anti-alkali mechanism was revealed. The Fe2O3-MoTiOx catalyst demonstrated the NO conversion was 90 % at a broad temperature range of 225-400 °C when the Fe2O3 loading ratio was 20 wt%. Besides, 80 % NO conversion rate of the Fe2O3-MoTiOx catalyst at 320 °C was still achieved even at a Na+ concentration of 300 μmol g-1. The phenomenon was attributed to the interaction between Fe and Ti species, which could generate more Lewis acid centers and facilitate the adsorption activation of NH3. During the Na poisoning process, Na ions predominantly adsorbed on the Fe2O3, effectively safeguarding the MoTiOx catalyst. Additionally, although the Brønsted acid sites were deteriorated because of Na poisoning, the abundant Lewis acid sites were preserved for NHx adsorption. Therefore, the Na-poisoned Fe2O3-MoTiOx catalyst maintained efficient NH3 adsorption and NO activation. The work provided theoretical guidance and data support for the development of novel alkali-resistant SCR catalysts.

Tuning the morphology of CoMn2O4 for efficient alkali-resistant catalytic reduction of NOx

To improve the NH3-SCR catalytic performance of CoMn2O4, we altered their geometric morphological and structural characteristics, developing the hollow spherical and hollow cubic morphologies of CoMn2O4 by employing hard-templating and self-templated strategy. The influence of catalyst morphology on NH3-SCR activities and alkali metal resistance was discussed specifically, and the physicochemical properties of fresh and used catalysts poisoned by alkali metals were investigated by XRD, FTIR, N2 physisorption, H2-TPR, NH3-TPD, XPS and in situ DRIFTS. Interestingly, the CoMn2O4 hollow spherical (CoMn2O4-S) outperformed hollow cubic (CoMn2O4-C) and the reference solid sphere with much better low-temperature activity, the widened temperature window and higher poisoning-resistant ability. Loss of acid sites, drop of active oxygen species and reducibility accounted for the poor low-temperature activity of alkali poisoned catalysts. CoMn2O4-S catalysts with abundant acid sites and hollow configuration can decelerate the process of alkali metal poisoning. With the aid of in situ DRIFTS investigation, it was confirmed that the SCR reaction over fresh or poisoned CoMn2O4 catalysts follows an Eley-Rideal (E-R) mechanism.

Modulating the Activity and SO2 Resistance of α-Fe2O3 Catalysts for NH3-SCR of NOx via Crystal Facet Engineering.

The development of Fe-based catalysts for the selective catalytic reduction of NOx by NH3 (NH3-SCR of NOx) has garnered significant attention due to their exceptional SO2 resistance. However, the influence of different sulfur-containing species (e.g., ferric sulfates and ammonium sulfates) on the NH3-SCR activity of Fe-based catalysts as well as its dependence on exposed crystal facets of Fe2O3 has not been revealed. This work disclosed that nanorod-like α-Fe2O3 (Fe2O3-NR) predominantly exposing (110) facet performed better than nanosheet-like α-Fe2O3 (Fe2O3-NS) predominantly exposing (001) facet in NH3-SCR reaction, due to the advantages of Fe2O3-NR in redox properties and surface acidity. Furthermore, the results of the SO2/H2O resistance test at a critical temperature of 250 °C, catalytic performance evaluations on Fe2O3-NR and Fe2O3-NS sulfated by SO2 + O2 or deposited with NH4HSO4 (ABS), and systematic characterization revealed that the reactivity of ammonium sulfates on Fe2O3 catalysts to NO(+O2) contributed to their improved catalytic performance, while ferric sulfates showed enhancing and inhibiting effects on NH3-SCR activity on Fe2O3-NR and Fe2O3-NS, respectively; despite this, Fe2O3-NR showed higher affinity for SO2 + O2. This work set a milestone in understanding the NH3-SCR reaction on Fe2O3 catalysts in the presence of SO2 from the aspect of crystal facet engineering.